Society is not just for the civilized. Bacteria, the simplest and most ancient class of organisms, are known to form communities too. Many types of bacteria actively aggregate on surfaces, producing sticky complex polymers that help them attach and embed more of their comrades into the ever growing biofilm. Just like human communities, bacterial biofilms have their shady characters as well. Widely called ‘cheaters’ and collectively called ‘cheater populations’, these bacteria refuse to partake in group behaviour such as secretion of sticky polymers, but still stay in the biofilm and reap the benefits. Recent research from the lab of Dr. Dipshikha Chakravortty at the Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore has revealed some interesting insights into the behaviour of biofilms and the cheaters within them.
Salmonella entericaare a class of bacteria that are the leading cause of food borne diseases worldwide. One of the main characteristics that makes this class of bacteria so virulent is their ability to form biofilms. “There is a thick matrix (of polysaccharides), and no therapy can penetrate them in their niche. They last for years together inside the host” says Dr. Chakravortty, describing the biofilms formed by Salmonella. Although it is widely recognized that every biofilm has its set of cheaters, very few studies have been able to show what happens to these cheaters and what effect their non-compliance has on the biofilm community.
In the recent publication, Dr. Chakravortty and her colleagues have used genetically engineered mutant bacteria that are unable to produce extracellular polymeric substances (EPS) as cheaters that do not contribute to the creation and maintenance of the biofilm. “We wanted to see if we introduce the genetic mutant of the matrix formers, do the wildtype (non-mutant) bacteria accept them or keep them at bay?” explains Dipshikha. Growing a mixture of wildtype and mutant bacteria and allowing them to form a biofilm, the scientists then measured the response of the biofilms to stresses like antibiotics and sodium hypochlorite. “They (cheaters) will to some extent take advantage of the matrix” explains Dipshikha, “but the biofilm becomes susceptible to detergents, antibiotics etc.”
Going further, the group tested the effect of the cheaters on the pathogenicity of Salmonella enterica on mice. Biofilms are integral to the ability of this pathogen to cause disease, and as expected, they found that the infiltration of cheaters reduces the pathogenesis of the bacteria in the initial stages of infection. Not only did this show how important a strong biofilm is for infection, but also hinted at a direct role for the mutated gene during virulence.
When the researchers tagged the bacteria with fluorescent proteins for the purpose of microscopy, they found that cheater bacteria were located in the periphery of the biofilm. Further investigation revealed that cellulose, a major component of the EPS, keeps the non-producers at bay from the main centre of the biofilm community. The results have opened up several new questions for the group. “Biofilms are very strong and sturdy structures, we do not know if the cheaters can infiltrate an existing biofilm, and if they do, to what extent can they disrupt it” says Dipshikha.
Apart from answering fundamental questions about the fate of cheaters and their effect on biofilms, Dr. Chakravortty’s work shows that genetically engineered cheaters have great potential for vaccine development. Apart from priming the immune system to future infections, the introduction of non-pathogenic cheater bacteria may provide an additional protective effect of destabilizing any biofilm that the invading bacteria may attempt to form. As most enteropathogenic bacteria form biofilms as a protective and virulence structure, the scope of the study extends to a multitude of pathogenic species that may be destabilized from within. In the near future, these Trojan horses may indeed become the shields that protect against biofilm forming pathogens like Salmonella.
About the authors:
Dr. Dipshikha Chakravortty is an Associate Professor at the Centre for Infectious Disease Research and the Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore. Chakravarthy S. Srinandan is associated with the Department of Microbiology and Cell Biology, IISc and Biofilm Biology Lab, Centre for Research on Infectious Diseases, School of Chemical and Biotechnology, SASTRA University, Thanjavur. Monalisha Elango is associated with the Department of Microbiology and Cell Biology, IISc. Divya P. Gnanadhas is associated with both Department of Microbiology and Cell Biology, IISc and the Department of Aerospace Engineering, IISc.
Contact: Dipshikha Chakravortty can be contacted at firstname.lastname@example.org
About the paper: The article ‘Infiltration of Matrix-Non-producers Weakens the Salmonella Biofilm and Impairs Its Antimicrobial Tolerance and Pathogenicity’ was published in the journal Frontiers in Microbiology in December, 2015.
The abstract can be accessed at http://dx.doi.org/10.3389/fmicb.2015.01468