Effect of colloidal association on the measured activity of alkylbenzyldimethylammonium chlorides against Pseudomonas aeruginosa.

The antibacterial activities of a homologous series (C8-C18) of alkylbenzyldimethylammonium cholorides (ABDAC) against Pseudomonas aeruginosa have been measured using both a minimum inhibitory concentration (MIC) procedure and a sterilization kinetics test carried out in deionized water. There was a log-linear relationship between activity measured by kinetics and carbon number. With MIC there was a log-linear relationship up to C14, when there was a turndown in activity. Consideration of the colloidal association of ABDAC in deionized water and in a simple salts growth media leads us to suggest that use of high concentrations of nutrient salts in MIC tests will lower the effective concentration of the surface active agents. This change may be responsible for the turndown in activity observed in MIC tests, and that in such circumstances the MIC test does not give a true reflection of the intrinsic activity of the compounds. Literature reports of parabolic relationships between ABDAC alkyl chain length and antimicrobial activity are reinterpreted on this basis.