Acquisition of daptomycin resistance in patients results in decreased virulence in Drosophila

Staphylococcus aureus can acquire antimicrobial resistance, which in turn may affect its pathogenic potential. Using a panel of paired clinical isolates collected before and after daptomycin resistance acquisition, most frequently through a single mprF mutation, we show a relationship between increasing daptomycin minimum inhibitory concentration and reduced virulence in a Drosophila systemic infection model. Analyzing toxin production, in vitro bacterial growth characteristics, and cell surface properties, we failed to link daptomycin resistance-related attenuated virulence to either reduced virulence factor production, reduced fitness, or any of the cell surface characteristics investigated. Competition assays in Drosophila also did not support any altered ability in immune evasion. Instead, using a panel of mutant flies defective for various immune components, we show that this daptomycin resistance-related attenuated virulence is mostly explained by greater susceptibility to the activity of Drosophila prophenoloxidase, a tyrosinase involved in melanization, but not to antimicrobial peptides or Bomanin antimicrobial effectors. Further investigation could not link daptomycin resistance-related attenuation of virulence to differential susceptibility to reactive oxygen species or quinones prominently associated with phenoloxidase bacterial-killing activity. Taken together, it appears that daptomycin resistance attenuates Staphylococcus aureus virulence through enhanced sensitivity to phenoloxidase based on a complex mechanism. Our study provides new insights into the understanding of the crosstalk between antimicrobial resistance, escape from immune killing, and virulence.