Failure to correlate ampicillin and amoxicillin resistance by standard disk susceptibility tests and minimal inhibitory concentration in Haemophilus species.

22 Haemophilus strains, isolated from lower respiratory tract infections, were tested for their susceptibility to ampicillin and amoxicillin using both MIC and Kirby Bauer methods. Ampicillin and amoxicillin resistance tested with MIC method was not exactly correlated with Kirby Bauer standard (10 mcg ampicillin and 25 mcg amoxicillin disks) inhibition zones method (error rate: 52.6%). Yet using a 2 mcg ampicillin and 5 mcg amoxicillin handmade disk, error range decreased to 14.8%. Beta-lactamase production on whole cells were for 6 strains of the 22 tested. After breaking the cell wall, 19 on the 22 strains were beta-lactamase producers, confirming the results obtained with the MIC method.