Phenol: a complex chemoeffector in bacterial chemotaxis.

Earlier observations that phenol is a repellent for Salmonella typhimurium but an attractant for Escherichia coli were confirmed. This behavioral difference was found to correlate with a difference in the effect phenol had on receptor methylation levels; it caused net demethylation in S. typhimurium but net methylation in E. coli. On the basis of mutant behavior and measurement of phenol-stimulated methylation, the attractant response of E. coli was shown to be mediated principally by the Tar receptor. In S. typhimurium, two receptors were found to be sensitive to phenol, namely, an unidentified receptor, which mediated the repellent response and showed phenol-stimulated demethylation; and the Tar receptor, which (as with E. coli) mediated the attractant response and showed phenol-stimulated methylation. In wild-type S. typhimurium, the former receptor dominated the Tar receptor, with respect to both behavior and methylation changes. However, when the amount of Tar receptor was artificially increased by the use of Tar-encoding plasmids, S. typhimurium cells exhibited an attractant response to phenol. No protein analogous to the phenol-specific repellent receptor was evident in E. coli, explaining the different behavioral responses of the two species toward phenol.