How pathogenic bacteria can enter the body
Via Science Daily, a report on how Streptococcus pneumoniae, a relatively innocuous bacterium that can cause conjunctivitis, is able to increase in virulence by breaching our bodies defenses. Researchers at a division of Harvard Medical School recently published in the medical journal PLoSONE, and described a novel enzyme made by virulent isolates of this bacterium.
The human body has two broad classes of surfaces which face the external world; our skin and our mucous membranes. Skin represents a formidable barrier to microorganisms, as the dry and salty environment inhibits the growth of most pathogens. Additionally, the dense, dead outer layers act as a physical barrier to microbial penetration. Mucous membranes, such as those that line the conjunctiva of the eye, the oral cavity, the gastrointestinal tract, and many other sites, presents a moist, nutrient rich environment that would at first glance promote the growth of microorganisms. The secretion of mucus however presents as significant a barrier to entry as the dry layers of skin; consequently the active secretion of mucus is essential to prevent colonization by pathogenic microorganisms.
Mucus secretions are composed of two types of proteins called “mucins.” The sticky properties of mucin solutions trap microorganisms, and the cells which line the mucous membranes actively move trapped microbes before they can penetrate into tissue and cause disease. The researchers cultured virulent (or more able to cause disease) isolates of Streptococcus pneumoniae in the presence of mucin-secreting cells.
What they found was that these isolates of the bacterium were significantly more able to break down mucin in comparison to less-virulent isolates of the same organism. The assay that they used to identify the enzyme responsible for this property was very straightforward: the ability to release mucin from human cells was measured in bacterial protein samples, and the protein responsible for the activity was identified from the S. pneumoniae genome. The protein has been named ZmpC, and disease causing isolates of the bacterium possess the gene, while non-disease causing isolates do not.
So the mechanism for virulence becomes easy to understand. The bacterium secretes an enzyme that breaks down the sticky mucin layers on our membranes, and is able to gain access to the tissue underneath it. This mechanism then provides a possible therapeutic approach to treating infections by these isolates. One could apply a competitive inhibitor which mimics how mucin binds to the active site on the ZmpC enzyme, preventing the ability of the organism to degrade the sticky barrier. Such a compound could easily be administered in aerosol form.