pressure exerted on microbes by the constant challenge of antimicrobial compounds favors the survival of organisms that are resistant to these drugs (Holmberg et al., 1987).

An organism's development of drug resistance through selective pressure usually begins with exposure to an antimicrobial drug. Antimicrobial drugs and other compounds designed to combat human pathogens work by killing or inhibiting the growth of susceptible microorganisms. Because of genetic variability, however, not every bacterium, virus, protozoan, helminth, or fungus is naturally susceptible to these drugs. The result is that the drugs leave untouched a small number of resistant microbes, effectively "selecting for" those organisms that can survive attack by the drugs. These resistant organisms pose a potentially serious threat to health. Although the role of selective pressure in antimicrobial resistance is clear, additional studies (using appropriate epidemiological and molecular biological methodologies) are needed to identify and investigate the risk factors that promote transmission of resistant pathogens in the hospital setting.

Many standard hospital procedures facilitate patient acquisition of nosocomial infections. The use of conventional medical devices is responsible for the greatest share of such infections: several hundred thousand cases of device-related disease occur each year. The most common of these (and the most frequent of all nosocomial infections) is urinary tract infection (UTI) (Harding et al., 1991). The great majority of hospital-acquired UTIs are the result of catheterization. In some cases, infection results from nonsterile insertion of the catheter; more frequently, however, it is the entry with the catheter of normal body bacteria (e.g., E. coli and Staphylococcus species, which are usually kept out of the bladder by the mucosal barriers of the urinary tract) that cause infection. Other devices, such as endotracheal tubes and mechanical ventilators, can cause infection in a similar fashion.

Pneumonia is the second most common hospital-acquired illness and the leading cause of death from nosocomial infection. Infections related to surgical wounds are the third most common type. Skin provides one of the body's natural defenses against microbial invasion, and it is also home to usually harmless staphylococcal bacteria. When the skin is broken, however, as happens during surgery or intravenous catheterization, staphylococci (including antibiotic-resistant hospital strains) can gain access to deeper tissues and cause infection. Bloodstream infections, the fourth most common type of nosocomial condition, occur when microbes make their way deep into the body—typically with the help of medical devices or the use of invasive procedures—and enter the bloodstream. On rare occasions, bloodstream infections, including transfusion-induced yersiniosis and HIV infection, can also result from the use of contaminated blood products (see Box 2-4) (Cover and Aber, 1989; Martone, 1990).

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