TRENDS IN FOOD MICROBIOLOGY
Hin-chung Wong
Department of Microbiology
Soochow University
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1. HISTORY OF FOOD MICROBIOLOGY
2. CONVENTIONAL SYLLABUS OF FOOD MICROBIOLOGY.
3. FOOD MICROBIOLOGY IN THE CHANGING WORLD
3.1. International Trade
3.2. Consumers Trends and Communication
3.3. Development of New Technology
3.4. Development of New Ingredients
3.5. Protection food supply against Food Terrorism Event
3.6. Green movement
4. EMERGING OF NEW FOOD-BORNE PATHOGENIC BACTERIA
5. ANTIMICROBIAL RESISTANCE
6. TRENDS IN RESEARCH ABOUT FOOD MICROBIOLOGY
7. TRENDS IN FOOD MICROBIOLOGY EDUCATION
8. REFERENCES
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1. HISTORY OF FOOD MICROBIOLOGY
In the 1930s, microbiologists were mainly concerned with food preservation
and spoilage. It means the study of natural flora of foods and the spoilage
organisms, and killing of microorganisms by various kinds of food
preservations, e.g. canning, low temperature, low available water, low pH, or
inhibitory chemicals, etc.
In the U.S., Foodborne diseases were little heard in the 1940s. However, the
only conventional food-poisoning bacteria mentioned in those days were
Clostridium botulinum and Staphylococcus aureus, and Salmonella spp.
Botulism was known quite clearly because of the works done to save the
canning industry. Staphylococcal food poisoning was a frequent problem with
cream-filled baked goods, cured ham, various salads, roast fowl, and
occasionally, certain types of cheese. Salmonellosis was believed to be
transmitted by sick animals.
Other highlights in food microbiology are listed in (Table 1)(Foster, 1989).
To name a few:
1959 Aflatoxins were discovered in 1959, when thousands of turkey died in
Great Britain after eating moldy peanut meal.
1960-1969 Type E botulism caused dealth in the early 1960s. Salmonella
continued to be an important pathogen in food.
1970-1979 A new and unfamiliar agent, Escherichia coli O27:H20, which
caused outbreak of gastroenteritis in 1971. Disease outbreaks
caused by Yersinia enterocolitica and Campylobacter jejuni were
heard of near the end of the decade, but received little attention
except from specialists in foodborne disease control.
1980-1989 C. jejuni emerged as the leading cause of gastroenteritis in U.S. Y.
enterocolitica was also identified in several outbreaks of
gastroenteritis, most of them from dairy products. E. coli O157:H7
first appeared in 1982 and ccaused serious outbreaks. Aeromonas
hydrophila also was recognized in the early part of the decade as a
possible cause of foodborne disease. Beside the conventional V.
cholerae and V. parahaemolyticus, other Vibrio species also
attrached attention. L. monocytogenes also re-emerged as an
important foodborne disease.
2. CONVENTIONAL SYLLABUS OF FOOD MICROBIOLOGY
Food microbiology is a course to study the relationship of habitat to
occurrence of microorganisms of foods, the effect of environment on growth of
various microorganisms in food, the microbiology of food spoilage and food
manufacture, the physical, chemical, and biological destruction of
microorganisms in foods, the microbiological examination of foodstuffs, and
public health and sanitation bacteriology (Anonymous, 1990).
The content of textbook "Microbiology of Foods" by Ayres et al. (Ayres et al.,
1980) published in 1980 can be divided into the following parts:
Part I. General considerations: a general discussion on microorganism
occurring in foods and the relationship of methods of food
processing and microorganisms.
Part II. Fermentations including single cell proteins and some oriental
fermentation.
Part III. Specific Food Products, on the microbial flora and spoilage of various
foods.
Part IV. Foodborne Illnesses
Other textbooks of food microbiology have similar contents. The emphasis of
these authors on different parts can be estimated in term of ratio of pages
covering each part to the whole volume (Table 2). A number of emerging
foodborne pathogens and the recent development of new detection technologies
are not included in these textbooks.
Table 2. Comparison of the contents of several textbooks of food
microbiology
Number of Pages (/whole volume)
________________________________________________________
Textbook
Part I
Part II
Part III
Part IV
Introduction
and
Fermentation Various Foodborne
Processing
Food
Diseases
________________________________________________________
Ayres et al. 146 (21%)
1980
Jay, 1984
251 (54%)
Frazier and
Westhoff,
311 (61%)
1978
Banwart,
325 (66%)
1981
102 (15%)
281 (41%)
152 (22%)
32 (7%)
85 (18%)
93 (20%)
87 (17%)
157 (31%)
67 (13%)
36 (7%)
30 (6%)
102 (21%)
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3. FOOD MICROBIOLOGY IN THE CHANGING WORLD
3.1. International Trade
International trade has been growing rapidly. Raw foods and also prepared
foods are distributing internationally. Also food industries have been seeking
international coorperation and have different plants all over the world and that
would enhance the distribution of local foods, e.g. the sales of Chinese or
Japanese foods have been rising rapidly in the U.S. Importance of the
microbiology of these foods will be increasing. To avoid importing foreign
foodborne pathogens or to avoid exporting foods containing pathogens, more
intensive monitor of foodborne pathogens is required, e.g. the V. cholerae in
exporting aquaculture to Japan, L. monocytogenes in exporting foods to U.S.A.
