The evolution of invasion by enteric bacteria

Roles of YcfR in Biofilm Formation in Salmonella Typhimurium ATCC 14028

An increasing number of foodborne diseases are currently attributable to farm produce contaminated with enteric pathogens such as Salmonella enterica. Recent studies have shown that a variety of enteric pathogens are able to colonize plant surfaces by forming biofilms and thereby persist for long periods, which can subsequently lead to human infections. Therefore, biofilm formation by enteric pathogens on plants poses a risk to human health. Here, we deciphered the roles of YcfR in biofilm formation by Salmonella enterica. YcfR is a putative outer membrane protein associated with bacterial stress responses. The lack of YcfR facilitated the formation of multicellular aggregates on cabbage leaves as well as glass surfaces while reducing bacterial motility. ycfR deletion caused extensive structural alterations in the outer membrane, primarily in lipopolysaccharides, outer membrane proteins, cellulose, and curli fimbria, thereby increasing cell surface hydrophobicity. However, the absence of YcfR rendered Salmonella susceptible to stressful treatments, despite the increased multicellular aggregation. These results suggest that YcfR is an essential constituent of Salmonella outer membrane architecture and its absence may cause multifaceted structural changes, thereby compromising bacterial envelope integrity. In this context, YcfR may be further exploited as a potential target for controlling Salmonella persistence on plants.

Examining the Link between Biofilm Formation and the Ability of Pathogenic Salmonella Strains to Colonize Multiple Host Species

Salmonella are important pathogens worldwide and a predominant number of human infections are zoonotic in nature. The ability of strains to form biofilms, which is a multicellular behavior characterized by the aggregation of cells, is predicted to be a conserved strategy for increased persistence and survival. It may also contribute to the increasing number of infections caused by ingestion of contaminated fruits and vegetables. There is a correlation between biofilm formation and the ability of strains to colonize and replicate within the intestines of multiple host species. These strains predominantly cause localized gastroenteritis infections in humans. In contrast, there are salmonellae that cause systemic, disseminated infections in a select few host species; these “invasive” strains have a narrowed host range, and most are unable to form biofilms. This includes host-restricted Salmonella serovar Typhi, which are only able to infect humans, and atypical gastroenteritis strains associated with the opportunistic infection of immunocompromised patients. From the perspective of transmission, biofilm formation is advantageous for ensuring pathogen survival in the environment. However, from an infection point of view, biofilm formation may be an anti-virulence trait. We do not know if the capacity to form biofilms prevents a strain from accessing the systemic compartments within the host or if loss of the biofilm phenotype reflects a change in a strain’s interaction with the host. In this review, we examine the connections between biofilm formation, Salmonella disease states, degrees of host adaptation, and how this might relate to different transmission patterns. A better understanding of the dynamic lifecycle of Salmonella will allow us to reduce the burden of livestock and human infections caused by these important pathogens.

Biofilm formation by enteric pathogens and its role in plant colonization and persistence

The significant increase in foodborne outbreaks caused by contaminated fresh produce, such as alfalfa sprouts, lettuce, melons, tomatoes and spinach, during the last 30 years stimulated investigation of the mechanisms of persistence of human pathogens on plants. Emerging evidence suggests that Salmonella enterica and Escherichia coli, which cause the vast majority of fresh produce outbreaks, are able to adhere to and to form biofilms on plants leading to persistence and resistance to disinfection treatments, which subsequently can cause human infections and major outbreaks. In this review, we present the current knowledge about host, bacterial and environmental factors that affect the attachment to plant tissue and the process of biofilm formation by S. enterica and E. coli, and discuss how biofilm formation assists in persistence of pathogens on the plants. Mechanisms used by S. enterica and E. coli to adhere and persist on abiotic surfaces and mammalian cells are partially similar and also used by plant pathogens and symbionts. For example, amyloid curli fimbriae, part of the extracellular matrix of biofilms, frequently contribute to adherence and are upregulated upon adherence and colonization of plant material. Also the major exopolysaccharide of the biofilm matrix, cellulose, is an adherence factor not only of S. enterica and E. coli, but also of plant symbionts and pathogens. Plants, on the other hand, respond to colonization by enteric pathogens with a variety of defence mechanisms, some of which can effectively inhibit biofilm formation. Consequently, plant compounds might be investigated for promising novel antibiofilm strategies.

