Enterococci are used as starter and probiotic cultures in foods, and they occur as natural food contaminants. The genus Enterococcus is of increased significance as a cause of nosocomial infections, and this trend is exacerbated by the development of antibiotic resistance. In this study, we investigated the incidence of known virulence determinants in starter, food, and medical strains of Enterococcus faecalis, E. faecium, and E. durans. PCR and gene probe strategies were used to screen enterococcal isolates from both food and medical sources. Different and distinct patterns of incidence of virulence determinants were found for the E. faecalis and E. faecium strains. Medical E. faecalis strains had more virulence determinants than did food strains, which, in turn, had more than did starter strains. All of the E. faecalis strains tested possessed multiple determinants (between 6 and 11). E. faecium strains were generally free of virulence determinants, with notable exceptions. Significantly, esp and gelE determinants were identified in E. faecium medical strains. These virulence determinants have not previously been identified in E. faecium strains and may result from regional differences or the evolution of pathogenic E. faecium. Phenotypic testing revealed the existence of apparently silent gelE and cyl genes. In E. faecalis, the trend in these silent genes mirrors that of the expressed determinants. The potential for starter strains to acquire virulence determinants by natural conjugation mechanisms was investigated. Transconjugation in which starter strains acquired additional virulence determinants from medical strains was demonstrated. In addition, multiple pheromone-encoding genes were identified in both food and starter strains, indicating their potential to acquire other sex pheromone plasmids. These results suggest that the use of Enterococcus spp. in foods requires careful safety evaluation.

Lactococcus lactis is of great importance for the nutrition of hundreds of millions of people worldwide. This paper describes the genome sequence of Lactococcus lactis subsp. cremoris MG1363, the lactococcal strain most intensively studied throughout the world. The 2,529,478-bp genome contains 81 pseudogenes and encodes 2,436 proteins. Of the 530 unique proteins, 47 belong to the COG (clusters of orthologous groups) functional category "carbohydrate metabolism and transport," by far the largest category of novel proteins in comparison with L. lactis subsp. lactis IL1403. Nearly one-fifth of the 71 insertion elements are concentrated in a specific 56-kb region. This integration hot-spot region carries genes that are typically associated with lactococcal plasmids and a repeat sequence specifically found on plasmids and in the "lateral gene transfer hot spot" in the genome of Streptococcus thermophilus. Although the parent of L. lactis MG1363 was used to demonstrate lysogeny in Lactococcus, L. lactis MG1363 carries four remnant/satellite phages and two apparently complete prophages. The availability of the L. lactis MG1363 genome sequence will reinforce its status as the prototype among lactic acid bacteria through facilitation of further applied and fundamental research.Lactococcus lactis, a mesophilic fermentative bacterium producing lactic acid from sugar (hexose) fermentation, is an important industrial microorganism with extensive and diverse uses in food fermentation. Strains of L. lactis are used as defined mixtures or in undefined combinations with other lactic acid bacteria (LAB) in the production of fermented milk products. The organism has adapted to growth in milk under stringent human selection for better performance with respect to taste, flavor, and texture of dairy products, and this process continues today (57,98,99). In 1985, the "dairy streptococci" were reclassified into two L. lactis subspecies, Lactococcus lactis subsp. lactis (previously Streptococcus lactis) and Lactococcus lactis subsp. cremoris (previously Streptococcus cremoris), to distinguish them from the streptococci sensu stricto, which contain a number of notorious human pathogens (82, 83).The strain used in this study, L. lactis subsp. cremoris MG1363, is the international prototype for LAB genetics, and the knowledge gained from fundamental research on this strain has been exploited for a wide variety of biotechnological applications. The large and unstable complement of plasmid DNA of the parent strain, L. lactis NCDO712, was eliminated by employing UV treatment and protoplast-curing strategies in the early 1980s (41). The resultant plasmid-free strain, L. lactis MG1363, is robust and genetically amenable, which has facilitated the analysis of introduced lactococcal and heterologous DNA. Sophisticated systems have been developed for the expression of proteins and peptides in this strain, and it has been used as a cell factory for a wide variety of heterologous products (e.g., antimicrobials, including bacteriocins [50], bacteriop...

Aims: To investigate the mode of action of vanillin, the principle flavour component of vanilla, with regard to its antimicrobial activity against Escherichia coli, Lactobacillus plantarum and Listeria innocua. Methods and Results: In laboratory media, MICs of 15, 75 and 35 mmol l )1 vanillin were established for E. coli, Lact. plantarum and L. innocua, respectively. The observed inhibition was found to be bacteriostatic. Exposure to 10-40 mmol l )1 vanillin inhibited respiration of E. coli and L. innocua. Addition of 50-70 mmol l )1 vanillin to bacterial cell suspensions of the three organisms led to an increase in the uptake of the nucleic acid stain propidium iodide; however a significant proportion of cells still remained unstained indicating their cytoplasmic membranes were largely intact. Exposure to 50 mmol l )1 vanillin completely dissipated potassium ion gradients in cultures of Lact. plantarum within 40 min, while partial potassium gradients remained in cultures of E. coli and L. innocua. Furthermore, the addition of 100 mmol l )1 vanillin to cultures of Lact. plantarum resulted in the loss of pH homeostasis. However, intracellular ATP pools were largely unaffected in E. coli and L. innocua cultures upon exposure to 50 mmol l )1 vanillin, while ATP production was stimulated in Lact. plantarum cultures. In contrast to the more potent activity of carvacrol, a well studied phenolic flavour compound, the extent of membrane damage caused by vanillin is less severe. Conclusions: Vanillin is primarily a membrane-active compound, resulting in the dissipation of ion gradients and the inhibition of respiration, the extent to which is species-specific. These effects initially do not halt the production of ATP. Significance and Impact of the Study: Understanding the mode of action of natural antimicrobials may facilitate their application as natural food preservatives, particularly for their potential use in preservation systems employing multiple hurdles.