Pre-exposure to phosphate-buffer saline (PBS) renders some <i>Pseudomonas aeruginosa</i> strains susceptible to triclosan on solid medium

Triclosan is an antibiotic frequently used to selectively isolate Pseudomonas aeruginosa from environmental and animal samples. Here, we report that while nine tested P. aeruginosa strains were indeed triclosan-resistant when grown on Luria Bertani (Lennox) broth agar supplemented with triclosan, all nine exhibited decreased CFU by plating following pre-exposure to phosphate-buffer saline (PBS). Compared to growth on lysogeny broth agar in the absence of PBS incubation, PBS pre-exposure was associated with mean CFU reductions ranging from ∼25% (clinical isolate PABL048) to ∼80% (laboratory strain PA14). The potential for triclosan agar to cause significant errors in experiments designed to quantify P. aeruginosa numbers was demonstrated using a mouse model of P. aeruginosa gastrointestinal carriage. In this experiment, P. aeruginosa carriage was underestimated by up to 20-fold when fecal samples were processed using PBS. These findings suggest that triclosan should be used with caution to quantify P. aeruginosa numbers in protocols that utilize PBS.