Invasion of eukaryotic cells by Legionella pneumophila: A common strategy for all hosts?

Legionella pneumophila is an environmental micro-organism capable of producing an acute lobar pneumonia, commonly referred to as Legionnaires' disease, in susceptible humans. Legionellae are ubiquitous in aquatic environments, where they survive in biofilms or intracellularly in various protozoans. Susceptible humans become infected by breathing aerosols laden with the bacteria. The target cell for human infection is the alveolar macrophage, in which the bacteria abrogate phagolysosomal fusion. The remarkable ability of L pneumophila to infect a wide range of eukaryotic cells suggests a common strategy that exploits very fundamental cellular processes. The bacteria enter host cells via coiling phagocytosis and quickly subvert organelle trafficking events, leading to formation of a replicative phagosome in which the bacteria multiply. Vegetative growth continues for 8 to 10 h, after which the bacteria develop into a short, highly motile form called the 'mature form'. The mature form exhibits a thickening of the cell wall, stains red with the Gimenez stain, and is between 10 and 100 times more infectious than agar-grown bacteria. Following host cell lysis, the released bacteria infect other host cells, in which the mature form differentiates into a Gimenez-negative vegetative form, and the cycle begins anew. Virulence of L pneumophila is considered to be multifactorial, and there is growing evidence for both stage specific and sequential gene expression. Thus, L pneumophila may be a good model system for dissecting events associated with the host-parasite interactions.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

Gene transfer between Salmonella enterica serovar Typhimurium inside epithelial cells

Virulence and antibiotic resistance genes transfer between bacteria by bacterial conjugation. Conjugation also mediates gene transfer from bacteria to eukaryotic organisms, including yeast and human cells. Predicting when and where genes transfer by conjugation could enhance our understanding of the risks involved in the release of genetically modified organisms, including those being developed for use as vaccines. We report here that Salmonella enterica serovar Typhimurium conjugated inside cultured human cells. The DNA transfer from donor to recipient bacteria was proportional to the probability that the two types of bacteria occupied the same cell, which was dependent on viable and invasive bacteria and on plasmid tra genes. Based on the high frequencies of gene transfer between bacteria inside human cells, we suggest that such gene transfers occur in situ. The implications of gene transfer between bacteria inside human cells, particularly in the context of antibiotic resistance, are discussed.

Pathoadaptive mutations that enhance virulence: genetic organization of the cadA regions of Shigella spp

Pathoadaptive mutations improve the fitness of pathogenic species by modification of traits that interfere with factors (virulence and ancestral) required for survival in host tissues. A demonstrated pathoadaptive mutation is the loss of lysine decarboxylase (LDC) expression in Shigella species that have evolved from LDC-expressing Escherichia coli. Previous studies demonstrated that the product of LDC activity, cadaverine, blocks the action of Shigella enterotoxins and that the gene encoding LDC, cadA, was abolished by large chromosomal deletions in each Shigella species. To better understand the nature and evolution of these pathoadaptive mutations, remnants of the cad region were sequenced from the four Shigella species. These analyses reveal novel gene arrangements in this region of the pathogens' chromosomes. Insertion sequences, a phage genome, and/or loci from different positions on the ancestral E. coli chromosome displaced the cadA locus to form distinct genetic linkages that are unique to each Shigella species. Hybridization studies, using an E. coli K-12 microarray, indicated that the genes displaced to form the novel linkages still remain in the Shigella genomes. None of these novel gene arrangements were observed in representatives of all E. coli phylogenies. Collectively, these observations indicate that inactivation of the cadA antivirulence gene occurred independently in each Shigella species. The convergent evolution of these pathoadaptive mutations demonstrates that, following evolution from commensal E. coli, strong pressures in host tissues selected Shigella clones with increased fitness and virulence through the loss of an ancestral trait (LDC). These observations strongly support the role of pathoadaptive mutation as an important pathway in the evolution of pathogenic organisms.

