~ ~~ ~~Gel electrophoresis of DNA from 95 clinical isolates of Shigella sonnei and Shigella JIexneri resistant to antibiotics revealed a heterogeneous plasmid population. Most of the plasmids were smaller than 6 megadaltons (Mdal). Six S. sonnei isolates with the most common antibiotic resistance pattern were characterized. They had two plasmids in common : one was a self-transmissible Fif plasmid of 46 Mdal encoding tetracycline resistance, while the other was a 5.5 Mdal non-conjugative plasmid encoding resistance to streptomycin and sulphafurazole. In addition, several cryptic plasmids ranging in size from 1.0 to 24.5 Mdal were present. Mobilization of the 5.5 Mdal SuSm plasmid and a 1.0 Mdal cryptic plasmid was demonstrated with all six S. sonnei isolates during conjugation. This mobilization was mediated by the 46 Mdal self-transmissible Fif R plasmid and also by a 24-5 Mdal Fiplasmid carrying no known drug resistance determinants.

SUMMARYInvestigation of a small series of cases of typhoid fever infected in a river between 1963 and 1970 revealed that all were caused by a single source, a carrier of a rare phage type of Salmonella typhi. The contamination of the river resulted from an incorrect sewage connexion with a surface water drain outfall into the river.

A genetic study has been performed on a set of mutations which prevent the replication of F-factors in Escherichia coli K-12. The gene affected is designated seg and has been located by transduction in the serB-thr segment of the chromosome. The seg gene does not appear to be related to the dnaC locus.

No correlation was found between phaseolotoxin production and the presence of any one plasmid in twelve Pseudomonas syringae pv. phaseolicola strains. The spontaneous loss of ability to produce phaseolotoxin by three of these strains was not associated with loss of a specific plasmid. Phaseolotoxin is a potent inhibitor of the enzyme ornithine carbamoyltransferase (EC 2.1.3.3) from bean and other plants (Patil et al., 1970; Ferguson & Johnston, 1980). Some strains of P. syringae pv. phaseolicola give rise to spontaneous variants which infect bean plants without producing chlorosis (Jensen & Livingston, 1944). Such variants do not produce phaseolotoxin (R. E. Mitchell, unpublished data). One explanation of this phenomenon may be that genes coding for phaseolotoxin production reside on a plasmid and that loss of this plasmid results in the inability of the variant to produce phaseolotoxin. Plasmid DNA has been demonstrated in strains of P. syringae pv. phaseolicola (Gasson, 1976; Gantotti et al., 1979;Panopoulos et al., 1979) and a correlation between loss of toxin production and the U.V. light-induced loss of a 22.5 megadalton (Mdal) plasmid has been suggested (Gantotti et al., 1979). This communication describes experiments which show that spontaneous loss of phaseolotoxin production is not associated with loss of a plasmid, and that the ability of P. syringae pv. phaseolicola strains to produce phaseolotoxin does not always coincide with the presence of a particular plasmid. M E T H O D S

The use of an agreed protocol proved to be very helpful to both human and veterinary public health agencies in the investigation and control of an outbreak of bovine tuberculosis. There is a clear need for better human screening tests.