Plasmids have a major role in the development of disease caused by enteric bacterial pathogens. Virulence plasmids are usually large (>40 kb) low copy elements and encode genes that promote ...host-pathogen interactions. Although virulence plasmids provide advantages to bacteria in specific conditions, they often impose fitness costs on their host. In this Review, we discuss virulence plasmids in Enterobacteriaceae that are important causes of diarrhoea in humans, Shigella spp., Salmonella spp., Yersinia spp and pathovars of Escherichia coli. We contrast these plasmids with those that are routinely used in the laboratory and outline the mechanisms by which virulence plasmids are maintained in bacterial populations. We highlight examples of virulence plasmids that encode multiple mechanisms for their maintenance (for example, toxin-antitoxin and partitioning systems) and speculate on how these might contribute to their propagation and success.
Acquisition of a single copy, large virulence plasmid, pINV, led to the emergence of Shigella spp. from Escherichia coli. The plasmid encodes a Type III secretion system (T3SS) on a 30 kb ...pathogenicity island (PAI), and is maintained in a bacterial population through a series of toxin:antitoxin (TA) systems which mediate post-segregational killing (PSK). The T3SS imposes a significant cost on the bacterium, and strains which have lost the plasmid and/or genes encoding the T3SS grow faster than wild-type strains in the laboratory, and fail to bind the indicator dye Congo Red (CR). Our aim was to define the molecular events in Shigella flexneri that cause loss of Type III secretion (T3S), and to examine whether TA systems exert positional effects on pINV. During growth at 37°C, we found that deletions of regions of the plasmid including the PAI lead to the emergence of CR-negative colonies; deletions occur through intra-molecular recombination events between insertion sequences (ISs) flanking the PAI. Furthermore, by repositioning MvpAT (which belongs to the VapBC family of TA systems) near the PAI, we demonstrate that the location of this TA system alters the rearrangements that lead to loss of T3S, indicating that MvpAT acts both globally (by reducing loss of pINV through PSK) as well as locally (by preventing loss of adjacent sequences). During growth at environmental temperatures, we show for the first time that pINV spontaneously integrates into different sites in the chromosome, and this is mediated by inter-molecular events involving IS1294. Integration leads to reduced PAI gene expression and impaired secretion through the T3SS, while excision of pINV from the chromosome restores T3SS function. Therefore, pINV integration provides a reversible mechanism for Shigella to circumvent the metabolic burden imposed by pINV. Intra- and inter-molecular events between ISs, which are abundant in Shigella spp., mediate plasticity of S. flexneri pINV.
Plasmids are diverse extrachromosomal elements significantly that contribute to interspecies dissemination of antimicrobial resistance (AMR) genes. However, within clinically important bacteria, ...plasmids can exhibit unexpected narrow host ranges, a phenomenon that has scarcely been examined. Here we show that pConj is largely restricted to the human-specific pathogen, Neisseria gonorrhoeae. pConj can confer tetracycline resistance and is central to the dissemination of other AMR plasmids. We tracked pConj evolution from the pre-antibiotic era 80 years ago to the modern day and demonstrate that, aside from limited gene acquisition and loss events, pConj is remarkably conserved. Notably, pConj has remained prevalent in gonococcal populations despite cessation of tetracycline use, thereby demonstrating pConj adaptation to its host. Equally, pConj imposes no measurable fitness costs and is stably inherited by the gonococcus. Its maintenance depends on the co-operative activity of plasmid-encoded Toxin:Antitoxin (TA) and partitioning systems rather than host factors. An orphan VapD toxin encoded on pConj forms a split TA with antitoxins expressed from an ancestral co-resident plasmid or a horizontally-acquired chromosomal island, potentially explaining pConj's limited distribution. Finally, ciprofloxacin can induce loss of this highly stable plasmid, reflecting epidemiological evidence of transient reduction in pConj prevalence when fluoroquinolones were introduced to treat gonorrhoea.
The genus Neisseria contains the important pathogens Neisseria meningitidis and Neisseria gonorrhoeae. These Gram-negative coccoid bacteria are generally thought to be restricted to humans and ...inhabit mucosal surfaces in the upper respiratory and genito-urinary tracts. While the meningococcus and gonococcus have been widely studied, far less attention has been paid to other Neisseria species. Here we review current knowledge of the distribution of commensal Neisseria in humans and other hosts. Analysis of the microbiome has revealed that Neisseria is an abundant member of the oropharyngeal flora, and we review its potential impact on health and disease. Neisseria also exhibit remarkable diversity, exhibiting both coccoid and rod-shaped morphologies, as well as environmental strains which are capable of degrading complex organic molecules.
Bacterial pathogens exploit a huge range of niches within their hosts. Many pathogens can invade non-phagocytic cells and survive within a membrane-bound compartment. However, only a small number of ...bacteria, including Listeria monocytogenes, Shigella flexneri, Burkholderia pseudomallei, Francisella tularensis and Rickettsia spp., can gain access to and proliferate within the host cell cytosol. Here, we discuss the mechanisms by which these cytosolic pathogens escape into the cytosol, obtain nutrients to replicate and subvert host immune responses.
