A hospital can be seen as a high-density population made up of unusually susceptible people where the most antimicrobial-resistant and virulent pathogens can potentially circulate. Considering this, it is not surprising that hospital-acquired infections, or nosocomial infections, have been a problem since hospitals began (nosocomial is derived from the Greek word for hospital). Modern medical practices, however, including the extensive use of antimicrobial drugs and invasive therapeutic procedures, have changed the nature of the problem. In the United States alone, it is estimated that 5% to 6% of patients admitted to the hospital develop a nosocomial infection, adding over $4.5 billion to the price of health care. Nosocomial infections are so common that they make up at least half of all cases of infectious disease treated in the very hospitals where they were contracted. Many of these are from the patient's own normal flora, but approximately one-third of these infections are potentially preventable. Figure 20.11 shows the relative frequency of different types of nosocomial infections.
Nosocomial infections may range from very mild to fatal. Sometimes, because of a long incubation period, the infection may not be discovered until after the patient has been discharged. Many factors determine which microorganisms or viruses are responsible for these infections. These include the length of time the person is exposed, the manner in which a patient is exposed, the virulence and number of organisms, and the state of the patient's host defenses. The bacteria most commonly implicated in nosocomial infections include the following:
■ Enterococcus species. Enterococci, a part of the normal intestinal flora, are a common cause of nosocomial
urinary tract infections as well as wound and blood infections. Some strains are resistant to all conventional antimicrobial drugs. ■ Enterococci, p. 275
■ Escherichia coli and other members of the Enterobacteriaceae. Escherichia coli is a part of the normal intestinal flora. It is the most common cause of nosocomial urinary tract infections. ■ E. coli, p. 636
■ Enterobacteriaceae, p. 282
■ Pseudomonas species. These bacteria grow readily in many moist, nutrient-poor environments such as the water in the humidifier of a mechanical ventilator. Pseudomonas species are resistant to many disinfectants and antimicrobial drugs. They are a common cause of hospital-acquired pneumonia, and infections of the urinary tract and burn wounds. ■ Pseudomonas, p. 697
■ Staphylococcus aureus. Many people including health care personnel are carriers of this organism. Because it survives for prolonged periods in the environment, it is readily transmissable on fomites. It is a common cause of nosocomial pneumonia and surgical site infections. Hospital strains are often resistant to a variety of antimicrobial drugs. ■ Staphylococcus aureus, p. 537
■ Staphylococcus species other than S. aureus. These normal skin flora can colonize the tips of intravenous catheters, small plastic tubes inserted into the veins. The resulting biofilms continuously seed organisms into the bloodstream and increase the likelihood of a systemic infection. ■ biofilm, p. 104
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