Standardization of bacterial numbers of penicylinders used in disinfectant testing: interlaboratory study

An interlaboratory study was conducted to evaluate a method of standardizing bacterial numbers on penicylinders used in the AOAC use-dilution method (4.007-4.015) of disinfectant testing. Eight participating laboratories followed a broth adjustment method using their media and stock cultures of Staphylococcus aureus ATCC 6538, Salmonella choleraesuis ATCC 10708, and Pseudomonas aeruginosa ATCC 15442. The culture broths that were used to inoculate the penicylinders were incubated for 48 h at 37 degrees C after several (4-6) 24 h passages. McFarland turbidity standards of 1.0 and 0.5 were used to adjust visually the cultures of S. aureus and P. aeruginosa, respectively. S. choleraesuis was used undiluted. The results showed significant variability in numbers of test bacteria which adhered to the penicylinders, with mean values of 1.6 X 10(6) for S. choleraesuis, 3.5 X 10(6) for S. aureus, and 8.2 X 10(6) for P. aeruginosa. The results from collaborating laboratories attempting standardization of bacterial numbers on penicylinders demonstrated significant interlaboratory and cylinder variation for all 3 test organisms.