Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 71

The alimentary tract is another major site of viral invasion and dissemination. Eating, drinking, kissing, and sexual contact routinely place viruses in the gut. Virus particles that infect by the intestinal route must, at a minimum, be resistant to extremes of pH, proteases, and bile detergents. Many enveloped viruses do not initiate infection in the alimentary tract, because viral envelopes are susceptible to dissociation by detergents, such as bile salts.

Figure 2.8 A picture is worth a thousand words. A group of applied mathematicians evaluated the distance and “hang time” of various-sized droplets produced after a sneeze, using the same strategies as ballistics experts studying gunfire. As many as 40,000 droplets can be released in a single sneeze, some traveling over 200 miles an hour. Heavier droplets (seen in the photo) succumb to gravity and fall quickly, while smaller droplets (less than 50μm in diameter) can stay in the air until the droplet dehydrates. Courtesy of CDC/Brian Judd/James Gathany, CDC-PHIL ID#11161.

As depicted in Fig. 2.4, the lumen of the alimentary tract, from mouth to anus, is “outside” of our bodies, and thus the anatomy of the alimentary tube possesses many features of the skin. Like the skin, the gut has physical, chemical, and protein-based barriers that collectively limit viral survival and infection: the stomach is acidic, the intestine is alkaline, and proteases and bile salts are present at high concentrations. In addition, mucus lines the entire tract, and the luminal surfaces of the intestines contain antibodies and phagocytic cells. Moreover, the small and large intestines are coated in a thick (50-μm) paste of symbiotic bacteria that not only aids in digestion and homeostasis but also imposes a formidable physical barrier for virus particles to access the cells beneath. As viruses make their way from mouth to anus, they are confronted with myriad obstacles that limit uptake. Interestingly, some intestinal microbes may actually facilitate viral infection (Box 2.5).

Saliva in the mouth presents an initial obstacle to virus entry. While saliva is mostly water, it does contain lysozymes and other enzymes that aid in the breakdown of food but can also destabilize viral particles. One type of antibody found in saliva, secretory IgA (Chapter 4), may directly bind and inactivate incoming viral particles. A protein known as salivary agglutinin has been reported to directly interfere with influenza virus and human immunodeficiency virus type 1, possibly accounting for why ingestion is not the traditional route of infection by these viruses.