EXTENDED INCUBATION OF LST AND BGB TUBES AND THE MPN ESTIMATES OF COLIFORMS

Extending the incubation time past the standard 48 h for presumptive l auryl sulfate tryptose (LST) broth and confirmed brilliant green bile (BGB) broth tests for coliforms resulted in increases in most probable number (MPN) estimates. Whereas 40% of the samples showed an increase in the presumptive MPN when the incubation was extended from 48 to 72 h, only 5% showed confirmed MPNs, which exceeded the 95% confidence l imits established for the 48-h confirmed MPNs. Extending the incubatio n of BGB tubes to 72 h resulted in less than 5% of the samples exhibit ing increased MPNs, which exceeded the 48-h 95% confidence limits. Som e loss in viability of coliforms was observed when LST tubes were incu bated beyond 72 h. The study demonstrates the importance of adhering t o standard incubation times for interlaboratory comparisons and ensuri ng regulatory compliance.