Interaction of chlorhexidine with trisEDTA or miconazole in vitro against canine meticillin-resistant and -susceptible Staphylococcus pseudintermedius isolates from two UK regions

Background Topical therapy is an important alternative to systemic antibacterial therapy for treatment of canine superficial pyoderma in light of the emergence of multidrug‐resistant staphylococci. Chlorhexidine is widely used in shampoo products alone or in combination with miconazole or tromethamine‐ethylenediaminetetraacetic acid (trisEDTA). Comparisons of these combinations have not been made. Hypothesis/Objectives To determine minimum inhibitory concentrations (MICs) of combinations of chlorhexidine/miconazole and chlorhexidine/trisEDTA in vitro in a collection of Staphylococcus pseudintermedius (SP) from northern (NUK) and southeastern (SEUK) United Kingdom (UK) sources. Methods MICs of chlorhexidine, miconazole, trisEDTA and combinations of chlorhexidine/miconazole (1:1) or chlorhexidine/trisEDTA (80:16:1 and 80:5:1) were determined for 196 canine SP isolates from NUK [49 meticillin‐resistant (MRSP), 50 meticillin‐susceptible (MSSP)] and fom SEUK (48 MRSP, 49 MSSP) using agar dilution. Results TrisEDTA alone did not inhibit growth. Chlorhexidine/miconazole MICs (median = 0.5 mg/L) were lower than those of either drug alone (P < 0.05) and lower than chlorhexidine/trisEDTA MICs (median = 1 mg/L; P < 0.0005) in each bacterial type and from both regions, except for miconazole in NUK MSSP. An additive interaction was noted between chlorhexidine and miconazole or trisEDTA (80:16:1 ratio) in 79 and 43 isolates, respectively, whereas antagonism between chlorhexidine and trisEDTA was noted for three isolates. NUK isolates were more susceptible than SEUK isolates (P < 0.05), except MRSP exposed to chlorhexidine and the chlorhexidine/trisEDTA (80:16:1) combination. Conclusions and Clinical Importance These low MICs are likely to be exceeded by topical therapy. Evaluation of the mechanisms by which chlorhexidine combinations interact to reduce MICs is warranted, in view of increasing concerns of biocide tolerance in staphylococci. Résumé Contexte Les traitements topiques sont une alternative importante au traitement antibiotique systémique pour le traitement des pyodermites bactériennes superficielles à la vue de l’émergence des staphylocoques multirésistantes. La chlorhexidine est largement utilisée en shampooing seule ou en combinaison avec le miconazole ou le tris‐EDTA (tromethamine‐ethylenediaminetetraacetic acid). Les comparaisons de ces combinaisons n'ont pas été réalisées. Hypothèses/Objectifs Déterminer les concentrations minimales inhibitrices (MICs) d