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UNLABELLED Listeria monocytogenes is an important foodborne pathogen that can cause infection in children, pregnant women, the immunocompromised and the elderly. Antibiotic resistance in this species would represent a significant public health problem since the organism has a high fatality/case ratio and resistance may contribute to failure of therapeutic treatment. This study was designed to explore whether the in vitro transferability of antibiotic resistance from enterococci to Listeria spp. could occur. It was found that 2/8 Listeria strains were able to acquire tetracycline resistance from Enterococcus faecium. Listeria monocytogenes GLM-2 acquired the resistance determinant tet(M) and additional streptomycin resistance through in vitro mating with Ent. faecium S27 isolated from commercial fermented dry sausage. Similarly, Listeria innocua became more resistant to tetracycline, but the genetic basis for this change was not confirmed. It has been suggested that enterococci may transfer antibiotic resistance genes via transposons to Listeria spp., and this may explain, in part, the origin of their antibiotic resistance. Thus, the presence of enterococci in food should not be ignored since they may actively contribute to enhanced antibiotic resistance of L. monocytogenes and other pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY Acquisition of antibiotic resistance by pathogenic bacteria in the absence of antibiotic pressure represents an unquantified threat to human health. In the present work resistance to tetracycline and streptomycin were transferred by nonplasmid-based conjugation from Enterococcus faecium isolated from fermented sausage to Listeria monocytogenes and Listeria innocua. Thus, natural transfer of antibiotic resistance to Listeria strains may occur in the future which reinforces the concern about the safety of enterococcal strains present in foods.

UNLABELLED Nanocomposite carboxymethyl cellulose films containing sinigrin (SNG) were prepared by stirring 2% (w/v) carboxymethyl cellulose (CMC) and 2% (w/v) glycerol (as a plasticizer) in distilled water with or without SNG (an antimicrobial precursor) as a 99% pure reagent (pSNG) or as a crude extract (cSNG). These films plus normal CMC film with or without SNG were tested on Escherichia coli O157:H7- inoculated beef for antimicrobial activity. Beef pieces measuring 6 × 5 × 2 cm(3) (L × W × H) were dipped in an E. coli O157:H7 broth suspension containing >8 log10 CFU ml(-1) and were drained for 3 min over a sterile cloth. They were wrapped in CMC or NCMC films, placed in a high oxygen barrier film (Deli *1), vacuum-packaged and stored at 8°C for 5, 8, 12 and 18 days. The CMC and NCMC films without SNG were not antimicrobial against E. coli O157:H7; however, NCMC and CMC films with SNG were highly antimicrobial. After 5 days at 8°C, E. coli O157:H7 was reduced more than 4 log10 by the NCMC•pSNG film and this reduction remained almost the same until 18 days at 8°C when E. coli O157:H7 was reduced >5 log10 CFU g(-1) meat. SIGNIFICANCE AND IMPACT OF THE STUDY Transparent nanoparticulate carboxymethyl cellulose (CMC) films containing sinigrin (SNG), an antimicrobial precursor, controlled surface contamination of packaged fresh beef by the pathogen Escherichia coli O157:H7 when stored at 8°C. Films with nanoparticulation that carried pure SNG or the naturally occurring SNG in Oriental mustard were significantly more antimicrobial than similar films without nanoparticulation. As films without sinigrin were not antimicrobial, the combinations studied showed that nanoparticulation of the packaging film enhanced delivery of the antimicrobial incorporated within the film.

Aim: To evaluate the effect of starvation, heat, cold, acid, alkaline, chlorine and ethanol stresses on the resistance of Enterobacter sakazakii in powdered infant milk formula (PIMF) towards gamma radiation. Methods and Results: Stressed cells of E. sakazakii ATCC 51329 and four other food isolate strains were mixed individually with PIMF, kept overnight at room temperature, and then exposed to gamma radiation up to 7·5 kGy. The D10-values were determined using linear regression and for the stressed E. sakazakii strains these values ranged from 0·82 to 1·95 kGy. Conclusions: Environmental stresses did not significantly change the sensitivity of most E. sakazakii strains to ionizing radiation. Significance and Impact of the Study: Data obtained established that most forms of environmental stress are unlikely to significantly enhance the resistance of E. sakazakii strains to lethal, low dose irradiation treatment.