Colilert® is one of the standard test methods widely accepted for the determination of absence/presence of coliforms. Each test normally is negative and production continues with a sigh of relief. However, there is sometimes the nagging doubt that maybe this test is not working for some reason.
There is an answer to this dilemma which is a calibration technique known as Quant-Cult®. This kit provides safe samples of E.coli, K.pneumoniae and Ps. aeruginosa and a method of testing which is straightforward. The kit comes with small vials of the bacteria which are contained in the caps of the vials. The vials contain rehydration fluid which comes into contact with the bacteria when the vials are inverted. After a brief incubation period of 10 mins, the vial contents are added to 100ml sample jars followed by the standard Coilert® powder. After the normal incubation time, the sample with Ps. aeruginosa should remain colourless, the K. pneumoniae should be yellow and also the E. coli. The E. coli will fluoresce under UV light.
Make sure that the UV lamp is rated at at least 6W otherwise seeing the fluorescent effect will be difficult.
The standard Colilert® Test is very sensitive with only one coliform giving a positive reading. It is important therefore to be very scrupulous with hygiene when using the technique. It is essential to use sterile sample jars.
A further issue with sample jars that sometimes causes confusion, is that the suppliers provide sample jars containing sodium thiosulphate. This is unnecessary with water samples in the bottling plant because the purpose of the thiosulphate is to neutralise the effect of chlorine in treated tap water.
There is another method of detecting coliforms which is much less expensive than the Colilert® route. This involves a filtration technique with inoculation of a nutrient medium. However, the Colilert® technique is much easier to use.
Another thing to watch out for is not to leave the sample jar in the incubator for longer than the standard time. A yellow colour will develop eventually thus giving a false positive.
A further reminder, if the Colilert® powder appears to have lost its finely divided, free-flowing appearance do not use it, this will be an indication of hydrolysis and degradation, which again will give a false positive.
Finally, if you find detection of a positive result is made difficult by a yellow background caused by other effects, and use of the comparator is border line, then consider the use of Colisure® instead. This will give you a magenta colour as a positive result.