INFECTION AND RESISTANCE

Infection describes the process whereby harmful microorganisms enter the body, multiply, and cause disease. Normally the defense mechanisms of the body's immune system keep infectious microorganisms from becoming established. Those organisms, however, that can evade or diffuse the immune system and therapeutic strategies (e.g., the application of antibiotics) are able to increase their population numbers faster than they can be killed. The population increase usually results in host illness.

There are a variety of ways by which harmful microorganisms can be acquired. Blood contaminated with microbes, such as the viral agents of hepatitis and acquired immunodeficiency syndrome, is one source. Infected food or water is another source that causes illness and death to millions of people around the world every year. A prominent example is the food and water-borne transmission of harmful strains of Escherichia coli bacteria. Harmful microbes can enter the body through close contact with infected creatures. Transmission of the rabies virus by an infected raccoon bite and of encephalitis virus via mosquitoes are but two examples. Finally, breathing contaminated air can cause illness. Bacterial spores of the causative bacterial agent of anthrax are readily aerosolized and inhaled into the lungs, where, if sufficient in large enough numbers, can germinate and cause severe illness and even death.

To establish an infection, microbes must defeat two lines of defense of the body. The first line of defense is at body surfaces that act as a barrier guard the boundaries between the body and the outside world. These barriers include the skin, mucous membranes in the nose and throat, and tiny hairs in the nose that act to physically block invading organisms. Organisms can be washed away from body surfaces by tears, bleeding, and sweating. These are non-specific mechanisms of resistance.

The body's second line of defense involves the specific mechanisms of the immune system, a coordinated response involving a variety of cells and protein antibodies, whereby an invading microorganism is recognized and destroyed. The immune system can be strengthened by vaccination, which supplies or stimulates the creation of antibodies to an organism that the body has not yet encountered.

An increasing cause of bacterial infection is the ability of the bacteria to resist the killing action of antibiotics. Within the past decade, the problem of antibiotic resistant bacteria has become a significant clinical issue. Part of the reason for the development of resistance has been the widespread and sometimes inappropriate use of antibiotics (e.g., use of antibiotics for viral illness because antibiotics are not effective against viruses).

Resistance can have molecular origins. The membrane(s) of the bacteria may become altered to make entry of the antibacterial compound more difficult. Also, enzymes can be made that will destroy or inactivate the antibacterial agent. These resistance mechanisms can be passed on to subsequent generations of bacteria that will then be able to survive in increasing numbers.

Bacteria can also acquire resistance to antibiotics and other antibacterial agents, even components of the immune system, by growing on body surfaces, passages, and tissues. In this mode of growth, termed a biofilm, the bacteria are enmeshed in a sticky polymer produced by the cells. The polymer and the slow, almost dormant, growth rate of the bacteria protect them from antibacterial compounds that would otherwise kill them, and can encourage the bacteria to become resistant to the compounds. Examples of such resistance includes the chronic Pseudomonas aeruginosa lung infections experienced by those with cystic fibrosis and infection of artificially implanted material, such as urinary catheters and heart pacemakers.

Bacteria and viruses can also evade immune destruction by entering host cells and tissues. Once inside the host structures they are shielded from immune recognition.