Coliform Group

Coliform Group

	Coliform Group The coliform group includes two genera of bacteria, the Escherichia and the Aerobacter. This group is defined as all aerobic and facultative gram negative non-spore-forming bacilli which ferment lactose with gas production. The identification of the group can be limited to the following characteristics: 1. (a) ability to ferment lactose with gas formation in nutrient lactose broth followed by (b) ability to ferment lactose in brilliant green bile lactose broth.
2. (a) ability to ferment lactose with gas formation in nutrient lactose broth followed by (b) ability to grow on surface of eosin-methylene blue agar. The organisms isolated from these mediums should ferment lactose in nutrient lactose broth. The organism should be gram negative and not form spores. For E.coli to be a useful indicator organism of faecal pollution, it must be differentiated readily from nonfaecal bacteria. The conventional test for the detection of faecal contamination involves a three-stage test procedure: the presumptive test, the confirmed test and the completed test. Presumptive Test The first step in the bacteriological analysis is the development of the coliform organisms in standard lactose broth. The coliform group ferments lactose with the production of gas. The production of gas from lactose is a presumptive test for the group but not a definitely positive reaction because a few other bacteria, not necessarily of sanitary significance, may also produce the same reaction. The most common bacteria other than the coliform group which produce gas are the sporeformers, both anaerobic and aerobic.
The amount of gas produced in a 48 hour period of incubation at 37°C has no particular significance. Some strains of the coliform organisms produce only slight amounts of gas. Any amount of gas should be considered a positive presumptive test if the gas appears within 48 hour of incubation at 37°C. Confirmed Test To demonstrate the aerobic nature of the coliforms, smears are made on EMB medium from the tubes showing gas. The smears should be made such that discrete colonies are produced.
It must be remembered that the growth in the lactose broth is due to many species of bacteria in addition to the coliform organisms. Unless discrete colonies are formed, it is impossible to make isolations representing pure cultures because the confluent growth is a mixed population. The appearance of the confluent growth as indicating coliform organisms may be very misleading owing to the multiplicity of reactions induced by the different species of bacteria that may be present. By smearing the surface of the agar plate, the coliform group is able to grow but the anaerobic sporeformers are eliminated as they are unable to grow aerobically. The dyes in the medium also act as a deterrent to many of the undesirable bacteria. To distinguish the coliform organisms from other organisms that might grow on the agar plate, the use of special media like EMB and Endo agar give the coliform colonies a distinctive appearance that aids in their identification.
	Completed Test To make sure that either typical or atypical colonies are members of the coliform group, the colonies fished are planted into lactose broth and on an agar slant. If the organisms produce gas in the fermentation tube, then the corresponding agar-slant culture should be examined for spores and gram's reaction. These tests eliminate the aerobic, lactose-fermenting, gram positive sporeformers. The only organisms that successfully carry through these tests are the members of the coliform group.

A rapid test is also done for identifying the coliforms. In this procedure the presumptive test is made as usual but instead of smearing eosin-methylene blue agar or endo's medium, plantings are made into one of the following mediums: brilliant green bile lactose broth or crystal violet broth. If possible, platings should be made from the presumptive lactose-broth tubes as soon as gas appears. The plantings are incubated for 48 hours at 37°C. If gas appears during this period of incubation, the test is reported as positive.

E.coli meets many of the criteria for an ideal indicatory organism but there are limitations to its use as such and various other species have been proposed as additional or replacement indicators of water safety.

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