A comparison between MacConkey broth and glutamic acid media for the detection of coliform organisms in water

Abstract

Folpmers's glutamic acid medium and the lactose modification described by Burman & Oliver (1952) have been compared with MacConkey broth in the pre-sumptive coliform test of 4421 samples of water examined at six different laboratories in England and Wales. As compared with MacConkey broth, both glutamic acid media gave between 8 and 10% more isolations of Bact. coli I; the glucose medium additionally gave an increase of 6% in other coliform organisms, but this was associated with a very heavy excess (232%) of false positive results; lactose glutamic acid satisfactorily controlled false positive reactions, giving a reduction of 36%, but also suppressed by 35% the isolations of coliform organisms other than Bact. coli I.MacConkey broth gave the largest early (18 hr.) yield of positive results, but the results at the end of 24 hr. were approximately the same with all three media. Preliminary warming of the inoculated tube had only a trivial effect on the rapidity with which the organisms grew, but the warming resulted in a slightly higher total yield of Bact. coli I in the glutamic acid media. Incubation at 42° C. for 18 hr. followed by 37° C. for 30 hr. was found to be inferior to incubation at 37° C. for 48 hr. in the case of lightly polluted waters, in that true coliform organisms were suppressed without any compensating advantage in the yield of Bact. coli I.