FAQ: Science of Pathogenicity

How many microorganisms does it take to cause illness?

The number of microorganisms necessary to cause illness, known as the pathogen’s infectious dose, varies greatly depending on the pathogen. It may take more than 100 million cells of one pathogen to cause illness, while the infectious dose of an extremely virulent pathogen might be less than 10 cells. In addition, other factors may affect the infectious dose. For example, the infectious dose is usually lower for people with compromised or underdeveloped immune system function, including children and the elderly. Sometimes the characteristics of the food affect infectious dose, e.g., a high fat content may protect a pathogen from stomach acid and decrease the number of microorganisms necessary to cause illness. In addition, certain medications may facilitate a pathogen’s ability to overcome the immune system defenses in the gastrointestinal tract.

Why do some pathogens make us sicker than others? Pathogens cause illness in a variety of ways, so the symptoms vary as well. Some invade the gastrointestinal tract, some invade other portions of the body, and some create toxins affect various systems in the body. The immune system responds differently to different types of pathogens, which can affect the symptoms too.

My grandmother did things a certain way, and no one got sick. Why isn’t that still ok?

In some cases, the pathogens during your grandmother’s day were quite different from those you face today. In addition, the food itself may be different. When a pathogen evolves to become more virulent or a food contains a new pathogen, our food safety practices must change. Sometimes the new pathogen is addressed through changes in food processing, but sometimes it is more effectively addressed by changes in food handling and preparation. Sometimes is it best addressed by a combination of efforts. As a result, activities that were safe in the past may now pose a food safety concern. Also, sometimes new information allows us to link illness to activities or foods that had previously been unrecognized as a source of foodborne illness. Once the issue is identified, action can be taken to mitigate illness.

How and why do pathogens evolve?

Pathogens evolve in two basic ways: mutation and genetic exchange.

When bacteria divide, most of the offspring look and act just like their parent, but a very small proportion of the offspring mutate, increasing the chance that some might survive in a new, hostile environment. Environmental forces may select a mutant that confers an advantage in the face of a particular condition, such as acidity or temperature. In addition, environmental stresses can create microorganisms with greatly enhanced mutation frequencies. The larger number of different mutations increases the chance of a mutation that will allow the microorganism to survive in the stressful environment. These hypermutable microorganisms also may more readily share genetic material with other microorganisms, even remotely related species.

Transfer of genetic material from one microorganism to another can result in quantum jumps in evolution. The genetic material that encodes virulence factors can move between bacteria, with the potential to rapidly create a new pathogen, even from a previously nonpathogenic organism. Recently sequenced bacterial genomes reveal more extensive exchange of genetic material between species than had been expected.

How quickly do pathogens evolve?

Pathogens can evolve in a matter of hours. At optimal growth conditions, a bacterial cell may divide every 10-20 minutes, so changes that provide the organism with an advantage can quickly become dominant.

How are viruses and parasites different from bacteria in terms of food safety?

Viruses do not multiply on food, and refrigeration temperatures may help preserve viruses. Although viruses can only multiply after invading the human body, viruses generally increase in numbers more quickly than bacteria; a single infected host cell may produce hundreds or thousands of new viruses within a few hours, each of which can infect a new host cell. Viruses generally have a low infectious dose.

Viruses have properties that make them environmentally stable to the extremes of pH and the enzymes in the gastrointestinal tract. This stability also enables virus particles to survive a variety of food production, processing, and storage conditions, so many food products can be potential vehicles for viral pathogens. Methods for detection and identification of viruses are not as well developed as those for many bacterial pathogens, so linking viruses to outbreaks of foodborne illness can be difficult.

Parasites also rarely multiply in or on human food. However, their host range is wider than viruses, and they can replicate in human and non-human animal hosts. In addition to having a low infectious dose, parasites in the cyst or oocyst form are extremely environmentally stable and resistant to many of the traditional methods (e.g., chlorine) used to control bacterial pathogens.

What are biotoxins, and what role do they play in food safety?

Biotoxins are produced by some bacteria, algae, and fungi. Marine biotoxins are highly potent substances that tend to accumulate in finfish and/or shellfish. The toxins are tasteless, odorless, and most often heat and acid stable, which means that routine food safety inspection and food preparation techniques will not prevent disease. Mycotoxins are produced by a wide variety of molds and other fungi. In addition to acute effects at high concentrations, chronic effects often result from prolonged ingestion of low to moderate levels of toxin that do not produce symptoms of illness at the time of consumption, making it difficult to link the chronic diseases to contaminated food.


Recognizing that food safety is a fundamental and continuing issue, the Institute of Food Technologists commissioned an expert panel to review the available scientific literature related to emerging microbiological food safety issues. The panel’s report is divided into seven sections: Introduction, Pathogenicity, Human Hosts, Microbial Ecology, Application of Science to Food Safety Management, Next Steps, and Conclusions. Copies of the report are available at www.ift.org. Founded in 1939, IFT is a 28,000 member nonprofit scientific society for food science and technology.