Salmonella effector proteins and host-cell responses.

Acute gastroenteritis caused by Salmonella enterica serovar typhimurium is a significant public health problem. This pathogen has very sophisticated molecular machinery encoded by the two pathogenicity islands, namely Salmonella Pathogenicity Island 1 and 2 (SPI-1 and SPI-2). Remarkably, both SPI-1 and SPI-2 are very tightly regulated in terms of timing of expression and spatial localization of the encoded effectors during the infection process within the host cell. This regulation is governed at several levels, including transcription and translation, and by post-translational modifications. In the context of a finely tuned regulatory system, we will highlight how these effector proteins co-opt host signaling pathways that control the ability of the organism to infect and survive within the host, as well as elicit host pro-inflammatory responses.

Changing Trends in the Landscape of Patients Hospitalized With Acute Myocardial Infarction (2001 to 2011) (from the Worcester Heart Attack Study).

During the past several decades, new diagnostic tools, interventional approaches, and population-wide changes in the major coronary risk factors have taken place. However, few studies have examined relatively recent trends in the demographic characteristics, clinical profile, and the short-term outcomes of patients hospitalized for acute myocardial infarction (AMI) from the more generalizable perspective of a population-based investigation. We examined decade long trends (2001 to 2011) in patient's demographic and clinical characteristics, treatment practices, and hospital outcomes among residents of the Worcester metropolitan area hospitalized with an initial AMI (n = 3,730) at all 11 greater Worcester medical centers during 2001, 2003, 2005, 2007, 2009, and 2011. The average age of the study population was 68.5 years and 56.9% were men. Patients hospitalized with a first AMI during the most recent study years were significantly younger (mean age = 69.9 years in 2001/2003; 65.2 years in 2009/2011), had lower serum troponin levels, and experienced a shorter hospital stay compared with patients hospitalized during the earliest study years. Hospitalized patients were more likely to received evidence-based medical management practices over the decade long period under study. Multivariable-adjusted regression models showed a considerable decline over time in the hospital death rate and a significant reduction in the proportion of patients who developed atrial fibrillation, heart failure, and ventricular fibrillation during their acute hospitalization. These results highlight the changing nature of patients hospitalized with an incident AMI, and reinforce the need for surveillance of AMI at the community level.

Salmonella enterica serovar Typhimurium mutants unable to convert malate to pyruvate and oxaloacetate are avirulent and immunogenic in BALB/c mice.

Previously, we showed that the Salmonella enterica serovar Typhimurium SR-11 tricarboxylic acid (TCA) cycle must operate as a complete cycle for full virulence after oral infection of BALB/c mice (M. Tchawa Yimga, M. P. Leatham, J. H. Allen, D. C. Laux, T. Conway, and P. S. Cohen, Infect. Immun. 74:1130-1140, 2006). In the same study, we showed that for full virulence, malate must be converted to both oxaloacetate and pyruvate. Moreover, it was recently demonstrated that blocking conversion of succinyl-coenzyme A to succinate attenuates serovar Typhimurium SR-11 but does not make it avirulent; however, blocking conversion of succinate to fumarate renders it completely avirulent and protective against subsequent oral infection with the virulent serovar Typhimurium SR-11 wild-type strain (R. Mercado-Lubo, E. J. Gauger, M. P. Leatham, T. Conway, and P. S. Cohen, Infect. Immun. 76:1128-1134, 2008). Furthermore, the ability to convert succinate to fumarate appeared to be required only after serovar Typhimurium SR-11 became systemic. In the present study, evidence is presented that serovar Typhimurium SR-11 mutants that cannot convert fumarate to malate or that cannot convert malate to both oxaloacetate and pyruvate are also avirulent and protective in BALB/c mice. These results suggest that in BALB/c mice, the malate that is removed from the TCA cycle in serovar Typhimurium SR-11 for conversion to pyruvate must be replenished by succinate or one of its precursors, e.g., arginine or ornithine, which might be available in mouse phagocytes.

Precolonized human commensal Escherichia coli strains serve as a barrier to E. coli O157:H7 growth in the streptomycin-treated mouse intestine.

Different Escherichia coli strains generally have the same metabolic capacity for growth on sugars in vitro, but they appear to use different sugars in the streptomycin-treated mouse intestine (Fabich et al., Infect. Immun. 76:1143-1152, 2008). Here, mice were precolonized with any of three human commensal strains (E. coli MG1655, E. coli HS, or E. coli Nissle 1917) and 10 days later were fed 10(5) CFU of the same strains. While each precolonized strain nearly eliminated its isogenic strain, confirming that colonization resistance can be modeled in mice, each allowed growth of the other commensal strains to higher numbers, consistent with different commensal E. coli strains using different nutrients in the intestine. Mice were also precolonized with any of five commensal E. coli strains for 10 days and then were fed 10(5) CFU of E. coli EDL933, an O157:H7 pathogen. E. coli Nissle 1917 and E. coli EFC1 limited growth of E. coli EDL933 in the intestine (10(3) to 10(4) CFU/gram of feces), whereas E. coli MG1655, E. coli HS, and E. coli EFC2 allowed growth to higher numbers (10(6) to 10(7) CFU/gram of feces). Importantly, when E. coli EDL933 was fed to mice previously co-colonized with three E. coli strains (MG1655, HS, and Nissle 1917), it was eliminated from the intestine (<10 CFU/gram of feces). These results confirm that commensal E. coli strains can provide a barrier to infection and suggest that it may be possible to construct E. coli probiotic strains that prevent growth of pathogenic E. coli strains in the intestine.

