Despite extensive surveillance, food-borne Salmonella enterica infections continue to be a significant burden on public health systems worldwide. As the S. enterica species comprises sublineages that differ greatly in antigenic representation, virulence, and antimicrobial resistance phenotypes, a better understanding of the species' evolution is critical for the prediction and prevention of future outbreaks. The roles that virulence and resistance phenotype acquisition, exchange, and loss play in the evolution of S. enterica sublineages, which to a certain extent are represented by serotypes, remains mostly uncharacterized. Here, we compare 17 newly sequenced and phenotypically characterized nontyphoidal S. enterica strains to 11 previously sequenced S. enterica genomes to carry out the most comprehensive comparative analysis of this species so far. These phenotypic and genotypic data comparisons in the phylogenetic species context suggest that the evolution of known S. enterica sublineages is mediated mostly by two mechanisms, (i) the loss of coding sequences with known metabolic functions, which leads to functional reduction, and (ii) the acquisition of horizontally transferred phage and plasmid DNA, which provides virulence and resistance functions and leads to increasing specialization. Matches between S. enterica clustered regularly interspaced short palindromic repeats (CRISPR), part of a defense mechanism against invading plasmid and phage DNA, and plasmid and prophage regions suggest that CRISPR-mediated immunity could control short-term phenotype changes and mediate long-term sublineage evolution. CRISPR analysis could therefore be critical in assessing the evolutionary potential of S. enterica sublineages and aid in the prediction and prevention of future S. enterica outbreaks.

Twelve strains of different Bacteroides species were tested for their efficiency of detection of bacteriophages from sewage. The host range of several isolated phages was investigated. The results indicated that there was a high degree of strain specificity. Then, by using Bacteroidesfragilis HSP 40 as the host, which proved to be the most efficient for the detection of phages, feces from humans and several animal species and raw sewage, river water, water from lagoons, seawater, groundwater, and sediments were tested for the presence of bacteriophages that were active against B. fragilis HSP 40. Phages were detected in feces of 10% of the human fecal samples tested and was never detected in feces of the other animal species studied. Moreover, bacteriophages were always recovered from sewage and sewage-polluted samples of waters and sediments, but not from nonpolluted samples. The titers recovered were dependent on the degree of pollution in analyzed waters and sediments. * Corresponding author. MATERIALS AND METHODS Bacterial strains. Twenty-five strains of Bacteroides were used in this study. Strains HSP 39, HSP 40, and HSP 44 were obtained from the Microbiology Service at the Hospital de Sant Pau (Barcelona, Spain). They were isolated from peritoneal exudates and were classified as Bacteroides fragilis according to substrate utilization, products of fermentation determined by gas-liquid chromatography, growth in 20% bile, and indole production (12).

The study of the effects of osmolarity and growth phase on Salmonella typhi adherence to and invasion of Henle 407 epithelial cells provides the first evidence of a clear overlap between these two environmental stimuli. High-osmolarity conditions are required in the late-log phase for optimum induction of the adherent and invasive phenotypes.

Bacteroidesfragilis HSP40 phages have been detected in waters with various levels of fecal contamination of human origin. The average numbers of B. fragilis phages present in sewage water reached 5.3 x 103 per 100 ml of water. We found a number 1,000 times lower in a river contaminated with domestic sewage only, in which the levels of fecal coliforms and fecal streptococci were 10,000 times lower than those found in raw sewage. In addition, B. fragilis phages were not found in significant numbers in slaughterhouse wastewaters. They were not present in fecal-polluted waters containing fecal contamination from wildlife only. Although the number of B. fragilis phages present in contaminated waters was lower than the number of coliphages, their presence indicated human fecal contamination. It is also shown that Bacteroides phages are only able to multiply under anaerobic conditions in the presence of nutrients, and they cannot multiply in natural waters and sediments.

Bacteriophages infecting Bacteroides fragilis 40 were found in all kinds of sewage polluted samples. In contrast, they were never recovered either in non-faecally polluted samples or in samples only polluted by wildlife. In simulated laboratory experiments, a selected bacteriophage, B40-8, active against B. fragilis 40 did not replicate either in aerobiosis or under anaerobic conditions, when the culture medium was substituted by either fresh water, seawater or sediments. Inactivation rates of the same bacteriophage were studied in comparison with other viruses such as poliovirus 1, simian rotavirus SA11 and f2 coliphage, showing decay rates similar or inferior to the ones of the other viruses studied. Results herein shown indicate that bacteriophages infecting B.fragilis may be good candidates for a surrogate indicator of human viruses in the environment.