Impact of different amino acid substitutions in penicillin-binding protein 3 on beta-lactam susceptibility in Haemophilus influenzae

Purpose Beta-lactam antibiotics in combination with a beta-lactamase inhibitor are the first-line treatment option for Haemophilus influenzae infections. However, beta-lactamase-independent resistance to beta-lactams is increasing. This resistance mechanism has been linked to amino acid substitutions in the penicillin-binding protein 3 (PBP3), but how these substitutions lead to decreased binding affinities to certain beta-lactam antimicrobials remains unknown. Methods We investigated beta-lactam resistance and amino acid substitutions in PBP3 from fifty-three clinical isolates of H. influenzae collected in Switzerland from January to April 2016. Identification of key polymorphisms and classification of strains into PBP3 amino acid substitution groups I, II, and M was done as previously described. Based on published PBP3 crystal structures, we investigated how the group-specific amino acid substitutions impact the beta-lactam binding site. Results We found that both group I and group II substitutions disrupt the Asn526-Arg517-Glu324 interaction, which might affect the configuration of the beta-lactam binding site. Amino acid substitutions in group M strains are distant from the active site and have most likely no impact on beta-lactam binding. In accordance with this observation, all group M strains showed minimal inhibitory concentrations (MICs) within the susceptible range for all tested antimicrobials and were not significantly different to the wild type (beta-lactamase producers excluded), while group I and group II strains showed significantly higher MICs for beta-lactam antimicrobials. Conclusion Group M strains are phenotypically equal to the wild type, while amino acid substitutions of group I and group II might affect the beta-lactam binding through a common mechanism by disrupting the Asn526-Arg517-Glu324 interaction.