Daptomycin is a last-resort antibiotic for the treatment of invasive diseases caused by methicillin resistant Staphylococcus aureus. It functions by targeting phosphatidylglycerol in the staphylococcal membrane, leading to disruption of membrane integrity and cell wall synthesis. However, despite potent activity in vitro, daptomycin fails to resolve up to 30 % of cases of staphylococcal bacteraemia, suggesting that the host environment reduces bacterial susceptibility to this antibiotic.
Using human serum as an ex vivo model of bacteraemia, work described in this thesis demonstrated that the host environment induced daptomycin tolerance in S. aureus. This tolerance was due to two independent mechanisms. Firstly, LL-37, a host defence peptide present in serum, induced tolerance by activating the GraRS two component system. This led to an increase in the peptidoglycan content of the cell wall, reducing the binding of daptomycin to its membrane target. Secondly, incubation in serum led to an increase in the abundance of cardiolipin in the membrane, resulting in a decreased phosphatidylglycerol content. Together, these two mechanisms fully explained the induced tolerance phenotype, with inhibition of both peptidoglycan and cardiolipin synthesis completely abolishing the acquisition of daptomycin tolerance in serum.
In summary, these data demonstrate that host factors compromise daptomycin efficacy by triggering changes in the staphylococcal cell envelope which enable the bacterium to survive exposure to the lipopeptide antibiotic. This host-induced reduction in daptomycin susceptibility may in turn contribute to treatment failure. Additionally, these data demonstrate that tolerance can be reduced in vitro with GraRS inhibitors or the cell wall synthesis inhibitor fosfomycin, providing potential novel therapeutic approaches to prevent daptomycin tolerance and improve patient outcomes.