Synergism within polyhexamethylene biguanide biocide formulations.

Polyhexamethylene biguanides (PHMB) are mixtures of polymeric biguanides with an average polymer length (n) of 5, but containing high (n greater than 15, mol. wt 3300) and low molecular weight material (n = 2, mol. wt 400). Studies involving discrete molecular weight fractions of PHMB have shown that antimicrobial activity of PHMB increases with increasing polymer length. Cell suspensions which had not been subjected to centrifugation and/or washing during their preparation were employed. Whilst activity was still observed to increase with n, the trend was much reduced as n exceeded six. Centrifugation and washing of cells markedly increased the activity of high but not low molecular weight materials and corresponded to losses upon centrifugation of envelope lipopolysaccharide (LPS). Such envelope LPS represented high affinity binding sites on the surfaces of the cells. Combinations of various molecular weight fractions of PHMB were evaluated against filter-washed cells and revealed a profound synergy between extremes of polymer length.