Adsorption of Salmonella enteritidis by cetylpyridinium-exchanged montmorillonite clays

Recent experiments in our laboratory have suggested that certain montmorill onite clays, when exchanged with the cationic surfactant cetylpyridinium (C P), may be useful in removing bacteria from aqueous solution. During an ini tial study, screening various CP-exchanged products for potential antibacte rial activity, three CP-exchanged clays - CP*AAM (acid-activated montmorill onite), CP*STx-1 (Ca++-montmorillonite), and CP*SWy-2 (Na+-montmorillonite) , proved to be the most effective. Binding studies were performed using 1 m g each of CP-exchanged AAM, STx-1, and SWy-2 with a standardized Salmonella enteritidis solution containing approximate to 40,000 colony forming units (CFU)/ml. The modified clays reduced bacterial numbers 98.1, 97.6, and 95. 2%, respectively. In contrast, the parent clays only produced reductions of 39.8, 16.9, and 16.6%, respectively. Attempts were made to desorb CP from the modified clays by washing in sterile physiological saline for 24 h. The resulting wash solutions failed to produce any significant reduction in ba cterial colony counts; while, the washed clays retained their full antimicr obial activity. These findings suggested that the antibacterial effect of t he clays is localized on the clay surface and is not due to CP dissociating from the clay. Electron microscopy revealed that the bacteria adhered to t he surface of the CP-exchanged clays, but not the parent clays. Results fro m timed binding studies showed that the antibacterial effect was stable ove r the period observed. Rates of binding were positively influenced by incre asing temperature, not affected by changes in pH, and negatively influenced by the presence of organic contaminants. The mechanism by which bacterial counts are reduced may involve the enhanced hydrophobicity and affinity of the CP-exchanged clay for Salmonella and the antibacterial activity of CP. (C) 2000 Published by Elsevier Science B.V. All rights reserved.