MicrobiologyBytes

Campylobacter jejuni, a Gram-negative bacterium, is one of the leading bacterial causes of food-borne human gastroenteritis. C. jejuni is currently estimated to cause 5–14% of diarrhoea worldwide, which translates into 400–500 million cases per year. Most cases of C. jejuni mediated gastroenteritis (campylobacteriosis) are characterized by nausea, abdominal cramps, diarrhea, and fatigue. While outbreaks of campylobacteriosis occur predominantly through consumption of contaminated milk and untreated water, most Campylobacter infections are sporadic in nature and linked to the improper handling and consumption of poultry. The linkage between human infection and the handling of raw poultry is not unexpected, as C. jejuni is a common commensal organism of chickens. In fact, C. jejuni colonize the intestinal tract of a variety of animals, including common livestock (cattle, sheep, pigs), domestic animals (dogs, cats), poultry, and wildlife (rabbits, pheasant, quail).

A number of methods (e.g. serotyping, short variable region [SVR] sequencing, pulsed-field gel electrophoresis [PFGE] and multilocus sequence typing [MLST]) are useful for the discrimination of C. jejuni isolates in epidemiological investigations. These methods have enabled investigators to identify the strain responsible for an outbreak. The use of MLST in particular has provided researchers with the benefit of a defined molecular fingerprint to compare strains. The recent explosion of genome sequences and comparative genomic data has increased our understanding of the epidemiology and metabolic capacity of this organism.

The identification of genetic markers predictive of ecological source and virulence potential are important to detecting and preventing the dissemination of C. jejuni via food sources. Comparative genomic studies have demonstrated that the C. jejuni population structure relates to ecological source (livestock versus non-livestock sources). Additionally, DNA sequence analysis implicates phase and allelic variation as possible mechanisms for altered gene expression and protein synthesis.

In spite of recent advances, significant gaps still exist in our knowledge of C. jejuni biology. First, researchers have yet to uncover a correlation between genomic diversity and disease severity. Second, C. jejuni virulence and disease pathology are not yet predictable on the basis of genotype. Third, the core genes necessary for disease and the variable (i.e. dispensable) genes whose products contribute to C. jejuni disease are not known. Fourth, on the basis of the observation that nucleotide changes in certain genes alter a strain’s pathogenicity, studies are needed to identify additional genes/proteins whose expression/function is influenced by nucleotide/residue variations. To address these questions, a small infectious disease animal model is needed to test the pathogenic potential of C. jejuni isolates. Continued work focusing on the relationship of genotype to phenotype is important in understanding this enigmatic organism.

Comparative studies of Campylobacter jejuni genomic diversity reveal the importance of core and dispensable genes in the biology of this enigmatic food-borne pathogen. Curr Opin Biotechnol. Apr 3 2009
MLST, DNA microarrays, and genome sequencing has allowed for a greater understanding of the metabolic capacity and epidemiology of Campylobacter jejuni. While strain-specific genes may provide an isolate a selective advantage in environments and contribute to the organism’s pathogenicity, recent work indicates that C. jejuni pathogenicity is dictated by variations in the nucleotide sequence of core genes. Challenges facing C. jejuni researchers include determining (a) the degree to which genomic diversity enables this bacterium to persist in particular environments; (b) if C. jejuni virulence and disease severity can be predicted on the basis of genotype; (c) the set of core and variable genes whose products contribute to virulence; and (d) the genes in which nucleotide changes can affect a strain’s pathogenicity.

Related:

• Campylobacter jejuni
• Animals farmed for meat are the main source of Campylobacter infections
• How Campylobacter jejuni survives within cells
• Protein promotes antibiotic resistance in Campylobacter

In the vast majority of systems, it is unknown how many individual pathogens are required to cause infection of a host organism. Estimates of this number are necessary to predict the likelihood that hosts are infected by multiple genotypes, and so predict the magnitude of genetic drift, the evolution of new pathogen genotypes and the interactions between pathogens and their hosts – all features which are highly relevant in the face of an influenza pandemic. These interactions include competition, complementation and recombination. There is experimental evidence that the number of virions causing an infection can be small, as shown by the data from two experimental approaches. First, genetic drift can be very strong when low virus doses are used, suggesting that only a small number of virions caused the infection. Second, results of dose–response experiments suggest that the number of virions causing infection is in some instances very small. Although the data suggest that the number may be small, there has been no absolute proof that one virus particle is enough be enough to cause infection of a host. Recent research with insect viruses has now shown that one virus particle is enough to cause infection and subsequent disease.

