The effect of hydrocarbon chain length, pH, and temperature on the binding and bactericidal effect of amphiphilic betaine esters on Salmonella typhimurium

Amphiphilic betaine esters are quaternary ammonium compounds (QAC) with rapid microbicidal action. They are often labeled 'soft antimicrobial agents', since the compounds hydrolyze spontaneously into betaine and fatty alcohols, thus not only losing their surface active properties and toxicity but also becoming amenable to metabolic use. The present results show that the bactericidal effects of 1-decyl (B10), 1-dodecyl (B12), and 1-tetradecyl (B14) betaine esters on Salmonella typhimurium 395 MS decreased with decreasing hydrocarbon chain lengths, decreased at pH below neutral, and were lower at 0 degrees C that at 30 degrees C. At least part of the decreased effect at pH 4.0 as compared to pH 6.0 can be explained by reduced binding. However, reduced binding cannot explain the decrease in the microbicidal effect at 0 degrees C since the binding of B 14 was the same at 0 degrees C and 30 degrees C although 10-30 times higher concentrations were required at 0 degrees C to achieve the same microbicidal effect as at 30 degrees C. Neither can differences in binding explain the great differences seen in microbicidal effect between QAC with different chain lengths. It is proposed that the membrane deformation resulting in killing of S. typhimurium is more efficiently achieved with QAC with longer hydrocarbon chains and that reduced fluidity of the outer membrane of the bacteria at lower temperatures antagonizes the bactericidal effect. Charge interaction seems to be more important for the binding and bactericidal effect for the QAC with shorter hydrocarbon chains. The different effects of pH, temperature, and hydrocarbon chain length on binding, bactericidal effect, and hydrolysis have to be taken into account when optimizing disinfection and the subsequent elimination of disinfectants.