Science of Pathogenicity

Pathogenicity is the ability to cause illness. Because pathogens are living microorganisms that rapidly adapt and evolve, the methods they use to cause illness are never static. Pathogen evolution is driven by a variety of forces, only some of which relate to human activities. The continual evolution of microorganisms forces us to change food production processes and products to continue to effectively control microbiological food safety.

Fortunately, genomic and improved molecular and imaging techniques have vastly expanded scientific understanding of the microorganisms that cause foodborne disease. These tools also have enabled scientists to attribute foodborne disease to microorganisms that had not previously been identified as pathogenic or as foodborne. However, researchers still have many questions to answer, and each new discovery opens new avenues of research.

Pathogen Characteristics

The characteristics of foodborne pathogens are part of what makes microbiological food safety issues so complex. One type of microorganism may thrive under conditions that are fatal to a different microbe. Some pathogens cause disease by infecting the human host, while others produce toxins that cause illness when consumed. Some pathogens can multiply in food during storage while others cannot. Because most microorganisms can reproduce within a matter of minutes, these pathogens can evolve quickly when environmental stresses select for strains with unique survival characteristics.

Although bacteria are perhaps the first type of microorganism that comes to mind when discussing microbiological food safety, they are by no means the only pathogenic foodborne microorganisms. Parasites, viruses, and biotoxins all present food safety challenges, and these agents have properties that make them quite different from the common bacterial agents of foodborne disease.


Our scientific understanding of the microbiology of foodborne pathogens continues to improve. Scientists are only just beginning to understand the factors that cause a particular microbial strain to be pathogenic while other strains of the same microorganism are not, the ways by which some microorganisms adapt and evolve to become pathogenic, and the mechanisms pathogens employ to adapt to differing environments. Further research is essential to understand microbial ecology and virulence sufficiently well to anticipate future microbial hazards and construct barriers to disease. Some pathogenic microorganisms are significantly more virulent than others.

Virulence may vary within species, subspecies, and even different strains. Understanding the many different virulence factors that microorganisms use to cause illness offers opportunities to develop better controls and therapeutics. Further research will enable scientists to classify pathogens based on specific virulence factors rather than based on name, serotype or other traits unrelated to virulence. This research will improve our evaluation of safety, which currently is focused on microbes that may or may not be pathogenic.

Evolution and Selection

Evolution and selection are closely related. Evolution produces microorganisms that are distinctly different from previous generations; selection gives these mutant strains an advantage and causes them to become predominant. Selection forces also can increase the prevalence of known pathogens. Together, evolution and selection have a profound impact on the emergence of foodborne pathogens.


Adverse changes in the microbial environment may cause bacteria to experience stress. Although the stress kills many bacteria, some may survive, because bacteria have elaborate systems to adapt to environmental stress. In addition to tolerance of the original stress, the surviving microorganisms also may be tolerant to other unrelated stresses, and these tolerant microorganisms may demonstrate increased virulence. Understanding these response mechanisms will provide the information necessary to refine food processing conditions or to develop other appropriate intervention strategies that enhance food safety.

Better Tools

Recent advances in genomics have contributed to the further understanding of virulence. Genomics also has greatly facilitated our understanding of the continuous and sometimes rapid evolution of pathogens. Improved analytical systems are needed to gather better data about pathogens in the food production and processing environments. More sensitive quantitative methods are necessary for assessing pathogen growth, survival, and inactivation, as well as for accurate risk assessments.

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