Endospores are unique among bacterial spores in that they are produced inside of another cell (the mother cell) and, upon maturation, are released as free spores by lysis of the mother cell. They are readily recognized under phase-contrast microscopy by their phase bright (refractile) appearance. They also exhibit diagnostic features under electron microscopy, such as a protein shell consisting of an inner coat and an electron dense, outer coat. Endospores are composed of numerous molecules found, thus far, only in bacterial endospores. These molecules include most of the proteins that encase the spore in a protective shell (called the coat), a family of DNA-protective proteins known as SASP that are bound to the chromosome, and a unique small molecule, dipicolinic acid. All previously known examples of endospore-forming bacteria are members of the low G+C group of Gram-positive bacteria (Firmicutes) belonging either to Bacilli or to Clostridia, and in all cases in which a genome sequence is available, orthologs of genes involved in endospore formation are readily seen. The Mycobacterium genus is a member of the high G+C group of Gram-positive bacteria (Actinobacteria) for which there are no prior claims of endospore formation. Certain members of the group, such as Streptomyces, do produce spores, but spores of a fundamentally different kind that are not produced inside a mother cell.
A recent publication in PNAS reported that M. marinum and M. bovis bacillus Calmette–Guérin produce a type of spore known as an endospore, which had been observed only in the low G+C group of Gram-positive bacteria. Evidence was presented that the spores were similar to endospores in ultrastructure, in heat resistance and in the presence of dipicolinic acid. This paper reports that the genomes of Mycobacterium species and those of other high G+C Gram-positive bacteria lack orthologs of many, if not all, highly conserved genes diagnostic of endospore formation in the genomes of low G+C Gram-positive bacteria. It also failed to detect the presence of endospores by light microscopy or by testing for heat-resistant colony-forming units in aged cultures of M. marinum. Finally, we failed to recover heat-resistant colony-forming units from frogs chronically infected with M. marinum. It concludes that it is unlikely that Mycobacterium is capable of endospore formation.
Historically, tuberculosis caused by Mycobacterium bovis (bovine tuberculosis, bTB) was a major public health issue in the UK. bTB in humans was widespread in the UK before the introduction of pasteurisation of milk in the 1960s: in the 1930s, 40% of dairy cows were infected and 0.5% had tuberculous mastitis. During this period, approximately 2500 people died annually from bTB. Therefore, measures were introduced to eliminate bTB from the UK. As a result, by the 1970s, bTB was eliminated from most of Britain, with persistent infection limited to the southwest. Subsequently, bTB has re-emerged: in 2007, there were 4172 new herd breakdowns in England and Wales. The resurgence of bTB has resulted in public expenditure now approaching £100 million annually. More and more extreme measures are being proposed to stop the spread of the disease such as widespread badger culling programmes, despite scientific studies casting doubt on the efficacy of such practices.
This article argues that, apart from milk pasteurisation, these measures no longer make economic sense and hence are now resulting in gross misallocation of public resources. We are therefore of the opinion that there is no public health rationale for the multimillion bTB control programme in the UK provided that milk continues to be pasteurised. The logical conclusion arising from this is that without a public health perspective, bTB is essentially an endemic animal disease and hence any control programme should be economically effective in terms of improvements in animal health and welfare and industry profitability or viability.
Public health and bovine tuberculosis: what’s all the fuss about? Trends in Microbiology, Nov 25 2009
Bovine tuberculosis (bTB) in UK cattle is increasing rapidly. Consequently, the UK Government is spending escalating sums of money in attempts at disease control. We propose that bTB control in cattle is irrelevant as a public health policy. In the UK, cattle-to-human transmission is negligible. Aerosol transmission, the only probable route of human acquisition, occurs at inconsequential levels when milk is pasteurised, even when bTB is highly endemic in cattle. Furthermore, there is little evidence for a positive cost benefit in terms of animal health of bTB control. Such evidence is required; otherwise, there is little justification for the large sums of public money spent on bTB control in the UK.
