IN BRIEF

Mechanism for Expulsion of DNA from NTHI Described

August 22, 2017
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Researchers at Nationwide Children’s publish breakthrough discovery revealing how DNA and DNABII proteins are released into the biofilm matrix.

Nontypeable Haemophilus influenzae (NTHI) bacteria, a common culprit in otitis media, are known for their ability to create dense biofilms. As the subject of much biofilm and vaccine research, they are increasingly understood as complex and surprising organisms.

Lauren Bakaletz, PhD, director of the Center for Microbial Pathogenesis in The Research Institute at Nationwide Children’s Hospital, lead the team that shook up the science community when they described a method of motility for these organisms previously thought to be nonmotile.

Now, Dr. Bakaletz and her team have published an important discovery in the Proceedings of the National Academy of Sciences about how NTHI are able to build the elaborate biofilms that enable them to persist despite attacks from antibiotics and the immune system.

“Biofilms are sophisticated, three-dimensional, towering communities of bacteria with communication systems, a division of labor and the ability to build and disrupt these structures at will. And the bacteria in biofilms are very clever,” says Dr. Bakaletz. “They use a variety of strategies that enable resistance to antibiotics. It’s like a fortress. Nearly every chronic or recurrent infection caused by bacteria involves a biofilm.”

In the case of NTHI, a scaffold of extracellular DNA and DNABII proteins comprise the central structure of the biofilm. But how do the DNA and DNABII get there? The bacteria in the community are putting them there, but how?

The team expected to observe a lytic mechanism, but what they found surprised everyone.

In the study, Dr. Bakaletz and her team describe a regulated mechanism of controlled DNA and DNABII protein release that contributes significantly to the structural stability and robust architecture of biofilms formed by NTHI as well as other similar pathogens. AND, they caught it on film.

“When we noticed the bacteria putting the DNA into the biofilm, we wondered if they could actually put DNA out of the little hole from which they typically extend the type IV pilus,” says Dr. Bakaletz. “This had never been shown. It has been described that the bacteria could take DNA in through those holes, but not that they could release it.”

Additionally, Dr. Bakaletz notes, there were no genes in the NTHI genome that would allow it to do this.

NTHI have remnants of some stolen genes, specifically coding for TraC and TraG, which enable transport of DNA across the inner membrane. Through a series of experiments, Dr. Bakaletz and her team demonstrated that both the inner membrane proteins TraC and TraG as well as the outer membrane pore (formed by ComE)  are essential for providing the cellular machinery needed to move DNA through the two membranes and out into the biofilm matrix.

The team also found that the DNA release increased with induction of competence and appears to be stimulated by attachment to the substratum. Though, whether or not the cells can do this when free floating remains to be determined.

This discovery generates a wellspring of additional questions: How do the cells know when to do this? What signaling pathways or environmental triggers are involved? Is the bacterium extruding all of its DNA or copies of genetic hotspots? Are the cells still alive after they release the DNA?

“This was just such a great finding in terms of our understanding,” says Dr. Bakaletz. “If this is happening in disease, and we believe that it is, it’s a formidable trick for bacteria to have up their sleeve. So we really have to understand that and get beyond demonstrating the release of DNA and DNABII proteins to understand the mechanism(s) that control it.”

In a world where scientists have been studying bacteria for hundreds of years, “It’s a joy to get at something this fundamental,” Dr. Bakaletz says. “This is why we do this. We’re advancing scientific knowledge for future generations.”

 

References:

  1. Bakaletz LO, Baker BD, Jurcisek JA, Harrison A, Novotny LA, Bookwalter JE, Humger R, Munson RS Jr. Demonstation of Type IV pilus expression and a twitching phenotype by Haemophilus influenzae. Infections and Immunology. 2005;73(3):1635-1643.
  2. Jurcisek JA, Brockman KL, Novotny LA, Goodman SD, Bakaletz LO. Nontypeable Haemophilus influenzae releases DNA and DNABII proteins via a T4SS-complex and ComE of the type IV pilus machineryProceedings of the National Academy of Sciences U.S.A. 2017 Aug;114(32)E6632-E6641.