Inhibitory mechanisms by chlorobenzoate mixtures in chlorobenzoate-degrading microorganisms

Abstract

A microbial consortium selected on 2-chlorobenzoate was shown to be able to also utilise 4-chlorobenzoate and 2,5-dichlorobenzoate as sole carbon source. The consortium adapted to grow on 4-chlorobenzoate, degraded the carbon source via protocatechuate, whereas the same consortium degraded 2-chlorobenzoate and 2,5-dichlorobenzoate via 1,2-or 1,6-dioxygenation of the chlorinated ring. Moreover, no significant effects on the microbial growth due to the presence of chlorobenzoate mixtures were observed when 4-chlorobenzoate was the carbon source. Instead, when meta-substituted chlorobenzoates were added as co-substrates to 2,5-dichlorobenzoate, the growth of the consortium was totally inhibited, just as when the culture utilised for growth 2-chlorobenzoate. Uptake experiments with 2-chlorobenzoate-grown resting cells showed that 3-chlorobenzoate, 2,3-dichlorobenzoate and 2,3,5-trichlorobenzoate competed with 2-chlorobenzoate, entering the resting cells both preferentially and faster than the growth substrate, that was also impeded to enter. Also 3,4-dichlorobenzoate and 3,5-dichlorobenzoate hindered the uptake of the growth substrate (2-chlorobenzoate and 2,5-dichlorobenzoate, respectively), but they did not enter themselves the cells. Finally, 3,5-dichlorobenzoate neither entered the 4-chlorobenzoate-grown cells nor hindered 4-chlorobenzoate uptake. The relationships between growth inhibiting effects and chlorosubstituent position on the aromatic ring of the chlorobenzoates supplied as co-substrates are discussed.