Staphylococcal response to daptomycin in implant-associated infections

The use of medical devices carries the risk of infection. Due to the development of multi-resistant bacteria, missing microcirculation, and biofilm embedded bacteria these implant-associated infections are difficult to treat. For their successful treatment, drugs should act independently of the bacterial physiological state, penetrate the biofilm, and prevent further bacterial adherence to surfaces and formation of bacterial biofilm.
Daptomycin (DAP) exhibits concentration-dependent bactericidal activity against Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA). Its target is the bacterial membrane in which it forms pores resulting in membrane depolarization and subsequent cell death. Calcium ions (Ca2+) are indispensable for its activity. DAP does not need cell division or active metabolism for bactericidal activity and is able to penetrate biofilms. Therefore, in this thesis we followed the question whether DAP is efficient in eradicating planktonic and adherent MRSA in an implant-associated infection.
In the first part, DAP was bactericidal against stationary grown planktonic MRSA in vitro. Because the physiology of biofilm embedded bacteria is similar to stationary grown planktonic bacteria, DAP alone or in combination with rifampin was applied to treat MRSA in a foreign-body infection model in guinea pigs. DAP in combination with rifampin showed the highest efficacy against planktonic and adherent MRSA compared to rifampin-containing combinations with vancomycin, linezolid and levofloxacin. Additionally, in this combination DAP prevented the emergence of rifampin resistance. However, DAP alone was inefficient against attached and biofilm embedded MRSA in implant-associated infections.
Therefore, in the second part of this thesis various factors, which could explain treatment failure, were investigated by evaluating the effect of cell wall components, biofilm, adherence and Ca2+ in vitro and in vivo upon DAP activity. We found that the physiological state of the bacteria had an impact on the efficacy of DAP because DAP was only effective as prophylaxis, i.e. before bacterial adherence. Furthermore, DAP was not able to kill adherent staphylococci in vitro, independently of biofilm, nucleases, adhesins, autolysins, and alanyl-lipoteichoic acids. Resistance of adherent staphylococci was not due to mutations of adherent bacteria, since staphylococci became DAP-susceptible after detachment. Increasing DAP or Ca2+ concentrations partially enhanced killing of adherent staphylococci in vitro and in a murine tissue cage infection model.
In summary, DAP alone is not active in an implant-associated infection. This might be due to bacterial adhesion, which is associated with a change of their physiological state. We demonstrated in an implant-associated infection that DAP treatment is improved with Ca2+ and only successful with 2 repeated doses of DAP before and after infection or in combination with rifampin.