HIGH-FREQUENCY OF AUXOTROPHY IN CLINICAL ISOLATES OF PROTEUS-MIRABILIS HARBORING AN R-PLASMID

We report a novel phenomenon of high genetic instability, related to a uxotrophy, in strains of Proteus mirabilis. Among P. mirabilis strains harboring the R plasmid kept in our laboratory collection, and some f reshly isolated strains from clinical material, 54% of the samples pre sented auxotrophy at frequencies higher than 10(-3). Prototrophic clon es gave rise to auxotrophic ones at frequencies not explainable by the usual mutation mechanisms. The instability mainly affected the carbam oyl phosphate synthetase gene (car), which leads to a double requireme nt for arginine aid uracil for growth in minimal medium. Other genes w ere also affected, at a lower frequency. The car mutation does not rev ert 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 tran sposon in the generation of auxotrophy.