Virus populations are usually composed of collections of variants. In order to investigate whether virus particles (virions) can cause an infection independently from each other, and therefore individually, the researchers set up an experiment with two marked virus variants. The results showed that exposure to a low dose of virus particles resulted in a small number host infections, around 20%. The majority of these hosts turned out to be infected by a single virus genotype. In contrast, exposure to a high dose of virus resulted in virtually all the hosts becoming infected. Here most of the hosts were infected by both types of virus and only 14% were infected by only one of the variants.

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Based on the assumption that every virus particle operates independently from all other virus particles, the researchers set up a probability model to predict how many virus particles have caused an infection and how many different virus genotypes are present in infected hosts. The results of the infection experiments show that virus particles can indeed act independently, and that a single virus particle can be enough to cause an infection.

In infection caused by few virus particles (as may occur frequently in natural situations), the number of virus particles determines the degree of virus diversity that will be present within the host. This is an important finding because the interactions between virus variants, such as competition and exchanging genetic information, determine the progression of disease and the evolution of the virus. This is highly significant in the case of the new H1N1 influenza strain which has just entered the human population. Although each of the eight gene segments in the new virus has been seen in pigs in the past 10 years, this virus has only been in humans for a few months, and is quite unstable. Although less pathogenic than H5N1 influenza, which now seems to be endemic in countries like China, Indonesia, Vietnam and Egypt, recombination resulting in a virus which combines the high transmissibility of H1N1 with the killing power of the H5N1 becomes more likely as both viruses circulate in human populations.

Hold on tight, it might be a bumpy ride.

An experimental test of the independent action hypothesis in virus–insect pathosystems. 2009 Proc. R. Soc. B 276: 2233-2242
The independent action hypothesis (IAH) states that each pathogen individual has a non-zero probability of causing host death and that pathogen individuals act independently. IAH has not been rigorously tested. In this paper, we (i) develop a probabilistic framework for testing IAH and (ii) demonstrate that, in two out of the six virus–insect pathosystems tested, IAH is supported by the data. We first show that IAH inextricably links host survivorship to the number of infecting pathogen individuals, and develop a model to predict the frequency of single- and dual-genotype infections when a host is challenged with a mixture of two genotypes. Model predictions were tested using genetically marked, near-identical baculovirus genotypes, and insect larvae from three host species differing in susceptibility. Observations in early-instar larvae of two susceptible host species support IAH, but observations in late-instar larvae of susceptible host species and larvae of a less susceptible host species were not in agreement with IAH. Hence the model is experimentally supported only in pathosystems in which the host is highly susceptible. We provide, to our knowledge, the first qualitative experimental evidence that, in such pathosystems, the action of a single virion is sufficient to cause disease.

Related:

• How influenza pandemics get started
• Influenza Questions

Hans Rosling unveils new data visuals that untangle the complex risk factors of one of the world’s deadliest (and most misunderstood) diseases: HIV. He argues that preventing transmissions – not drug treatment – is the key to ending the epidemic.

Related:

• HIV in China
• HIV treatment reduces death rates in Africa
• Why is HIV a pathogen?
• How does HIV cause AIDS?

The traditional view of mutualism is one of a reciprocally beneficial interaction between two species. However, it is becoming increasingly evident that the distinction between mutualistic and parasitic interactions is far from clear and, also, that the nature of these relationships can vary for a single interaction. Moreover, although the net result of a mutualistic interaction must by definition be positive for the interacting species, these species usually also pay a price. For example, gut bacteria usually provide benefits to their host and, therefore, must be categorized as mutualistic, but the benefits come at a cost for the host. This explains why swine farmers add antibiotics to livestock feed (i.e. to suppress their gut flora).

Bark beetles are among the most destructive tree pests on the planet. Their symbiosis with fungi has consequently been studied extensively for more than a century. A recent study has identified actinomycete bacteria that are associated with the southern pine beetle and produce specific antibiotics against an antagonist of the beetles’ mutualistic fungus. In addition to highlighting the ecological complexity of bark-beetle-microbial symbioses, this work reveals a potential source of novel antibiotics.

Biological pest control in beetle agriculture. Trends Microbiol. Apr 16 2009

Related:

• Evolution-Proof Insecticides Against Malaria

Melioidosis, which can present as an acute septic illness or as a chronic low-grade infection, was first described in Burma more than 100 years ago. Largely due to its recognition as a biological threat agent, current knowledge on melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, has increased tremendously over recent years as more and more research groups are working on this disease. The global distribution boundaries of melioidosis continue to expand well beyond the traditionally recognized endemic region of southeast-Asia and northern-Australia. For instance, new cases have been described in Brazil and southeast-Africa, although it has to be clarified whether melioidosis is indeed endemic in these regions. Of interest, phylogenetic studies have recently hypothesized that B. pseudomallei has originated in tropical Australia with subsequent spread to southeast-Asia (dubbed as the Gondwana hypothesis). Pneumonia with bacterial dissemination to distant sites and abscess formation is a common presentation. Current treatment recommendations on the basis of clinical trial evidence are parental ceftazidim or carbapenem for 10–14 days or longer as clinically indicated, followed by oral trimethoprim–sulfamethoxazole ± doxycycline for at least 12–20 weeks. This review summarizes present knowledge on the molecular characterization of B. pseudomallei and the immunology of melioidosis with a special emphasis on its potential therapeutic implications.

