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Evaluation of the impact of tromethamine edetate disodium dihydrate on antimicrobial susceptibility of Pseudomonas aeruginosa in biofilm in vitro.

BACKGROUND
Biofilm formation by Pseudomonas aeruginosa has been documented in canine otic isolates. An increase in minimal inhibitory concentration (MIC) for specific antibiotics has been noted for biofilm-embedded bacteria. Tromethamine edetate disodium dihydrate buffered to pH 8 with tromethamine hydrochloride and deionized water (Triz-EDTA(®)) has been documented to potentiate bactericidal activity when used in combination with topical antibiotics, but the impact on biofilm-embedded bacteria is unknown.


HYPOTHESIS/OBJECTIVES
The objective of this study was to evaluate the impact of Triz-EDTA(®) use on in vitro antimicrobial susceptibility of biofilm-embedded P. aeruginosa.


METHODS
Biofilm formation was documented using a microtitre plate assay. Broth microdilution was used to assess the MIC of neomycin, polymyxin B, enrofloxacin and gentamicin for the biofilm-embedded bacteria. The microtitre plate assay was again used to assess the MIC of neomycin, polymyxin B, enrofloxacin and gentamicin for biofilm-embedded bacteria with added Triz-EDTA(®).


RESULTS
Thirty-one isolates from dogs with otitis were tested. Addition of Triz-EDTA(®) significantly reduced MICs for neomycin (P < 0.003) and gentamicin (P < 0.02) but not for polymyxin B (P = 0.3). Enrofloxacin MICs increased in the presence of Triz-EDTA (P < 0.036).


CONCLUSIONS AND CLINICAL IMPORTANCE
Triz-EDTA(®) may be a useful adjunctive treatment for chronic cases of Pseudomonas otitis where biofilms may have developed, if gentamicin or neomycin is to be used as a topical treatment. In vivo study is required to confirm this effect.