AXIO Proficiency Testing | Coliform Test Results in LGC Proficiency Testing Schemes
Reasons for testing for coliforms
Challenges in testing for coliforms Coliforms are defined not by their taxonomy but rather a biochemical reaction defined in the method. Due to the differing definitions of coliforms and the many test methods available, it is possible for some organisms to give inconsistent results, leading to problems with interpretation. Some methods simply rely on the morphological appearance of the colonies on selective agar, and give a presumptive result only, whilst other methods require evidence of gas production to give a confirmed result. Each method is designed to detect or enumerate typical coliforms which make up the majority of the classification. Unfortunately not all coliforms behave as expected under test conditions, some possess the enzyme β - galactosidase and therefore the potential to ferment lactose, but under test conditions may or may not ferment lactose to produce acid and gas. These include weak lactose fermenters and delayed lactose fermenters. Weak and delayed lactose fermenters produce low levels of the enzymes β - galactosidase or permease, both necessary to ferment lactose. Over time in a high lactose environment, delayed lactose fermenters will more rapidly ferment the lactose to produce acid and gas. Weak and delayed fermenters may not always behave as a coliform under laboratory conditions, they can give variable results and appear negative after 24 hours using standard methods.
Coliforms may be found in a number of habitats, such as soil, aquatic environments, and the intestines of humans and animals. The presence of coliforms in food or water is therefore considered to be an indicator of cross- contamination from these habitats, and therefore a sign of poor hygiene or quality. In such circumstances the food or water may also have been contaminated by enteric pathogens. Coliforms are generally present in greater numbers and are easier to test for than pathogens, so the coliform test is used as an initial screen to assess the quality of foods and water and the possible presence of pathogens, hence the name ‘indicator organisms’. Test methods Most methods are based on the resistance of coliforms to the presence of bile salts and their ability to ferment lactose with the production of acid and/or gas. ISO 4832:2006 – Horizontal method for the enumeration of coliforms describes use of VRBL pour plates with typical coliform colonies appearing as purplish red colonies with a diameter of at least 0.5mm. Confirmation is carried out on atypical colonies using Brilliant Green Lactose Bile Broth (BGBB) and observing for formation of gas in a Durham tube. Alternative culture methods include chromogenic agars, most probable number (MPN), rehydratable dried agar film, and many more.
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