Synthetic peptides, which have sequences identical to the natural ones isolated from the bacteria, prepared in the lab showed antimicrobial activity. This makes culturing of bacteria for peptides redundant and developing new antimicrobial compounds a step closer.

Bacteria found on the skin are known to harbour a large repertoire of antimicrobial agents. A new bacterial strain of Staphylococcus capitis identified by scientists at Delhi’s CSIR-Institute of Genomics and Integrative Biology (CSIR-IGIB) has a strong antibacterial activity against Gram-positive bacteria, including Staphylococcus aureus. The work reconfirms the growing understanding that bacteria found on the skin may be a “rich source” of novel antimicrobial molecules. The results of the study were published in the journal Scientific Reports.

The bacteria were isolated from the skin surface of a healthy human foot; the bacteria are specifically found near the toes. Different bacteria are found in different niches of the skin. For instance, bacteria found in the arm pit are different from those found on the feet.

Seven new peptides

The team led by Dr. Bhupesh Taneja and Dr. Rakesh Sharma sequenced the genome of the bacteria and identified all the possible peptides that have antibacterial activity. In all, the new strain of bacteria has nine antimicrobial peptides, of which two (epidermicin and gallidermin) have already been characterised from other bacteria. “The other seven new peptides have been found to have antimicrobial activity,” says Dr. Sharma.

“To be absolutely certain about the antimicrobial activity that we see is from the peptides and not from any other biological material as a result of contamination, we tested the seven purified synthetic peptides against a set of select microbes. It was a qualitative test,” says Dr. Taneja from CSIR-IGIB and one of the corresponding authors of the paper.

Synthetic peptides

Synthetic peptides with sequences identical to the natural ones isolated from the bacteria were synthesised by the team. The synthetic peptides were found to possess antibacterial activity, opening the window to developing new antimicrobial compounds. “Since the purified synthetic peptides are inhibitory, it not only confirms the antimicrobial activity but also shows that the synthetic peptides can be used directly without actually culturing the microbes,” says Dr. Taneja.

The researchers would next study the minimum inhibitory concentration (the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism) required by the peptides and test it against more species of Gram-positive bacteria and specifically against drug-resistant S. aureus.

Besides isolating the peptides responsible for antimicrobial activity, the researchers have identified the genes responsible for other functions such as adhesion, acid stress tolerance, colonisation and survival on human skin. “We studied the bacteria to understand the different adaptation strategies and unique features that allow them to thrive on the skin,” says Dr. Sharma from CSIR-IGIB and one of the corresponding authors of the paper.

The antimicrobial activity helps the bacteria to secure their niche environment by preventing other bacteria, including pathogenic bacteria, from colonising. “The new bacterial strain identified by us and S. aureus are closely related and can thrive in the same niche on the skin. And this drives the competition between the two bacteria,” says Dr. Sharma. Staphylococci are common colonizers of human skin and the third largest genera identified in human skin microbiome.

The team has been isolating bacteria from the skin and studying their roles. The researchers had earlier reported another bacteria from human skin with antimicrobial activity. And in a paper published in May this year, they reported the discovery of a new Gram-positive bacterial genus — Auricoccus indicus.  The bacteria were isolated from the external ear lobe of a healthy individual.

Published in The Hindu on September 7, 2017