MglA and mglB of Treponema denticola; similarity to ABC transport and spa genes

The mglA and mglB genes (td-mglA and td-mglB) of the oral spirochete Treponema denticola were sequenced. These two T. denticola genes are highly homologous to the E. coli and Treponema pallidum mglA and mglB genes which are part of the three gene beta-methylgalactoside transport operon, mglBAC. Both Td-mglA and td-mglB are also homologous to the high affinity ABC-type transporters for ribose and arabinose, and surface presentation antigens (spa) locus, part of the type III secretion systems in enteropathogens. Td-mglB and td-mglA are co-transcribed as a single mRNA in T. denticola as well as in E. coli cells as determined by reverse transcription PCR (RT-PCR). Homology to td-mglB and its expressed protein was found in other oral spirochetes as determined by Southern and western blot analysis.

Salmonella host cell invasion emerged by acquisition of a mosaic of separate genetic elements, including Salmonella pathogenicity island 1 (SPI1), SPI5, and sopE2

Salmonella spp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of the Salmonella chromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverse Salmonella spp., we show here that effector protein genes located in the core of SPI1 are present in all Salmonella lineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of the Salmonella chromosome, namely, sopB (SPI5, centisome 20), sopD (centisome 64), and sopE2 (centisomes 40 to 42). Our data demonstrate that sopB, sopD, and sopE2, along with SPI1, were already present in the last common ancestor of all contemporary Salmonella spp. Analysis of Salmonella mutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case of Salmonella enterica serovar Typhimurium SL1344, by SopE: a sopB sopE sopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], and sopE2) and that the last common ancestor of all contemporary Salmonella spp. was probably already invasive.

Promiscuous origin of a chimeric sequence in the Escherichia coli O157:H7 genome

A novel sequence of 2.9 kb in the intergenic region between the mutS and rpoS genes of Escherichia coli O157:H7 and closely related strains replaces a sequence of 6.1 kb in E. coli K-12 strains. At the same locus in Shigella dysenteriae type 1, a sequence identical to that in O157:H7 is bounded by the IS1 insertion sequence element. Extensive polymorphism in the mutS-rpoS chromosomal region is indicative of horizontal transfer events.

Comparative genetics of the inv-spa invasion gene complex of Salmonella enterica

The chromosomal region containing the Salmonella enterica pathogenic island inv-spa was present in the last common ancestor of all the contemporary lineages of salmonellae. For multiple strains of S. enterica, representing all eight subspecies, nucleotide sequences were obtained for five genes of the inv-spa invasion complex, invH, invE, invA, spaM, and spaN, al of which encode proteins that are required for entry of the bacteria into cultured epithelial cells. The invE, invA, spaM, and spaN genes were present in all eight subspecies of S. enterica, and for invE and invA and their products, levels of sequence variation among strains were within the ranges reported for housekeeping genes. In contrast, the InvH, SpaM, and SpaN proteins were unusually variable in amino acid sequence. Furthermore, invH was absent from the subspecies V isolates examined. The SpaM and SpaN proteins provide further evidence of a relationship (first detected by Li et al. [J. Li, H. Ochman, E. A. Groisman, E. F. Boyd, F. Solomon, K. Nelson, and R. K. Selander, Proc. Natl. Acad. Sci. USA 92:7252-7256, 1995]) between the cellular location of the products of the inv-spa genes and evolutionary rate, as reflected in the level of polymorphism within S. enterica. Invasion proteins that are membrane bound or membrane associated are relatively conserved in amino acid sequence, whereas those that are exported to the extracellular environment are hypervariable, possibly reflecting the action of diversifying selection.

Unique Salmonella choleraesuis surface protein affecting invasiveness. Possible inv related sequence

TnphoA mutagenesis of a Salmonella choleraesuis isolate recovered from septicemic infection of feeder pigs resulted in 56 PhoA+ KnR StrR mutants. Thirty-five mutants exhibited reduced levels of invasion in the Hep-2 cell model and were examined by SDS-PAGE Western Blot analysis using an anti-alkaline phosphatase antibody to visualize the insertion gene products. A mutant which produced a gene fusion product of 95 kDa and exhibited > 90% reduction in invasion was subcloned. A 10 Kb BamHI fragment of the chromosome containing the phoA insert was detected by hybridization and cloned into a pGEM vector. The resulting 1657 base sequence contained a 1104 bp ORF with two short regions of homology with S. typhimurium invF and invG. one region of homology with lcrD of Yersinia pseudotuberculosis but contained largely unique sequences not contained in Gene Bank. The full length sequence was not obtained as there was no stop codon detected. The % G+C was 44%, considerably lower than that of the Salmonella chromosome, but compatible with the proposed Yersinia origin of the inv genes. The NH2 387 a.a. sequence includes 5 transmembrane regions, resembling the model derived from the hydrophobicity plot of S. typhimurium InvA.

