Tuberculosis (TB), caused by Mycobacterium tuberculosis, has remained a major global health threat for over a century. Despite effective antibiotics and widespread vaccination, the disease continues to claim lives. In 2023, TB sickened about 10.8 million people and caused 1.25 million deaths worldwide. India shoulders the highest burden, reporting more than 2.6 million cases in 2024. These figures underscore that TB is far from eradicated and highlight the urgency to understand why it persists.
Dormant TB: A Hidden and Persistent Threat
One of the main reasons TB is difficult to control is its ability to enter a dormant or latent state. During this phase, bacteria stay alive but inactive for years. People with latent TB show no symptoms and cannot transmit the disease. However, when immunity weakens—due to HIV, other infections, or immunosuppressants—the bacteria can reactivate. Because most antibiotics target fast-growing bacteria, dormant TB cells often survive treatment. These slowly dividing or non-dividing bacteria display antibiotic tolerance, enabling them to persist in the body.
New Study Investigates Why Dormant TB Survives Antibiotics
A new study led by Prof. Shobhna Kapoor (IIT Bombay) and Prof. Marie-Isabel Aguilar (Monash University) sought to uncover why dormant TB bacteria remain largely unaffected by antibiotics. Published in Chemical Science, the research identifies survival mechanisms that contribute to drug tolerance and suggests ways to make existing drugs more effective.
Researchers Examine the Role of Bacterial Membranes
Prof. Kapoor’s team hypothesised that the bacterial membrane, composed mainly of lipids, may hold the key to drug tolerance. Therefore, they examined how the membrane changes as TB transitions from an active to a dormant state, and whether these changes influence antibiotic entry into the cell.
Because handling TB bacteria is hazardous, the researchers used Mycobacterium smegmatis, a harmless close relative. They cultured the bacteria under two conditions: an active phase with rapid division, and a late phase mimicking dormancy.
Dormant Bacteria Require Higher Drug Concentrations
As reported by iitb.ac.in, the team exposed the bacteria to four commonly used TB drugs—rifabutin, moxifloxacin, amikacin, and clarithromycin. They found that dormant bacteria required two to ten times higher drug concentrations to inhibit 50% of their growth. “The same drug that works well early in the disease needs a much higher dose to affect dormant TB cells,” Prof. Kapoor explains. Importantly, the bacterial strain used had no resistance-causing genetic mutations, confirming that reduced drug sensitivity stemmed from physiological changes related to dormancy rather than genetic resistance.
Dormancy Causes Major Shifts in Membrane Lipids
To understand these changes, the researchers analysed the membrane using advanced mass spectrometry. They identified over 270 lipid molecules. Lead author Ms. Anjana Menon (IITB-Monash Research Academy) reports that active bacteria contained glycerophospholipids and glycolipids, while dormant bacteria were dominated by fatty acyls—long, waxy molecules that stiffen the membrane.
Next, the team measured membrane fluidity. Active bacteria had loose, fluid membranes, whereas dormant bacteria had rigid, tightly packed structures. Levels of cardiolipin, a key lipid that keeps membranes flexible, dropped sharply during dormancy. “When cardiolipin decreases, the membrane becomes tighter and less permeable,” Ms. Menon explains.
Rigid Membranes Block Antibiotic Entry
The researchers then tested how rifabutin interacts with these membranes. They observed that rifabutin easily entered active cells but struggled to penetrate dormant ones. Prof. Kapoor notes, “The rigid outer layer becomes the main barrier and forms the bacterium’s first line of defence.” This physical barrier largely explains why dormant cells evade antibiotic action.
Weakening the Membrane May Boost Treatment Success
Because the membrane blocks drug entry, the team explored whether weakening it could restore drug sensitivity. Since TB treatment currently lasts at least six months and dormant bacteria often survive the regimen, improving existing drugs could significantly shorten therapy. “Even old antibiotics can work better if paired with a molecule that loosens the membrane,” says Prof. Kapoor. This strategy improves drug access without promoting permanent resistance.
Antimicrobial Peptides Offer a Promising Approach
The researchers are now studying antimicrobial peptides—small proteins that can make bacterial membranes slightly leaky. Alone, these peptides do not kill bacteria; however, when combined with antibiotics, they enhance drug penetration and effectiveness.
Next Steps: Testing Findings in Actual TB Bacteria
Because the study used a safe model organism, the next step is to validate the findings in Mycobacterium tuberculosis under higher biosafety conditions. Ms. Menon explains that their detailed lipid analysis can be readily applied in laboratories that work with actual TB strains, paving the way for more targeted interventions.
This research advances understanding of how dormant TB bacteria survive antibiotic treatment and lays the groundwork for therapies that could finally improve TB control worldwide.