In Taiwan, the introduction of fast-food chain service systems has gradually
changed the attitude of the local consumers. Microbiological quality control of
the central kitchens of those chain systems is becoming very important.
Foodborne infections may increase in the coming years as a consequence of
increased globalization of our food supply (Onwulata et al., 2008).
3.2. Consumers Trends and Communication
Changes of consumer attitudes are significant (Table 3) since World War II.
Women are no longer "traditional"-- a career wife, mother, and shopper.
Consumers play less time to prepare food and shifted to emphasis on
cost/benefit rather than price alone. Also play more attention on the
nutrition/health and well-being perspective (Breidenstein, 1988). Since
foodservice operation (restaurants) is becoming more and more important,
microbiological quality assurance should be enforced to prevent outbreaks of
food poisonings.
Residues, cholersterol, salt, sugar, artificial coloring, additives, etc. (Table 4)
are the hot topics of food risks to consumers and it is highly affected by the
mass communication and education (Jolly et al., 1989). News emphasizes
extraordinary events and skews risk perception (Lee, 1989). Perceptions of the
public and expert of risks from eating food do not coincide. The experts rank
microbial safety, over-nutrition and non-microbial safety (contaminants, natural
toxins, agricultural chemicals and food additives) on the top of the rank of
hazards, while the public rank pesticides, new food chemicals, familiar hazards
(fat and cholesterol, microbial spoilage, junk foods) on the top of this list (Lee,
1989).
From the study of foodborne pathogens, we know that foodborne pathogens
are widely distributing and it is not realistic to avoid absolutely the
contamination of any of these pathogens. What we could do is to minimize the
risk of each of these pathogens. So we have to determine the risk of pathogens
in different foods under different conditions.
The consumption of fruits and vegetables in the U.S. is growing. Fifty years
ago, the total annual per capita consumption of fresh, canned, frozen, and dried
fruits and vegetables was about 335 lb (fresh-weight basis); in 1985, it was
about 406 lb. Between 1970 and 1985, per capita consumption of fruits and
vegetables increased by 23%, and fruit juice by 20%. The factors responsible
for this growth include new, high-quality products, higher disposable personal
income, the desire of health-conscious Americans to include fruits and
vegetables in their diet, and improved distribution systems. The U.S. fruit and
vegetable processing industry has changed in terms of product mix and total
product use. Consumption of most canned fruit and vegetable products has
declined, while consumption of frozen and dried products has increased. The
growing availability and use of fresh fruits and vegetables year-round has
affected the demand or many traditional processed fruit and vegetable products
(Pearl, 1990).
The shift to "lite" is apparent. People are looking for lower-calorie, lower-fat,
lower-salt alternatives in the marketplace. They are also looking for foods
perceived to have a high degree of nutrition, such as high-fiber and
high-calcium foods. Most importantly, people are changing their food selection
patterns to achieve what they perceive to be a "healthier" diet. Grain-based
products fit the "new" emerging dietary patterns. They are perceived to be
relatively low in calorie, low in fat, high in fiber, and high in complex
carbohydrates. Wheat flour consumption in the U.S., on a per capita base,
decreased significantly from the early part of the century to the mid-1970s, but
from that point on a significant rise is seen. Consumption of rice-based products
has been increasing since about 1960 (Leveille, 1988).
Public health professionals say the need for light dairy products is clear.
Light dairy product is a dairy product which offers a material difference in a
nutrition property which is of significant interest to consumers. It is not
restricted to "reduced-calorie" dairy products, it may mean "low sodium"
products or other types. Between 1980 and 1989 the share of fluid milk
consumption accounted for by lowfat and nonfat milk rose from approximately
40% to almost 60%. Nonfat yogurts were introduced in 1988, lowfat yogurts
perhaps ten year before that. In 1989 the combination of lowfat and nonfat
yogurt constituted 80% of the entire yogurt market segment. It may be soon
outstripped by the nonfat ice cream segment. "Nonfat" cottage cheese is
catching on in the western U.S. (Thompson, 1990).
Recently, consumers concern on the food allergens and other health issues
(Table 5) (Swientek, 2008).
