Factors influencing a suspension test method for antimicrobial activity of disinfectants.

Factors influencing the numbers of Escherichia coli DSM 682 and Staphylococcus aureus ATCC 6538 surviving exposure to disinfectants were evaluated by factorial design. Aerobic conditions during pre-cultivation rendered E. coli more resistant to the lethal activity of benzalkonium chloride (BC) and a disinfectant containing grape fruit extract (GSE), whereas Staph. aureus became more sensitive. The degree of shaking and the pre-growth medium (tryptone soy broth or Mueller-Hinton broth) did not influence the result of the bactericidal test. The number of E. coli surviving BC treatment was significantly lower if the neutralizing broth contained thiosulphate, plate pouring was used instead of plate spreading, or the plates were incubated at 37 instead of 30 degrees C. The negative effect of plate pouring was also found with Staph. aureus. The use of filtration without prior neutralization of the disinfectant decreased the numbers of chlorine-treated, but not BC-treated, E. coli. The results showed that rigorous standardization is necessary to obtain good reproducibility of bactericidal suspension tests.