Bacteria mainly reproduce by a process referred to as binary fission.
Generation time ( Gt)= doubling time (Td).
• Is the time taken usually expressed in minutes
(sometimes hours or even days) for one bacterium to double or for a bacterial population to double.
• The shorter the generation time the faster the reproduction, vice versa.
1  2  4  8  16  32  64  128  256
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The study of microbial ecology related to foods
 the effect of the environment on food spoilage and manufacture
 the chemical, physical and biological destruction of microorganisms (M.O) in foods
 the microbiological examination of food stuff
 public health and sanitation microbiology.
The study of the interactions between the chemical, physical, and structural aspects of a niche and the composition of its specific microbial population.
Gt = 0.3 t log
10 z – log
10 x
0.3 = constant t = the duration of study (min) log
10 z = final cell numbers per ml or CFU log
10 x = initial cell numbers per ml or CFU
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Example:
If, under a a given growth conditions, the initial population of 10 to 10 time
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4 cells/ml of bacteria species increased cells/ml in 120 min, what was the generation
• t = 120 min
• log10 z = 10 6
• log10 x = 10 4 cells/ml cells/ml
• Gt = 0.3 (120) = 18 min
6–4

Reproduction = Death.

Cells adjust to new environment (acclimation) induction or repression of enzymes
 initiate chromosome and plasmid reproduction
• Length of lag phase depends on various conditions:
 temperature

 inoculum size physiological history of the microorganism.
Y= X 2 n
Y = the final number of bacteria
X = the initial number of bacteria n = number of generations ( doublings)
Example :
• Suppose you have Escherichia coli with a generation time of 20 minutes. If the initial concentration of E. coli in ground beef was 1 colony forming unit (cfu)/g how many E . coli would you expect after 2 h?
• Y = X 2n
Y = 1 x 2 6
= 1x (2x 2 x 2 x 2 x 2 x 2)
= 64 cfu/g
• Suppose the initial number of E. coli was 100,000 (10 5 after 4 hours?
). How many cell would you expect
Y= X 2n
Y=100,000 x 2 12
• Reproduction > Death.
• Vigorous growth via binary fission.
• First order kinetics .
• Number of microorganisms at any time is directly proportional to the initial number of microorganisms.
• Reproduction = Death.
• Substrate limitation or adverse environment such as acid production or oxygen limitation.
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• Death > Reproduction.
• Severe substrate limitation or accumulation of waste products or antimicrobials.
• By the action of autolytic enzymes, may lyse and release the cellular enzymes in food, which then act on food component
1- Lactic acid bacteria
Produce Lactic acid from fermentation of sugars
The production of lactic acid could be desirable or undesirable.
Desirable : sauerkraut and cheese
Undesirable : wine and distilled beverages
The major genera belong to the families
Lactobacillaceae and Streptococcaceae
Leuconostoc Lactobacillus
Streptococcus Pediococcus
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Bacteria can grow in food and make different changes
The changes include hydrolysis of carbohydrates, proteins, and fats (spoilage).
Some bacteria are useful in the industrial
(fermentation)
Some other bacteria are pathogenic to human beings
(disease)
Acetic acid bacteria belong to the genera
Acetobacter and Gluconobacter
Oxidize ethyl alcohol to acetic acid.
Can further oxidize acetic acid to carbon dioxide
Acetobacter aceti causes sliminess in vinegar generators.
Groups of Bacteria Important in Food
Bacteriology
Bacteria important in foods often are grouped on the basis of the common characteristics regardless for their classification.
Some bacteria might be included in two or more groups
• Produce butyric acid in relatively large amounts.
• Most bacteria of this group are spore-forming anaerobes of the genus Clostridium butyricum
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Most bacteria of this group are in the genus
Propionibacterium
It is important in the production of Swiss cheese
Responsible for changing lactic acid to propionic acid acetic acid and carbon dioxide
(forming holes)

 Degradation of proteins by extra-cellular protease or proteinases bacterial enzymes and produce foulsmelling compounds such as
 hydrogen sulfide
 mercaptans
 amines
 indole
 Putrefactive by Clostridium (sporeformers)
 Putrefactive by Pseudomonas and Proteus
(non-sporeformers)
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 Produce extra-cellular protease or proteinases enzymes.
 These enzymes hydrolyze a variety of proteins causing spoilage
 Many species of the genera Bacillus, Clostridium,
Pseudomonas and Proteus are proteolytic
:
 M. O that produce extra-cellular lipases triglycerides  glycerol + FFA milk, cream, butter, cheese, sausages
 FFA especially the short chained (C3 to C6) are very volatile.
 The FFA may be further oxidized to aldehydes and ketones giving rise to further flavor / odor compounds
 Pseudomonas spp is highly proteolytic and is capable of degrading milk protein (casein) resulting in coagulation and spoilage
Acid proteolytic:
 Some bacteria called acid proteolytic (carry on acid fermentation and proteolysis) such as
Streptococcus faecalis and Micrococcus caseolyticus
 Many of the aerobic proteolytic bacteria are also lipolytic
 Pseudomonas fluorescens is strongly lipolytic and proteolytic
 Example of the lipolytic bacteria belongs to the genera
 Pseudomonas
 Alcaligenes
 Serratia
 Micrococcus
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 These bacteria hydrolyze disaccharides or polysaccharides to simpler sugar
 Amylolytic bacteria: produce amylase which hydrolyze starch outside the cell
 Example:
Bacillus subtilis
- Clostridium butyricum



