Evaluation of Chlorhexidine-Containing Shampoos: In Vitro Efficacy Against Staphylococcus pseudintermedius and Lathering Ability.

Background: The antibacterial efficacy of chlorhexidine shampoo is directly affected by formulation and bathing factors.

Hypothesis/objective: To evaluate the in vitro antibacterial efficacy of chlorhexidine-containing shampoos at various dilutions and to compare their lathering ability.

Animals: No animals were utilised in this study.

Materials and methods: Eight chlorhexidine-containing shampoos, three non-chlorhexidine shampoos, and a 2% chlorhexidine gluconate solution were tested against one American Type Culture Collection (ATCC) meticillin-sensitive Staphylococcus pseudintermedius and one institutional meticillin-resistant S. pseudintermedius isolate. The effect of formulation on minimum inhibitory concentration (MIC) was determined using a broth microdilution method. The first dilution that had no visible growth and four preceding dilutions were plated on blood agar to determine the minimum bactericidal concentration (MBC). Lathering ability and lather stability were assessed using a modified cylinder shake method. MIC and MBC were compared for the dilution ratio and chlorhexidine gluconate/digluconate concentration using a Kruskal-Wallis test with Bonferroni correction (p < 0.001).

Results: All products had a detectable MIC. Statistically significant differences between MIC and MBC were observed between shampoos that were not based on chlorhexidine concentration alone. Two non-chlorhexidine shampoos had no detectable MBC. Over time, all shampoos had a significant decrease in lather height. Lathering ability significantly differed between some shampoos.

Conclusions and clinical relevance: This preliminary study suggests that shampoo formulation and not just chlorhexidine concentration impacts efficacy. Further investigation with more robust numbers of bacterial isolates and large-scale head-to-head clinical trials is required to determine if the reported in vitro variance has clinical significance.