Saturday, March 28, 2009

The temperature's rising (but why?)

I was recently asked to explain some of my comments on detection of faecal coliforms. <See Coliforms and Faecal Contamination Wednesday, July 18, 2007>

I have extracted the essentials of the questions below:

“Since the coliforms when tested… are grown at optimum growth temperature of 37C, why would you need to raise the faecal coliform test temperature to 44.5C to show thermotolerant E. coli are present? Why would food safety people be looking for thermotolerant bacteria in food rather than bacteria that grow at normal temperatures?”

In my article, I provided some background information on "faecal indicator microorganisms", which we use to show that a food or water sample may have been contaminated with faeces. These tests originated in the early days of public health services and safety of public water supply. The coliforms are easier to detect and enumerate than are Salmonella or Shigella or faecal viruses. (Salmonella and Shigella are not coliforms). The reason for incubating at 44.5C to demonstrate the presence of Escherichia coli dates from 1904 when Eijkman suggested it as a means of separating the "B. coli" originating in the faeces of warm blooded animals from the strains characteristic of cold blooded animals and thus providing us with a means to detect faecal contamination of water supplies by warm blooded animals, including humans.

So the answer to the first question is “we are not particularly interested in thermotolerant coliforms; rather we want to show that the water supply may have been contaminated by human wastes and hence potentially contains faecal pathogens”.

Having said that, of course there are other areas of food microbiology where we are very interested in the presence of thermotolerant or thermophilic bacteria. If we pasteurize milk with plate heat exchangers (the standard method) we may find that the cooling stages become colonized by thermotolerant streptococci, which may cause spoilage. In the case of canned foods, we find that some sporeforming bacteria can survive even a very rigorous thermal process. These bacteria are of no public health significance, but they may cause spoilage if the cans are held at high temperatures (greater than about 40C) as might be found in storage facilities in very hot countries or in a restaurant kitchen.

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