Emergence and prevalence of β-lactamase producing Haemophilus influenzae in Finland and susceptibility of 102 respiratory isolates to eight antibiotics

Abstract

A survey of 102 consecutive clinical isolates of Haemophilus influenzae mainly from otolaryngological patients revealed 13 ampicillin resistant ones, while 2 years earlier none were found. All the 13 strains which were resistant according to the broth dilution minimal inhibitory concentration (MIC) could be shown to produce β-lactamase using the chromogenic cephalosporin 87/312. Routine disc diffusion susceptibility testing had under-estimated the resistance and five of the β-lactamase producing strains had been reported as ampicillin susceptible. Amoxycillin and azidocillin were equally active and slightly less active than ampicillin against β-lactamase negative strains. The 13 ampicillin resistant strains were also resistant to amoxycillin and azidocillin, and also the MICs of cephalothin and cephalexin were significantly increased by β-lactamase production, while cefuroxime, erythromycin and chloramphenicol were not affected. A marked inoculum effect was demonstrated with the β-lactam antibiotics, and only a narrow (about tenfold) range of inoculum concentrations distinguished reliably between susceptible and resistant strains. The MICs of the penicillins against β-lactamase positive H. influenzae increased markedly with incubation time.