Invasive Group A Streptococcal Penicillin Binding Protein 2× Variants Associated with Reduced Susceptibility to β-Lactam Antibiotics in the United States, 2015–2021

Abstract

All known group A streptococci [GAS] are susceptible to β-lactam antibiotics. We recently identified an invasive GAS (iGAS) variant (<i>emm43.4</i>/PBP2x-T553K) with unusually high minimum inhibitory concentrations (MICs) for ampicillin and amoxicillin, although clinically susceptible to β-lactams. We aimed to quantitate PBP2x variants, small changes in β-lactam MICs, and lineages within contemporary population-based iGAS. PBP2x substitutions were comprehensively identified among 13,727 iGAS recovered during 2015-2021, in the USA. Isolates were subjected to antimicrobial susceptibility testing employing low range agar diffusion and PBP2x variants were subjected to phylogenetic analyses. Fifty-five variants were defined based upon substitutions within an assigned PBP2x transpeptidase domain. Twenty-nine of these variants, representing 338/13,727 (2.5%) isolates and 16 <i>emm</i> types, exhibited slightly elevated β-lactam MICs, none of which were above clinical breakpoints. The <i>emm43.4</i>/PBP2x-T553K variant, comprised of two isolates, displayed the most significant phenotype (ampicillin MIC 0.25 μg/ml) and harbored missense mutations within 3 non-PBP genes with known involvement in antibiotic efflux, membrane insertion of PBP2x, and peptidoglycan remodeling. The proportion of all PBP2x variants with elevated MICs remained stable throughout 2015-2021 (<3.0%). The predominant lineage (<i>emm4</i>/PBP2x-M593T/<i>ermT</i>) was resistant to macrolides/lincosamides and comprised 129/340 (37.9%) of isolates with elevated β-lactam MICs. Continuing β-lactam selective pressure is likely to have selected PBP2x variants that had escaped scrutiny due to MICs that remain below clinical cutoffs. Higher MICs exhibited by <i>emm43.4</i>/PBP2x-T553K are probably rare due to the requirement of additional mutations. Although elevated β-lactam MICs remain uncommon, <i>emm43.4</i>/PBP2x-T553K and <i>emm4</i>/PBP2x-M593T/<i>ermT</i> lineages indicate that antibiotic stewardship and strain monitoring is necessary.