Chapter 3: Sterilization, Disinfection, and Infection Control

From the time of debates about the germ theory of disease, killing microbes before they reach patients has been a major strategy for preventing infection. In fact, Ignaz Semmelweis successfully applied disinfection principles decades before bacteria were first isolated. This chapter discusses the most important methods used for this purpose in modern medical practice. Understanding how these methods work has become of increasing importance in an environment that includes immunocompromised patients, transplantation, indwelling devices, and AIDS.

Death/killing as it relates to microbial organisms is defined in terms of how we detect them in culture. Operationally, it is a loss of ability to multiply under any known conditions. This is complicated by the fact that organisms that appear to be irreversibly inactivated may, sometimes, recover when appropriately treated. For example, ultraviolet (UV) irradiation of bacteria can result in the formation of thymine dimers in the DNA with loss of ability to replicate. A period of exposure to visible light may then activate an enzyme that breaks the dimers and restores viability by a process known as photoreactivation. In addition, mechanisms exist for repair of the damage without light. Such considerations are of great significance in the preparation of safe vaccines from inactivated virulent organisms.

Sterilization is complete killing, or removal, of all living organisms from a particular location or material. It can be accomplished by incineration, nondestructive heat treatment, certain gases, exposure to ionizing radiation, some liquid chemicals, and filtration.

Pasteurization is the use of heat at a temperature sufficient to inactivate important pathogenic organisms in liquids such as water or milk, but at a temperature lower than that needed to ensure sterilization. For example, heating milk at a temperature of 74°C for 3 to 5 seconds or 62°C for 30 minutes kills the vegetative forms of most pathogenic bacteria that may be present without altering its quality. Obviously, spores are not killed at these temperatures.

Disinfection is the destruction of pathogenic microorganisms by processes that fail to meet the criteria for sterilization. Pasteurization is a form of disinfection, but the term is most commonly applied to the use of liquid chemical agents known as disinfectants, which usually have some degree of selectivity. Bacterial spores, organisms with waxy coats (eg, mycobacteria), and some viruses may show considerable resistance to the common disinfectants. Antiseptics are disinfectant agents that can be used on body surfaces such as the skin or vaginal tract to reduce the numbers of microbiota and pathogenic contaminants. They have lower toxicity than disinfectants used environmentally, but are usually less active in killing vegetative organisms. Sanitization is a less precise term with a meaning somewhere between disinfection and cleanliness. It is used primarily in housekeeping and food preparation contexts.

Asepsis describes processes designed to prevent microorganisms from reaching a protected environment. It is applied in many procedures used in the operating room, in the preparation of therapeutic agents, and in technical manipulations in the microbiology laboratory. An essential component of aseptic techniques is the sterilization of all materials and equipment used.

Killing of bacteria by heat, radiation, or chemicals is usually exponential with time; that is, a fixed proportion of survivors are killed during each time increment. Thus, if 90% of a population of bacteria are killed during each 5 minutes of exposure to a weak solution of a disinfectant, a starting population of 10⁶/mL is reduced to 10⁵/mL after 5 minutes, to 10³/mL after 15 minutes, and theoretically to one organism (10⁰)/mL after 30 minutes. Exponential killing corresponds to a first-order reaction or a “single-hit” hypothesis in which the lethal change involves a single target in the organism, and the probability of this change is constant with time. Thus, plots of the logarithm of the number of survivors against time are linear (Figure 3–1A); however, the slope of the curve varies with the effectiveness of the killing process, which is influenced by the nature of the organism, lethal agent, concentration (in the case of disinfectants), and temperature. In general, the rate of killing increases exponentially with arithmetic increases in temperature or in concentrations of disinfectant. If the microbial population includes a small proportion of more resistant forms (spores), the later stages of the curve are flattened (Figure 3–1B), and extrapolations from the exponential phase of killing may underestimate the time needed for achieving complete sterility.

The availability of reliable methods of sterilization has made possible the major developments in surgery and intrusive medical techniques that have helped to revolutionize medicine over the last century. Furthermore, sterilization procedures form the basis of many food preservation procedures, particularly in the canning industry. The various modes of sterilization described in the text are summarized in Table 3–1.

Heat

The simplest method of sterilization is to expose the surface to be sterilized to a naked flame, as is done with the wire loop used in microbiology laboratories. It can be used equally effectively for emergency sterilization of a knife blade or a needle. Of course, disposable material is rapidly and effectively decontaminated by incineration. Carbonization of organic material and destruction of microorganisms, including spores, occur after exposure to dry heat of 160°C for 2 hours in a sterilizing oven. This method is applicable to metals, glassware, and some heat-resistant oils and waxes that are immiscible in water and, therefore, cannot be sterilized in the autoclave. A major use of the dry-heat sterilizing oven is in preparation of laboratory glassware.