Gender: biological and behavioral differences determining “the state of being male or female”)
Sex: biological differences in males and females
In most countries, tuberculosis (TB) notification is twice as high in men as in women. Although there is clear evidence that socioeconomic and cultural factors leading to barriers in accessing health care may cause undernotification in women, particularly in developing countries, biological mechanisms may actually account for a significant part of this difference between male and female susceptibility to TB. The role of biological gender has been determined in a number of infectious and noninfectious diseases. However, there is an absence of information on the role of biological gender in TB. Thus, investigations should be conducted to clearly understand the role of sexual hormones, sex-related genetic background and genetic regulations, and metabolism, among other factors, in susceptibility differences between men and women. This research may help not only to fully understand the obviously biased gender distribution among TB cases, but also to better adapt future intervention strategies at the community level. In this review, we expand on the various issues relating to TB notification and gender bias.
Large prevalence surveys have suggested that the sex bias observed in pulmonary TB cases may result partly from genuine biological differences in male and female susceptibility to M. tuberculosis infection or the development of TB disease. This finding would not be particularly surprising, as many studies in humans and experimentally infected animals have established clear links between sex-specific factors, including steroid hormones and genetic variants, and the differential susceptibility of males and females to a number of other infectious and noninfectious diseases. In particular, gender bias among pulmonary microbial diseases is not restricted to TB, and important sex differences in the incidence and severity of a number of respiratory tract bacterial infections have been reported in the literature. As a selected example, it has been shown that men have a four-times higher risk of developing nosocomial Legionella pneumophila infection than women. Only 5% to 10% of individuals exposed to M. tuberculosis develop TB, and up to 70% of those who do develop the disease are male. In other words, the human population as a whole is remarkably resistant to M. tuberculosis, but women seem to be even more resistant to the bacillus than men. So, why do only a minority of individuals, other than patients with HIV/AIDS, fail to control infection? Why are women less likely to develop TB than men? Why are some women more resistant to TB than other women exposed to a similar extent? Field research consortia including not only microbiologists, immunologists, and human geneticists, but also epidemiologists and sociologists, should be established to unravel the many faces of sexual inequality in TB, and to decipher the delicate mechanisms involved in natural and sex-associated resistance to TB. Such work would facilitate the design of future intervention strategies for combating the disease and the development of useful tools for evaluating prognosis and protection in future clinical trials.
Dr Helen McShane at the University of Oxford and the Oxford-Emergent Tuberculosis Consortium Ltd have received a Wellcome trust Strategic Award to part-fund the first trial to test efficacy of the new TB vaccine candidate, MVA85A, for potential efficacy in South African infants. In this film Dr McShane discusses the new vaccine candidate and these trials. www.wellcome.ac.uk
The causative agents of tuberculosis, grouped in the Mycobacterium tuberculosis complex (MTBC), have infected one-third of the present human population and a wide range of other mammals. However, questions, such as why, where and when the disease began and expanded, have largely remained unanswered. A new study provides genetic evidence indicating that the common ancestor of the tuberculosis complex emerged some 40,000 years ago in East Africa, the region from where modern human populations disseminated around the same period. This initial step was followed 10,000 to 20,000 years later by the radiation of two major lineages, one of which spread from human to animals. In more recent years (approximately 180 years ago), coinciding with the human population explosion and the industrial revolution, the human-associated pathogen lineages have strongly expanded. These results thus reveal the strikingly parallel demographic evolution between humans and one of their primary pathogens.
Using mycobacterial tandem repeat sequences as genetic markers, the authors show that the MTBC consists of two independent clades, one composed exclusively of M. tuberculosis lineages from humans and the other composed of both animal and human isolates. The latter also likely derived from a human pathogenic lineage, supporting the hypothesis of an original human host. These findings unveil the dynamic dimension of the association between human host and pathogen populations.
Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is responsible for dramatic health problems globally. It is estimated that this pathogen infects one third of the human population and causes three million deaths annually. Most individuals remain asymptomatic for years before developing an active disease. In such individuals, the bacilli are not cleared but rather persist in a dormant state. Major goals of TB research are to (i) understand how the bacilli remain alive for years within infected individuals, and (ii) find how to prevent their reactivation and hence clinical disease. During dormancy, most of the bacilli are confined to granulomas that consist of well-defined aggregates of different host immune cells. Granulomas prevent spreading of bacilli. Researchers analyzed the role of a particular cell population found within granulomas, the ‘‘foamy macrophages’’. These cells are filled with droplets of lipids, a well-known nutrient for persistent bacilli. They found that within these cells, the bacilli do not replicate, but remain alive and seem to internalize host lipids. The foamy macrophages might thus constitute a reservoir for persisting bacilli within their human host, and could provide a relevant model for screening of new antimicrobials against non-replicating persistent mycobacteria.
