Additional Information:
Microbial risk assessment due to municipal solid waste dumps
[Source:http://www.fda.gov/downloads/food/foodborneillnesscontaminants/ucm297627.pdf:
http://www.phac-aspc.gc.ca/lab-bio/res/psds-ftss/aspergillus-spp-eng.php]
Infective dose and Probability of transmission to rag picker from dump site
1. Staphylococcus aureus
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Infective dose: The intoxication dose of SE is less than 1.0 microgram. This toxin level is
reached when S. aureus populations exceed 100,000 organisms/g in food. This level is
indicative of unsanitary conditions in which the product can be rendered injurious to
health. In highly sensitive people, ingestion of 100 to 200 ng of enterotoxin can cause
symptoms. The population of S. aureus at the time of analysis may be significantly
different, and not representative of, the highest population that occurred in the product.
This should be taken into consideration when examining foods.
Target populations: All people are believed to be susceptible to this type of bacterial
intoxication; however, intensity of symptoms may vary.
COMMUNICABILITY: Communicable period is as long as a purulent lesion is present or
carrier state persists.
MODE OF TRANSMISSION: Ingestion of food containing enterotoxins. Vertical transmission
during vaginal delivery is uncommon . Person-to-person transmission occurs through
contact with a purulent lesion or with a carrier. Unsanitary conditions and crowded
community settings increase exposure to S. aureus . Infection may be spread from personto-person through health care workers or patients. Nasal colonization can lead to autoinfection
2. Enteropathogenic Escherichia coli
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Infective dose: Volunteer feeding studies showed that a high dose, ranging from 10 million
to 10 billion ETEC cells, may be needed to cause an infection in adults. Children may be
affected by a smaller dose.
Target Populations: Infants and travelers to underdeveloped countries are most at risk of
ETEC infection. As with other infections, people with weak immune systems are more likely
than others to suffer severe, even life-threatening cases.
COMMUNICABILITY: Can be transmitted through direct contact with infected persons
MODE OF TRANSMISSION: Contaminated food, water, and fomites serve as vehicles for
the fecal/oral transmission of EPEC.
3. Enterotoxigenic Escherichia coli
1
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Infective dose: Volunteer feeding studies showed that a high dose, ranging from 10 million
to 10 billion ETEC cells, may be needed to cause an infection in adults. Children may be
affected by a smaller dose.
Target Populations: Infants and travelers to underdeveloped countries are most at risk of ETEC
infection. As with other infections, people with weak immune systems are more likely than
others to suffer severe, even life-threatening cases.
COMMUNICABILITY: Person-to-person infection can occur, but is uncommon because the
infectious dose is high.
MODE OF TRANSMISSION: ETEC are spread primarily by the fecal-oral route and are most
common in developing countries that lack appropriate sanitation and drinking-water
treatment facilities. The most important mode of transmission is contaminated, improperly
treated drinking water. Fruits and vegetables that are washed with contaminated water and
not cooked also serve as vehicles of transmission. Person-to-person spread is uncommon.
4. Enterohemorrhagic Escherichia coli
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Infective dose: The infective dose of EHEC is estimated to be very low, in the range of 10 to
100 cells.
Target Populations: All people are believed to be susceptible to hemorrhagic colitis, but
young children and the elderly are more susceptible and at higher risk for the illness to
progress to more severe complications. Others with weak immune systems also are at risk,
such as people with some chronic diseases or AIDS, and people on immunosuppressive
medications; for example, some drugs used for arthritis and cancer chemotherapy.
COMMUNICABILITY: Communicable for duration of fecal excretion (7-9 days); 3 weeks in
one third of children.
MODE OF TRANSMISSION: Ingestion of contaminated food (undercooked hamburger
meat, unpasteurized milk); fecal-oral transmission; person-to-person transmission
(extremely high)
5. Enteroinvasive Escherichia coli
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Infective dose: The infective dose of EIEC is thought to be in the range of 200 to 5,000 cells,
somewhat higher than that of Shigella. The difference in the dose may depend on which
virulence plasmid these pathogens harbor.
Target Populations: All populations are susceptible to EIEC infections.
COMMUNICABILITY: Yes. Person-to-person transmission is possible but is uncommon.
MODE OF TRANSMISSION: EIEC are spread by the fecal/oral route. Contaminated food and
water are the usual vehicles for the spread. Food-borne outbreaks have occurred. Personto person transmission can also occur
6. Streptococcus species
2
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Infective dose: The infectious dose for group A Streptococcus probably is fewer than 1,000
organisms.
Target Populations: All people are susceptible. However, children, immune compromised
people, and people 65 years or older, in nursing homes, are more vulnerable. Scarlet fever
and rheumatic fever are more common among children 5 to 15 years old than among
adults.
7. Klebsiella species
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Infective dose: Unknown
COMMUNICABILITY: Members of Klebsiella spp. can be transmitted from person-toperson; however, the communicability period is unknown. Approximately one-third of
people carry Klebsiellae in their stoolshttp://www.phac-aspc.gc.ca/lab-bio/res/psdsftss/klebsiella-eng.php - footnote1; detection rates according to different studies vary from
5% to 36%. Detection rates in nasopharynx vary from 1% to 6%. Hospital personnel have
been shown to frequently carry Klebsiellae on their hands.
MODE OF TRANSMISSION: Klebsiella spp. can be transmitted through skin contact with
environmentally contaminated surfaces and/or objects; examples include Loofah
spongeshttp://www.phac-aspc.gc.ca/lab-bio/res/psds-ftss/klebsiella-eng.php - footnote1, medical
equipment, and blood products. Fecal transmission has also been suggested for some cases
of bacteremia caused by Klebsiella spp..