Moist heat in the form of water or steam is far more rapid and effective in sterilization than dry heat because reactive water molecules denature protein irreversibly by disrupting hydrogen bonds between peptide groups at relatively low temperatures. Most vegetative bacteria are killed within a few minutes at 70°C or less, although many bacterial spores can resist boiling for prolonged periods.

In effect, the autoclave is a sophisticated pressure cooker (Figure 3–2). In its simplest form, it consists of a chamber in which the air can be replaced with pure saturated steam under pressure. Air is removed either by evacuation of the chamber before filling it with steam or by displacement through a valve at the bottom of the autoclave, which remains open until all air has drained out. The latter, which is termed a downward displacement autoclave, capitalizes on the heaviness of air compared with saturated steam. When the air has been removed, the temperature in the chamber is proportional to the pressure of the steam; autoclaves are usually operated at 121°C. Under these conditions, spores directly exposed are killed in less than 5 minutes, although the normal sterilization time is 10 to 15 minutes to account for variation in the ability of steam to penetrate different materials and to allow a wide margin of safety.

The effectiveness of autoclaves depends on the absence of air, pure saturated steam, and access of steam to the material to be sterilized. Pressure per se plays no role in sterilization other than to ensure the increased temperature of the steam. “Flash” autoclaves, which are used in operating rooms, often use saturated steam at a temperature of 134°C for 3 minutes. Air and steam are removed mechanically before and after the sterilization cycle to ensure that metal instruments may be available rapidly.

Gas

A number of articles, particularly certain plastics and lensed instruments that are damaged or destroyed by autoclaving, can be sterilized with gases. Ethylene oxide is an inflammable and potentially explosive gas. It is an alkylating agent that inactivates microorganisms by replacing labile hydrogen atoms on hydroxyl, carboxy, or sulfhydryl groups, particularly of guanine and adenine in DNA. Ethylene oxide sterilizers resemble autoclaves and expose the load to 10% ethylene oxide in carbon dioxide at 50°C to 60°C under controlled conditions of humidity. Exposure times, usually, are approximately 4 to 6 hours and must be followed by a prolonged period of aeration to allow the gas to diffuse out of substances that have absorbed it. Aeration is essential, because absorbed gas can cause damage to tissues or skin. Ethylene oxide is an effective sterilizing agent for heat-labile devices such as artificial heart valves that cannot be treated at the temperature of the autoclave. Other alkylating agents such as formaldehyde vapor can be used without pressure to decontaminate larger areas such as rooms.

Ultraviolet Light and Ionizing Radiation

Ultraviolet (UV) light in the wavelength range of 240 to 280 nm is absorbed by nucleic acids and causes genetic damage, including the formation of the thymine dimers discussed previously. The practical value of UV sterilization is limited by its poor ability to penetrate. Its main application has been in irradiation of air in the vicinity of critical hospital sites and as an aid in the decontamination of facilities used for handling particularly hazardous organisms.

Ionizing radiation carries far greater energy than UV light. It, too, causes direct damage to DNA and produces toxic free radicals and hydrogen peroxide from water within the microbial cells. Cathode and gamma rays from cobalt-60 are widely used in industrial processes, including the sterilization of many disposable surgical supplies such as gloves, plastic syringes, specimen containers, some foodstuffs, and the like, because they can be packaged before exposure to the penetrating radiation.

Physical Methods

Filtration

Both live and dead microorganisms can be removed from liquids by positive- or negative-pressure filtration. Membrane filters, usually composed of cellulose esters (eg, cellulose acetate), are available commercially with variable pore sizes (0.005-1 μm). For removal of bacteria, a pore size of 0.2 μm is effective for disinfection of large volumes of fluid, especially fluid containing heat-labile components such as serum. Filtration is not considered effective for removing viruses.

Pasteurization

Pasteurization involves exposure of liquids to temperatures in the range 55°C to 75°C to remove all vegetative bacteria. Spores are unaffected by the pasteurization process. Pasteurization is used commercially to render milk safe and to extend its storage quality. With the outbreaks of infection due to contamination with enterohemorrhagic Escherichia coli (see Chapter 33); this has been extended (reluctantly) to fruit drinks. To the dismay of some of his compatriots, Pasteur proposed application of the process to wine-making to prevent microbial spoilage and vinegarization. Pasteurization in water at 70°C for 30 minutes has been effective and inexpensive when used to render plastics, such as those used in inhalation therapy equipment, free of organisms that may, otherwise, multiply in mucus and humidifying water.

Microwaves

The use of microwaves in the form of microwave ovens or specially designed units is another method of disinfection. These systems are not under pressure, but they can achieve temperatures near boiling if moisture is present. In some situations, they are being used as a practical alternative to incineration for disinfection of hospital waste. These procedures cannot be considered sterilization only because heat-resistant spores may survive the process.

