Detection of Enteric Bacteria in Raw Food Samples from Vietnam ...

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ABSTRACT
isolates were detected to contain integrons, with some of them containing 2 integrons.
Sequencing results revealed that integrons of Salmonella spp. and E. coli isolates harboured
varying gene cassettes, including aadA1, aadA2, aadA5, aacA4, dhfrXII, drfA1 and dhfrA17,
blaPSE1 and catB3. Other mobile elements, that were plasmids, were detected in all 23
antibiotic resistant Salmonella spp. and 33 E. coli tested isolates, in which 35% of Salmonella
spp. isolates and 76% of E. coli isolates harboured plasmids of more than 95 kb. Conjugation
experiments has showed the successful transfer of all or part of the antibiotic resistance
phenotypes among the food isolates, which involved the transfer of one, or more than one,
plasmids. Conjugation was demonstrated to occur between the same and different species
(Salmonella spp. and E. coli) of food isolate collections. In addition, the transfer of resistance
genes by transduction was also demonstrated through the phage P22 HT105/1 int-201 where a
pathogenic isolate from chicken showed the ability to receive nalidixic acid resistance and
streptomycin resistance determinants from donors. The ability of food isolates to exchange
resistance genes by different mechanisms demonstrated in this study imply the possible health
risk caused by food bacteria, and virulence strains might even become more resistant.
Salmonella genomic island 1 (SGI1), a 43-kb genomic region contains a 13-kb antibiotic
resistance gene cluster, has been reported in multiresistant S. Typhimurium DT104 and in
other S. enterica serovars worldwide. PCR mapping identified a variant SGI1 antibiotic
resistance gene cluster in an isolate of S. Albany from chicken, in which aadA2 gene in SGI1
was replaced by dfrA1 and orfF gene cassettes. Macrorestriction analysis by pulsed field gel
electrophoresis of this strain digested by XbaI showed that it is genetically distinct from SGI1
containing S. Typhimurium DT104, suggesting the horizontal transfer of this region.
However, the resistance gene cluster could not be transferred to recipient by typical in vitro
conjugation experiment.