Sales of functional products are rising across all food and drink categories in
the U.S. and Europe alike, and growth rates are appreciably faster than those
shown in mainstream food and drinks markets. The U.S. functional food market
was worth $18.9 billion in 2004, with annual sales growth averaging 7.2% over
the 1999–2004 period. It is forecast to grow at a slightly slower but still
significant 5.7% over 2004–09, far outstripping growth in the overall food and
beverage market. Dairy products are the second most popular nutraceutical
category in the U.S., with consumers spending $5.0 billion on functional dairy
food in 2004. Most adults have been told from childhood that milk is an
essential part of a healthy diet. This makes it particularly worthwhile to launch
functional milk variants targeted at improving bone health, as well as functional
yogurt variants targeted at gut health (Vierhile, 2006)
Similar changes of consumers' attitude are also significant in Taiwan. People
in Taiwan are consuming more and more the prepared refrigerated or frozen
foods which are usually made of mixed meats, seafood, vegetables and other
raw materials. So, there is a risk of mixing bacteria from different origin and
spreading of certain pathogens.
3.3. Development of New Technology
The advancement of modern biotechnology has great impacts on the food
industry (Fig. 1) (Harlander, 1989).
Detection Technologies
The first priority of the food industry is ensuring a wholesome food supply
that is free of pathogens and toxins. Quick detection methods have been
developed for various toxins and pathogens based mainly on the immunoassays
and molecular methods. Rapid and sensitive methods based on the development
of DNA probes and poly-and monoclonal antibodies have begun to replace
classical microbiological techniques for detection of potentially pathogenic
microorganisms. Kits are currently available for detection of common
foodborne pathogens and toxins. Polymerase chain reaction (PCR) will
dramatically improve the sensitivity of DNA probe-based assay systems.
Biosensors are developed for preventing the potential threat of bioterrorism
and emerging diseases (Fig. 2, 3) (Blyn, 2006).
Nanotechnology has been employed in the development of biohazard detection
method, such as the “super cloth”. Super cloth is a high-surface area and
high-absorbency non-woven fabric made from very fine fibers incorporated
with bio-recognition agents such as DNA strands, RNA strands, or antibodies.
The detection systems are based on biotin-protein binding. Polylactic acid and
streptavidin are used as the fiber and protein, respectively. The bio-recognition
agents provide high specificity for detecting target biohazards (Bugusu, 2008).
Production of Food Ingredients
Many potential food ingredients including enzymes, pigments, aromatic and
flavor compounds, etc. may be produced by natural or engineered
microorganisms (Table 6, 7) (Anonymous, 1988b; Gatfield, 1988).
Biotechnology has been applied to change the constituents of food crops.
Low-linolenic soybeans were introduced in 2005. Also now available are a
naturally stable sunflower and a low-linolenic canola oil. These crops produce
oils that do not have to be partially hydrogenated, thereby reducing trans fatty
acids. Adding hydrogen is necessary to keep certain oils stable, but the process
results in trans fats. As research increasingly has raised health concerns about
trans fats, the UDA now requires food manufacturers to list the amount of trans
fats on their labels. To avoid such listing, some manufacturers have switched to
oil from new low-linolenic soybean varieties, which enable them to continue
using soybean oil while avoiding trans fats and high amounts of saturated fats
that substitute oils contain (Magin, 2006).
Improvement of Traditional Fermentations
The most advanced dairy fermentations function at a level of sophistication
considerably lower than that of pharmaceutical fermentations. Meat
fermentations are another order of magnitude lower, and vegetable
fermentations are yet another order of magnitude lower (Table 8, 9)
(Anonymous, 1988b). Genetic engineering provides an alternative to classical
mutation and selection for improving microbial starter cultures. To date,
primary emphasis has been placed on construction of multifunctional
food-grade cloning and expression vectors and development of high-efficiency
gene-transfer systems. The ultimate goal is to construct strains with improved
metabolic properties (Harlander, 1989).
Control of mycotoxin production
The potential for using microorganisms to detoxify mycotoxins shows
promise. Exposure of deoxynivalenol (DON) to microbes contained in the
contents of the large intestines of chickens resulted in complete transformation
in vitro to de-epoxy-DON, which is 24 times less toxic than DON. Similar
findings were demonstrated with the microflora of cow intestines.
Genetic modification of mold susceptible plants via a number of transgenic
approaches holds great promise. One approach involves increasing production
of compounds such as antifungal proteins or secondary metabolites that reduce
infection by the microorganism. This may be accomplished by introducing a
novel gene to express the target compound. Another option is to enhance
expression of such a compound by the existing gene, capitalizing on the plant’s
own defense mechanisms. Alternatively, methods to increase production of
enzymes that degrade mycotoxins are also being pursued (Murphy et al., 2006).
Use of bacteriophages
Phages are found in enormous numbers in the environment and in our food.
