Indicator tests; which to use?
The Enterobactericeae, coliform and E.coli enumeration tests are all examples of hygiene indicator tests. But which test should you use?
Indicator tests provide useful information about the effectiveness of heat processing of food, and the hygienic conditions of food production. There are several advantages of using indicator tests:
Simple laboratory tests with results available in 24 hours
Indicator organisms are easier to detect than pathogens
The source and behaviour of the indicator organisms is similar to that of enteric pathogens
The presence of indicator organisms gives an early warning of a problem
The most commonly used of the indicator tests is the Enterobacteriaceae test (or Entero for short). The Enterobacteriaceae group is a large family of bacteria which include those originating from plants, water and the intestinal tract of humans and animals. The Entero group includes enteric pathogens such as Salmonella, Shigella and harmful strains of E.coli, as well as many, harmless environmental organisms.
Since the Entero test covers such a wide range of organisms, it gives a more general indication of the hygienic quality of food production. Enteros are heat sensitive, so a negative Entero result gives confidence that food has been cooked correctly and made in hygienic conditions.
Most coliforms are members of the Enterobacteriaceae family. Coliforms are bacteria that ferment lactose in the presence of bile salts. Why is this relevant? Since lactose is the sugar found in milk, and bile salts are found in human and animal intestines, so coliforms are designed to grow in human and animal intestines. Coliform media used by laboratories contains lactose and bile salts to encourage growth of coliforms.
So the presence of coliforms in food is an indication of possible faecal contamination, but not a guarantee, since you can also find coliforms in the environment, in water and on food equipment.
A limitation of coliform testing is that some of the enteric pathogens do not ferment lactose, so they will not grow on a coliform plate; a negative coliform result does not mean absence of pathogens. Nevertheless, coliform testing is still used in the dairy industry as a useful tool for evaluating the hygienic quality of milk.
E.coli is a natural component of human and animal gut flora. The vast majority of E.coli strains are harmless, but there are some dangerous strains such as E.coli O157, and other emerging serotypes. The presence of E.coli in food indicates faecal contamination. The usefulness of E.coli as an indicator test is questionable, since its presence does not necessarily correlate with the presence of other enteric pathogens such as Salmonella, and the testing for E.coli is more involved than the other tests.
The Entero test is the preferred indicator method. Entero testing is recommended by EFSA (European Food Safety Authority) and is listed as the indicator test in the HPA (Health Protection Agency) guidelines for ready-to-eat food. Most of the major retailers have Enteros listed on their own-brand product microbiological specifications.
In some instances, you may wish to test for E.coli as well; if there is potential for cross contamination, a history of a problem, or your product contains fresh fruit and vegetables that could be a source of E.coli.
Satisfactory results for ready-to-eat food:
Enteros or coliforms <100 cfu/g
E.coli <10 cfu/g or <20 cfu/g.
Any result exceeding this criteria must be investigated. It is recommended that if high Entero results persist, testing should be carried out for E.coli and possibly Salmonella.
EC 2073-2005 lays down criteria for Enteros in pasteurised milk which should have <1 cfu/ml
Higher Entero results are expected in un-processed fruit and veg
E.coli can be present in high numbers in mould ripened cheese. This is normal due to the nature of the cheesemaking process. However, the risk of contamination and growth from harmful strains of E.coli must be evaluated and monitored.