COMPARISON OF THE TAXONOMY, SEROLOGY, DRUG-RESISTANCE TRANSFER, AND VIRULENCE OF CITROBACTER-FREUNDII STRAINS FROM MAMMALS AND POIKILOTHERMIC HOSTS

In this study, the phenotypic, antigenic, and virulence characteristic s of 32 Citrobacter freundii strains of fish, human, and veterinary or igin were comparatively analyzed. In addition, the spread of drug resi stance factors by conjugation was investigated. Regardless of the sour ce of isolation, the strains exhibited variable reactions mainly for a rginine dihydrolase, ornithine decarboxylase, and fermentation of sucr ose, melibiose, amygdalin, and salicin. Total fatty acid methyl ester analysis by gas chromatography proved to be useful for an intratypic d ifferentiation within the C. freundii strains studied. In fact, althou gh all of the isolates exhibited similar fatty acid methyl ester profi les, significant differences in the major fatty acids 16:1 and 16:0 an d in the 17:0 Delta region were observed between the isolates from sal monids and the remaining strains. Serological studies using agglutinat ion tests, analysis of lipopolysaccharides (LPS), and the correspondin g immunoblots with 13 antisera indicated a great antigenic diversity a mong the strains. Common LPS patterns were shared only by some isolate s shelving high cross-agglutination titers. In contrast, although all strains exhibited very similar surface protein patterns, only two comm on outer membrane proteins of 54 and 58 kDa were immunologically relat ed. Infectivity trials performed in mice and rainbow trout indicated t hat all of the C. freundii strains were not pathogenic for mice (50% l ethal dose of >5 x 10(7)). Although the isolates displayed a low degre e of virulence for trout, inoculated strains were always recovered fro m the survivors in pure culture. The detection of plasmid DNA by agaro se gel electrophoresis revealed that poikilothermic isolates harbored one or more large-molecular-size plasmid bands ranging from 110 to 25 MDa, whereas practically no plasmids were detected in the isolates fro m homoiotherms. Although all C. freundii strains were multidrug resist ant, only the isolates from fish (13 of 14) displayed resistance to te tracycline and oxytetracycline, drugs that are commonly used to treat fish in aquaculture facilities. Mating assays indicated that 7 of 13 s trains selected as potential donors possessed transferable R factors t o Escherichia coli coding mainly for tetracyclines alone or in associa tion with chloramphenicol and streptomycin. Restriction endonuclease c leavage analysis allowed us to determine the genetic similarities amon g some of the plasmids transferred. The positive results of secondary matings demonstrated the potential dissemination of the conjugative R factors in the environment.