Factors influencing the assessment of the pseudomonacidal activity of disinfectants by a quantitative suspension test. I. Influence of the growth conditions of the test organism.

A matter of special concern in disinfectant testing is the preparation of the bacterial suspension that shall be exposed to the action of the disinfectant. In the present paper attention is drawn to three details of the testing method being determinative for the resistivity of the test organism and for the degree of standardization of the test results. - In the first place the influence of the nutrient medium used in the cultivation of the test organism has been investigated. Sixteen culture media (Table 1) have been compared. The resistance of Pseudomonas aeruginosa to phenol depends on the formulation of the preparatory medium: significantly more bacteria survive if the test organism is grown on the nutrient agar following the German pharmacopoeia (Table 2). Further the author has investigated the effect of the temperature and the length of the initial culture incubation period (Tables 3 and 4). Here also the differences among temperatures (37, 32 and 22 degrees C) or incubation periods (48, 24 and 18 h) are statistically significant. The highest numbers of survivors are found when the initial culture is incubated at 37 degrees C and for 48 h. Whereas the resistance of the test culture is determined by these three factors, the variability of the test results is not. The coefficients of variation calculated for the different initial culture media, the three incubation temperatures or the three incubation periods show no differences among these test conditions.