Manner and meaning of susceptibility testing of ampicillin-resistant Haemophilus influenzae

We examined ampicillin-resistant strains of Haemophilus influenzae to compare the percentage of resistant organisms in each strain with the susceptibility to ampicillin by an agar dilution method. Using an inoculum of 10(4) colony-forming units, the minimal inhibitory concentration (MIC) increased with the percentage of resistant organisms in the strain. Laboratory-manipulated strains composed of different proportions of a susceptible and a resistant strain behaved similarly. The survival of isolated colony-forming units (colony MIC) was then determined by spreading inocula over the surface of a set of MIC plates, resulting in separation of individual colonies. This modification of the susceptibility test to the colony level gave end points that were clear and reproducible and that did not vary with changes in incubation time or temperature. True differences in susceptibility among strains were demonstrated by this method, whereas results of the conventional MIC test may reflect only the number of resistant organisms present in the inoculum.