Natural Antibiotic Casts a Net Against Bacteria

A “natural antibiotic” protects the body against bacteria by tangling them in a net, not poking holes in them, UC Davis researchers have found. Experiments with genetically-modified, or transgenic mice were crucial to the discovery, along with cell cultures, biochemistry and sophisticated studies of how small proteins assemble together.

It’s an entirely new mechanism of action for defensins, a group of molecules with natural antibiotic activity found in the gut, on the skin and in white blood cells. Most defensins studied so far work by punching holes in the membranes around bacterial cells. The discovery, published June 22 in the journal Science, also points to possible causes of Crohn’s disease and other inflammatory bowel diseases.

The researchers led by Hiutung Chu and Charles Bevins at the UC Davis School of Medicine were studying human defensin 6 (HD6), one of six related small proteins made by humans. HD6 is secreted by cells deep in the folds of the small intestine and seems to help keep the gut microbes in balance.

Other defensins kill bacteria in a lab dish, but early research studies repeatedly showed that HD6 just does not kill bacteria in culture.

However, when the researchers used transgenic mice that make HD6 in their intestines, they found that the HD6 mice could not be infected with bacterium Salmonella entericum through the gut. In control mice, the infection spread to other organs. This result indicated that a novel anti-microbial mechanism was at work.

The authors tested if HD6 blocked infection by interfering with proteins that form the mechanism through which S.entericum invades cells, but when they found that the transgenic mice expressing HD6 were also protected against invasion by the bacterium  Yersinia enterocolitica – which uses a different mechanism to invade cells – it became clear that HD6 must block infection by a different method.

Through a series of other experiments in vivo and using the transgenic HD6 mice, the team found that when HD6 encounters a bacteria like Salmonella, the small proteins rapidly link up to form a net or web that tangles the bacteria. This forms a barrier lining that stops hostile bacteria from crossing the gut lining and infecting the rest of the body.

In order to determint what part of the bacteria HD6 interacted with, the Bäumler laboratory created bacterial mutants lacking surface structures known as flagella and type I fimbriae. When those structures were removed HD6 was unable to form the fibrils on the bacterial surface, indicating that these structures act as anchoring points that trigger HD6 nanonet formation.

Put together, the molecular, cell culture, and transgenic mouse experiments build a case to show how HD6 works and protects the gut from infections.

Indeed in a commentary in this week’s issue of Science Professor Andre Ouelette  and Professor Michael Selsted of the University of Southern California note that this study highlights the need to undertake in vivo animal studies to complement and understand the relevance of in vitro observations.

“Aside from delineating the role of HD6 and its unusual mechanism of action, the report by Chu et al. should give us pause for reflection. HD6 has no evident activity in in vitro assays, yet it affects mucosal immunity profoundly. Thus, in vitro bactericidal assays, although useful for comparing peptides in structure-activity studies, may not predict or even hint at the potential impact of a peptide in vivo. Given the diversity of in vitro biological functions that have been associated with defensins and other hostdefense peptides, the challenge is to establish the physiological relevance of those activities.”

Inflammatory bowel disease

People with Crohn’s Disease have unusually large amounts of bacteria in the crypts of the small intestine, causing chronic inflammation. It may be that the gut defensins HD5 and HD6 are defective in these people, so the bacteria can invade and colonize the gut folds more easily. Understanding how these defenses work could lead to insights and new treatments for Crohn’s and similar conditions. No doubt animal models such as transgenic mice will play an important role in that research, too, along with other techniques.

Read more about this study at the UC Davis Health system Newsroom