Characterization of BAS-1, a novel class A β-lactamase encoded in Bacteroides thetaiotaomicron

We characterized a novel class A β-lactamase (BAS-1) in a pan-β-lactam-resistant Bacteroides thetaiotaomicron clinical isolate. Whole-genome sequencing identified bla BAS-1 within a mobile genetic element associated with IS 4351 . Cloning, susceptibility testing, β-lactam inactivation method, and half-maximal inhibitory concentration assay confirmed resistance to penicillins, cephalosporins, monobactams, and reduced susceptibility to the β-lactamase inhibitors. To date, no class A β-lactamase from Bacteroides has been shown to display such a resistant profile, challenging the assumed reliability of β-lactam/β-lactamase inhibitor combinations.