Foamy Macrophages from Tuberculous Patients’ Granulomas Constitute a Nutrient-Rich Reservoir for M. tuberculosis Persistence. PLoS Pathog 4(11): e1000204
Tuberculosis (TB) is characterized by a tight interplay between Mycobacterium tuberculosis and host cells within granulomas. These cellular aggregates restrict bacterial spreading, but do not kill all the bacilli, which can persist for years. In-depth investigation of M. tuberculosis interactions with granuloma-specific cell populations are needed to gain insight into mycobacterial persistence, and to better understand the physiopathology of the disease. We have analyzed the formation of foamy macrophages (FMs), a granuloma-specific cell population characterized by its high lipid content, and studied their interaction with the tubercle bacillus. Within our in vitro human granuloma model, M. tuberculosis long chain fatty acids, namely oxygenated mycolic acids (MA), triggered the differentiation of human monocyte-derived macrophages into FMs. In these cells, mycobacteria no longer replicated and switched to a dormant non-replicative state. Electron microscopy observation of M. tuberculosis–infected FMs showed that the mycobacteria-containing phagosomes migrate towards host cell lipid bodies (LB), a process which culminates with the engulfment of the bacillus into the lipid droplets and with the accumulation of lipids within the microbe. Altogether, our results suggest that oxygenated mycolic acids from M. tuberculosis play a crucial role in the differentiation of macrophages into FMs. These cells might constitute a reservoir used by the tubercle bacillus for long-term persistence within its human host, and could provide a relevant model for the screening of new antimicrobials against non-replicating persistent mycobacteria.
The discovery of the earliest known cases of human tuberculosis (TB) in bones found submerged off the coast of Israel shows that the disease is 3000 years older than previously thought. Direct examination of this ancient DNA confirms the latest theory that bovine TB evolved later than human TB. The new research sheds light on how the TB bacterium has evolved over the millennia and increases our understanding of how it may change in the future.
The bones, thought to be of a mother and baby, were excavated from Alit-Yam, a 9000 year-old Neolithic village, which has been submerged off the coast of Haifa, Israel for thousands of years. Scientists found characteristic bone lesions that are signs of TB in skeletons from the settlement, one of the earliest with evidence of domesticated cattle. An international team conducted detailed analyses of the bones using scientific techniques that revealed DNA and cell wall lipids from Mycobacterium tuberculosis, the principal agent of human TB. The DNA was sufficiently well-preserved for molecular typing to be carried out and the analysis of the bacterial cell wall lipids by high performance liquid chromatography provided direct, confirmatory evidence of tuberculosis.
This is the best evidence yet that in a community with domesticated animals but before dairying, the infecting strain was actually a human pathogen. The presence of large numbers of animal bones shows that animals were an important food source, and this probably led to an increase in the human population that helped the TB to be maintained and spread. The DNA of the strain of TB in these skeletons had lost a particular piece of DNA which is characteristic of a common family of strains present in the world today. The fact that this deletion had occurred 9000 years ago gives us a much better idea of the rate of change of the bacterium over time, and indicates an extremely long association with humans.
Examining ancient human remains for the markers of TB is very important because it helps to aid our understanding of prehistoric tuberculosis and how it evolved. This then helps us improve our understanding of modern TB and how we might develop more effective treatments.
