Effect of cloned inhibitor-resistant TEM β-lactamases on the susceptibility of Haemophilus influenzae to amoxicillin/clavulanate

Objectives To determine the effect of cloned inhibitor-resistant TEM β-lactamases (IRTs) on the susceptibility of Haemophilus influenzae to amoxicillin/clavulanate. Methods IRT-2, -4 and -5 genes with various promoters were cloned into control strains of H. influenzae and the amoxicillin/clavulanate MICs were measured using Etests. Results IRT enzymes were able to raise the amoxicillin/clavulanate MICs to between 0.38/0.19 and 4.0/2.0 mg/L depending on the IRT and promoter genotype, compared with MICs of 0.19/0.09 to 0.5/0.25 mg/L for the corresponding strains with TEM-1. Strains with an IRT and altered penicillin-binding proteins had amoxicillin/clavulanate MICs as high as 8.0/4.0 mg/L. Conclusions Cloned IRT enzymes in H. influenzae raise the amoxicillin/clavulanate MICs to an extent comparable to naturally occurring strains with decreased amoxicillin/clavulanate susceptibility.