- Bacillus : responsible for flat sour spoilage of canned foods
- C. thermosaccharolyticum : gaseous spoilage
- Lactobacillus thermophilus : obligatory thermophilic lactic acid bacterium
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 Species of Clostridium are classified into
1-Proteolytic: attack protein and may or may not attack sugar
( C. lentoputrescens )
2- Saccharolytic: attack sugar but not proteins ( C. butyricum
)
These bacteria grow at temperature below 15°C
These bacteria are important in refrigerated foods
Example:
The genera:
• Pseudomonas
• Flavobacterium
• Alcaligenes
• Some species of the genera:
» Micrococcus
» Lactobacillus
» Enterobacter
» Arthrobacter
 These bacteria produce pectinase which can degrade pectin ( complex carbohydrates ) and cause softening of plant tissues
 Example of pectolytic bacteria:
– Erwinia
– Bacillus
– Clostridium
Slightly halophilic bacteria grow at 2 to 5% sodium chloride
Moderately halophilic bacteria grow at 5 to 20% sodium chloride
Extremly halophilic bacteria grow at 20 to30% sodium chloride
Salt tolerant bacteria can grow with or without salt
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Halophilic bacteria are important in salted food
Examples
The genera:
– Halobacterium
– Sarcina
– Micrococcus
– Pseudomonas
– Vibrio
– Pediococcus
– Alcaligenes
Examples:
Alcaligenes viscolactis and Enterobacter aerogenes : causing ropiness of milk
Leuconostoc spp.
: producing slime in sucrose solutions
Some species of Streptococcus and Lactobacillus cause milk slimy or ropy
Lactobacillus plantarum : causes ropiness of fruit and vegetables products (sauerkraut and beer)
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These bacteria grow at high concentrations of sugar
Example: Leuconostoc
Many kinds of bacteria produce small amounts of gas
(heterofermentative lactis)
Examples of genera produce carbon dioxide:
Lactobacillus, Leuconostoc (Heterofermentative) and Propionibacterium
Examples of genera produce carbon dioxide and hydrogen:
Escherichia, Enterobacter, Proteus, Bacillus and clostridium
These bacteria produce color in foods
Example:
–
–
–
–
Flavobacterium
Serratia
Pseudomonas
Micrococcus
(yellow to orange)
(red)
(blue-green)
(different colors)
16- Coliform group
Considered as a good indicator M.O for sanitary quality of foods and water
Indicator M.O: are those M.O or their metabolic products whose presence in a given food at a certain level may be used to assess the microbiological quality or to predict the shelf life or suggest the possibility of microbial hazard.
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1- easily and rapidly debatable
2- could be distinguished from the other food flora
3- their growth and numbers have a direct negative correlation with product quality.
4- their growth should not be affect adversely by other food microflora
 group of bacteria belonging to the family
Enterobacteriaceae
 gram negative, asporogenous, motile
 facultative-anaerobic
 capable of growth in the presence of bile salts or similar surface active agents
 produce acid and gas from lactose within 48 hours at 44 ± 0.5ºC.
 sensitive for heating and freezing
5-have a growth and survival rate in food as that the enteric pathogen
6- non pathogenic
7- should not suffer sub lethal injury more than the pathogen
8- their presence has a correlation with the presence of pathogen
1. Fecal coliforms originate primarily from feces of man and warm blooded animal.
Example is Escherichia coli .
2. Non-fecal coliform normally originates from soil or water.
Example is Enterobacter aerogenes.
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• Escherichia
• Enterobacter
• Klebsiella
• Citrobacter and probably Aeromonas and Serratia
• Non-fecal coliforms are eliminated by using a high incubation temp. (44.5 ± 0.2 or 45.0 ± 0. ºC) for 24 h in selective broths containing lactose
• In heat-processed foods, their presence (even in small numbers) is viewed with caution
• Their presence is considered as post heattreatment contamination from improper sanitation after heat treatment
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