An insertional mutagenesis system that uses transposons carrying unique DNA sequence tags was developed for the isolation of bacterial virulence genes. The tags from a mixed population of bacterial mutants representing the inoculum and bacteria recovered from infected hosts were detected by amplification, radiolabeling, and hybridization analysis. When applied to a murine model of typhoid fever caused by Salmonella typhimurium, mutants with attenuated virulence were revealed by use of tags that were present in the inoculum but not in bacteria recovered from infected mice. This approach resulted in the identification of new virulence genes, some of which are related to, but functionally distinct from, the inv/spa family of S. typhimurium.

SummaryBackgroundBCG vaccination provides incomplete protection against tuberculosis in infants. A new vaccine, modified Vaccinia Ankara virus expressing antigen 85A (MVA85A), was designed to enhance the protective efficacy of BCG. We aimed to assess safety, immunogenicity, and efficacy of MVA85A against tuberculosis and Mycobacterium tuberculosis infection in infants.MethodsIn our double-blind, randomised, placebo-controlled phase 2b trial, we enrolled healthy infants (aged 4–6 months) without HIV infection who had previously received BCG vaccination. We randomly allocated infants (1:1), according to an independently generated sequence with block sizes of four, to receive one intradermal dose of MVA85A or an equal volume of Candida skin test antigen as placebo at a clinical facility in a rural region near Cape Town, South Africa. We actively followed up infants every 3 months for up to 37 months. The primary study outcome was safety (incidence of adverse and serious adverse events) in all vaccinated participants, but we also assessed efficacy in a protocol-defined group of participants who received at least one dose of allocated vaccine. The primary efficacy endpoint was incident tuberculosis incorporating microbiological, radiological, and clinical criteria, and the secondary efficacy endpoint was M tuberculosis infection according to QuantiFERON TB Gold In-tube conversion (Cellestis, Australia). This trial was registered with the South African National Clinical Trials Register (DOH-27-0109-2654) and with ClinicalTrials.gov on July 31, 2009, number NCT00953927FindingsBetween July 15, 2009, and May 4, 2011, we enrolled 2797 infants (1399 allocated MVA85A and 1398 allocated placebo). Median follow-up in the per-protocol population was 24·6 months (IQR 19·2–28·1), and did not differ between groups. More infants who received MVA85A than controls had at least one local adverse event (1251 [89%] of 1399 MVA85A recipients and 628 [45%] of 1396 controls who received the allocated intervention) but the numbers of infants with systemic adverse events (1120 [80%] and 1059 [76%]) or serious adverse events (257 [18%] and 258 (18%) did not differ between groups. None of the 648 serious adverse events in these 515 infants was related to MVA85A. 32 (2%) of 1399 MVA85A recipients met the primary efficacy endpoint (tuberculosis incidence of 1·15 per 100 person-years [95% CI 0·79 to 1·62]; with conversion in 178 [13%] of 1398 infants [95% CI 11·0 to 14·6]) as did 39 (3%) of 1395 controls (1·39 per 100 person-years [1·00 to 1·91]; with conversion in 171 [12%] of 1394 infants [10·6 to 14·1]). Efficacy against tuberculosis was 17·3% (95% CI −31·9 to 48·2) and against M tuberculosis infection was −3·8% (–28·1 to 15·9).InterpretationMVA85A was well tolerated and induced modest cell-mediated immune responses. Reasons for the absence of MVA85A efficacy against tuberculosis or M tuberculosis infection in infants need exploration.FundingAeras, Wellcome Trust, and Oxford-Emergent Tuberculosis Consortium (OETC).

SummaryThe type III secretion system of Salmonella pathogenicity island 2 (SPI-2) is required for systemic infection of this pathogen in mice. Cloning and sequencing of a central region of SPI-2 revealed the presence of genes encoding putative chaperones and effector proteins of the secretion system. The predicted products of the sseB, sseC and sseD genes display weak but significant similarity to amino acid sequences of EspA, EspD and EspB, which are secreted by the type III secretion system encoded by the locus of enterocyte effacement of enteropathogenic Escherichia coli. The transcriptional activity of an sseA::luc fusion gene was shown to be dependent on ssrA, which is required for the expression of genes encoding components of the secretion system apparatus. Strains carrying nonpolar mutations in sseA, sseB or sseC were severely attenuated in virulence, strains carrying mutations in sseF or sseG were weakly attenuated, and a strain with a mutation in sseE had no detectable virulence defect. These phenotypes were reflected in the ability of mutant strains to grow within a variety of macrophage cell types: strains carrying mutations in sseA, sseB or sseC failed to accumulate, whereas the growth rates of strains carrying mutations in sseE, sseF or sseG were only modestly reduced. These data suggest that, in vivo, one of the functions of the SPI-2 secretion system is to enable intracellular bacterial proliferation.

Mapping the insertion points of 16 signature-tagged transposon mutants on the Salmonella typhimurium chromosome led to the identification of a 40-kb virulence gene cluster at minute 30.7. This locus is conserved among all other Salmonella species examined but is not present in a variety of other pathogenic bacteria or in Escherichia coli K-12. Nucleotide sequencing of a portion of this locus revealed 11 open reading frames whose predicted proteins encode components of a type Ill secretion system. To distinguish between this and the type III secretion system encoded by the inv/spa invasion locus known to reside on a pathogenicity island, we refer to the inv/spa locus as Salmonella pathogenicity island (SPI) 1 and the new locus as SPI2. SPI2 has a lower G+C content than that of the remainder of the Salmonella genome and is flanked by genes whose products share greater than 90%o identity with those of the E. coli ydhE and pykF genes. Thus SP12 was probably acquired horizontally by insertion into a region corresponding to that between theydhE and pykF genes of E. coli. Virulence studies of SP12 mutants have shown them to be attenuated by at least five orders of magnitude compared with the wild-type strain after oral or intraperitoneal inoculation of mice.Salmonella typhimurium is a principal agent of gastroenteritis in humans and produces a systemic illness in mice that serves as a model for human typhoid fever (1). After oral inoculation of mice with S. typhimurium, the bacteria pass from the lumen of the small intestine through the intestinal mucosa via enterocytes or M cells of the Peyer's patch follicles (2). The bacteria then invade macrophages and neutrophils, enter the reticuloendothelial system, and disseminate to other organs, including the spleen and liver, where further reproduction results in an overwhelming and fatal bacteremia (3). To invade host cells, to survive and replicate in a variety of physiologically stressful intracellular and extracellular environments, and to circumvent the specific antibacterial activities of the immune system, S. typhimurium employs a sophisticated repertoire of virulence factors (4).To gain a more comprehensive understanding of virulence mechanisms of S. typhimurium and other pathogens, we recently developed a transposon mutagenesis system called signature-tagged mutagenesis, which combines the strength of mutational analysis with the ability to follow simultaneously the fate of a large number of different mutants within a single animal (5). By using this approach, we identified 43 mutants with attenuated virulence from a total of 1152 mutants that were screened. The nucleotide sequences of DNA flanking the insertion points of transposons in 5 of these mutants showed that they were related to genes encoding type III secretion systems of a variety of bacterial pathogens (6, 7). The products of the inv/spa gene cluster of S. typhimurium (8,9) are proteins that form a type III secretion system required for the assembly