A topical antimicrobial test system (TOPITEST): Nathan's Agar Well revisited

Nathan's Agar Well Diffusion (NAWD) assay is a topical antimicrobial susceptibility predictor originally presented in 1977. It provided the burn surgeon information on the appropriate topical antimicrobial necessary to control burn wound infection. A new topical antimicrobial assay (TOPITEST) was compared in two phases. Phase I was designed to compare the new TOPITEST which uses Brain Heart Infusion (BHI) agar as its basal medium with a modified NAWD (M:NAWD) which uses Mueller-Hinton Agar (MHA). Clinical isolates from burned wound biopsies previously tested by the M:NAWD assay were tested in replicate. The use of this procedure provides very important clinical information as it is our routine topical susceptibility assay which we rely upon to provide our topical therapeutic approach in the treatment of localized burn wound infections. ATCC strains of Staphylococcus aureus (SAUR), Pseudomonas aeruginosa (PSAR) and Escherichia coli (ECOL) were used in both systems for quality control (QC). We compared both systems on how well they agreed regarding bacterial sensitivity or resistance. All 178 gram-negative isolates were in perfect agreement (perfect agreement of 100%, p > 0.0001) however, the zones of inhibition were appreciable enhanced for the MHA base. Similar results were observed for the 147 gram-positive isolates, with the exception of Bacitracin and Polymyxin combination (BP). Phase II compared the TOPITEST in the same manner as Phase I substituting MHA in the pre-packaged system. Of the 239 gram-negative and positive isolates all were in 100% agreement. Comparisons among the specific pathogens selected and tested with TOPITEST using both basal media revealed that the majority of responses were in perfect agreement with the exception of BP for QC.ECOL, QC.SAUR and Klebsiella species (KLEB). These data suggest that susceptibility is dependent on the basal medium used for diffusion assays or on the vehicle they are compounded in.