Millions of phages exist in our digestive systems, and we regularly consume
millions more with food and water. While this alone provides overwhelming
evidence of their safety.
Decades of extensive use in medical applications—with exposure to phages not
limited to oral uptake, but rather systemic application being common—showed
no adverse effects whatsoever. Specific safety-related research has also been
performed. An oral toxicity study with rats receiving high doses of
Listeria-phage Listex P100 did not reveal side effects, and a study in humans,
with E. coli-specific phages able to infect commensal E.coli strains as well as
pathogenic strains, failed to show any adverse effects. Two simple safety rules
should be kept in mind: temperate phages—easily discernible through genome
analysis—should be avoided. Also, phages capable of generalized transduction
should be avoided unless the production host is non-pathogenic.
The U.S. food industry is mostly concerned with the “big four” food pathogens:
Listeria monocytogenes, Salmonella, Campylobacter, and pathogenic E. coli.
Of these, only Listeria regularly colonizes production facilities and thus is able
to contaminate food very late in the production process. Therefore, phage
treatment at the stage where this contamination is likely to take place is the
logical conclusion.
For many foods, this will be at some point prior to packaging, but cheese, for
example, may be vulnerable to contamination throughout the ripening stage. A
number of studies on successful phage treatment of various foodstuffs
contaminated with Listeria have been published. The other three organisms do
not regularly colonize facilities, and it is usually raw products that introduce
contamination. As a matter of fact, these organisms colonize animals whose
meat is used for human consumption. Therefore, one possible treatment is
application of phages during livestock farming, in addition to—or as an
alternative to—treating the meat after slaughter. Studies have been undertaken
to treat chickens with phages against Salmonella and Campylobacter and to
treat ruminants with phages targeted against pathogenic E. coli.
Treatment of facility surfaces is also a possibility. Food-contact surfaces, in
particular, could be cleaned effectively using phages, even during production,
without interrupting manufacturing processes. In the United States, several
bacteriophage applications have been approved for use and approved as GRAS.
These applications are against Listeria, methicillin-resistant Staphylococcus
aureus (MRSA), Salmonella, E. coli, and Campylobacter (Hagens and
Offerhaus, 2008).
Waste Management
Creative handling of food-processing waste streams could produce variable
products, e.g. biofuel, specialty chemicals which could be used in the industrial
sectors or as pharmaceutical products (Harlander, 1989).
Development of New Methods of Preservation and Processing
New technologies are being developed, that include controlled atmospheres
in the packaging of meats produces or other foods, asceptic packaging,
extrusion, ultrafiltration, etc. New technologies lead to the generation of novo
food products, and also new microbiological problems.
3.4. Development of New Ingredients
A number of new ingredients are produced or introduced into the food
systems, e.g. the non-caloric sweeteners, enzymes, colors, flavor compounds,
low-calorie oils, antimicrobials, cyclodextrins, et. Microbiological quality,
growth and survival of certain bacteria may be altered by some of these
compounds in food systems.
Some examples are listed as follows:
Acesulfame-K is an artificial sweeteners manufactured by Hoechst Co., and
it has been approved to use by a number of countries since 1983.
Sucralose is derived by the selective chlorination of sucrose at the 4,1', and
6' positions. It yields a white crystalline powder that has intense sweetness (400
to 800 times that of sucrose) and high solubility in water approximately 28% at
20C). It is extraordinarily stable and easy to incorporate into food and beverage
products using traditional food processes. It is nontoxic, and that it is not
carcinogenic, teratogenic, mutagenic, or caloric (Anonymous, 1988a).
Isomalt is an energy-reduced bulk sweetener, it is actually a sugar alcohol or
polyol produced from sucrose. In small intestine, isomalt is only partially
hydrolyzed and absorbed. Similar properties are observed in xylitol, another
sugar alcohol (Pepper and Olinger, 1988). Therefore, it does not lead to any
appreciable increase in blood sugar or insulin levels, which makes isomalt
suitable for diabetics. Other sugar alcohols include sorbitol, maltitol,
maltotriitol and hydrogenated oligosaccharides (Anonymous, 1988a). Xylitol
may also contribute to improved dental health by interfering with the formation
of new cavities and the progression of existing caries. It is not fermented by oral
bacteria and it has inhibitory effect on Streptococcus mutans (Pepper and
Olinger, 1988). Major suppliers of alternative sweeteners are listed in Table 10.
Low-caloric Oils Low-caloric oils are being investigated by substitute for or
reduce the caloric value of oil-like substances in food. Water-soluble
compounds (such as polydextrose, tapioca dextrin, maltodextrins), modified
triglycerides (e.g. carbohydrate fatty acid esters, sucrose polyester),
polycarboxylic acid esters, sterically hindered esters, medium-chain
triglycerides and polyglycerins, or conversion of the ester linkages of
triglycerides to ethers, etc. (LaBarge, 1988). Sucrose polyester may only cause
mild side effects on gastrointestinal problems. Flatulence, soft stools, anal
leakage, diarrhea, and increased urgency or frequency of bowel movements
have been reported (Toma et al., 1988).
