Metabolism of esters of phydroxybenzoic acid by a strain of pseudomonas aeruginosa

ABSTRACT

Abacterial contaminant identified as Pseudomonas aeruginosa [No. 396] was able to grow in a carbon-free fluid mineral salts I [basal medium] supplemented with propyl - p-hydroxybenzoate or with both the propyl-and the methyl-p-hydroxybenzoates in combiration at concentrations higher than those commonly recommended for the preservation of multi-dose pharmaceutical preparations. This strain failed to grow on the basal medium when it was supple [mented with the methyl ester alone except after prolonged incubation [15 days or more] where selection of resistant cells occurred. Evidence was presented to show that the microorganism was able to utilize the propyl-but not the methylmoiety of the paraben ester for growth. The first step in the metabolism of the propyl ester by this strain involves hydrolysis into p-hydroxyberzoic acid [PHBA] by both infra and extra-cellular esterases. These enzmyes were inducible in nature and can hydrolyze the methyl ester too. The activity of these esterases were significantly affected by minor changes in the functional groups on the benzene ring as compound structurally related to parabens were not hydrolyzed. Cell suspension experiments showed that the PHBA resulting from hydrolysis underwent hydroxylation into protocatechuic acid [PCA] only after the complete disappearance of the paraben substrate. This I would involve a 4-hyaroxybenzoate -3-mono-oxygenase which was shown to be also inducible but was strictly intracellular. The propyl paraben competitively inhibited this degradative step probably because of the structural similarity with PHBA. PCA did not appear in amounts corresponding to the loss of PHBA and growth continued indicating possible further metabolism of PCA to other metabolites which were in turn utilized for growth