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The feasibility of using two primers internal to the stdA gene (which encodes the fimbrial major subunit of the std fimbrial gene cluster in Salmonella enterica serovar Typhi) to detect Salmonella by PCR was explored. The 518-bp stdA specific sequence was conserved among 268 strains from 45 serovars of S. enterica. One Salmonella bongori CCUG 30042 strain and 34 non-Salmonella strains did not possess this sequence. A sensitivity test revealed that the stdA-specific primer set detected 3.4 x 10(-1) pg of genomic DNA and 3.0 x 10(5) CFU/ml with serial dilutions of Salmonella Typhimurium cells. In vitro testing for specificity using pig carcass sponge samples contaminated with Salmonella Typhimurium also was performed. An initial Salmonella Typhimurium inoculum of 4.4 x 10(1) CFU/ml in pig carcass exudates reached the stdA primer detection level after preenrichment in buffered peptone water at 37 degrees C for 18 h in the presence of indigenous non-Salmonella flora at 4.0 X 10(7) CFU/ml, but the detection level decreased to 4.4 x 10(0) CFU/ml after selective enrichment in Rappaport-Vassiliadis R10 broth for 18 h at 42 degrees C. The PCR method with primers specific for stdA is a quick and sensitive tool for detecting S. enterica, which is an important cause of foodborne disease.

Background Type 1 fimbriae are the most commonly found fimbrial appendages on the outer membrane of Salmonella enterica serotype Typhimurium. Previous investigations indicate that static broth culture favours S. Typhimurium to produce type 1 fimbriae, while non-fimbriate bacteria are obtained by growth on solid agar media. The phenotypic expression of type 1 fimbriae in S. Typhimurium is the result of the interaction and cooperation of several genes in the fim gene cluster. Other gene products that may also participate in the regulation of type 1 fimbrial expression remain uncharacterized. Results In the present study, transposon insertion mutagenesis was performed on S. Typhimurium to generate a library to screen for those mutants that would exhibit different type 1 fimbrial phenotypes than the parental strain. Eight-two mutants were obtained from 7,239 clones screened using the yeast agglutination test. Forty-four mutants produced type 1 fimbriae on both solid agar and static broth media, while none of the other 38 mutants formed type 1 fimbriae in either culture condition. The flanking sequences of the transposons from 54 mutants were cloned and sequenced. These mutants can be classified according to the functions or putative functions of the open reading frames disrupted by the transposon. Our current results indicate that the genetic determinants such as those involved in the fimbrial biogenesis and regulation, global regulators, transporter proteins, prophage-derived proteins, and enzymes of different functions, to name a few, may play a role in the regulation of type 1 fimbrial expression in response to solid agar and static broth culture conditions. A complementation test revealed that transforming a recombinant plasmid possessing the coding sequence of a NAD(P)H-flavin reductase gene ubiB restored an ubiB mutant to exhibit the type 1 fimbrial phenotype as its parental strain. Conclusion Genetic determinants other than the fim genes may involve in the regulation of type 1 fimbrial expression in S. Typhimurium. How each gene product may influence type 1 fimbrial expression is an interesting research topic which warrants further investigation.

One hundred fifty-eight Salmonella strains isolated from pork carcasses in a nationwide screening program in Taiwan from 2000 through 2003 were analyzed for serotype distribution and antimicrobial susceptibility. Twenty Salmonella serotypes were obtained, among which Derby, Anatum, Typhimurium, and Schwarzengrund were the most frequently isolated, accounting for 76% of the strains. Antimicrobial susceptibility tests with the microdilution method were performed on these serotypes to determine the MIC. All strains tested were sensitive to ceftriaxone, with an MIC90 (minimum concentration inhibiting 90% of isolates tested) of 0.25 to 8 microg/ml. More than 60% of the strains were resistant to ampicillin, chloramphenicol, tetracycline, nalidixic acid, and sulfamethoxazole, with MIC90 values of 128 to >512 microg/ml. More than 80% of the Salmonella Schwarzengrund strains were resistant to ciprofloxacin (MIC90 = 8 microg/ml) and enrofloxacin (MIC90 = 16 microg/ml). The Salmonella Typhimurium strains exhibited 17 and 23% resistance to ciprofloxacin and enrofloxacin, respectively, with an MIC90 of 8 microg/ ml, and these two antibiotics also were active against Salmonella Derby and Salmonella Anatum. Cephalothin, gentamicin, and trimethoprim had limited activity against Salmonella Anatum and Salmonella Schwarzengrund, with MIC90 values of 256 to >512 microg/ml. Cephalothin and gentamicin were moderately active against Salmonella Derby and Salmonella Typhimurium, but 30 to 40% of these strains were resistant to trimethoprim. The Salmonella strains isolated from pork carcasses in Taiwan were relatively resistant to the antimicrobial agents tested, with the exception of ceftriaxone. Although a variety of MIC values were obtained, generally these values were high.