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High frequency of auxotrophy in clinical isolates of Proteus mirabilis harboring an R plasmid

Salete M.C. Newton; Sergio Olavo P. da Costa

Departamento de Microbiologia, Instituto de Ciências Biomédicas da Universidade de São Paulo, 05508-900 São Paulo, SP, Brasil. Send correspondence to S.O.P.C.

ABSTRACT

We report a novel phenomenon of high genetic instability, related to auxotrophy, in strains of Proteus mirabilis. Among P. mirabilis strains harboring the R plasmid kept in our laboratory collection, and some freshly isolated strains from clinical material, 54% of the samples presented auxotrophy at frequencies higher than 10^-3. Prototrophic clones gave rise to auxotrophic ones at frequencies not explainable by the usual mutation mechanisms. The instability mainly affected the carbamoyl phosphate synthetase gene (car), which leads to a double requirement for arginine and uracil for growth in minimal medium. Other genes were also affected, at a lower frequency. The car mutation does not revert to prototrophy. A similar phenomenon of instability was induced in Escherichia coli strain HB 101 upon introduction of a drug-resistance plasmid from P. mirabilis. We have ruled out the hypothesis of a transposon in the generation of auxotrophy.

Keywords: auxotrophy; clinical isolates; Proteus mirabilis; harboring; R plasmid.

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