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

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. 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. 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. 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.