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

Objectives The epidemiology of invasive Haemophilus influenzae has changed in recent years. β-Lactamase-negative 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 β-lactam antibiotics indicated in the treatment of invasive infections. Methods The antibiotic susceptibility of 307 invasive H. influenzae isolates to seven β-lactam antibiotics was 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 epidemiology of invasive BLNAR isolates by PFGE and MLST, and the time–kill curves of two isolates with PBP3 mutations conferring reduced susceptibility to aminopenicillins and cephalosporins. Results Of the invasive isolates, 86.6% were non-typeable and 62% were isolated from adults. Decreased susceptibility to β-lactams was due to the BLNAR genotype (gBLNAR; 19.2%) and to β-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 disease was mainly due to non-typeable isolates infecting adults, and the most common mechanism of β-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. [ABSTRACT FROM PUBLISHER]