In vitro and in vivo efficacy of vancomycin against Elizabethkingia species and the impact of increased vancomycin MICs

ABSTRACT This study aimed to evaluate the concordance of vancomycin susceptibility testing methods, its in vivo and in vitro efficacy, and the mechanisms underlying elevated MICs in Elizabethkingia spp. Vancomycin susceptibilities of 18 E. anophelis isolates were determined using multiple assays. The efficacy of vancomycin against five clinical isolates and one laboratory-induced mutant with an elevated vancomycin MIC was evaluated using time-kill assays and Galleria mellonella and murine models. Vancomycin MICs (16–32 mg/L) determined by broth microdilution were consistent with agar dilution, Etest, and MBC assay results. All isolates had zone diameters < 17 mm and were, thus, categorized as non-susceptible according to the CLSI criteria for Enterococcus spp. Time-kill assays of five clinical isolates demonstrated that vancomycin at a clinically relevant concentration (4 mg/L) exhibited poor bactericidal activity similar to that of teicoplanin. Vancomycin improved Galleria mellonella survival in a dose-dependent manner, whereas teicoplanin, dalbavancin, oritavancin, and daptomycin were ineffective. Murine models revealed that vancomycin at a human-equivalent dose (25 mg/kg twice daily) prolonged survival in most infections and modestly reduced bacterial load, while teicoplanin remained ineffective. Vancomycin efficacy was significantly reduced in G. mellonella and mice infected with a mutant strain exhibiting an elevated MIC (128 mg/L), which was attributable to spontaneous mutations in pbp 4. In conclusion, E. anophelis were consistently non-susceptible to vancomycin as determined by multiple in vitro assays. However, vancomycin demonstrated unique in vivo activity among glycopeptides although this effect was abrogated by spontaneous mutations leading to elevated MICs. IMPORTANCE Elizabethkingia anophelis is a multidrug-resistant pathogen associated with limited treatment options and high mortality. Most commonly considered agents, including fluoroquinolones, piperacillin/tazobactam, and trimethoprim/sulfamethoxazole, are increasingly compromised by resistance, toxicity, or inconsistent efficacy. Although vancomycin is not routinely used for Gram-negative infections due to limited outer membrane permeability, case reports have suggested potential benefit in Elizabethkingia infections under critical conditions. In this study, we show that E. anophelis isolates are uniformly non-susceptible to vancomycin in vitro and exhibit minimal bactericidal activity. However, vancomycin conferred a modest but statistically significant survival benefit in two independent animal models. Importantly, this effect was lost in strains with vancomycin-induced MIC elevation, and genome analysis identified pbp4 mutations as a potential underlying mechanism. These findings suggest vancomycin may offer therapeutic benefit when no preferred options are available. They support cautious use in selected cases and highlight the need for continued monitoring of susceptibility and resistance development.