Standard methods for the examination of water and sewage

by American Public Health Association. Laboratory Section; American Chemical Society; Association of Official Agricultural Chemists

Counting shall in all cases be done with a lens of 2½ diameter’s magnification, 3½X. The Engraver’s Lens No. 146 made by the Bausch & Lomb Optical Company fills the requirements, and is a convenient lens for the purpose.

8. THE TEST FOR THE PRESENCE OF MEMBERS OF THE B. COLI GROUP.

It is recommended that the B. coli group be considered as including all non-spore-forming bacilli which ferment lactose with gas formation and grow aërobically on standard solid media.

The formation of 10 per cent. or more of gas in a standard lactose broth fermentation tube within 24 hours at 37° C. is presumptive evidence of the presence of members of the B. coli group, since the majority of the bacteria which give such a reaction belong to this group.

The appearance of aërobic lactose-splitting colonies on lactose-litmus-agar or Endo’s medium plates made from a lactose broth fermentation tube in which gas has formed confirms to a considerable extent the presumption that gas-formation in the fermentation tube was due to the presence of members of the B. coli group.

To complete the demonstration of the presence of B. coli as above defined, it is necessary to show that one or more of these aërobic plate colonies consists of non-spore-forming bacilli which, when inoculated into a lactose broth fermentation tube, form gas.

It is recommended that the standard tests for the B. coli group be either (A) the Presumptive, (B) the Partially Confirmed, or (C) the Completed test as hereafter defined, each test being applicable under the circumstances specified.

A. PRESUMPTIVE TEST.

1. Inoculate a series of fermentation tubes with appropriate graduated quantities of the water to be tested. In every fermentation tube there must always be at least three times as much medium as the amount of water to be tested. When necessary to examine larger amounts than 10 cc. as many tubes as necessary shall be inoculated with 10 cc. each.

2. Incubate these tubes at 37° C. for 48 hours. Examine each tube at 24 and 48 hours, and record gas-formation. The records should be such as to distinguish between: