In vitro antimicrobial efficacy of beta-defensin 3 against Staphylococcus pseudintermedius isolates from healthy and atopic canine skin

beta-Defensins (BDs) are highly conserved antimicrobial peptides important in innate defence against bacteria. beta-Defensin 3 has a specific role in protecting the skin. This study quantified the minimal inhibitory concentration (MIC) of human (h)BD3 against Staphylococcus pseudintermedius isolates from atopic and healthy dogs. Single colony isolates (1 x 105 colony-forming units/mL log phase) were cultured with doubling dilutions of hBD3 in sodium phosphate buffer from 0.8 to 50 mu g/mL at 37 degrees C for 2 h, before adding 100 mu L of tryptone soy broth and incubating for a further 20 h. Bacterial growth was assessed as the mean optical density at 540 nm corrected for background. The median MIC was 12.5 mu g hBD3/mL (range 3.125-25 mu g/mL; n = 22). Forty-five percent of the isolates were inhibited at < 6.25 mu g hBD3/mL, and 90% were inhibited at < 12.5 mu g hBD3/mL. Bacterial growth was not inhibited at < 1.6 mu g hBD3/mL. There were no significant differences in the inhibition by hBD3 of isolates from atopic (median MIC 12.5 mu g/mL, range 6.25-25 mu g/mL, n = 14) and healthy dogs (median MIC 9.4 mu g/mL, range 3.125-12.5 mu g/mL, n = 8); from noninfected colonized sites (median MIC 12.5 mu g/mL, range 3.125-25 mu g/mL, n = 16) and infected lesions (median MIC 9.4 mu g/mL, range 6.25-12.5 mu g/mL, n = 6); or between sample sites (nose median MIC 12.5 mu g/mL, range 6.25-25 mu g/mL, n = 5; perineum median MIC 12.5 mu g/mL, range 3.125-25 mu g/mL, n = 7; ear median MIC 6.25 mu g/mL, range 6.25-12.5 mu g/mL, n = 4; lesions median MIC 9.4 mu g/mL, range 6.25-12.5 mu g/mL, n = 6). In conclusion, hBD3 inhibited the growth of canine S. pseudintermedius isolates in vitro irrespective of origin.