Background
GBS is intrinsically resistant to aminoglycosides. High-level aminoglycosides resistance abrogates the synergistic bactericidal activity of the combined use of ampicillin with gentamicin in the empirical therapy for neonatal sepsis and GBS meningitis. No recommendation for aminoglycosides susceptibility testing for beta-haemolytic streptococci are present in the guidelines issued by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST). Only the French EUCAST reports breakpoints for gentamicin and streptomycin for beta-haemolytic streptococci.
Methods
Seventy-five GBS strains isolated from different sources in adult disease were tested for both gentamicin and streptomycin by disk diffusion (disk content for gentamicin 120 µg and streptomycin 300 µg).
Results
High-level gentamicin resistance was identified in three GBS strains according to the interpretative criteria for zone diameter for enterococci of CLSI. EUCAST and French EUCAST criteria could not be used due to a different recommended gentamicin disk content (30 µg for EUCAST and 500 µg for the French EUCAST). A PCR screening of the acquired aminoglycoside-modifying enzymes is ongoing to correlate the presence of genes to zone diameters.
Streptomycin resistance was identified in three GBS strains (4%) according to CLSI for enterococci, fifty-two GBS strains (69.3%) according to EUCAST for enterococci and in almost the totality of GBS strains (72 out of 75 strains) according to the French EUCAST for beta-haemolytic streptococci.
Conclusions
The proportion of high-level resistance to streptomycin in GBS greatly differed between different guidelines. HLAR GBS should be routinely checked in clinical microbiology laboratories provided the introduction of shared microbiological breakpoints.