Immunity to Burkholderia pseudomallei. Curr Opin Infect Dis. 2009 22(2): 102-108
Largely due to its recognition as a biological threat agent, current knowledge on melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, has increased tremendously over the last years. This review summarizes current understanding on the molecular characterization of B. pseudomallei and the immunology of melioidosis. The genome of B. pseudomallei is composed of two chromosomes of which the largest part represents the B. pseudomallei core genome, whereas the remaining accessory genome has been associated with bacterial virulence. Virulence factors, most notably quorum sensing, type III secretion system, lipopolysaccharide and other surface polysaccharides, flagella and various factors essential for the intracellular life cycle of B. pseudomallei, have been further characterized. The neutrophils play a critical in host defense, which is initiated by the Toll-like receptors. The proinflammatory immune response – including the activation of coagulation – and its regulation have been further dissected. Severe melioidosis can probably be seen as the clinical manifestation of a pathogen recognition receptor mediated dysregulation of the immune response to invading B. pseudomallei. B. pseudomallei employs numerous tactics to evade the immune response. Studies on host-pathogen interactions in melioidosis have identified a whole range of potential new treatment targets.

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• Glanders and Melioidosis – coming soon to a crowded shopping centre near you?
• Pathogenic soil bacterium is influenced by land management

Applications are invited for a PhD Scholarship commencing in October 2009 to support research on the following project:

A new family of sigma-factor binding proteins; role and mechanism of transcriptional regulation.
Principal Supervisor: Dr Helen O’Hare, Department of Infection, Immunity & Inflammation.

The Scholarship will start on October 1st, 2009, run for three years and provide a waiver of University tuition fees and a student stipend equivalent to that of a UK Research Council award, £13,290 from October, plus an annual £1,000 research training support grant and £300 student conference/travel allowance. Candidates must hold a First or Upper Second Class honours degree (or equivalent) in a relevant discipline. This Scholarship is only available to candidates who are eligible to pay the Home/EU tuition fee, i.e. permanently resident in the UK or another EU country.

Further details about each project are available from the Principal Supervisor. Applications should be submitted as soon as possible by post or online.

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Chagas disease, also known as American Trypanosomiasis, is the most relevant parasitic disease in Latin America, being a major burden that affects mostly poor human populations living in rural areas. The kissing-bugs of the Triatominae family transmit the parasite Trypanosoma cruzi by infectious blood-sucking. Triatoma infestans is the vector of major relevance in the southern Cone of South America. Current control strategies, heavily based on residual insecticide spraying, are threatened by the emergence of pyrethroid-resistant bug populations. Ensuring the long-term and sustainable control of this overwhelming disease remains a major challenge.

Entomopathogenic fungi may be a safe and efficient means of controlling Triatoma infestans, the bug that helps spread Chagas disease, according to new research conducted in Argentina. A new study shows the success of the fungi to kill bugs resistant to current control methods. This work is based on the understanding of the initial contact interactions between a mycoinsecticide agent – the fungus Beauveria bassiana – and the host defense barrier, the bug cuticle. The proposed methodology is also supported by data showing a relationship between the triatomine cuticle width and its hydrocarbon surface components, with insecticide resistance.

The researchers performed both laboratory and field experiments showing that the fungus Beauveria bassiana is virulent against bug populations from pyrethroid-resistant foci in the Argentina/Bolivia border. An attraction-infection trap was developed and tested during a 15-day period in field assays performed in two rural villages, demonstrating that more than 50% of the bugs detected were killed by fungal infection. By existing vector population models, the bug population reduction was estimated to reduce the risk of acquiring the parasite infection. This approach might also prove useful at different settings, e.g. environments where current tactics and procedures are reported to fail, and rural communities located in remote areas inaccessible to sanitary control teams. The authors emphasize that these results might help to provide a safe and efficient alternative to overcome bug pyrethroid-resilience in the short term, and might be useful to control other Chagas disease vectors as well. These results will help to provide a safe and efficient alternative to overcome pyrethroid-resilience of these noxious bugs. A high transfer potential to immediate application in rural communities located in remote areas inaccessible to sanitary control teams, and to the control of other Chagas disease vectors as well, is also envisaged.

