Comparison of the In vitro Activity of Five Antimicrobial Drugs against Staphylococcus pseudintermedius and Staphylococcus aureus Biofilms

Resistance in canine pathogenic staphylococci is necessitating re-evaluation of the current antimicrobial treatments especially for biofilm-associated infections. Long, repeated treatments are often required to control such infections due to the tolerance of bacteria within the biofilm. To comply with the goal of better antibiotic stewardship in veterinary medicine, the efficacies of the available drugs need to be directly assessed on bacterial biofilms. We compared the activities of amoxicillin, cefalexin, clindamycin, doxycycline, and marbofloxacin on in vitro biofilms of Staphylococcus pseudintermedius and Staphylococcus aureus. Exposure of biofilms for 15 h to maximum concentrations of the antibiotics achievable in canine plasma only reduced biofilm bacteria by 0.5–2.0 log(10) CFU, compared to the control, except for marbofloxacin which reduced S. aureus biofilms by 5.4 log(10) CFU. Two-antibiotic combinations did not improve, and even decreased, bacterial killing. In comparison, 5 min-exposure to 2% chlorhexidine reduced biofilms of the two tested strains by 4 log(10) CFU. Our results showed that S. pseudintermedius and S. aureus biofilms were highly tolerant to all the drugs tested, consistent with the treatment failures observed in practice. Under our in vitro conditions, the use of chlorhexidine was more efficacious than antimicrobials to reduce S. pseudintermedius biofilm.