Antibacterial agents benzoic acid

The degree of ionization of acidic and basic antimicrobial agents depends on pH. Some compounds are active only in the unionized state (e.g., phenolics) whereas others are preferentially active as either the anion or cation. It therefore follows that the activity of a particular concentration of an agent will be enhanced at a pH that favors the formation of the active species. Thus, cationic antibacterials such as acridines and quaternary ammonium compounds are more active under alkaline conditions. Conversely, phenols and benzoic acid are more active in an acid medium. Chlorbutol is less active above pH 5 and unstable above pH 6. Phenylmercuric nitrate is only active at above pH 6 whereas thiomersal is more active under acid conditions. The sporicidal activity of glutaraldehyde is considerably enhanced under alkaline conditions whereas hypochlorites are virtually ineffective at above pH 8. 

The majority of the agents that fall into this category are antimetabolites. An antimetabolite may be defined as a compound, structurally related to a metabolite, which prevents its further utilization by competing with it for an enzyme. The recognition that a compound, closely related to a metabolite, could prevent its participation in an essential biosynthetic pathway, arose from studies on the antibacterial effect of sulfanilamide and its reversal by the structurally related p-amino-benzoic acid (PAB) ... 

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