Effect of extended-spectrum â-lactamases on the susceptibility of Haemophilus influenzae to cephalosporins

The extended-spectrum β-lactamases (ESBLs) TEM-3, TEM-4 and TEM-5 were cloned into Haemophilus influenzae. These recombinant strains exhibited cefotaxime MICs of 0.5, 0.25 and 0.12 mg/L for TEM-3, -4 and -5, respectively, and the MIC of cefaclor was 4.0 mg/L. These MICs are higher than those of β-lactamase-negative strains, or those producing simple wild-type TEM-1 β-lactamase, but not high enough to be categorized as resistant according to the breakpoints of the NCCLS. The clones were also categorized as susceptible using NCCLS disc diffusion methodology and interpretive criteria. This study shows that current NCCLS susceptibility testing methods may have difficulty in detecting ESBLs if they were to occur in H. influenzae.