Loss of bactericidal capacity of long-chain quaternary ammonium compounds with protein at lowered temperature

Amphiphilic betaine esters are quaternary ammonium compounds (QACs) with rapid microbicidal effect, which spontaneously hydrolyze into nontoxic products, thus being referred to as soft antimicrobial agents. The bactericidal effect of l-decyl (B10), 1-dodecyl(B12), and 1-tetradecyl(B14) betaine esters on Salmonella typhimurium was strongly influenced by temperature, pH and length of hydrocarbon chain. At pH 6.0, presence of 1.5 mM (10% w/v) BSA raised the concentration of B14 for 99% killing (BC2) from 0.006 mM to 1.8 mM. There was a stoichiometric relationship between concentration of BSA and BC2 of B14, indicating that one molecule of B14 was bound per BSA molecule when 99% killing was achieved. When the temperature was lowered to 0 degrees C only minor killing was seen in 1.5 mM BSA at the highest concentration of B14 tested, 57 mM. With B10 at 30 degrees C and pH 6.0, the presence of 1.5 mM BSA raised the bactericidal concentration (BC2) from 0.69 mM to 4.1 mM, and at 0 degrees C and 1.5 mM BSA the BC2 was 11 mM. Thus, the impairment caused of the bactericidal effect of B10 by BSA and lower temperature was less than for B14, since B14 is much more active than B10 at 30 degrees C in the absence of BSA, somewhat mon active than B10 at 30 degrees C in the presence of 1.5 mM BSA and much less active than B10 at 0 degrees C in the presence of BSA. B12 showed properties intermediate between B10 and B14. Lowered pH reduced the bactericidal effect particularly when reduced from pH 5,0 to 3.0 with B10. In the presence of 1.5 mM BSA the bactericidal effect of 1-dodecyl (DTAB) and 1-hexadecyl(CTAB) trimethylammonium bromide decreased in the same manner as for B10 and B14, respectively. Increasing the time of incubation at 0 degrees C to 50 min, a 99% killing effect was seen with 17 mM CTAB, whereas the same killing effect was reached in 8 min with 17 mM DTAB. Binding of[H-3]CTAB to S, typhimurium ,2 was also reduced at 0 degrees C in the presence of BSA. Thus, in the presence of 1.5 mM BSA, QACs with the longer hydrocarbon chain were en most efficient at 30 degrees C whereas at 0 degrees C those with the shorter hydrocarbon chain were most active. Consequently, QACs with shorter tails should be used for disinfection in the presence of proteins at lower temperatures.