Objectives: Methicillin-resistant Staphylococcus aureus (MRSA) and multiresistant Staphylococcus pseudintermedius (MRSP) have emerged as important pathogens in animal infections. Associated therapeutic problems and the zoonotic potential of staphylococci have renewed interest in topical antibiotics for treatment and carrier decolonization. Fusidic acid and mupirocin are used topically in humans and animals but resistant strains isolated from people are increasing. This study investigates the in vitro activity of fusidic acid and mupirocin against coagulase-positive staphylococci from pets.
Methods: A collection of 287 staphylococci was examined, comprising 102 MRSA, 102 methicillin-susceptible S. aureus, 71 S. pseudintermedius and 12 MRSP from canine and feline infections and carrier sites isolated in the UK and Germany. MICs were determined by the agar dilution method according to CLSI (formerly NCCLS) standards.
Results: The majority (89.7%) of all MICs were </=0.25 mg/L. High MICs were observed for seven MRSA isolates (five with an MIC of fusidic acid of 512 mg/L, one with an MIC of fusidic acid of 1024 mg/L and one with with an MIC of mupirocin of 16 mg/L). MICs of both antibiotics were </=2 mg/L for all MRSP. Infection isolates had higher MICs than those isolated from carriage sites for both antibiotics (P </= 0.001).
Conclusions: In all but seven MRSA isolates, MICs were below the concentrations achievable experimentally at application sites suggesting therapeutic efficacy of both antibiotics in infections involving multiresistant staphylococci and for decolonization of carriers. However, the seven MRSA with high MICs, all of the dominant UK human hospital lineages, highlight the importance of monitoring treatment success as resistant strains may occur in animals.