The role of a 120-kb plasmid in relation to virulence and drug resistance factor in Shigella dysenteriae was studied. For characterization of plasmids, the mating system is a useful and efficient means of transferring both large and small plasmids to a new host. The conjugative transfer of a 120-kb (pCAT120) ampicillin-resistant plasmid of S. dysenteriae to E. coli K-12 was not successful. Introduction of an E. coli fertility factor plasmid F, did not help to mobilize the plasmid. Low transfer frequencies of antibiotic markers to E. coli were achieved by treatment of the donor S. dysenteriae with N-methyl-N'-nitro-N-nitrosoguanidine. The transconjugants showed resistance to ampicillin, chloramphenicol, tetracycline and cadmium. A transconjugant carrying the 120-kb plasmid of S. dysenteriae produced keratoconjunctivitis in guinea pigs. Repeated subculture of Clmr transconjugant (pCAT120) on tryptic soya agar plates became Clms and showed four distinct DNA bands ranging from 3 to 10 kb in size on agarose gel electrophoresis. Utilization of organic acids, metal resistance (Cd), dye-binding properties (Crb+, Ebr+) and drug resistance (Amp, Tet) were identified on 10, 7, 4 and 3-kb plasmid DNA fragment of pCAT120 respectively. Crb+ 4-kb DNA fragment of pCAT120 was isolated, purified and transferred to an avirulent E. coli K12 by transformation. However, transformant (pET4) showed poor growth on solid media and its growth in liquid culture was only possible after supplementation of the unknown low-molar-mass thermolabile factor(s) secreted by the recipient strain.(ABSTRACT TRUNCATED AT 250 WORDS)

National Institute of Cholera and Enteric Diseases Beliaghata Calcutta India