Detection of beta-lactamase-negative ampicillin resistance in Haemophilus influenzae in Belgium.

Haemophilus influenzae, a frequent colonizer of the respiratory tract, is the causative agent of several clinically important infections. In cases that require therapeutic intervention, laboratory susceptibility testing can detect beta-lactam antibiotic resistance and guide the best treatment course. In the absence of a beta-lactamase, beta-lactam resistance may be due to an altered form of the PBP3 protein, encoded by the ftsI gene. While these so-called beta-lactamase-negative ampicillin-resistant (BLNAR) strains are of serious clinical interest, identification in the clinical laboratory is not always straightforward. In the current study, the ftsI genes of a set of phenotypic BLNAR H. influenzae isolates taken from samples collected in the UZ Brussel hospital in Belgium were sequenced and re-tested at the National Reference Laboratory (NRC). Non-silent mutations in the ftsI gene were found in 100% of the isolates. Although 30% of the isolates were classified by the NRC as beta-lactamase-negative ampicillin-sensitive (BLNAS) strains based on the EUCAST guidelines on ampicillin minimal inhibitory concentration (MIC), all isolates showed MIC values ≥1 mg/L. These relatively high MIC values indicate a decreased susceptibility to ampicillin, and suggest that sequencing of the ftsI gene should be used as part of an antibiotic susceptibility testing (AST) algorithm in the clinical laboratory. This would allow clinicians to make better informed decisions regarding patient treatment.