Virulence and Risk of HPC Bacteria in Human Population Groups

THE ISSUE

plate count is a means of assessing the concentration of these bacteria in foods, water, and water filtration systems. Methods vary, but are designed to enumerate bacteria that have evolved an environmental lifestyle. Most commonly, low nutrient, low ionic strength culture media are employed. The group of environmental bacteria enumerated depends on the media formulation and incubation conditions but are commonly known as heterotrophic plate count (HPC) bacteria; in Europe this group is also referred to as autochthonous flora. While HPC inhabit an environmental niche, there has been concern that at some concentration they may be a human health risk.

RESEARCH STRATEGY

A peer-reviewed research paper was prepared by Martin J. Allen, AWWA Research Foundation and Stephen C. Edberg, Ph.D., A.B.M.M., Yale University School of Medicine.

MAJOR FINDINGS AND SIGNIFICANCE

A review of the literature, including animal and human feeding studies, analysis of virulence factors, and outbreaks demonstrates that HPC bacteria as enumerated on HPC culture media have not been established as a human health threat at any concentration in drinking water or foods. The evidence can be divided into three categories:

• Laboratory evidence: HPC bacteria have not been shown to possess virulence factors in significant amounts that are associated with human disease. Studies to date have shown the 1-2% of HPC bacteria possess possible virulence factors, and these in bacterial species not associated with disease.
• Animal and human evidence: the one large study, which examined the effects of high concentrations of HPC bacteria inoculated into highly immunocompromised, mice failed to demonstrate disease. Human feeding studies have not demonstrated significant pathogenic potential with Aeromonas and Pseudomonas. At best, transient colonization in humans have been established in patients receiving broad spectrum antibiotics; this colonization ceases once the antibiotic therapy ends.
• Epidemiological evidence: Three large (Calderon, 1988; Calderon, 1991; Hilliard, 2001) and one small (Colford, 2002) epidemiological studies failed to show any association between HPC and gastroenteritis. One (Payment, 1991) showed equivocal results, with an association between HPC and gastroenteritis but no association between the amount of drinking water consumed and gastroenteritis.
  

Edberg, S.C., and Allen, M.J., Virulence and Risk of HPC Bacteria in Human Population Groups, April 2002, submitted for publication in the International Journal of Food Protection.

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