E-tests of the strains O35E, 300 and 415 and their respective isogenic m35 mutants with ampicillin (A), amoxicillin (B) and amocixillin-clavulanate (C). Bacteria were cultured on agar plates together with E-test strips overnight. Bars show the minimum inhibitory concentration (MIC) for each antibiotic. Data are presented as means ± 1 SD (n = 3). * p < 0.05 for wild-type vs. respective mutant.