1. From the Endo’s medium or lactose-litmus-agar plate made as
prescribed under “B,” fish at least two typical colonies, transferring
each to an agar slant and a lactose broth fermentation tube.
2. If no typical colonies appear upon the plate within 24 hours, the
plate should be reincubated another 24 hours, after which at least two
of the colonies considered to be most likely B. coli, whether typical or
not, shall be transferred to agar slants and lactose broth fermentation
tubes.
3. The lactose broth fermentation tubes thus inoculated shall be
incubated until gas formation is noted; the incubation not to exceed 48
hours. The agar slants shall be incubated at 37° C. for 48 hours, when a
microscopic examination shall be made of at least one culture, selecting
one which corresponds to one of the lactose broth fermentation tubes
which has shown gas-formation.
The formation of gas in lactose broth and the demonstration of
non-spore-forming bacilli in the agar culture shall be considered a
satisfactory completed test, demonstrating the presence of a member of
the B. coli group.
The absence of gas-formation in lactose broth or failure to demonstrate
non-spore-forming bacilli in a gas-forming culture constitutes a
negative test.