No abstract

From a collection of 2,800 Tn5-TC1 transposon mutants of Salmonella typhimurium F98, 18 that showed reduced intestinal colonization of 3-week-old chicks were identified. The sites of transposon insertion were determined for most of the mutants and included insertions in the lipopolysaccharide biosynthesis genes rfaK, rfaY, rfbK, andrfbB and the genes dksA, clpB,hupA, and sipC. In addition, identification was made of an insertion into a novel gene that encodes a protein showing similarity to the IIC component of the mannose class of phosphoenolpyruvate-carbohydrate phosphotransferase systems, which we putatively called ptsC. Transduction of most of the transposon mutations to a fresh S. typhimurium F98 genetic background and construction of defined mutations in therfbK, dksA, hupA, sipC, and ptsC genes of S. typhimurium F98 supported the role in colonization of all but the pts locus. The virulence of the rfbK, dksA, hupA,sipC, and ptsC defined mutants andclpB and rfaY transductants in 1-day-old chicks was tested. All but the ptsC and rfaY mutants were attenuated for virulence. A number of other phenotypes associated with some of the mutations are described.

Twenty-four-hour-old, aerobically grown, Luria-Bertani broth cultures of Salmonella typhimurium F98 suppressed the growth of a spectinomycin-resistant (Spc r ) derivative of the same strain inoculated at 10 3 CFU ml ؊1 . This growth suppression is genus specific and RpoS independent, and it is not solely a result of nutrient depletion (P. A. Barrow, M. A. Lovell, and L. Zhang-Barber, J. Bacteriol. 178:3072-3076, 1996). Mutations in three genes are shown here to significantly reduce growth suppression under these conditions. The mutations were located in the nuo, cyd, and unc operons, which code for the NADH dehydrogenase I, cytochrome d oxidase, and F 0 F 1 proton-translocating ATPase complexes, respectively. When cultures were grown under strictly anaerobic conditions, only the unc mutant did not suppress growth. Prior colonization of the alimentary tract of newly hatched chickens with the S. typhimurium F98 wild type or nuo or cyd mutants suppressed colonization by an S. typhimurium F98 Spc r derivative inoculated 24 h later. In contrast, the S. typhimurium unc mutant did not suppress colonization. The nuo and unc mutants showed poorer growth on certain carbon sources. The data support the hypothesis that growth suppression operates because of the absence of a utilizable carbon source or electron acceptor.The normal microbial flora of the alimentary tract is a major factor protecting animals and humans against colonization by pathogenic bacteria (17). Newly hatched chickens without an established flora are particularly susceptible to Salmonella infection and colonization. Colonized young chicks often excrete large numbers of salmonellae into their environment for long periods, thus becoming a potential reservoir of infection (7). When chickens reach several weeks of age, the lower alimentary tract is colonized by large numbers of obligate anaerobes which confer increased resistance to the establishment of Salmonella infection. The resistance of young chicks to infection may be artificially increased by oral administration, immediately after hatching, of a suspension or culture of cecal contents obtained from healthy adult birds (23). This phenomenon has been termed "competitive exclusion," and its discovery has led to a search for individual strains of bacteria which can inhibit colonization by pathogens to the same extent as the normal flora. Oral administration of an avirulent Salmonella typhimurium strain to day-old chicks can confer protection against oral infection by a virulent strain (known as colonization inhibition) (8, 10). Colonization inhibition is normally genus specific, is not caused by bacteriophage or bacteriocin activity, and is not a consequence of immunity induced by the first strain (8). Attempts are currently under way to elucidate the mechanisms responsible for this effect. An analogous phenomenon in vitro has been observed with 24-h-old stationaryphase nutrient broth cultures of S. typhimurium (6,8,11). When such cultures are inoculated with small numbers of genetically tagged (antibiot...

No abstract

No abstract