Enzymes: Heat-stable protease from Thermomonospora fusca may be used in
the production of protein hydrolysates from either plant, fish, or animal proteins,
or for the cleaning of protein-solid ultrafiltration or reverse osmosis membranes
(Gusek and Kinsella, 1988). Novel lipase can be used in cheese production and
other dairy products modifying flavor of the products.
Cyclodextrins are cyclic molecules that have been derived enzymatically
from starch and have the ability to encapsulate other molecules within their
ringed structures. Cyclodextrins or modified ones could be used in controlling
flavor release, masking odors and tastes, stabilizing emulsions, increasing
foaming power, controlling or masking color, debittering of grapefruit juice, etc.
(Anonymous, 1988c)
Antimicrobial Substances: Antimicrobial substances such as hydrogen
peroxide, diacetyl, bacteriocins, and reuterin are produced from lactic acid
bacteria. Bacteriocins have been recognized and have been the subject of much
recent investigations. Hydrogen peroxide is approved for use in applications
such as bleaching and modification of food starch but not as an antimicrobial
agent in foods. Diacetyl and lactic and acetic acids are listed as GRAS. Nisin,
after 25 years of safe use in many European countries, was affirmed by FDA
(1988) as GRAS for use as an antimicrobial agent to inhibit the outgrowth of C.
botulinum spores and action was based on the accumulated body of scientific
data indicating that nisin is nontoxic, nonallergenic, and a safe effective
antimicrobial (Daeschel, 1989).
3.5. Protection food supply against Food Terrorism Event
Food may be a source of risk to human by accidental (food safety) and an
intentional (food defense) contamination. Accidental food contaminations are
typically associated with innate pathogenic microorganisms and their natural
proliferation pathways. Intentional contamination, on the other hand, is
associated with a select group of unfamiliar agents that typically have high
mortality rates. While both have the potential to inflict harm and cause
significant economic losses. A terrorist attack against the food supply chain
would target access points that would render the greatest impact—the goal
being to cause high morbidity and mortality, widespread economic disruption,
and fear. Microorganisms concerned in accidental and intentional
contamination events are listed in Table 11 (Takhistov and Bryant, 2006).
3.6. Green movement
Both manufacturers and consumers are embracing the green movement. The
percentage of consumers who purchased products made with recycled
packaging and/or manufactured in an energy-efficient, environmentally friendly
way jumped from 12% in August 2006 to 36% in December 2007 in Mintel, a
company. Mintel noted that while some of the food products labeled as
eco-friendly were organic and natural, others focused on different
environmental issues such as Green Energy Credits logos on packaging or
support for health associations. European consumers, too, are purchasing more
products positioned as eco-friendly, including those that feature reduced
packaging, are made with biodegradable packaging, and are labeled certified
organic and/or fair trade. About 27% of European consumers bought these
products in 2006, and in Europe, more than 60% of new product launches in
2007 were of these environmentally friendly products (Nachay, 2008).
4. EMERGING OF NEW FOOD-BORNE PATHOGENIC BACTERIA
Foodborne illness is not a simple problem in need of a solution; it is a
complex combination of factors that must be managed on a continual basis. A
number of factors will drive the emergence of new food safety concerns,
including changes in the characteristics of the consuming public, changes in the
foods we manufacture and sell, changes in the hazards themselves, and changes
in the ability of public health officials to identify illnesses as foodborne and to
trace the illnesses to their food source (Arthur, 2002).
In addition to the well-known food poisoning microorganisms
(Enteropathogenic E. coli, Campylobacter, Yersinia, Clostridium botulinum,
Staphylococcus aureus, Salmonella, Shigella, Bacillus cereus, etc), the
importance of some new microoganisms are recognized or being evaluated.
Aeromonas species
During the past several years there has been increasing speculation regarding
the possible role of A. hydrophila, A. sorbria and A. caviae as a cause of human
gastroenteritis. There is sufficient evidence to indicate that A. hydrophila can
produce fatal septicemia in individuals debilitated by some other disease or
condition.
Edwardsiella tarda
It is known as pathogen of catfish or other fishes especially in the warm
water. It is also caused epidemic in the cultured crimson sea breams in Japan. It
has also been implicated in gastroenteritis in humans, wound infections and
meningitis. The incidence of E. tarda in catfish is somewhat uncertain. It is
reported the organism present on 79% of the domestic fish and 30% of the
imported fish examined.
