Pathogenic Bacteria

This short course makes no attempt to provide comprehensive training on food safety with respect to cheese manufacture. However, some food safety principles will be discussed in the context of other topics, for example, acid control and food plant sanitation. Here, we mention only some characteristics of a few pathogens which are particularly significant to cheese making. We begin with definitions to distinguish between food infection and food intoxication.

Food infections are caused by organisms which grow in the gastro intestinal track. Illness occurs after ingestion of an infectious dose which depends on many factors including the health status of the person.

Food intoxication results from toxins produced by bacteria. Toxins may be present within the bacteria (endotoxin) or excreted outside the bacteria (exotoxin). The organism need not be alive or even present to cause illness. A good example, is Staphylococcus aureus. Like all the other pathogenic bacteria listed below, Staphylococcus aureus is destroyed by pasteurization but its enterotoxin survives pasteurization.

Pathogens: common before 1940

  • Corynebacterium diptheriae: causes diphtheria
  • Brucella abortus: causes brucellosis in cows and undulant fever in people.
  • Mycobacterium tuberculosis: causes TB in people.
  • Coxiella burneti: causes Q fever in people.

Pathogens which emerged during 1940 - 1970

  • Staphylococcus aureus: food intoxication caused by heat stable enterotoxins of which enterotoxin A is the most common.
  • Salmonella species: Salmonellosis is an infection caused by many species and strains of species of Salmonella. Salmonellosis is of great concern to the dairy industry, especially the cheese and milk powder sectors. Infectious doses can be extremely low, perhaps as low as a single organism.
  • Enteropathogenic E. Coli produce enterotoxins, some of which are heat stable. Of numerous species and strains, the most famous is E. Coli 0157 H7, which occurs frequently in raw milk. E. Coli0157 H7 is of particular concern because it is quite acid tolerant and is able to grow at refrigeration temperatures.

Recent Pathogens

  • Yersina enterocolitica is a psychrotrophic infectious agent.
  • Campylobacter jejuni, is an infectious agent which has passed Salmonella as the leading cause of diarrhoea all over the world.
  • Listeria monocytogenes is a psychrotrophic infectious agent, which requires special caution because it is acid tolerant and more heat stable than most pathogens, although it does not survive proper pasteurisation.
  • Bacillus cereus is mainly important as spoilage agent. However, some strains are mildly pathogenic which is problematic because Bacillus cereus forms heat stable spores which survive pasteurization and are able to grow at refrigeration temperatures.