Dormant and actively dividing TB bacteria form distinct groups with very different susceptibility to anti-TB drugs. Now, researchers from the Institute of Genomics and Integrative Biology (CSIR-IGIB), Delhi, have found that even among the actively dividing bacteria, the essentiality of TB genes varies depending on whether the bacteria reside in fat-rich environment or not. The team led by Dr. Sheetal Gandotra also found inherent synergy between fat and iron in host cells providing the bacteria resilience to oxidative stress.
Tuberculosis bacteria are known to also reside in lipid-rich environments, both within and outside the cells, where they end up once they multiply within and bring about cell death. To better understand the physiology of the bacteria in such an environment, the researchers studied fat cells (adipocytes) and their precursors (preadipocytes) which have relatively less fat content.
Besides thriving and multiplying inside both cell types and killing them, the bacteria also thrives on dead cellular environment. “This is similar to the extracellular environment that supports bacterial growth in TB lesions,” says Dr. Gandotra who is the corresponding author of a paper published in the journal Infection and Immunity.
Since the fat content in both the cell types (adipocytes and preadipocytes) are very different, the researchers questioned whether pathways that bacteria employ to survive in these cell types are also different. They undertook gene-expression studies to answer this question. Their analysis showed that genes responsible for iron intake were less expressed in bacteria found in adipocytes than in preadipocytes, suggesting higher iron concentration in fat cells.
High fat and iron
A series of investigations showed that indeed the high fat content is associated with higher iron also. “But as high iron also induces oxidative stress, we hypothesised that bacteria in the adipocyte environment might be making the bacteria resilient to oxidative stress,” says Dr. Gandotra.
The researchers experimentally tested their hypothesis by using TB mutant bacteria which are sensitive to iron-mediated oxidative stress. “We found the mutants growing unhindered in adipocytes though they are rich in iron but unable to grow in preadipocytes which are not iron-rich. This proved that the adipocyte environment was providing protection to TB bacteria from iron-mediated oxidative stress,” says Ananya Nandy from IGIB and first author of the paper.
The researchers do not yet know the complete mechanism by which TB bacteria mitigates oxidative stress. “But fat from the adipocytes may be involved in providing resistance to oxidative stress,” says Dr. Gandotra.
When there is excess cell necrosis (death of cells) there is accumulation of lipids within the granuloma. The researchers carried out mouse infection studies to test the link between lipid accumulation and iron storage in the granuloma. “The mouse infection studies showed that when there is excess fat there is excess accumulation of iron-storage protein in the granuloma. This provided a clue to the link between lipid accumulation and iron storage,” says Nandy.
Nutrients
“Our work sheds light on the link between macro (fat) and micro (iron) nutrients in a tissue. And different regions of a tissue will have different levels of availability of these nutrients. Tuberculosis bacteria have the ability to adapt to each condition,” says Dr. Gandotra. “This probably is the reason why it is difficult to treat tuberculosis because the genes essential for survival of the bacteria in one environment will not be essential in another region of the granuloma.”
Also, anti-tuberculosis drug isoniazid, which kills TB bacteria by inducing oxidative stress, is not effective against bacteria that grow in fat cells (adipocyte). So inhibiting the pathways essential for reducing oxidative stress in fat-rich environment can possibly make isoniazid drug more efficacious.