Control of Pyrethroid-Resistant Chagas Disease Vectors with Entomopathogenic Fungi. PLoS Negl Trop Dis 3(5): e434
Triatoma infestans-mediated transmission of Tripanosoma cruzi, the causative agent of Chagas disease, remains as a major health issue in southern South America. Key factors of T. infestans prevalence in specific areas of the geographic Gran Chaco region – which extends through northern Argentina, Bolivia, and Paraguay – are both recurrent reinfestations after insecticide spraying and emerging pyrethroid-resistance over the past ten years. Among alternative control tools, the pathogenicity of entomopathogenic fungi against triatomines is already known; furthermore, these fungi have the ability to fully degrade hydrocarbons from T. infestans cuticle and to utilize them as fuel and for incorporation into cellular components. Here we provide evidence of resistance-related cuticle differences; capillary gas chromatography coupled to mass spectrometry analyses revealed that pyrethroid-resistant bugs have significantly larger amounts of surface hydrocarbons, peaking 56.266.4% higher than susceptible specimens. Also, a thicker cuticle was detected by scanning electron microscopy. In laboratory bioassays, we showed that the virulence of the entomopathogenic fungi Beauveria bassiana against T. infestans was significantly enhanced after fungal adaptation to grow on a medium containing insect-like hydrocarbons as the carbon source, regardless of bug susceptibility to pyrethroids. We designed an attraction-infection trap based on manipulating T. infestans behavior in order to facilitate close contact with B. bassiana. Field assays performed in rural village houses infested with pyrethroid-resistant insects showed 52.4% bug mortality. Using available mathematical models, we predicted that further fungal applications could eventually halt infection transmission. This low cost, low tech, ecologically friendly methodology could help in controlling the spread of pyrethroid-resistant bugs.

Related:

• New vector for Chagas disease

We all know that China is changing – fast. With the largest population in the world and the speed of urbanization, the potential for the spread of human immunodeficiency virus (HIV) is worrying. By some estimates, over a million people in China could be infected with HIV, but reliable figure on the prevalence of HIV/AIDS in China have always been hard to come by. The most recent data from CDC estimates that in 2007, an estimated 700,000 people in China were living with HIV. An estimated 50,000 new HIV infections and 20,000 deaths related to acquired immunodeficiency syndrome (AIDS) occurred in 2007, and an estimated 71% of persons with HIV infection were unaware of their HIV status. In 2007, 40% of those living with HIV had been infected through heterosexual transmission and 38% through injection-drug use (HIV Infection – Guangdong Province, China, 1997-2007. MMWR 58(15) 396-400 April 24, 2009).

Guangdong Province in southeastern China is the country’s most heavily populated region, with an estimated 76 million permanent residents and 17 million migrants. Guangdong has undergone rapid economic development in recent years. The number of HIV infections in Guangdong increased from 102 recorded in 1997 to 4,593 in 2007, although this increase is partly due to increased testing and surveillance. Among males classified by HIV transmission category, 82% of newly diagnosed infections were attributed to injection-drug use, and among females, 53% engaged in high-risk heterosexual conduct.

During the ten years from 1997-2007, an aggregated total of 22,571 newly diagnosed HIV cases were reported in Guangdong, 82% in males and 18% in females. Every year, injection-drug use was the most commonly reported HIV transmission category.

The recent increase in reported HIV cases attributed to high-risk heterosexual contact and the decline in cases attributed to injection-drug use might suggest a shift in Guangdong’s HIV epidemic similar to the national trend, in which heterosexual transmission is the main transmission category in China. In the central region of Guangdong Province, where approximately 80% of the province’s HIV cases were reported in 2007, rapid economic growth has led to an influx of migrant workers. Migrant women who lack appropriate job skills might seek to supplement their income by becoming sex workers, and migrant men living apart from their spouses might become clients of sex workers.

The findings in this report are subject to at least three limitations. First, large percentages of the data were missing key elements. For example, in approximately 22% of cases, the patient’s age group was unknown, and approximately 38% of patients were not classified by transmission category. Second, because definitions for sex worker and injection-drug user were institution based, verification was not possible. Finally, because HIV-positive people in China are required by law to report their names and national identification numbers, those consenting to HIV testing likely represent a sample that is biased in unpredictable ways.

More community-based sampling of populations at high risk are being planned to provide a more complete picture of the HIV epidemic in Guangdong. Surveillance methods should be redefined so that they rely less extensively on institutions and more accurately represent those populations at greatest risk. Finally, because an estimated 71% of people with HIV infection in China are unaware of their status, more HIV counseling and testing should be urgently provided.

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Related:

• The changing face of HIV in China. 2008 Nature 455: 609-11
• HIV treatment reduces death rates in Africa
• Why is HIV a pathogen?
• How does HIV cause AIDS?