Direct damage to the host is a general mechanism utilized by pathogenic organisms to ensure infection and destruction of the host cell. The pathogenic organism typically causes damage due to its own growth process. The promotion of disease is characterized by the ability of a pathogenic organism to enter a host and inflict damage and destruction onto the host cell. The pathogenic organism must exhibit specific characteristics that promote its growth into a host cell including, but not limited to, the ability to invade, colonize, and attach to host cells.
The ability of a pathogen to gain entrance to a host cell is fundamental in the ability of the pathogen to promote and cause disease. The ability to manipulate the process of phagocytosis is a mechanism often utilized by bacteria to ensure they effectively invade a host. Phagocytosis is a process utilized by phagocytes (white blood cells) as a defense mechanism to protect from foreign bodies. The phagocytes engulf invaders and present them to additional factors within the immune system that result in their destruction. However, a successful and destructive pathogen often exhibits the ability to evade phagocytosis.
The mechanism(s) utilized by pathogens to avoid phagocytosis include avoiding both contact and engulfment. Pathogens that exhibit the ability to avoid contact utilize various processes to accomplish this, including: the ability to grow in regions of the body where phagocytes are incapable of reaching; the ability to inhibit the activation of an immune response; inhibiting and interfering with chemotaxis which drives the phagocytes to site of infection; and 'tricking' the immune system to identify the bacteria as 'self.' Additional mechanism(s) by which bacteria can avoid destruction is by avoiding engulfment. This is accomplished by the ability of the bacteria to exhibit produce molecules that interfere with the phagocytes ability to internalize the bacteria. Molecules that interfere with this process include certain types of proteins and sugars that block engulfment (Figure 1).
Once the pathogen has successfully evaded engulfment and destruction by the immune system, it is detrimental because the bacteria then multiply. Often times, bacteria will directly attach themselves to host cells and utilize nutrients from the host cell for their own cellular processes. Upon the use of host nutrients for its own cellular processes, the bacteria may also produce toxins or enzymes that will infiltrate and destroy the host cell. The production of these destructive products results in the direct damage of the host cell. The waste products of the microbes will also damage to the cell. Examples of bacteria that will damage tissue by producing toxins, include, Corynebacterium diphtheriae and Streptococcus pyogenes. Specifically, Corynebacterium diphtheriae causes diphtheria, which isa disease of the upper respiratory tract. It produces a toxin, diphtheria toxin, which alters host protein function. The toxin can then result in damage to additional tissues including the heart, liver, and nerves. Streptococcus pyogenes is associated with strep throat and "flesh-eating disease." The bacteria produce enzymes which function in disrupting fibrin clots. Fibrin clots will form at sites of injury, in this case, at the site of foreign invasion. The enzymes, capable of digesting fibrin, will open an area within the epithelial cells and promote invasion of the bacteria into the tissues.