Biochemical characterization of a Pseudomonas fluorescens strain isolated from a benzalkonium chloride solution

A bacterium isolated as the contaminant of a batch of commercial benzalkonium chloride (BAC) solution (10% (w/v)) stored in a loosely capped bottle in the Department of Pharmacy Shinshu University Hospital was identified as Pseudomonas fluorescens belonging to biotype G of Stanier, et al. The strain was highly resistant to BAC, and the lowest concentration of BAC that inhibited visible growth of the strain as measured on nutrient agar plates was > or = 5000 micrograms/ml. BAC is a typical quaternary ammonium detergent. Thus we examined the tolerable growth concentration of various strains on surfactants. We were able to confirm growth of P. fluorescens of BAC resistance strain (PFRB) in 5% concentration, but the other strains were not able to grow in 0.1% concentration. We investigated the relationship between biotype and resistance to BAC. PFRB and three clinical isolated strains were found to be the same biotype G. However, no apparent correlation was found between the same biotypes and minimum inhibitory concentration (MIC) of disinfectant or growth permissible concentration on surfactants. The strain was unable to decompose BAC, as no growth occurred in the minimum medium containing BAC as the sole source of carbon, nitrogen or both. Our finding caused us to realize that P. fluorescens might also be a contaminant of disinfectants, as we have seen in Pseudomonas cepacia.