Studies on the mechanisms of the antibacterial action of ortho-phthalaldehyde

The reaction of ortho-phthalaldehyde (OPA) with amino acids and proteins wa s investigated as a possible mode of action. Bacterial pellets (obtained by centrifugation) changed colour after exposure to OPA. These colours were m ore intense at alkaline than acidic pH. Acidic and alkaline OPA reacted wit h primary amino acids to form coloured products. The reaction rate accelera ted with increasing pH. OPA increased the optical density of bacterial cell suspensions (an indication of protein coagulation or microbial surface or other changes in the opacity of cell constituents). The inhibition of ethyl enediaminetetraacetic acid- and sodium lauryl sulphate-induced lysis was no t as great as for glutaraldehyde (GTA), possibly indicating less cross-link ing of amines. Interactions with primary amino groups of the outer envelope or cell wall probably play a part in the action of OPA but the level of cr oss-linking associated with the outer membrane does not appear to be as ext ensive as that of GTA. The aromatic component might allow OPA to penetrate the outer layers of cells, thus helping to explain the very high activity o f OPA against Gram-negative vegetative organisms even though the degree of cross-linking seems to be less than that seen with GTA. Thus, OPA reacts st rongly with primary amines and stabilizes, to some extent, the outer membra ne and cell walls of vegetative organisms and this probably accounts for pa rt, but not necessarily all, of its lethal action.