Pseudomonas pellicle in disinfectant testing: electron microscopy, pellicle removal, and effect on test results

Pseudomonas aeruginosa ATCC 15442 is a required organism in the Association of Official Analytical Chemists use-dilution method for disinfectant efficacy testing. When grown in a liquid medium, P. aeruginosa produces a dense mat or pellicle at the broth/air interface. The purpose of this investigation was to examine the pellicle by scanning electron microscopy, to evaluate three pellicle removal methods, and to determine the effect of pellicle fragments on disinfectant efficacy test results. The efficacies of three methods of pellicle removal (decanting, vacuum suction, and filtration) were assessed by quantifying cell numbers on penicylinders. The Association of Official Analytical Chemists use-dilution method was used to determine whether pellicle fragments in the tubes used to inoculate penicylinders affected test results. Scanning electron micrographs showed the pellicle to be a dense mass of intact, interlacing cells at least 10 microns thick. No significant differences in pellicle removal methods were observed, and the presence of pellicle fragments usually increased the number of positive tubes in the use-dilution method significantly.