Pronounced heterogeneity observed in high-level daptomycin-resistant viridans group streptococci

Viridans group streptococci (VGS) have demonstrated high-level daptomycin resistance (HLDR) upon daptomycin exposure. This study evaluated the extent of heterogeneity and whether dose escalation or combination therapy could prevent resistance development. Five VGS strains (daptomycin MICs 0.25-2mg/L) were evaluated. In vitro models utilised simulated daptomycin dosages of 4, 6, 8 and 12mg/kg with estimated fC_max of 4.1, 6.6, 8.6 and 12.9mg/L, respectively. Time-kill studies included fC_max simulations of daptomycin alone or combined with ceftriaxone, gentamicin, linezolid, rifampicin or vancomycin. Population analyses were performed on daptomycin-containing and non-containing agar plates. Extreme heterogeneity was observed in four strains with daptomycin population MICs 4-512-fold higher than broth microdilution. Whilst Streptococcus gordonii 1649 did not consistently develop HLDR, its population MIC was above the established daptomycin breakpoint. In vitro modelling demonstrated initial kill by daptomycin in all strains within 8h, with substantial re-growth by 24h despite increasing daptomycin. Daptomycin kill curves also displayed resistance development by 24h. However, synergy or additivity was noted for most regimens and strains. Synergy was most notable with daptomycin plus linezolid or rifampicin. Overall, daptomycin plus ceftriaxone or gentamicin were the most potent regimens. Gentamicin or rifampicin with daptomycin were least additive. For combination regimens with colonies isolated at 24h, HLDR was reduced 16-64-fold (MICs 4-16mg/L). Daptomycin monotherapy for VGS led to rapid development of HLDR likely due to extreme heterogeneity. Combination therapy suppressed or minimised the degree of resistance although the mechanism remains unknown.