In vitro evaluation of topical biocide and antimicrobial susceptibility of Staphylococcus pseudintermedius from dogs

Staphylococcus pseudintermedius is an important canine pathogen, and the emergence and widespread dissemination of meticillin-resistant strains (MRSP) is of significant concern. Multidrug-resistant infections may require alternative approaches, such as the use of topical therapy. There is minimal information about the in vitro susceptibility of meticillin-susceptible S. pseudintermedius (MSSP) and MRSP to biocides and topical antimicrobials.

The hypothesis was that clinical isolates of MSSP and MRSP would not have universal susceptibility to topical biocides and antimicrobials. The goal of this study was to assess the susceptibility of a collection of S. pseudintermedius isolates to selected antimicrobials and biocides.

The study was performed on clinical isolates of MSSP and MRSP from dogs with skin and soft tissue infections collected throughout North America between 2006 and 2008.

The minimal inhibitory concentrations (MICs) of chlorhexidine digluconate, benzalkonium chloride, triclosan, accelerated hydrogen peroxide, geranium oil, tea tree oil and grapefruit seed extract were tested for 25 MRSP and 25 MSSP isolates from dogs using the agar dilution method. The MICs of fusidic acid, bacitracin and mupirocin were determined using Etests.

Triclosan demonstrated excellent activity against all bacterial isolates, with no growth at the lowest concentration evaluated (MIC ≤ 0.5 μg/mL). Conversely, grapefruit seed extract did not inhibit growth at the highest concentration tested (MIC > 3.84 μg/mL). All isolates were susceptible to mupirocin, fusidic acid and bacitracin. There were no significant differences noted in the range, MIC(50) or MIC(90) between MSSP and MRSP isolates.

While isolates were susceptible to most of the tested compounds, universal susceptibility to all compounds with potential antimicrobial activity cannot be assumed, and specific testing is required.