Factors affecting the bactericidal action of disinfectants. Implications for selection of resistant strains.

The efficacy of some disinfectants was studied in the presence of known interfering factors by the AFNOR test (NFT 72-150, 170 and 171) with five bacterial strains: P. aeruginosa, E. coli, S. aureus, S. faecalis and M. smegmatis. These factors included proteins (albumin and yeast extract), hard water and pH (5 and 9). The effect of each interfering factor was found to depend on the bacterial strain and on the chemical composition of the disinfectant. For Gram-negative bacteria, especially P. aeruginosa, and for M. smegmatis the minimal bactericidal concentration rose significantly in the presence of hard water and of proteins. This phenomenon may explain the selective effect of disinfectants at their commonly used concentrations.