Enteric Viruses
Human enteric viruses appear to be the major cause of shellfish-associated
viral disease. Presently, there are more than 100 known enteric viruses which
are excreted in human feces and ultimately find their way into domestic sewage
(Table 12) (Gerba, 1988). A few have been shown epidemiologically to be
transmitted by shellfish: hepatitis A, non-A, non-B hepatitis, Norwalk, Snow
Mountain agent, astroviruses, caliciviruses and small round viruses. Lack of
methods for virus detection and the difficulty of recognizing viral disease
outbreaks have the difficulty of recognizing viral disease outbreaks have
probably precluded the list from being longer. Some epidemiological studies
have suggested that shellfish may play a significant role in the transmission of
hepatitis A virus (Gerba, 1988). In laboratory studies, enteric viruses have been
reported to survive from 2 to 130 days in seawater, and they generally survive
longer in such environments than do coliform bacteria.
Plesiomonas shigelloides
P. shigelloides (formerly Aeromonas shigelloides) has been implicated in
human gastroenteritis for 40 years. Symptoms of the infection include diarrhea,
abdominal pain, nausea, chills, fever, headache, and vomiting, etc. (Hackney
and Dicharry, 1988). It is widespread in nature, being mostly associated with
fresh surface water, but may also be found in seawater. It is more often isolated
during the warmer months. Some strains can grow at refrigerating temperature
(Hackney and Dicharry, 1988; Ward, 1989).
Vibrio species
A number of Vibrio species cause gastroenteritis, wound infection, ear
infection, or septicemia in human beings (Table 13) (Hackney and Dicharry,
1988). These pathogenic Vibrio species include V. cholerae O1, O139 and
non-O1, V. parahaemolyticus, V. vulnificus, V. mimicus, V. hollisae, V. furnissii,
V. fluvialis, etc. Fewer than 5% of the non-O1 strains from human sources in the
U.S. produce cholera toxin. Associated symptoms of gastroenteritis have
included diarrhea, abdominal cramps, fever, nausea, vomiting. Almost all of the
cases of non-O1 V. cholerae infections in the U.S. have been associated with
eating raw oysters; but egg and asparagus salad and potatoes have also been
vehicles for the bacteria.
Since 1992, a new pandemic strain of V. cholerae O139 occurred and rapidly
spread over many countries (Faruque et al., 2003; Wong et al., 2002). Also, the
occurrence of O3:K6 strains of V. parahaemolyticus widely spread all over the
world and are recognized as the first pandemic strain of this species (Chiou et
al., 2000; Wong et al., 2000).
Enterobacter sakazakii
Infection by E. sakazakii is an extremely rare event. Six of the 58 reported
cases of E. sakazakii infection worldwide involved individuals more than four
years of age, and the median age was 74. The vast majority (83%) of cases have
been reported in infants less than one year of age, where the fatality rate ranged
from 30% to 80% despite antibiotic treatment. Infants born at less than 36
weeks gestational age were at risk until six weeks post-term. Illness symptoms
normally appear a few days after birth, and the health of the infant rapidly
deteriorates. Infection may result in meningitis (58%), necrotizing enterocolitis
(29%), or sepsis (17%). In one outbreak in which 11 infants were positive for E.
sakazakii, one developed meningitis and four others had clinical signs of severe
sepsis, although the microorganism could not be isolated from the blood.
Bacteremia is often, but not necessarily, confirmed. The severe consequences of
infection in some cases may be linked to the production of enterotoxin by E.
sakazakii. More than 20% of the 18 tested strains produced enterotoxin. When
infection does not result in death, the affected infant may have permanent
neurological or developmental deficiencies. In one case, mortality was averted
by months of antibiotic treatment, but the patient was mentally retarded and
paralyzed by age two. Infants may be colonized with E. sakazakii without
developing symptoms (Gurtler et al., 2005).
5. ANTIMICROBIAL RESISTANCE
Use of antimicrobials, more specifically antibiotics, can create selective
pressure that leads to emergence of antimicrobial-resistant microorganisms.
Bacterial strategies for resisting antimicrobials include impaired uptake,
modification or overproduction of the target site, bypass of sensitive steps,
absence of enzymes or metabolic pathways, efflux, enzymatic degradation,
receptor alteration, and change in membrane permeability.
Bacteria may also experience stress adaptation (resistance stemming from
exposure to subinhibitory levels of stress that trigger stress-response protein
transcription and translation), co-selection (resistance to antimicrobials having
unrelated targets, stemming from separate genes transferred together),
cross-resistance (resistance to antimicrobials having the same molecular targets),
and crossprotection (in which adaptation to one stress is associated with
increased resistance to another, unrelated stress).
Antibiotic resistance among foodborne pathogens may create an increased
burden on human health in different ways. For example, resistant pathogens
contaminating food animals have the potential to reach humans; human use of
antibiotics may increase the risk of acquiring an infection with an
antimicrobial-resistant pathogen; human infection with a resistant microbe may
limit illness treatment options (in the uncommon instances of foodborne illness
in which antibiotic use is warranted); and antibiotic-resistant foodborne
pathogens may develop increased virulence.
