Mycobacterium leprae
Mycobacterium leprae is one
of the two species of bacteria that cause Hansen's disease, a chronic but curable infectious disease that damages the peripheral nerves and targets the skin, eyes, nose, and muscles.
It is an acid-fast, Gram-positive, rod shaped bacterium and an obligate intracellular parasite, which means, unlike its relative Mycobacterium tuberculosis, it cannot be grown in cell-free laboratory media. This is likely due to gene deletion and decay that the genome of the species has experienced via reductive evolution, which has caused the bacterium to depend heavily on its host for nutrients and metabolic intermediates. It has a narrow host range and apart from humans, the only other natural hosts are nine-banded armadillo and red squirrels.
The bacteria infect mainly macrophages and Schwann cells, and are typically found congregated as a palisade.
Mycobacterium leprae was sensitive to dapsone as a treatment alone, but since the 1960s, it has developed resistance against this antibiotic. Currently, a multidrug treatment is recommended by the World Health Organization, including dapsone, rifampicin, and clofazimine. The species was discovered in 1873 by the Norwegian physician Gerhard Armauer Hansen, and was the first bacterium to be identified as a cause of disease in humans.
Microbiology
Mycobacterium leprae is an intracellular, pleomorphic, non-sporing, non-motile, acid-fast, pathogenic bacterium. It is an aerobic bacillus with parallel sides and round ends, surrounded by the characteristic waxy coating of mycolic acid unique to mycobacteria. It is Gram-positive by Gram staining, but Mycobacterium leprae was traditionally stained with carbol fuchsin in the Ziehl–Neelsen stain. Because the bacilli are less acid-fast than Mycobacterium tuberculosis, the Fite-Faraco staining method, which has a lower acid concentration, is used now. In size and shape, it closely resembles MTB. The bacteria are found in the granulomatous lesions and are especially numerous in the nodules. This bacteria often occur in large numbers within the lesions of lepromatous leprosy and are usually grouped together as a palisade.By optical microscopy of host cells, Mycobacterium leprae can be found singly or in clumps referred to as "globi", the bacilli can be straight or slightly curved, with a length ranging from 1–8 μm and a diameter of 0.3 μm. The bacteria grow best at 27 to 30 °C, making the skin, nasal mucosa and peripheral nerves primary targets for infection by Mycobacterium leprae.
Host range
Mycobacterium leprae has a narrow host range and apart from humans the only other hosts are nine-banded armadillos and red squirrels, and armadillos have been implicated as a source of zoonotic leprosy in humans. In the laboratory, mice can be infected and this is a useful animal model.Cultivation
Mycobacterium leprae has an unusually lengthy doubling time, as well as its inability to be cultured in the laboratory. Because the organism is an obligate intracellular parasite, it lacks many necessary genes for independent survival, causing difficulty in culturing the organism. The complex and unique cell wall that makes members of the genus Mycobacterium difficult to destroy is also the reason for its extremely slow replication rate. Mycobacterium leprae prefers cool temperatures, slightly acidic microaerophilic conditions, and prefers the use of lipids as an energy source over sugars. The growth conditions needed for Mycobacterium leprae are known, but an exact axenic medium to support the growth of Mycobacterium leprae still has yet to be discovered. Since in vitro cultivation is not generally possible, it has instead been grown in mouse foot pads, and in armadillos due to their low core body temperature.Metabolism
The reductive evolution experienced by the Mycobacterium leprae genome has impaired its metabolic abilities in comparison to other Mycobacterium, specifically in its catabolic pathways.Catabolism
Mycobacterium lepraes inability to be grown in axenic media indicates its reliance on nutrients and intermediates from its host. Many of the catabolic pathways present in other Mycobacterium species are compromised, due to the absence of enzymes that play key roles in degradation of nutrients. Mycobacterium leprae has lost the ability to use common carbon sources, such as acetate and galactose, in its central energy metabolism pathways. Additionally, lipid degradation is impaired, with deficits in key lipase enzymes, and other proteins involved in lipolysis. Functional carbon catabolic pathways continue to exist in the species, such as the glycolytic pathway, the pentose phosphate pathway, and the TCA cycle. These deficiencies extensively restricts the microbe's growth to a limited number of carbon sources, such as host-derived intermediates.Anabolism
Mycobacterium leprae's anabolic pathways have been largely unaffected by its reductive evolution. The species retains its ability for the synthesis of genetic material, such as purines, pyrimidines, nucleotides, and nucleosides, as well as the synthesis of all amino acids, except for methionine and lysine.Genome
The first genome sequence of a strain of Mycobacterium leprae was completed in 2001, revealing 1604 protein-coding genes and another 1,116 pseudogenes. The genome sequence of a strain originally isolated in Tamil Nadu, India, and designated TN, was completed in 2013. This genome sequence contains 3,268,203 base pairs and an average G+C content of 57.8%, which is significantly less than M. tuberculosis, which has 4,441,529 bp and 65.6% G+C.Comparing the genome sequence of Mycobacterium leprae with that of MTB reveals an extreme case of reductive evolution. Less than half of the genome contains functional genes. It is estimated approximately 2000 genes from Mycobacterium leprae genome has been lost. Gene deletion and decay appear to have eliminated many important metabolic activities, including siderophore production, part of the oxidative and most of the microaerophilic and anaerobic respiratory chains, and numerous catabolic systems and their regulatory circuits. This reductive evolution is largely linked to the organism's development into an obligate intracellular bacterium.
