Evaluation of fosfomycin combined with vancomycin against vancomycin-resistant coagulase negative staphylococci

The pathogenic potential of coagulase-negative staphylococci (CoNS) is increasingly recognized worldwide. The aim of the present study was to evaluate different combinations of fosfomycin (FOS) with vancomycin (VAN) or oxacillin (OXA) against vancomycin-resistant clinical isolates of CoNS. Characterization of VAN resistance in selected isolates was also sought. Antibiotic susceptibility of isolates was tested by disc diffusion method, MICs of antibiotics were determined by the agar dilution method, and fosfomycin combinations were evaluated by a time-kill assay according to the guidelines of the CLSI. Moreover, oxacillin and glycopeptides (vancomycin and teichoplanin) resistances were also characterized phenotypically and genotypically in this study. Out of 258 staphylococci, 52 were CoNS (20.2%). All isolates were multidrug resistant with 75% (n = 39) oxacillin-resistant, most of them with oxacillin MIC levels of >32 mg/L. Moreover, vancomycin non-susceptibility was observed in 46.2% (n = 24) of the tested isolates with MIC range of 4-32 mg/L. Identification of selected isolates revealed that S. epidermidis was the most abundant among tested CoNS, followed by S. hominis, S. saprophyticus, and interestingly S. caseolyticus. Furthermore, Synergistic and bactericidal effects of FOS + VAN combination were observed in 3 of 9 isolates after 6 h of treatment and in all studied isolates at 24 h. On the other hand, FOS + OXA combinations were ineffective. This study provides evidence that fosfomycin combination with vancomycin could be considered as a therapeutic alternative for CoNS infections. It also sheds some light on the possible emergence of the otherwise harmless bacterium S. caseolyticus as a human pathogen.