The extent to which antibiotic use in food animals produces clinically
important antibiotic resistant infections in humans is unknown (Fig. 4). There is
evidence that points to but does not prove that antibiotic use in food animals
poses a human health threat. There are very few data regarding food
animal-to-human transfer of antimicrobial resistance to indicate more frequent
or severe infections or increased morbidity and mortality.
Food product modifications, such as changes in formulation or processing
conditions, may lead to sublethal stressing of microbes. Surviving
microorganisms may have increased resistance or virulence. Some
antimicrobial treatments may lead to dominance of acid-resistant pathogens.
For example, spraying meat carcasses with organic acids may select for survival
of acid-tolerant E. coli O157:H7 (Doyle, 2006).
6. TRENDS IN RESEARCH ABOUT FOOD MICROBIOLOGY
We will increase our reliance on food processors and we now know it is
impossible to eliminate all risks. Thus the best we can do is to minimize the risk.
According to the USDA, investigations are mostly concerning harmful bacteria
including Salmonella, Shigella, Campylobacter, E. coli, Yersinia enterocolitica,
and Listeria monocytogenes. Quicker and less expensive methods of detecting
harmful microorganisms and chemical residues are being studied. Studies focus
on the most pathogenic strains of microorganisms; and development of ways to
prevent mycotoxin formation in growing plants (Carter, 1999; Hackney and
Dicharry, 1988).
Papers in food microbiology conducted in departments of food science or
other institutions published during the period 1988-1989 on the Journal of
Applied Bacteriology and also during the 1988-1990 on the Applied and
Environmental Microbiology were briefly analyzed. It indicated the research
activities about food microbiology at that time. These two journals are well
respected journals concerning food microbiology. A number of papers about the
contamination, survival of food pathogens or spoilage microorganisms are also
published in other journals such as Journal of Food Protection, Journal of Food
Science, International Journal of Food Microbiology, etc.
Methodology in isolation and detection, genetics & molecular studies,
occurrence & spoilage, and toxin studies are the most popular fields of research
in food microbiology. Some papers about the toxin and molecular studies also
appear on other journals.
Papers on the incidence of poilage or pathogens in food were analyzed. This
kind of study most concern about different pathogenic bacteria in various kinds
of foods usually of high risk, e.g. Vibrio in seafood, Aeromonas in water or
aquaculture, Listeria in dairy products, Campylobacter in meat, etc. Papers on
the growth and survival of microorganisms included the survival of
microorganisms in various kinds of food and also the effects of bacteriostatic or
bacteriocidic chemicals (e.g. nisin, disinfectants, acid), or processes (heat,
controlled air..). Physiological and biochemical behaviour were also concerned
by some papers, e.g. the production of enzymes (proteolytic enzymes,
endonuclease, pectinase, lipase, etc.) during the growth of certain spoilage or
pathogenic microorganisms.
Most of the studies on the phage, plasmid, and other molecular aspects of
food-related microorganisms concerned about lactic acid bacteria, namely,
Streptococcus (Lactococcus), Lactobacillus, Pediococcus, etc. Methods
involving various techniques were developed, e.g. immunoelectrical method in
the detection of S. aureus (the enzyme-immuno detection signal was amplified
by electrical method). Various formats were employed in the immunological
methods, e.g. latex agglutination, reverse phase latex agglutination, enzyme
immunosorbent assay (ELISA), filter-method, immunomagnetic separation, etc.
Monoclonal antibodies were used in some studies. Protease was also detected
by ELISA. Molecular methods were developed in the 90’s.
Overall speaking, the Listeria, Staphylococcus, Vibrio, Clostridium, E. coli
were the most intensively studied bacteria in addition to mycotoxin producing
fungi in the 90’s.
Similar research subjects and microorganisms are also frequently investigated
in recent year (2008) (Table 14, 15).