K. rhinoscleromatis can be transmitted from person-to-person via airborne secretions;
however, prolonged contact with infected individuals is required for infection.
K. granulomatis are sexually transmitted. They may also be vertically transmitted (from
mother to child) or by accidental inoculation. Transmission rates between partners are
low (<50%) compared to other sexually transmitted diseases.
8. Proteus
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Infective dose: Unknown
Target Populations: All people may be susceptible to pathogenic forms of these bacteria.
Acute gastrointestinal illness may occur more frequently in undeveloped areas of the
world. The chronic illness is common in malnourished children living in unsanitary
conditions in tropical countries. Protracted illness is more commonly experienced by the
very young. Immuno compromised people may be more susceptible to illness from these
pathogens than are immune competent people, but that may also depend on the bacterial
strain (how virulent it is) and how much of it is consumed.
COMMUNICABILITY: Proteus spp. are not known to be transmitted from person-to-person.
MODE OF TRANSMISSION: Proteus spp. are part of the human intestinal flora and can
cause infection upon leaving this location. They may also be transmitted through
contaminated catheters (particularly urinary catheters) or by accidental parenteral
inoculation. The specific mode of transmission, however, has not been identified.
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9. Salmonella species
Non typhoidal Salmonellosis
 Infective dose: As low as one cell, depending on age and health of host and strain differences
among members of the genus.
 Target Populations: Anyone, of any age, may become infected with Salmonella. Particularly
vulnerable are people with weak immune systems, such as the very young and the elderly,
people with HIV or chronic illnesses, and people on some medications; for example,
chemotherapy for cancer or the immunosuppressive drugs used to treat some types of
arthritis. People with HIV are estimated to have salmonellosis at least 20 times more than does
the general population and tend to have recurrent episodes.
 COMMUNICABILITY: Communicable throughout course of infection; several days to several
weeks; temporary carriers can continue for several months; antibiotic therapy can prolong
period of communicability; 1% of infected adults and 5% of infected children excrete organism
for over 1 year.
 MODE OF TRANSMISSION: By ingestion of directly or indirectly contaminated food, from
infected animals or food by infected animal or person; from animal feeds and fertilizers
prepared from contaminated meat scraps; fecal-oral transmission from person to person;
direct contact with pets such as reptiles, birds, turtles, tortoises.
10. Pseudomonas aeruginosa
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Infectious dose: Unknown for humans.
COMMUNICABILITY: Spread of infection from person-to-person is speculated to be highly
possible during infection, especially amongst cystic fibrosis patients.
MODE OF TRANSMISSION: P. aeruginosa have been found to survive within droplet nuclei
and can remain in aerosols for long periods of time, thus there is evidence of potential
airborne transmission. Contact with contaminated water is also a major route, but since
the oral infectious dose is thought to be very high, routes that pose the greatest health risk
are skin exposure (for example, in contaminated hot tub water) and lung exposure from
inhaling aerosols discharged from infected respiratory tracts . The bacterial can often enter
the body through injuries and wounds. The use of contaminated mechanical respiratory
ventilators in hospital settings is also a common source of nosocomial infections
11. Aspergillus sp.
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Infective dose: Unknown
COMMUNICABILITY: No evidence of human to human transmission.
MODE OF TRANSMISSION: Inhalation of airborne conidia, through contaminated water
(exposure to conidia during showering), and nosocomial infections (hospital fabrics and
plastics may serve as importance source of Aspergillus spp.
12. Candida sp.
4
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Infective dose: Unknown
COMMUNICABILITY: Communicable for duration of lesions.
MODE OF TRANSMISSION: Endogenous spread (part of normal human flora); by contact
with excretions of mouth, skin, and feces from patients or carriers; from mother to infant
during childbirth; disseminated candidiasis may originate from mucosal lesions, unsterile
narcotic injections, catheters
Probability of transmission to rag picker from dump site via all modes
BACTERIA/FUNGI
Aspergillus sp.
Bacillus sp.
Candida sp.
Escherichia coli
Klebisiella sp.
Proteus sp.
Pseudomonas aeruginosa
Salmonella sp.
Staphylococcus aures
Candida sp
Psedomonas aeruginosa
Strept sp.
Probability of transmission to rag picker from dump site
via all modes
0.001
0.003
0.002
0.004
0.002
0.001
0.003
0.001
0.001
0.003
0.002
0.001
Note: Understanding relationship between cfu/g (mention in the case study per gram in relation
to the unit of “ cells” mentioned for infective dose
Colony-forming
unit (CFU)
is an estimate of viable bacterial or fungal numbers. Unlike
direct microscopic counts where all cells, dead and living, are counted, CFU estimates viable cells. The
appearance of a visible colony requires significant growth of the initial cells plated - at the time of counting
the colonies it is not possible to determine if the colony arose from one cell or 1,000 cells. Therefore, the
results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units
per gram) for solids to reflect this uncertainty (rather than cells/mL or cells/g).
The purpose of plate counting is to estimate the number of cells present based on their ability to give rise
to colonies under specific conditions of nutrient medium, temperature and time. Theoretically, one viable
cell (viable defined as able to multiply via binary fission under the controlled conditions) can give rise to
a colony through multiplication. However, solitary cells are the exception in nature, and most likely the
progenitor of the colony was a mass of cells deposited together. In addition, many bacteria grow in chains
(e.g. Streptococcus) or clumps (e.g. Staphylococcus). Estimation of microbial numbers by CFU will, in most
cases, undercount the number of living cells present in a sample for these reasons.
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