Among 48,901 samples of ready-to-eat food products at the Danish retail market, 0.5% had counts of Bacillus cereus-like bacteria above 10(4) cfu g(-1). The high counts were most frequently found in starchy, cooked products, but also in fresh cucumbers and tomatoes. Forty randomly selected strains had at least one gene or component involved in human diarrhoeal disease, while emetic toxin was related to only one B. cereus strain. A new observation was that 31 out of the 40 randomly selected B. cereus-like strains could be classified as Bacillus thuringiensis due to crystal production and/or content of cry genes. Thus, a large proportion of the B. cereus-like organisms present in food may belong to B. thuringiensis.

Two wild-type strains of Lactobacillus plantarum previously isolated from fermented dry sausages were analysed for their ability to transfer antibiotic resistance plasmids in the gastrointestinal tract. For this purpose, we used gnotobiotic rats as an in vivo model. Rats were initially inoculated with the recipient Enterococcus faecalis JH2-2 at a concentration of 10(10) CFU mL(-1). After a week, either of the two donors L. plantarum DG 522 (harbouring a tet(M)-containing plasmid of c. 40 kb) or L. plantarum DG 507 [harbouring a tet(M)-containing plasmid of c. 10 kb and an erm(B)-containing plasmid of c. 8.5 kb] was introduced at concentrations in the range of 10(8)-10(10) CFU mL(-1). Two days after donor introduction, the first transconjugants (TCs) were detected in faecal samples. The detected numbers of tet(M)-TCs were comparable for the two donors. In both cases, this number increased to c. 5 x 10(2) CFU g(-1) faeces towards the end of the experiment. For erm(B)-TCs, the number was significantly higher and increased to c. 10(3) CFU g(-1) faeces. To our knowledge, this is the first study showing in vivo transfer of wild-type antibiotic resistance plasmids from L. plantarum to E. faecalis.

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Tetracycline-resistant gram-negative bacteria were isolated from four different marine sediments in Scandinavia and analyzed with DNA probes for the determinant classes A to E. Colony hybridizations of 429 isolates revealed that class E is the dominating resistance determinant in these marine sediments. Comparison of fecally polluted and unpolluted sediments showed few determinant classes in unpolluted sediment and a complex composition of several determinant classes in polluted sediment. Total DNA extraction and analysis with DNA probes for determinant classes A to E resulted in no hybridization signal, because of the low number of gram-negative tetracycline-resistant bacteria. Identification of class E isolates revealed that this determinant is present not only in Aeromonas hydrophila, Escherichia coli, and Vibrio salmonicida but also in additional strains.

The transfer of the plasmids pJKJ5 and TOL (pWWO) from Pseudomonas putida to the indigenous bacterial community on alfalfa sprouts was studied. Tagging with fluorescent protein markers allowed direct quantification of the introduced donor bacteria and of indigenous bacteria that had received the plasmids. The sprouts were observed for 9 days; during this time alfalfa seeds, inoculated with donor bacteria, developed to edible and subsequently decaying sprouts. The first transconjugants were detected on day 6 after donor inoculation and occurred at frequencies of 3.4 ؋ 10 ؊4 and 2.0 ؋ 10 ؊6 transconjugant cells per donor cell for pKJK5::gfp and TOL::gfp, respectively. Confocal laser scanning microscopy revealed that the sprouts were heavily colonized with donors and that most transconjugants were located around the hypocotyl and root areas. Randomly selected members of the indigenous bacterial community from both inoculated and uninoculated sprouts, as well as a representative part of the community that had received the plasmids, were characterized by polymorphisms of PCR-amplified ribosomal DNA (rDNA) spacer regions between the 16S and 23S genes, followed by partial 16S rDNA sequencing. This showed that the initially dominating genera Erwinia and Paenibacillus were gradually replaced by Pseudomonas on the fully developed sprouts. Transconjugants carrying either of the investigated plasmids mainly belonged to the genera Pseudomonas and Erwinia. The numbers of transconjugant cells did not reach detectable levels until 6 days after the onset of germination, at which point these species constituted the majority of the indigenous bacteria. In conclusion, the alfalfa sprouts provided an environment that allowed noteworthy frequencies of plasmid transfer from P. putida in the absence of selective pressure that could favor the presence of the investigated plasmids.