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[link] lists selected foodborne pathogens and their ID 50 values in humans (as determined from epidemiologic data and studies on human volunteers). Keep in mind that these are median values. The actual infective dose for an individual can vary widely, depending on factors such as route of entry; the age, health, and immune status of the host; and environmental and pathogen-specific factors such as susceptibility to the acidic pH of the stomach. It is also important to note that a pathogen’s infective dose does not necessarily correlate with disease severity. For example, just a single cell of Salmonella enterica serotype Typhimurium can result in an active infection. The resultant disease, Salmonella gastroenteritis or salmonellosis , can cause nausea, vomiting, and diarrhea, but has a mortality rate of less than 1% in healthy adults. In contrast, S. enterica serotype Typhi has a much higher ID 50 , typically requiring as many as 1,000 cells to produce infection. However, this serotype causes typhoid fever , a much more systemic and severe disease that has a mortality rate as high as 10% in untreated individuals.
| ID 50 for Selected Foodborne Diseases Food and Drug Administration. “Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins.” 2nd ed. Silver Spring, MD: US Food and Drug Administration; 2012. | |
|---|---|
| Pathogen | ID 50 |
| Viruses | |
| Hepatitis A virus | 10–100 |
| Norovirus | 1–10 |
| Rotavirus | 10–100 |
| Bacteria | |
| Escherichia coli , enterohemorrhagic (EHEC, serotype O157) | 10–100 |
| E. coli , enteroinvasive (EIEC) | 200–5,000 |
| E. coli , enteropathogenic (EPEC) | 10,000,000–10,000,000,000 |
| E. coli , enterotoxigenic (ETEC) | 10,000,000–10,000,000,000 |
| Salmonella enterica serovar Typhi | <1,000 |
| S. enterica serovar Typhimurium | ≥1 |
| Shigella dysenteriae | 10–200 |
| Vibrio cholerae (serotypes O139, O1) | 1,000,000 |
| V. parahemolyticus | 100,000,000 |
| Protozoa | |
| Giardia lamblia | 1 |
| Cryptosporidium parvum | 10–100 |
Pathogens can be classified as either primary pathogens or opportunistic pathogens. A primary pathogen can cause disease in a host regardless of the host’s resident microbiota or immune system. An opportunistic pathogen , by contrast, can only cause disease in situations that compromise the host’s defenses, such as the body’s protective barriers, immune system, or normal microbiota. Individuals susceptible to opportunistic infections include the very young, the elderly, women who are pregnant, patients undergoing chemotherapy, people with immunodeficiencies (such as acquired immunodeficiency syndrome [AIDS]), patients who are recovering from surgery, and those who have had a breach of protective barriers (such as a severe wound or burn).
An example of a primary pathogen is enterohemorrhagic E. coli ( EHEC ), which produces a virulence factor known as Shiga toxin . This toxin inhibits protein synthesis, leading to severe and bloody diarrhea, inflammation, and renal failure, even in patients with healthy immune systems. Staphylococcus epidermidis , on the other hand, is an opportunistic pathogen that is among the most frequent causes of nosocomial disease. M. Otto. “ Staphylococcus epidermidis --The ‘Accidental’ Pathogen.” Nature Reviews Microbiology 7 no. 8 (2009):555–567. S. epidermidis is a member of the normal microbiota of the skin, where it is generally avirulent. However, in hospitals, it can also grow in biofilms that form on catheters, implants, or other devices that are inserted into the body during surgical procedures. Once inside the body, S. epidermidis can cause serious infections such as endocarditis, and it produces virulence factors that promote the persistence of such infections.
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