A bacterium that looks very similar to the causative agent of tuberculosis has been found in a sea sponge
Tuberculosis, which is caused by Mycobacterium tuberculosis, is one of the most dangerous and difficult to cure infectious diseases. Quite unexpectedly, a close relative of this pathogen was found in a marine sponge living off the coast of Australia. The authors of the new paper studied the genome of the M. spongiae they described and infected laboratory mice with it to assess the pathogenic potential of the new bacterium.
Among the dangerous and common infectious diseases is tuberculosis, which is caused by related mycobacteria from the Mycobacterium tuberculosis complex (primarily M. tuberculosis itself). Also known as Koch's bacilli, the bacteria parasitize inside the cells of humans and some animals, causing a protracted and poorly treatable infection. Approximately 1.5 million people die each year from the effects of tuberculosis, and the disease has haunted humans for several thousand years.
Treatment of tuberculosis causes great problems due to the peculiarities of M. tuberculosis - resistance to antibiotics and other chemical drugs, transition to a quiescent state, lack of plasmids and so on. Therefore, to successfully fight the disease, it is important to understand what made the mycobacterium so unique and how it evolved in the past.
M. tuberculosis has a number of close relatives - mycobacteria that infect cows, goats, other animals and sometimes also humans. One of them, M. leprae, causes the frightening disease leprosy. The bacteria share a very similar (in some places identical) genome and a parasitic lifestyle. In contrast, however, other members of the same genus are harmless and live in the environment, differing markedly from the genomes of Koch's bacillus.
A valuable addition to the “family tree” of mycobacteria is a new species described in the journal PLOS Pathogens. Mycobacterium spongiae was discovered inside the marine sponge Fascaplysinopsis reticulata, found off the coast of Australia (Great Barrier Reef). On the one hand, the bacterium lives in the most dissimilar to the human body environment, on the other - its genome is 80 percent the same as the genome of M. tuberculosis. It turns out that the discovered microorganism is the closest relative of harmful bacteria, which leads a completely different way of life.
Therefore, mycobacterium from the sponge may well turn out to be a “transitional link” that will help to understand how these microbes switched from life in the environment to specialized parasitism.
The biologists compared the FSD4b-SM strain they isolated and the bacteria of the pathogenic complex by examining their genome, protein and lipid set. It turned out that the bacterium from the sponge is extremely slow to divide even against the background of other mycobacteria - it takes about 64 days to double the number of cells in the colony. The optimal temperature of the medium for its cultivation was 28 degrees Celsius. At the same time, M. spongiae has a standard for this group bacilliform shape with a length of two micrometers and a thickness of 0.4 micrometers.
Next, the authors evaluated the ability of the bacterium to infect laboratory mice. After injection through the nose, M. spongiae was able to settle in the lungs of the rodents and lived there for 56 days, but they did not show any signs of disease. This is quite unexpected, given the almost complete overlap between the DNA sequences responsible for infection in M. spongiae and pathogenic mycobacteria. Scientists hope that the strain they have isolated will help in obtaining a vaccine against tuberculosis. In particular, it will serve as a genetic “donor”: its DNA will probably be able to improve the weakened strain of bovine tubercle bacillus (Mycobacterium bovis), which is widely used to produce the BCG vaccine.