A Salmonella enterica serovar typhimurium succinate dehydrogenase/fumarate reductase double mutant is avirulent and immunogenic in BALB/c mice.

Previously we showed that the tricarboxylic acid (TCA) cycle operates as a full cycle during Salmonella enterica serovar Typhimurium SR-11 peroral infection of BALB/c mice (M. Tchawa Yimga et al., Infect. Immun. 74:1130-1140, 2006). The evidence was that a DeltasucCD mutant (succinyl coenzyme A [succinyl-CoA] synthetase), which prevents the conversion of succinyl-CoA to succinate, and a DeltasdhCDA mutant (succinate dehydrogenase), which blocks the conversion of succinate to fumarate, were both attenuated, whereas an SR-11 DeltaaspA mutant (aspartase) and an SR-11 DeltafrdABCD mutant (fumarate reductase), deficient in the ability to run the reductive branch of the TCA cycle, were fully virulent. In the present study, evidence is presented that a serovar Typhimurium SR-11 DeltafrdABCD DeltasdhCDA double mutant is avirulent in BALB/c mice and protective against subsequent infection with the virulent serovar Typhimurium SR-11 wild-type strain via the peroral route and is highly attenuated via the intraperitoneal route. These results suggest that fumarate reductase, which normally runs in the reductive pathway in the opposite direction of succinate dehydrogenase, can replace it during infection by running in the same direction as succinate dehydrogenase in order to run a full TCA cycle in an SR-11 DeltasdhCDA mutant. The data also suggest that the conversion of succinate to fumarate plays a key role in serovar Typhimurium virulence. Moreover, the data raise the possibility that S. enterica DeltafrdABCD DeltasdhCDA double mutants and DeltafrdABCD DeltasdhCDA double mutants of other intracellular bacterial pathogens with complete TCA cycles may prove to be effective live vaccine strains for animals and humans.

A Salmonella nanoparticle mimic overcomes multidrug resistance in tumours.

Salmonella enterica serotype Typhimurium is a food-borne pathogen that also selectively grows in tumours and functionally decreases P-glycoprotein (P-gp), a multidrug resistance transporter. Here we report that the Salmonella type III secretion effector, SipA, is responsible for P-gp modulation through a pathway involving caspase-3. Mimicking the ability of Salmonella to reverse multidrug resistance, we constructed a gold nanoparticle system packaged with a SipA corona, and found this bacterial mimic not only accumulates in tumours but also reduces P-gp at a SipA dose significantly lower than free SipA. Moreover, the Salmonella nanoparticle mimic suppresses tumour growth with a concomitant reduction in P-gp when used with an existing chemotherapeutic drug (that is, doxorubicin). On the basis of our finding that the SipA Salmonella effector is fundamental for functionally decreasing P-gp, we engineered a nanoparticle mimic that both overcomes multidrug resistance in cancer cells and increases tumour sensitivity to conventional chemotherapeutics.

The interaction of gut microbes with host ABC transporters.

ATP binding cassette (ABC) transporters are increasingly recognized for their ability to modulate the absorption, distribution, metabolism, secretion and toxicity of xenobiotics. In addition to their essential function in drug resistance, there is also emerging evidence documenting the important role ABC transporters play in tissue defense. In this respect, the gastrointestinal tract represents a critical vanguard of defense against oral exposure of drugs while at the same time functions as a physical barrier between the lumenal contents (including bacteria) and the intestinal epithelium. Given emerging evidence suggesting that multidrug resistance protein (MDR) plays an important role in host-bacterial interactions in the gastrointestinal tract, this review will discuss the interplay between MDR of the intestinal epithelial cell barrier and gut microbes in health and disease. In particular, we will explore host-microbe interactions involving three apically restricted ABC transporters of the intestinal epithelium; P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP2) and cystic fibrosis transmembrane regulator (CFTR).

Role of motility and the flhDC Operon in Escherichia coli MG1655 colonization of the mouse intestine.

Previously, we reported that the mouse intestine selected mutants of Escherichia coli MG1655 that have improved colonizing ability (M. P. Leatham et al., Infect. Immun. 73:8039-8049, 2005). These mutants grew 10 to 20% faster than their parent in mouse cecal mucus in vitro and 15 to 30% faster on several sugars found in the mouse intestine. The mutants were nonmotile and had deletions of various lengths beginning immediately downstream of an IS1 element located within the regulatory region of the flhDC operon, which encodes the master regulator of flagellum biosynthesis, FlhD(4)C(2). Here we show that during intestinal colonization by wild-type E. coli strain MG1655, 45 to 50% of the cells became nonmotile by day 3 after feeding of the strain to mice and between 80 and 90% of the cells were nonmotile by day 15 after feeding. Ten nonmotile mutants isolated from mice were sequenced, and all were found to have flhDC deletions of various lengths. Despite this strong selection, 10 to 20% of the E. coli MG1655 cells remained motile over a 15-day period, suggesting that there is an as-yet-undefined intestinal niche in which motility is an advantage. The deletions appear to be selected in the intestine for two reasons. First, genes unrelated to motility that are normally either directly or indirectly repressed by FlhD(4)C(2) but can contribute to maximum colonizing ability are released from repression. Second, energy normally used to synthesize flagella and turn the flagellar motor is redirected to growth.