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 auxotrophy, in strains of Proteus mirabilis. Among P. mirabilis strains harboring the R plasmid Kept in our laboratory collection, and some freshly isolated strians from clinical material, 54 per cent of the samples presented auxotrophy at frequencies higher than 10(-3). Prototrophic closes 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.

Expression of cholera toxin epitopes inserted in flagellin by attenuated Salmonella strains

A novel antigen expression system has recently been developed by means of bacterial flagella, which are potent immunogens (Newton et al., Science 244:70-72,1989). Here we show the insertion of two epitopes from the cholera toxin B-subunit in Salmonella flagelin, CTP-1 (residues 8-20) and CTP-3 (residues 50-64) (Jacob et al., Proc. Natl Acad. Sci. USA 81: 7893-7896, 1984). First, we inserted CTP-1 for expression as a flagellar fusion; a hybrid flagellin expressing CTP-1 and CTP-3 in the hypervariable region of the flagellin gene was also constructed in an attempt to increase the cholera toxin neutralization potential, as well as to address some practical questions concerning the flagellar antigen expression system. The resulting constructs were non-motile although expression of chimeric flagellin was detected by immunoblotting and electron microscopy. One of the constructs (CTP-l + CTP-3) severely affected flagellin expression.