Bacteria Testing

In the microbiology laboratory, water is tested for bacteria. The most common bacteria that is tested for are coliform bacteria. The coliform group consists of several genera of bacteria belonging to the family Enterobacteriaceae. The historical definition of this group has been based on the method used for the detection (lactose fermentation) rather than on the tenets of systematic bacteriology. When the fermentation technique is used, this group is defined as all facultative anaerobic, gram-negative, non-spore forming, rod-shaped bacteria that ferment lactose with gas and acid formation within 48 hours at 35ºC.

Presence/absence of fecal coliforms

The test for the presence/absence of fecal coliforms is a very useful test for the private individual who is concerned about drinking water. During this test the water is incubated with the appropriate media. After 24 hours, there are three possible results. If the sample appears unchanged, there are no coliform bacteria present. If the sample turns yellow, it indicates the presence of coliform bacteria. The third possibility occurs when the sample is exposed to an ultraviolet lamp. If the sample fluoresces, it indicates that there is Escherichia coli, a fecal coliform, is present as part of the total coliform.

Total coliform enumeration

Total coliform enumeration is performed by request on samples that are positive in the presence/absence for fecal coliform test. A small amount of the sample is spread on an agar plate and allowed to incubate. The colonies of bacteria are then counted and recorded.

Fecal coliform enumeration

Fecal coliform enumeration is performed by request on samples that are positive in the presence/absence for fecal coliform test. A small amount of the sample is spread on an agar plate and allowed to incubate. The colonies of bacteria are then counted and recorded.

Identification of E. coli, Salmonella, Shigella, Aeromonas, and Psuedomonas

Identification of E. coli, Salmonella, Shigella, Aeromonas, and Pseudomonas is performed only on positive samples from the presence/absence coliform test and is done at the request of the client.