Isolates of β-lactamase-negative ampicillin-resistant haemophilus influenzae causing invasive infections in Spain remain susceptible to cefotaxime and imipenem

Abstract

Objectives: The epidemiology of invasive Haemophilus influenzae has changed in recent years. b-Lactamasenegative ampicillin-resistant (BLNAR) invasive isolates have recently been described in Europe but their clinical significance is unclear. Our main goal was to determine whether invasive H. influenzae remains susceptible to b-lactam antibiotics indicated in the treatment of invasive infections. Methods: Theantibiotic susceptibility of307invasive H. influenzae isolatesto sevenb-lactam antibioticswas determined by microdilution and interpreted by EUCAST and CLSI breakpoints. We also identified the bla genes, the amino acid substitutions in the transpeptidase domain of penicillin-binding protein 3 (PBP3), the molecular epidemiologyof invasiveBLNARisolatesbyPFGEandMLST, andthe time-kill curves oftwoisolates withPBP3mutations conferring reduced susceptibility to aminopenicillins and cephalosporins. Results: Of the invasive isolates, 86.6%were non-typeable and62%were isolated fromadults. Decreased susceptibility to b-lactams was due to the BLNAR genotype (gBLNAR; 19.2%) and to b-lactamase production (16.9%). Susceptibility rates to amoxicillin/clavulanic acid, cefotaxime, cefixime and imipenem were greater than 98%. Of 18 gBLNAR non-typeable isolates studied by MLST, 15 different STs were obtained. Amoxicillin and cefotaxime were bactericidal after 2 and 4 h of incubation, respectively. Conclusions: Invasive H. influenzae diseasewas mainlydue to non-typeable isolates infecting adults, and the most common mechanism of b-lactam resistance was mutations in the transpeptidase domain of PBP3. The gBLNAR non-typeable isolates were genetically diverse. The majority of invasive H. influenzae remained susceptible to third-generation cephalosporins; amoxicillin and cefotaxime were bactericidal in two gBLNAR isolates. © The Author 2013. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.