Table 14. Subjects of papers published in 2008 in Journal of Applied
Microbiology and International Journal of Food Microbiology
Topics
JAM # IJFM # Total
Incidence, prevalence, 24
43
67
spoilage
Growth, survival,
42
42
84
Processing
Fermentation
24
45
69
Enzyme
3
7
10
Bacterial Spore
3
2
5
Antimicrobials
21
24
45
Genetics & molecular
22
21
43
Isolation and detection 26
30
56
Culture
10
10
Immunology
4
3
Molecular
12
17
Pathogens
29
11
40
Bacterial toxin
8
7
15
Health food
13
14
27
Bacteriocin
12
7
19
Mycotoxin
2
26
28
%
13
17
14
2
1
9
8
11
8
3
5
4
6
Table 15. Microorganisms studied in papers published in 2008 in Journal of
Applied Microbiology and International Journal of Food Microbiology
Microorganisms
Acetic acid bacteria
Aeromonas
Alicybacillus
Aureobasidium
Bacillus
Brucella
Burkholderia cepacia
Campylobacter
Clostridium
Edwardsiella
JAM #
0
2
0
1
10
1
3
6
2
1
IJFM #
13
1
1
0
8
0
0
12
9
0
Total
13
3
1
1
18
1
3
18
11
1
%
3
1
0
0
4
0
1
4
2
0
E. coli
Enterobacter sakazakii
Enterococcus
Klebsiella
Helicobacter
Lactic acid bacteria
Listeria
Salmonella
Staphylococcus
Vibrio
Yersinia
Fungus
Yeast
Viruses
29
5
3
0
2
52
11
9
4
10
0
12
20
5
18
3
7
1
1
63
36
17
13
9
4
36
26
12
47
8
10
1
3
115
47
26
17
19
4
38
46
17
10
2
2
0
1
25
10
6
4
4
1
8
10
4
Food Industry Research Interests
•
•
•
•
•
•
•
Industry research needs are varied. Specifically, the research needs are:
Structure-function/nutrition of health-promoting wholesome foods with multi
health benefits and new processing technologies to protect and concentrate
nutrients such as phytonutrients and folic acid and flavor/aroma phenols.
Enhance quality and new uses (product innovations). Develop and implement
methods to improve processing and end-product quality and develop rapid
measurements for functionality and nutrient prediction.
Develop healthy and flavorful, value-added products (e.g., bran, oil, protein)
to maximize health benefits through processing and address health/obesity.
Develop new delivery techniques for nutrients (e.g., delivery of probiotics)
and develop new processing technologies for nutrient identification,
characterization, stabilization, and delivery.
Develop knowledge and understanding of bio-metabolism—nutrient/food
interaction.
Improve quality. Conduct enhanced value-added research for food and feed,
improving quality of harvested and processed produce, quality of produce in
controlled atmosphere, and reducing quality loss in storage. Develop
postharvest practices for optimizing quality through improved monitoring.
New technologies for processing streams. Develop processes to recover more
feed and fiber from waste by the removal of harmful substances such as
allergens, gossypol, or acrylamide, and through process enhancement and
recovery of food-based byproducts.
• Food security. Provide more quality food through new technologies and
enhanced nutrient retention. Also, nanotechnology has the potential to
generate new products for the food industries with numerous benefits in
smart packaging, nanosensors for food safety, food nutrient delivery systems,
nanoemulsions, etc. (Onwulata et al., 2008).
7. TRENDS IN FOOD MICROBIOLOGY EDUCATION
Food microbiology was an important course in Bacteriology or Microbiology
departments several decades ago in the western world. Nowadays, nearly all the
food microbiology course is conducted in food science or related departments.
Considering the facts or trends discussed above, the syllabus of food
microbiology for undergraduate or graduate levels should cover the following
contents:
Part I General Considerations
1. A brief introduction to the world of microbiology.
2. Microorganisms occurring in foods, especially in some new food systems.
3. Relationship of conventional and new methods of food processing and
microorganisms, including the introduction of new chemicals.
4. An introduction to biotechnology and the advance of conventional or new
identification methods. Numerical or computer-assisted classification should
also be introduced.
5. Microbiologic quality assurance of food processing plants or restaurant
should be discussed.
Part II Food Fermentations
1. Application of biotechnology in culture and process improvements.
2. Fermentation and microbiology of fermented foods of Oriental or African
countries should be discussed in addition to traditional food fermentation.
3. Production of enzymes or other food ingredients by microorganisms should
be discussed.
4. Waste treatment of food processing plants by microorganisms should also be
discussed.
5. Include the application of different microorganisms in health food.
Part III Microbiology of Specific Food Products
1. Discuss on the microflora of raw food materials and traditional foods.
2. Also discuss on the microflora and spoilage of new food systems, e.g. light
dairy products, low-calorie foods, vacuum packed foods, foods packed in
controlled atmosphere, etc.
Part IV Foodborne Illnesses
1. Should discuss on the occurrence and incidence, growth and survival at
different environments or in foods, toxin production and briefly on the
pathogenicity of well-known and emerging pathogensk, and also the effect
of environmental stresses.
2. Since the identification of pathogenic or toxigenic strains employs
sophisticated techniques, brief discussions on the immunology and
molecular biology should be included.
3. Should play more attention on some local foodborne pathogens e.g. the
Vibrio, Aeromonas, enteroviruses and mycotoxin producing fungi are
important in tropical or subtropical coastal countries.
4. Should also discusson the emerging foodborne pathogens, and global climate
changes on the foodborne pathogens.
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