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 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 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 activity of Drosophila prophenoloxidase, a tyrosinase involved in melanization, but not to antimicrobial peptides or to Bomanin antimicrobial effectors. 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, escape from immune killing, and virulence. Author summaryAcquiring antimicrobial resistance can increase or decrease bacterial virulence. However, the mechanisms causing these resistance-virulence linked effects are unclear. Here, we bring new insights on the crosstalk between antimicrobial resistance and virulence. We characterized a panel of Staphylococcus aureus strains isolated from patients before and after resistance acquisition to the antibiotic daptomycin. Relative to the parental strain, resistant isolates most often varied by one single mutation, in a gene involved in the composition of the bacterial membrane, and these strains were much less virulent when fruit-flies were infected. Our results indicate that the difference of virulence is unrelated to changes in bacterial toxin production, bacterial growth, immune evasion or cell surface properties. Instead, resistant strains were more vulnerable to a host proenzyme involved in the antibacterial melanization response, an important response deployed throughout the arthropods. We predict that daptomycin resistance forces staphylococci to alter the composition of their cell surface. This alteration causes the bacteria to become more vulnerable to killing by melanization. Our results contribute to our understanding of the link between antimicrobial resistance and pathogenicity.