Factors influencing the assessment of the pseudomonacidal activity of disinfectants by a quantitative suspension test. II. The post-disinfection recovery of surviving organisms.

Bacteria which have been treated by a disinfectant may be partially damaged so that they will be able to survive and to multiply only if all environmental factors are kept as optimal as possible. Otherwise no growth at all is obtained and the disinfectant solution will be wrongly considered as active. Therefore the technique of the post-medication recovery of surviving microorganisms is a matter of great importance. Since our aim is to elaborate a quantitative test, the counting technique using pour or streak plates is investigated here. In contrast to the results of viable counts of normally cultivated P. aeruginosa (Tables 1 and 2), the colony counts of phenol-treated bacteria gain higher figures when the spread plate method is used instead of the pour plate technique (Table 3). - Out of nine solid media examined (Table 4), four culture media are considered to be superior to the others (Table 5). They are Davis Agar, the Nutrient Agar of the German pharmacopoeia, the Nutrient Agar proposed by the APHA, and Tryptone Soy bean peptone Agar. Further investigation on the influence of the recovery medium is needed since there is stated that significant differences are obtained when three preparations of the same formulation are compared (Table 6). Finally the author observed that the volume of the culture medium affects the survival rate (Table 8). The recovery of disinfectant-treated bacteria is found to be very complex; further research on this domain should be encouraged.