Retron reverse transcriptase (rrtT) can be lost in multidrug resistant Salmonella enterica serovar Typhimurium DT 104 strains and influences virulence for mice

In Salmonella enterica serovar Typhimurium, retron reverse transcriptase (rrtT), which is part of St85 retron, is quite ubiquitous and is located in the thdF - yidY intergenic region. In this study, we showed that rrtT is relatively unstable in multidrug resistant, Salmonella genomic island 1 (SGI1) positive strains. Out of 365 field strains, 55 were free of retron. In 54 of the rrtT negative strains, the excision must have occurred by the same mechanism in which the rrtT together with five other genes was excised. Altogether 8164 bp was missing in the chromosome of the rrtT negative strains. Since the deletion happened exactly between the right inverted repeat of IS6100 and inside the yieE gene, we propose that intramolecular transposition of IS6100 followed by homologous recombination was responsible for the excision. Excision of retron together with the right end of SGI1 may also result in its stabilisation in the Salmonella typhimurium genome. Experimental deletion of rrtT resulted in an accelerated course of infection in orally infected mice. Since the retron excision occured exclusively in multidrug resistant S. typhimurium, it cannot be excluded that such strains may increase their virulence in the future.