Vitamin C inhibits the synthesis of a molecule essential for bacteria survival

That vitamin C, an anti-oxidant agent, boosts and strengthens immunity is well known. Its ability to speed-up recovery from tuberculosis and impede the TB causing bacteria from causing disease, and even kill the bacteria in culture at high concentration are also known. Now, a study by a team of researchers at the Indian Institute of Science (IISc), Bengaluru has found the molecular mechanism by which vitamin C impedes and even kills Mycobacterium smegmatis, a non-pathogenic bacterium that belongs to the same genus as the TB-causing mycobacteria. The results were published in the journal FEMS Microbiology Letters.

Stress response

During times of stress or hostile conditions, such as increased temperature and presence of antibiotics, bacteria tend to come together and form a biofilm to protect themselves. The stress response pathway is crucial for bacteria to survive during hostile conditions. So blocking this pathway is a sure way of killing the bacteria.

In mycobacterium, the (p)ppGpp (Guanosine pentaphospahte or Guanosine tetraphosphate) is a key molecule in the stress response pathway. The (p)ppGpp is synthesised by Rel protein, which in turn is made by the Rel gene.

The team led by Dipankar Chatterji from the Molecular Biophysics Unit at IISc looked at the effects of vitamin C on the stress response pathway. “We chose vitamin C because its structure is similar to (p)ppGpp,” says Prof. Chatterji. “So we hypothesised that vitamin C should be competing to bind to the Rel enzyme and inhibiting (p)ppGpp synthesis.”

To test their hypothesis, the researchers conducted experiments using M. smegmatis. M. smegmatis is used as a model organism for TB-causing Mycobacterium tuberculosis.

Role of vitamin C

In vitro studies showed “significant” inhibition of (p)ppGpp synthesis in the presence of vitamin C. The inhibition level was seen to increase as the vitamin C concentration increased. The more the vitamin C concentration, the greater the possibility of vitamin C binding to the Rel enzyme, thus inhibiting (p)ppGpp synthesis. At about 10 mM concentration, the synthesis of (p)ppGpp molecule was completely inhibited.

The binding of vitamin C to the Rel enzyme is weak and this explains why high concentration of vitamin C is needed to inhibit (p)ppGpp synthesis.

“Using Mycobacterial cells we found that 1 mM of vitamin C produced 50% inhibition in (p)ppGpp synthesis. Vitamin C is able to get inside cells and inhibit (p)ppGpp synthesis,” says Kirtimaan Syal from IISc, the first author of the paper.

When 2 mM of vitamin C was added, “significant” defect in biofilm formation was seen. There was more than 50% reduction in viability of cells in a matter of four days when M. smegmatis was treated with 2mM of vitamin C. The viability of cells reduced even further with time, raising the possibility of therapeutic implications.

Therapeutic potential

“This suggests that vitamin C can act as a precursor for more potential inhibitors; it can be chemically modiﬁed into more potential derivatives,” they write. “Vitamin C is natural, and it can form one of the nutrient-based treatments of the disease. Vitamin C is water soluble and has no toxic effect,” says Dr. Syal.

“We are trying to synthesise derivatives of vitamin C to enhance inhibition of (p)ppGpp synthesis even at lower concentration,” Dr. Syal says.