Microorganisms are all around us. They are in the air, on our hands, on items we touch, and even in the food we eat. Many microorganisms are helpful (i.e., gut bacteria that aid in digestion); however, some microorganisms known as pathogens are capable of causing disease within humans. In health care settings, pathogens are very common and appropriate measures must be taken to protect our patients, our coworkers, and ourselves. The goal of this chapter is to

There are several types of organisms capable of producing disease (also known as pathogens): bacteria, virus, fungus, parasite, and prion. Bacteria are very small, single-celled organisms that lack a nucleus. They are extremely diverse and often very specialized. They are capable of utilizing many different energy sources such as oxygen, sunlight, and even geothermal heat. A single bacterium may trade DNA with another bacterium and confer traits to the recipient. A particularly important example of this capability is when one bacterium confers resistance to an antibiotic to another bacterium. Bacteria cause disease by producing toxins that interfere with cells, eliciting an inflammatory response, or invading host cells. Examples of bacteria include Staphylococcus aureus (methicillin-resistant Staphylococcus aureus [MRSA] is a common form of antibiotic-resistant bacteria found in health care settings), Streptococcus pneumoniae (the most common cause of bacterial pneumonia), Clostridium difficile (common cause of diarrhea), and Mycobacterium tuberculosis (the cause of tuberculosis). Bacterial illness is generally treated with antibiotic medications (e.g., penicillin, vancomycin, azithromycin, and ciprofloxacin) that work by attacking specific bacterial cell targets that are not found in humans.

A virus is even smaller than a bacterium and much simpler (Fig. 24.1). It is composed of RNA or DNA (genetic material) surrounded by a protein shell and a lipid envelope. Unlike other organisms, a virus does not contain the cellular machinery to produce energy, manufacture proteins, or even reproduce. In order to perform these functions, it must take over a host cell. A virus “infects” a cell by attaching to the host cell membrane and becoming incorporated into the cell. Once inside, the virus can use host enzymes and proteins to replicate its own genetic material and produce viral proteins. The virus eventually kills the host cell and is released. It is capable of infecting all kinds of living cells, including bacteria, plants, and animals. Examples of viruses that infect humans are influenza (cause of the flu), herpes virus (a viral class that may cause cold sores, chicken pox, genital herpes, and even a form of cancer), human immunodeficiency virus (the cause of acquired immunodeficiency syndrome or AIDS), and hepatitis viruses (cause liver inflammation and eventually cirrhosis). Antiviral medications (e.g., acyclovir, highly active antiretroviral therapy (HAART) are available to treat many illnesses caused by viruses.

A fungus is a cellular organism containing a nucleus that shares many features of animals (i.e., they lack chlorophyll and require organic compounds as energy sources) and plants (i.e., presence of a cell wall and ability to reproduce sexually and asexually). It exists in many forms, including molds, yeast, and mushrooms. Only a
very small percentage actually causes disease in humans. Examples of fungal infections are tinea pedis (causes athlete’s foot), apergillosis (causes lung infections), and candidiasis (causes skin and mucous membrane infections). Antifungal drugs are available and include amphotericin, fluconazole, and caspofungin.

Parasitic infections are often caused by organisms from the Protozoa kingdom. Protozoans are single-celled organisms with a nucleus that resemble yeast. Examples of Protozoan infections include malaria (an infection of red blood cells caused by Plasmodium) and amoebic dysentery (caused by Entamoeba histolytica). Other parasitic infections are caused by multicellular worms including hookworms (intestinal worm that can cause severe diarrhea, intestinal obstruction, and anemia) and tapeworms (intestinal worms usually from eating undercooked meat that can invade into tissues of the body such as muscles and brain). A variety of antiparasitic medications exist including quinine (for malaria infection) and mebendazole (for worm infections).

Prions are the most recently discovered class of pathogens. They are proteins that have been folded into an abnormal shape. Once a prion has infected a cell, it induces host proteins to become misfolded, thereby producing disease. Creutzfeldt-Jakob disease is an example of a prion infection that causes progressive brain degeneration. In fact, all known prions affect brain and neural tissue. More importantly, prion infections are universally fatal. Transmission of a prion can occur by grafting infected tissue (i.e., cornea transplant, dural grafts) or ingestion. Prions are extremely resistant to standard medical sterilization protocols. Presently, treatments for prion disease are only experimental.

A health care-associated infection (HAI) is an infection that occurs during hospital admission with no evidence that the infection was present at the time of admission. In 2002, there were 1.7 million HAIs in the United States that led to 99,000 deaths. The most common types of HAIs are urinary tract infection (34%), surgical site infection (17%), pneumonia (13%), and bloodstream infection (14%). Remarkably, many of these infections are preventable. The United States Department of Health and Human Service and other international health care organizations have launched major initiatives to reduce or eliminate HAIs.

One of the most common sources of HAIs are bloodstream infections caused by indwelling venous catheters, particularly central venous catheters. A bloodstream infection (also known as septicemia

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