Antibiotic-resistant strains of Aeromonas hydrophila have been isolated from the natural environment in the Chesapeake Bay and areas surrounding Dacca and the Matlab region of Bangladesh. The Bangladesh strains carried resistance to chloramphenicol, streptomycin, and tetracycline, and 57% of them had a multiple streptomycin-tetracycline resistance phenotype correlated with the presence of a large plasmid. The Chesapeake Bay strains were resistant to polymyxin B ane tetracycline, but showed neither multiple resistance nor R-factor carriage. Twenty-five percent of the environmental strains were toxigenic in a Y-1 adrenal cell assay. Toxigenicity showed no positive correlation with drug resistance or with plasmid carriage. Environmental areas of heavy human impact appear to be associated with a higher incidence of antibiotic-resistant strains of aeromonads.

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Vibrio cholerae strains of the 01 serovar, isolated from both clinical and environmental sources, had a much lower frequency of plasmid carriage (2/112 = 2%) than clinical and environmental non-01 serovar isolates (46/187 = 25%). The cryptic plasmids found in non-01 strains were all of low molecular weight and were shown by hybridization analysis to consist of two unrelated subgroups. Each subgroup was observed to be present in strains isolated from both clinical and environmental sources. One 01 serovar (ATCC 14033) carried a small cryptic plasmid, belonging to one of these plasmid groups, while a second (25728) contained a high molecular weight multiple antibiotic resistance plasmid which did not hybridize to either plasmid subgroup.

Two group F vibrio organisms have been identified among a collection of vibrio strains isolated from the aquatic environment in Bangladesh. Neither group F strain produced a cholera-like enterotoxin. One of the isolates, BV12, contained an R plasmid conferring resistance to streptomycin and chloramphenicol.

As a first step toward developing a system of genetic exchange between Vibrio parahaemolyticus strains, spontaneously arising auxotrophic and Kanagawa phenomenon-negative (KP-) mutants were isolated and characterized. Auxotrophic mutants were selected by nalidixic acid enrichment of parental cultures. Some Cysand Argmutants of a KP+ strain were found to be KP-. Reversion to prototrophy by these strains was not accompanied by a return to the parental KP+ phenotype. Additionally, two prototrophic KP-mutants were isolated. No detectable levels of vibriolysin were found in supernatant extracts of KPmutants by slide gel immunodiffusion analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, or assay for lethal activity in mice. All Cys-, Arg-, and Purmutants tested reverted to a different auxotrophy (phenotypic interconversion) as well as to prototrophy. The possible role of insertion sequence-like elements in vibriolysin production and phenotypic interconversion is discussed.