Possible relationship of a 36-megadalton Salmonella enteritidis plasmid to virulence in mice

All of the Salmonella enteritidis strains isolated from diseased animals (61 strains) and from beef (2 strains) in Japan and in West Germany (1 strain), except for 2 strains isolated from ducks, harbored either a 36-megadalton (Md) plasmid alone or in combination with several other plasmids of different sizes. It is likely that these 36-Md plasmids from various S. enteritidis strains were derived from the same origin because their plasmid DNAs showed the same cleavage patterns obtained with EcoRI, HindIII, and BamHI. We also suggested that this plasmid is native to S. enteritidis. Tests carried out on two strains isolated from ducks which naturally lacked this plasmid and one strain whose plasmid was artificially cured showed that the strains without the 36-Md plasmid showed less virulence compared to a wild-type strain harboring the 36-Md plasmid, suggesting that this 36-Md plasmid might be associated with virulence for mice.