Distribution of pathogenicity islands in Salmonella spp

We investigated the phylogenetic distribution of the SPI-1 and SPI-2 pathogenicity islands in Salmonella spp. SPI-1 was present in representatives of all eight subspecific groups, but no SPI-2-hybridizing sequences were detected in group V (S. bongori). Our data suggest that SPI-2 was acquired by S. enterica after its split from S. bongori.

Two pathogenicity islands in uropathogenic Escherichia coli J96: cosmid cloning and sample sequencing

Many of the virulence genes of pathogenic strains of Escherichia coli are carried in large multigene chromosomal segments called pathogenicity islands (PAIs) that are absent from normal fecal and laboratory K-12 strains of this bacterium. We are studying PAIs in order to better understand factors that govern virulence and to assess how such DNA segments are gained or lost during evolution. The isolation and sample sequencing of a set of 11 cosmid clones that cover all of one and much of a second large PAI in the uropathogenic E. coli J96 are described. These PAIs were mapped to the 64- and 94-min regions of the E. coli K-12 chromosome, which differ from the locations of three PAIs identified in other pathogenic E. coli strains. Analysis of the junction sequences with E. coli K-12-like DNAs showed that the insert at 94 min is within the 3' end of a phenylalanine tRNA gene, pheR, and is flanked by a 135-bp imperfect direct repeat. Analysis of the one junction recovered from the insert at 64 min indicated that it lies near another tRNA gene, pheV. To identify possible genes unique to these PAIs, 100 independent subclones of the cosmids were made by PstI digestion and ligation into a pBS+ plasmid and used in one-pass sample DNA sequencing from primer binding sites at the cloning site in the vector DNA. Database searches of the J96 PAI-specific sequences identified numerous instances in which the cloned DNAs shared significant sequence similarities to adhesins, toxins, and other virulence determinants of diverse pathogens. Several likely insertion sequence elements (IS100, IS630, and IS911) and conjugative R1 plasmid and P4 phage genes were also found. We propose that such mobile genetic elements may have facilitated the spread of virulence determinants within PAIs among bacteria.

Identification of a pathogenicity island required for Salmonella survival in host cells

We have identified a region unique to the Salmonella typhimurium chromosome that is essential for virulence in mice. This region harbors at least three genes: two (spiA and spiB) encode products that are similar to proteins found in type III secretion systems, and a third (spiR) encodes a putative regulator. A strain with a mutation in spiA was unable to survive within macrophages but displayed wild-type levels of epithelial cell invasion. The culture supernatants of the spi mutants lacked a modified form of flagellin, which was present in the supernatant of the wild-type strain. This suggests that the Spi secretory apparatus exports a protease, or a protein that can alter the activity of a secreted protease. The "pathogenicity island" harboring the spi genes may encode the virulence determinants that set Salmonella apart from other enteric pathogens.

Molecular mechanisms of bacterial virulence: type III secretion and pathogenicity islands

Recently, two novel but widespread themes have emerged in the field of bacterial virulence: type III secretion systems and pathogenicity islands. Type III secretion systems, which are found in various gram-negative organisms, are specialized for the export of virulence factors delivered directly to host cells. These factors subvert normal host cell functions in ways that seem beneficial to invading bacteria. The genes encoding several type III secretion systems reside on pathogenicity islands, which are inserted DNA segments within the chromosome that confer upon the host bacterium a variety of virulence traits, such as the ability to acquire iron and to adhere to or enter host cells. Many of these segments of DNA appear to have been acquired in a single step from a foreign source. The ability to obtain complex virulence traits in one genetic event, rather than by undergoing natural selection for many generations, provides a mechanism for sudden radical changes in bacterial-host interactions. Type III secretion systems and pathogenicity islands must have played critical roles in the evolution of known pathogens and are likely to lead to the emergence of novel infectious diseases in the future.