A Comparison Between Known Organic Materials and ∝-Casein Used in the Bacteriological Evaluation of Benzalkonium Chloride

Conventional tests have used yeast suspension, milk and other types of organic material as soiling agents and it was considered that the variability of these natural products could be overcome by the use of a more standard type of organic material. For this reason ∝-casein was compared with yeast suspension and milk concentrations on Benzalkonium chloride efficiency.

Using a suspension test method with Escherichia coli (NCTC 8196) and Staphylococcus aureus (NCTC 4163) as test organisms ∝-casein was compared with 2% yeast suspension and reconstituted skimmed milk (2, 4 and 6%). Taking bacterial survivors as the index of disinfectant activity, it was found that 2% ∝-casein gave more consistent results than when 2% yeast was died. This latter material showed considerable variation due to the age of suspension, older suspension caused less reduction in germicidal efficiency. Using milk as the soiling agent it was found that 6% milk corresponded approximately with the effect produced by 0.5% ∝-casein, Experimental determination of protein in these products has been used as a basis for the possible explanation of these similarities.

The Kelsey-Sykes capacity test was used to compare ∝-casein with yeast as the organic soil and it was shown that 2% yeast was equivalent to 0.5% -casein in the effect on disinfectant efficiency. Attempts to determine the amount of disinfectant 'bound' to the organic material were carried out by an agar-diffusion technique. The results obtained are discussed in relation to the sensitivities and resistances of various test organisms.

The nature of the disinfectant binding is discussed in relation to the types of soiling agents used in the investigation and suggestions are made concerning the future use of ∝-casein as a test soil material.