Microbiology, Lec 20

Nonspecific Host Defenses

One of our most important non-specific defenses is our normal microbiota as discussed above and previously - see Probiotics article and Yogurt.

Our Indigenous Microbiota discourage pathogens by:

1) Colonizing surfaces and thereby keeping pathogens from attaching.
2) Out-competing pathogens for nutrients and space.
3) Producing antibiotics and Bacteriocins that are toxic to other bacteria.

Bacteriocins are plasmid-encoded peptides or proteins that are toxic to bacteria related to the producing organism. For example, E. coli produces bacteriocins called colicins which are toxic to other strains of E. coli and some other enteric bacteria.

A. Physical barriers

1. Skin and mucosal linings

2. Flushing by fluids e.g. in the mouth and urinary tract.

3. Trapping and movement of cilia in the respiratory tract (pg. 594).

4. Low pH, e.g. in the stomach, skin and vagina.

B. Chemical defenses.

1. Lysozyme in bodily fluids (see lec. 3) (see Fig. 29.8)

2. Other enzymes such as Beta lysin (kills gram + bacteria) which is released by blood platelets, and lactoperoxidase in the saliva (produces singlet oxygen that is toxic to many bacteria).

C. Phagocytes.

One of our most important defenses against microbe are the phagocytes in our blood and lymph systems. There are two major types of phagocytes, the granulated polymorphonuclear neutrophils (PMNs) and the macrophages (monocytes).

PMNs or Neutrophils are circulating cells that phagocytize foreign particles including microbes. They are especially important in rapid response to invasion and are the first phagocytes on the scene during the inflammatory response (see below).

Macrophages (monocytes) can be fixed or circulating and are the second line of phagocytic defense and are thus more important in chronic infections. They also line the lymph nodes of the lymphatic system, where they remove microbes from the passing lymph fluids (Figure 29.10).

Figure 29.14 shows the process of phagocytosis of invading bacteria.

D. The Inflammatory Response (inflammation)

results from a chain of events in response to injury and/or microbial invasion. The basic idea is to remove invaders from an injured area via one or more of the following: dilation of capillaries (allows more phagocytes to get to the invaders), release of chemotactic factors (attracts phagocytes to the area), increased permeability of capillary walls (allows phagocytes to get into damaged tissues), clot formation (blocks further invasion in large wounds) See figure 29.12 for an overview of this process.