Pseudogenes
Many of the genes that were present in the genome of the common ancestor of Mycobacterium leprae and M. tuberculosis have been lost in the Mycobacterium leprae genome. Due to Mycobacterium leprae's reliance on a host organism, many of the species' DNA repair functions have been lost, increasing the occurrence of deletion mutations. Because the products supplied by these deleted genes are typically present in the host cells infected by Mycobacterium leprae, the impact that the mutations have on the microbe is minimal, allowing for survival within the host despite its reduced genome. Consequently, Mycobacterium leprae has undergone a dramatic reduction in genome size with the loss of many genes. Over half of the pathogen's genome is now made up by pseudogenes due to the pathogen undergoing what is known as reductive evolution. Among published genomes, Mycobacterium leprae contains the highest number of pseudogens. Many of these pseudogenes arose from insertions of stop codons which may have been caused by sigma factor dysfunction or the insertion of transposon- derived repetitive sequences. Some of the Mycobacterium leprae pseudogens expression levels will alter upon infection of macrophages, which suggests that some Mycobacterium leprae pseudogens are not all "decayed" genes, but could also function in infection, intracellular replication, and replication. This genome reduction is not complete. Downsizing from a genome of 4.42 Mbp, such as that of M. tuberculosis, to one of 3.27 Mbp would account for the loss of some 1200 protein-coding sequences.Essential enzymes
There are eight essential enzymes for Mycobacterium leprae, and one of them is alanine racemase. This enzyme is significant because it is found in D-alanine—D-alanine ligase and alanine/Aspartate metabolism. Other essential enzymes include putative dTDP4deydrorhamnose 3, 5epimerase which plays an important role in both Nucleotide sugar metabolism and polyketide sugar unit biosynthesis. Petidoglycan biosynthesis also require murG, murF, MurE, murY, murC, and murD, the remaining six essential enzymes for ''mycobacterium leprae.''Distribution
The bacterium has a global distribution in humans but the highest prevalence is in sub-Saharan Africa, Asia and South America. The geographic occurrences of Mycobacterium leprae include: Angola, Brazil, Central African Republic, the Democratic Republic of Congo, Federated States of Micronesia, India, Kiribati, Madagascar, Nepal, Republic of Marshall Islands, and the United Republic of Tanzania.Since the introduction of multidrug therapy in the 1980s, the prevalence of leprosy cases has declined by 95%. This decline led the World Health Organization to declare leprosy eliminated as a public health problem, defined as a prevalence of less than one leprosy patient per 10,000 population. Aside from Mycobacterium leprae transmission from infected humans, environmental sources could also be an important reservoir. Mycobacterium leprae DNA was detected in soil from houses of leprosy patients in Bangladesh, armadillos' holes in Suriname and habitats of lepromatous red squirrels in the British Isles. One study found numerous reports of leprosy cases with a history of contact with armadillos in the United States. A zoonotic transmission pathway from exposure to armadillos has been proposed, with human patients from a previous study in southeastern United States shown to be infected with the same armadillo-associated Mycobacterium leprae genotype. High rates of Mycobacterium leprae infection were observed in armadillos in the Brazilian state of Pará, and individuals who frequently consumed armadillo meat showed a significantly higher titres of the M. leprae-specific antigen, phenolic glycolipid I compared with those who did not or ate them less frequently.