Characterization of a small cryptic plasmid from Salmonella enteritidis that affects the growth of Escherichia coli

We examined the plasmid content of 25 clinical isolates of Salmonella enteritidis, and detected the presence of small plasmids (3-5.3 kb) in 9 of them, alone, or in addition to the large, so-called virulence plasmid. A 5.3-kb plasmid isolated as unique extrachromosomal DNA from a strain responsible for a high-mortality outbreak was characterized by restriction mapping and cloning. The plasmid replicon was localized in a 1.7-kb fragment, that hybridized with three of the small plasmids detected in S. enteritidis, and with another small plasmid from Salmonella typhimurium. A strain of Escherichia coli carrying this plasmid, or a cloned 3.7-kb PvuII restriction fragment, showed a slower growth rate, especially in minimal medium, as well as a noticeable increase in DNA methyltransferase activity.