Detection and Molecular Characterization of 9000-Year-Old Mycobacterium tuberculosis from a Neolithic Settlement in the Eastern Mediterranean. PLoS ONE 3(10): e3426 Mycobacterium tuberculosis is the principal etiologic agent of human tuberculosis. It has no environmental reservoir and is believed to have co-evolved with its host over millennia. This is supported by skeletal evidence of the disease in early humans, and inferred from M. tuberculosis genomic analysis. Direct examination of ancient human remains for M. tuberculosis biomarkers should aid our understanding of the nature of prehistoric tuberculosis and the host/pathogen relationship. We used conventional PCR to examine bone samples with typical tuberculosis lesions from a woman and infant, who were buried together in the now submerged site of Atlit-Yam in the Eastern Mediterranean, dating from 9250-8160 years ago. Rigorous precautions were taken to prevent contamination, and independent centers were used to confirmauthenticity of findings. DNA from five M tuberculosis genetic loci was detected and had characteristics consistent with extant genetic lineages. High performance liquid chromatography was used as an independent method of verification and it directly detected mycolic acid lipid biomarkers, specific for the M. tuberculosis complex. Human tuberculosis was confirmed by morphological and molecular methods in a population living in one of the first villages with evidence of agriculture and animal domestication. The widespread use of animals was not a source of infection but may have supported a denser human population that facilitated transmission of the tubercle bacillus. The similarity of the M. tuberculosis genetic signature with those of today gives support to the theory of a long-term co-existence of host and pathogen.
According to a new analysis, neglected tropical diseases (NTDs) as a group may have surpassed HIV/AIDS, tuberculosis and malaria as the most prevalent infectious diseases in Latin America and the Caribbean. The work found that NTDs are the most common infections of approximately 200 million of the poorest people in the region. They include tens of millions of cases of intestinal worm infections, and almost 10 million cases of Chagas disease, as well as schistosomiasis, trachoma, dengue fever, leishmaniasis, lymphatic filariasis (LF), and onchocerciasis. NTDs produce extreme poverty by adversely impacting child development, pregnancy outcomes and worker productivity. In some cases in Latin America and the Caribbean, NTDs also represent a living legacy of slavery, because they were first introduced into the region through the global slave trade, and even today they predominantly affect people of African descent and indigenous groups, as well as other vulnerable groups such as women and children.
In the coming years, schistosomiasis transmission could be eliminated in the Caribbean, and that transmission of lymphatic filariasis and onchocerciasis could be eliminated in Latin America and the Caribbean with proven successful, cost effective and low-cost treatments. The most burdensome NTDs, such as Chagas disease, intestinal worm infections, and schistosomiasis may first require scale-up of existing resources and/or the development of new tools in order to achieve wider control and/or elimination. Ultimately, successful wide-scale efforts for NTD elimination will require an inter-sectoral approach that bridges public health with social services and environmental interventions.
Neglected diseases impose a huge burden on developing countries, constituting a serious obstacle for socioeconomic development and quality of life. They mostly affect people living either in shantytowns, indigenous communities or poor rural and agricultural areas. Last week, UK government officials announced that they will be contributing £50 million over the next five years toward the control and elimination of NTDs, including Guinea worm. In addition, the World Health Organziation announced that in 2007 alone, 546 million of the world’s poorest people received treatment for lymphatic filariasis at a cost of 10 cents per person, enabling them to live healthier more productive lives. After rainfall-induced disasters like Hurricane Ike, respiratory and intestinal infections usually increase and there is increased risk of breeding of the mosquito that transmits lymphatic filarisis in Haiti. While around three million people will be treated in Haiti in 2008 for lymphatic filariasis, additional resources are needed to step up and maintain treatment coverage in Haiti with its population of 9.5 million people, particularly in the wake of the Hurricane.
Endospores are unique among bacterial spores in that they are produced inside of another cell (the mother cell) and, upon maturation, are released as free spores by lysis of the mother cell. They are readily recognized under phase-contrast microscopy by their phase bright (refractile) appearance. They also exhibit diagnostic features under electron microscopy, such as a protein shell consisting of an inner coat and an electron dense, outer coat. Endospores are composed of numerous molecules found, thus far, only in bacterial endospores. These molecules include most of the proteins that encase the spore in a protective shell (called the coat), a family of DNA-protective proteins known as SASP that are bound to the chromosome, and a unique small molecule, dipicolinic acid. All previously known examples of endospore-forming bacteria are members of the low G+C group of Gram-positive bacteria (Firmicutes) belonging either to Bacilli or to Clostridia, and in all cases in which a genome sequence is available, orthologs of genes involved in endospore formation are readily seen. The Mycobacterium genus is a member of the high G+C group of Gram-positive bacteria (Actinobacteria) for which there are no prior claims of endospore formation. Certain members of the group, such as Streptomyces, do produce spores, but spores of a fundamentally different kind that are not produced inside a mother cell.
