Saline and our defensins

If one were to look at our airway from a bacteria's point of view we would see an enormous layer of sticky mucus that we would try to avoid as much as possible because if we got stuck we would be trapped forever.

• This mucus covers the airway both in the back of the nose and in the lungs.

• It is swept out by the action of microscopic hairs called cilia that beat 8-12 times every second.
  • In the lungs they sweep it up

  • In the nose they sweep it back.

  • In both cases it winds up in the stomach where the mucus and the bacteria are respectively broken down for digestion and killed.

• If you look with a microscope at this process you would see virtually all of the bacteria trapped in the mucus.

In the section on helping there are some tips on how to help this very effective defense system.

If we could get under the mucus we would be able to get closer to the actual cells that make up the airway. But even here there is a fluid covering these cells that has some chemicals called "defensins" that also kill bacteria. Most of the following information is taken from the work of people at the University of Iowa. Direct references to this work or quotes from it will be in a different color.

• There are several of these defensins that working both individually and together provide another line of defense against invading bacteria.

• But they are salt sensitive.
  • The antibacterial activity of most of these agents is salt sensitive; an increase in salt concentration inhibits the activity of individual factors and attenuates synergy between agents.(11614)

  • This contributes to the problem cystic fibrosis children have with so many respiratory infections.

  • It also suggests that there may be a problem with our common use of saline to moisten the nose. [This topic is also discussed further in the section on helping.]

These researchers used a 5% solution of xylitol sprayed into the nose in order to reduce the saline concentrations and enhance the activity of these defenses. They found:

• That in a cell model it took 12 hours for 40% of the xylitol to diffuse into the cell from the airway.
  • This means that in the normal airway, where the mucus is swept out and replaced every 15 minutes, that the absorption of xylitol of xylitol into the nasal tissues is negligible. [GOOD SAFETY FACTOR]

  • That it makes a good osmotic agent, able to pull fluid from the cell to decrease the saline concentration in the airway fluid.

• Xylitol Added to the airway surface decreased Chloride ion concentrations in the Airway surface fluid in tissue from cystic fibrosis patients.
  • The increase of chloride ions, characteristic of patients with CF, blocks the defensins.

  • Again, saline, made up of sodium and chloride ions, may not be too good an idea for the airway.

• Xylitol does not affect bacterial growth and doesn't interfere with defensin activity. Major CF pathogens, P. aeruginosa and S. aureus, as well as several others cannot use xylitol for their own growth, nor does it kill them or interfere with antibiotics that do.

• Xylitol applied to nasal epithelia in live subjects reduces the number of normal bacteria. Compared to saline it removes more than 5 times as many bacteria. It does this without killing them.

THIS WORK WAS DONE WITH A 5% SOLUTION OF XYLITOL, LESS THAN HALF THE CONCENTRATION WE HAVE BEEN TALKING ABOUT. 

A 5% SOLUTION IS NOT CONCENTRATED ENOUGH TO STIMULATE OUR OWN NORMAL WASHING MECHANISM.

THE BENEFITS SEEN IN THIS STUDY SHOULD BE ENHANCED BY USING A MORE CONCENTRATED SOLUTION OF XYLITOL.

 

Proc Natl Acad Sci U S A. 2000 Oct 10;97(21):11614-9.

 
The osmolyte xylitol reduces the salt concentration of airway surface liquid and may enhance bacterial killing.

Zabner J, Seiler MP, Launspach JL, Karp PH, Kearney WR, Look DC, Smith JJ, Welsh MJ.

Howard Hughes Medical Institute, Departments of Internal Medicine, Pediatrics, and Physiology and Biophysics, and Nuclear Magnetic Resonance Facility, University of Iowa College of Medicine, Iowa City, IA 52242, USA. joseph-zabner@uiowa.edu

The thin layer of airway surface liquid (ASL) contains antimicrobial substances that kill the small numbers of bacteria that are constantly being deposited in the lungs. An increase in ASL salt concentration inhibits the activity of airway antimicrobial factors and may partially explain the pathogenesis of cystic fibrosis (CF). We tested the hypothesis that an osmolyte with a low transepithelial permeability may lower the ASL salt concentration, thereby enhancing innate immunity. We found that the five-carbon sugar xylitol has a low transepithelial permeability, is poorly metabolized by several bacteria, and can lower the ASL salt concentration in both CF and non-CF airway epithelia in vitro. Furthermore, in a double-blind, randomized, crossover study, xylitol sprayed for 4 days into each nostril of normal volunteers significantly decreased the number of nasal coagulase-negative Staphylococcus compared with saline control. Xylitol may be of value in decreasing ASL salt concentration and enhancing the innate antimicrobial defense at the airway surface.