Characterization of cytotoxicity and R-plasmid in Salmonella krefeld

From 1981 to 1984, 254 isolates of Salmonella krefeld were isolated from newborns and infants with acute gastroenteritis in southern Taiwan. All the crude enterotoxin preparations of S. krefeld caused the cytotoxic elongation reaction in Chinese hamster ovary K1 (CHO-K1) cells. Cytotoxic enterotoxin was also produced by S. krefeld inducing Vero cells to round up and appear partially detached from the culture plate. It was noted that S. krefeld showed internalization and multiplication in CHO-K1 cells. S. krefeld exhibited 12 different resistant patterns. And the predominant patterns were found to be resistant to kanamycin and ampicillin (Ka-Amr), and resistant to kanamycin, ampicillin, chloramphenicol and tetracycline (Ka-Am-Cm-Ter). It was found that two distinct plasmids of 34 megadalton (Md) and 120 Md were commonly present in these strains. S. krefeld haboured 34 Md and 120 Md R-plasmid, which conferred resistance to Ka-Amr and Ka-Am-Cm-Ter, respectively. From the resistance transferred patterns, Ka-Amr was the most common resistance among transconjugants. The frequency of transfer of the 34 Md R-plasmid (2.71 x 10(-3) transconjugants/donor cell) from S. krefeld to E. coli K-12 14R525 was 20 times higher than that of the 120 Md R-plasmid (1.48 x 10(-4) transconjugants/donor cell). In analysis of the restriction endonuclease digest of the 34 Md plasmid obtained from different bacterial sources, their specific identical DNA fragment pattern suggested that the outbreak infection due to S. krefeld had a common origin.