During colonization and disease, bacterial pathogens must survive the onslaught of the host immune system. A key component of the innate immune response is the generation of reactive oxygen and ...nitrogen species by phagocytic cells, which target and disrupt pathogen molecules, particularly DNA, and the base excision repair (BER) pathway is the most important mechanism for the repair of such oxidative DNA damage. In this Review, we discuss how the human-specific pathogens Mycobacterium tuberculosis, Helicobacter pylori and Neisseria meningitidis have evolved specialized mechanisms of DNA repair, particularly their BER pathways, compared with model organisms such as Escherichia coli. This specialization in DNA repair is likely to reflect the distinct niches occupied by these important human pathogens in the host.
It is increasingly being recognised that the interplay between commensal and pathogenic bacteria can dictate the outcome of infection. Consequently, there is a need to understand how commensals ...interact with their human host and influence pathogen behaviour at epithelial surfaces. Neisseria meningitidis, a leading cause of sepsis and meningitis, exclusively colonises the human nasopharynx and shares this niche with several other Neisseria species, including the commensal Neisseria cinerea. Here, we demonstrate that during adhesion to human epithelial cells N. cinerea co-localises with molecules that are also recruited by the meningococcus, and show that, similar to N. meningitidis, N. cinerea forms dynamic microcolonies on the cell surface in a Type four pilus (Tfp) dependent manner. Finally, we demonstrate that N. cinerea colocalises with N. meningitidis on the epithelial cell surface, limits the size and motility of meningococcal microcolonies, and impairs the effective colonisation of epithelial cells by the pathogen. Our data establish that commensal Neisseria can mimic and affect the behaviour of a pathogen on epithelial cell surfaces.
During colonisation of the upper respiratory tract, bacteria are exposed to gradients of temperatures. Neisseria meningitidis is often present in the nasopharynx of healthy individuals, yet can ...occasionally cause severe disseminated disease. The meningococcus can evade the human complement system using a range of strategies that include recruitment of the negative complement regulator, factor H (CFH) via factor H binding protein (fHbp). We have shown previously that fHbp levels are influenced by the ambient temperature, with more fHbp produced at higher temperatures (i.e. at 37°C compared with 30°C). Here we further characterise the mechanisms underlying thermoregulation of fHbp, which occurs gradually over a physiologically relevant range of temperatures. We show that fHbp thermoregulation is not dependent on the promoters governing transcription of the bi- or mono-cistronic fHbp mRNA, or on meningococcal specific transcription factors. Instead, fHbp thermoregulation requires sequences located in the translated region of the mono-cistronic fHbp mRNA. Site-directed mutagenesis demonstrated that two anti-ribosomal binding sequences within the coding region of the fHbp transcript are involved in fHbp thermoregulation. Our results shed further light on mechanisms underlying the control of the production of this important virulence factor and vaccine antigen.
Neisseria meningitidis is a common human commensal which occasionally causes invasive meningococcal disease (IMD). The bacterium recruits the negative complement regulator complement factor H (CFH) ...to its surface by expressing factor H-binding protein (fHbp); this protects the meningococcus from the human complement system.fHbp is regulated in vivo by environmental cues (such as oxygen levels and temperature), and interactions between fHbp and CFH and other host complement proteins (encoded by the cfh locus) are central to host susceptibility to IMD.A recent bacterial genome-wide association study highlighted the role of fHbp determining whether strains harmlessly colonize an individual or cause IMD.Knowledge of fHbp structure:function is being exploited to develop next-generation vaccines against the meningococcus, and to understand why infection can result in harmless colonization or IMD in certain individuals.
Neisseria meningitidis is a human-adapted pathogen that causes meningitis and sepsis worldwide. N. meningitidis factor H-binding protein (fHbp) provides a mechanism for immune evasion by binding human complement factor H (CFH) to protect it from complement-mediated killing. Here, we discuss features of fHbp which enable it to engage human CFH (hCFH), and the regulation of fHbp expression. Studies of host susceptibility and bacterial genome-wide association studies (GWAS) highlight the importance of the interaction between fHbp and CFH and other complement factors, such as CFHR3, on the development of invasive meningococcal disease (IMD). Understanding the basis of fHbp:CFH interactions has also informed the design of next-generation vaccines as fHbp is a protective antigen. Structure-informed refinement of fHbp vaccines will help to combat the threat posed by the meningococcus, and accelerate the elimination of IMD.
Shigella spp. are Gram‐negative enteric pathogens and the leading cause of bacterial dysentery worldwide. Since the discovery more than three decades ago that the large virulence plasmid of Shigella ...is essential for pathogenesis, our understanding of how the bacterium orchestrates inflammation and tissue destruction at the mucosal surface has been informed by studies employing the rabbit ileal loop model. Here, we outline how Phillippe Sansonetti, together with his co‐workers and collaborators, exploited this model to provide a holistic view of how Shigella survives in the intestinal tract, traverses the intestinal epithelial barrier, and manipulates the host immune system to cause disease.
PDF
