The Role of Lipid Bodies in Mycobacterium Tuberculosis Infection
Keywords:Lipid bodies, Mycobacterium tuberculosis, Non-replicative state, Phynotipic drug tolerance, dormant tubercle bacilli
Lipid bodies are dynamic organelles with key roles in regulating storage turnover of lipids in different cells and organisms. Triacylglycerols (TAGs) are the dominating storage lipid in higher eukaryotes and are frequently found in eukaryotic micro organisms, like moulds, yeasts and algae. The objective of this review was to document the formation and occurrence of lipid bodies within Mycobacteria in particular and other bacteria in general. Mycobacterium tuberculosis is able to persist in the human host for decades in an apparently dormant state where it is presumed to reside in a hypoxic environment. Mycobacterium tuberculosis under stress stores triacylglycerol (TAG). There is strong evidence that fatty acids are the energy source used by the pathogen for its long-term survival during the persistence phase of infection. Triacylglycerol synthase (tgs) genes have been found to be highly induced as the pathogen enters a non-replicating state upon slow withdrawal of oxygen and upon treatment with NO. Based on the enzymatic activities of the expressed tgs gene products and the degree of their induction, tgs1 appears to have the potential to be a major contributor to TAG synthesis induced by hypoxia and NO. Along with several stress factors, such as hypoxia, nutrient deprivation, production of NO and IFN-γ by host cells, acidic pH is also considered to be one of the major stress factors which bacteria may encounter inside the host.
Sputum has been traditionally thought to contain active growing tubercle bacilli. However, recent studies rejected the commonly held belief that smear-positive sputum is dominated by aerobically replicating Mycobacterium tuberculosis. A survey on clinical samples revealed that lipid bodies were universal features of tubercle bacilli in sputum, and lipid body positive tubercle bacilli were tolerant to the bactericidal action of antibiotics and resistant to multiple stresses.