Summary: During growth on benzoate-minimal medium mt-2 (PaW1) segregates derivative (“cured”) strains which have lost the ability to use the pathway encoded by its resident catabolic plasmid pWWO. Experiments with two plasmids identical to pWWO but each with an insert of Tn401, which confers resistance to carbenicillin, suggested that the “benzoate caring” occurs far more frequently by the specific deletion of the 39 kbp region carrying the catabolic genes than by total plasmid loss. This effect was not pH-dependent, and was not produced during growth on other weak organic acids, such as succinate or propionate, or when benzoate was present in the medium with an alternative, preferentially used carbon source such as succinate. Growth on benzoate did not cause loss from strain PaW174 of the plasmid pWWO-174, a derivative of pWWO which has deleted the 39 kbp region but carries Tn401. Similarly the naphthalene-catabolic plasmid pWW60-1, of the same incompatibility group as pWWO, was not lost from PaW701 during growth on benzoate. Competition between wild-type PaW1 and PaW174, which has the “cured” phenotype, showed that the latter has a distinct growth advantage on benzoate over the wild-type even when initially present as only 1% of the population: when PaW174 was seeded at lower cell ratios, spontaneously “cured” derivatives of PaW1 took over the culture after 60-80 generations, indicating that they are present in PaW1 cultures at frequencies between 10 and 10. We conclude that the progressive takeover of populations of PaW1 only occurs when benzoate is present as the sole growth source and that neither benzoate, nor other weak acids, affect plasmid segregation or deletion events: a sufficient explanation is that the “cured” segregants grow faster than the wild-type using the chromosomally determined β-ketoadipate pathway.


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