The Disinfectant Properties of Quaternary Ammonium Compounds and Sodium Hypochlorite: A Comparison of Results Using Different Test Methods

Abstract

SUMMARY: On exposing a strain of Bacterium coli 28.D.10 in a surface film at atmospheric temperature to atmospheres of different moisture contents, it was found that for relative humidities between 100 and 66% the numbers of survivors decreased with decreasing humidity. There was also some evidence of a slight increase in survivors for a decrease in relative humidity from 43 to 0%. The percentage of survivors of Bact. coli after exposure to quaternary ammonium disinfectants also decreased with relative humidity between 100 and 66% but no significant differences were found for changes in relative humidity below 66%. The numbers of survivors of a culture of Staphylococcus aureus were the same after storage at a relative humidity of 43% as at 100%; drying did not appear to affect the sensitivity of Staph. aureus to quaternary ammonium compounds. Tests of the effect of storage time in a saturated atmosphere gave results which were not entirely consistent, but where differences were observed, there was a lower percentage of survivors for freshly inoculated films than for films which had been stored for 3 hr, presumably because a fresh film was more easily removed to the disinfectant. When either Bact. coli or Staph. aureus was exposed to a disinfectant, the percentage of survivors was higher when the organisms were in a surface film than when they were inoculated directly into the disinfectant. Agitation during exposure reduced the numbers of survivors from a surface film. Neither the glass nor the metal coming in contact with the disinfectants affected the level of survivors. Under the conditions of testing, sodium hypochlorite was a more effective disinfectant than the quaternary ammonium compounds used.

Laboratory and Field Evaluation of Chemical Sterilizers for Dairy Farms

“…There are several reasons which make it desirable that tests shall include a comparison of the test material with a standard. Garvie & Clarke (1955), and earlier workers, have shown that any result can be obtained by altering the conditions of the test. The mixed culture inoculum of the can test is subject to slight variation from day to day and this could affect the result seriously if absolute standards were imposed.…”

“…The inhibitor used for QAC solutions is 2% lecithin in 3% Lubrol W. Lubrol is effective in stopping the bactericidal and bacteriostatic action of a QAC with Bact. coli as the test organism, but Garvie & Clarke (1955) have shown that this is not the case with Staph. aureus.…”

“…This is a test that can be done on the laboratory bench. The details have been published in the Society's Journal (Garvie & Clarke, 1955), together with the method of statistical appraisal of the results (Clarke & Garvie, 1955). Briefly the details are as follows:…”

“…A 24 hr agar slope culture of Bacterium coli type I or, where desirable, Stuphylococcw aureus between the fourth and eighth daily transfer (Garvie, 1955) is washed off and suspended in a mixture of 9 ml of sterile glass-distilled water and 1 ml of heated milk. A dropping pipette is used to deliver 0.06 ml of the suspension (containing approximately 200 x lo6 organisms) on to a 3 x 1 in.…”

Methods for the Detection of Survivors on Milk Handling Equipment with Reference to the Use of Disinfectant Inhibitors

“…It has been shown that bacteria suspended in a disinfectant solution are much more readily killed than when they are deposited on a surface (Garvie & Clarke, 1955), where presumably they are protected in some way. Similarly, organisms on milking equipment treated with normal concentrations of disinfectant survive because'they have not come in contact with it.…”