Bacterial numbers on penicylinders used in disinfectant testing: use of 24 hour adjusted broth cultures

The current AOAC use-dilution methods of disinfectant efficacy testing require the use of 48-54 h unadjusted broth cultures of Salmonella choleraesuis, Staphylococcus aureus, and Pseudomonas aeruginosa for the inoculation of stainless steel penicylinders. The use of unadjusted broth cultures contributes to noncomparable numbers of organisms on penicylinders among the test strains due to relative efficacy of bacterial attachment to penicylinders and to bacterial numbers in broth. To achieve comparable numbers of cells on the penicylinders among the 3 test strains, the cell densities of S. aureus and P. aeruginosa in broth culture were visually adjusted. Growth studies were conducted using S. choleraesuis and P. aeruginosa to determine the numbers of cells in broth at timed intervals and the corresponding numbers of cells attaching to the penicylinders. Results showed that the use of the 24 h broth cultures for all 3 test strains, with adjustment of S. aureus and P. aeruginosa broths, contributes to more comparable numbers of organisms attached to the penicylinders used in disinfectant testing.