Acquisition of daptomycin resistance results in decreased virulence

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 single mprF mutation, we show a relationship between increasing daptomycin minimum inhibitory concentration and reduced virulence in a Drosophila systemic infection model. Analysing 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 to any of the cell surface characteristics investigated. Competition assays in a 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 Drosophila prophenoloxidase activity, a tyrosinase involved in melanization, but not to antimicrobial peptides or to Bomanin antimicrobial effector. Further investigation could not link daptomycin resistance-related attenuation of virulence to a differential susceptibility to reactive oxygen species or to quinones prominently associated with phenoloxidase bacterial-killing activity. Taken together, it appears that daptomycin resistance attenuates Staphylococcus aureus virulence through an enhanced sensitivity to phenoloxidase based on a complex mechanism. Our study provides new insights in the understanding of the crosstalk between antimicrobial resistance and virulence and highlights that daptomycin resistance can contribute to decipher the mechanism of action of phenoloxidase antimicrobial activity.