Chemical Methods

Given access and sufficient time, chemical disinfectants cause the death of pathogenic vegetative bacteria. Most of these substances are general protoplasmic poisons and are not used in the treatment of infections other than very superficial lesions, having been replaced by antimicrobial agents. Some disinfectants such as the quaternary ammonium compounds, alcohol, and the iodophors reduce the superficial flora and can eliminate contaminating pathogenic bacteria from the skin surface. Other agents such as the phenolics are valuable only for treating inanimate surfaces or for rendering contaminated materials safe. All are bound and inactivated to varying degrees by protein and dirt, and they lose considerable activity when applied to other than clean surfaces.

Chemical disinfectants are classified on the basis of their ability to sterilize. High-level disinfectants kill all agents, except the most resistant of bacterial spores. Intermediate-level disinfectants kill all agents, but not spores. Low-level disinfectants are active against most vegetative bacteria and lipid-enveloped viruses.

Alcohol

The alcohols are protein denaturants that rapidly kill vegetative bacteria when applied as aqueous solutions in the range of 70% to 95% alcohol. They are inactive against bacterial spores and many viruses. Solutions of 100% alcohol dehydrate organisms rapidly but fail to kill, because the lethal process requires water molecules. Ethanol (70-90%) and isopropyl alcohol (90-95%) are widely used as skin decontaminants before simple invasive procedures such as venipuncture. Their effect is not instantaneous, and the traditional alcohol wipe, particularly when followed by a vein-probing finger, is more symbolic than effective because insufficient time is given for significant killing. Isopropyl alcohol has largely replaced ethanol in hospital use because it is somewhat more active and is not subject to diversion to parties.

Halogens

Iodine is an effective disinfectant that acts by iodinating or oxidizing essential components of the microbial cell. Its original use was as a tincture of 2% iodine in 50% alcohol, which kills more rapidly and effectively than alcohol alone. Tincture of iodine has now been largely replaced by preparations in which iodine is combined with carriers (povidone) or nonionic detergents. These agents, termed iodophors, gradually release small amounts of iodine. They cause less skin staining and dehydration than tinctures, and are widely used in preparation of skin before surgery.

Chlorine exists as hypochlorous acid in aqueous solutions that dissociate to yield free chlorine over a wide pH range, particularly under slightly acidic conditions. In concentrations of less than one part per million, chlorine is lethal within seconds to most vegetative bacteria, and inactivates most viruses; this efficacy accounts for its use in rendering supplies of drinking water safe and in chlorination of water in swimming pools. Chlorine is the agent of choice for decontaminating surfaces and glassware that have been contaminated with viruses or spores of pathogenic bacteria. For these purposes, it is usually applied as a 5% solution called hypochlorite.

Hydrogen Peroxide

Hydrogen peroxide is a powerful oxidizing agent that attacks membrane lipids and other cell components. Although it acts rapidly against many bacteria and viruses, it kills bacteria that produce catalase and spores less rapidly. Hydrogen peroxide has been useful in disinfecting items such as contact lenses, which are not susceptible to its corrosive effect.

Surface-Active Compounds

Surfactants are compounds with hydrophobic and hydrophilic groups that attach to and solubilize various compounds or alter their properties. Anionic detergents such as soaps are highly effective cleansers, but they have little direct antibacterial effect, probably because their charge is similar to that of most microorganisms. Cationic detergents, particularly the quaternary ammonium compounds (“quats”) such as benzalkonium chloride, are highly bactericidal in the absence of contaminating organic matter. Their hydrophobic and lipophilic groups react with the lipid of the cell membrane of the bacteria, alter the membrane’s surface properties and its permeability, and lead to loss of essential cell components and death. These compounds have little toxicity to skin and mucous membranes, and thus they have been used widely for their antibacterial effects in a concentration of 0.1%. They are inactive against spores and most viruses. Care is needed in the use of quats because they adsorb to most surfaces as well as cotton, cork, and even dust. As a result, their concentration may be lowered to a point at which certain bacteria, particularly Pseudomonas aeruginosa, can grow in the quat solutions and serve as a source of infection.

Phenolics

Phenol is a potent protein denaturant and bactericidal agent. Substitutions in the ring structure of phenol have substantially improved activity and have provided a range of phenols and cresols that are the most effective environmental decontaminants available for use in hospital hygiene. Concern about their release into the environment in hospital waste and sewage has created some pressure to limit their use. Phenolics are less “quenched” by protein than are most other disinfectants, have a detergent-like effect on the cell membrane, and are often formulated with soaps to increase their cleansing property. They are too toxic to skin and tissues to be used as antiseptics, although brief exposures can be tolerated. They are the active ingredient in many mouthwash and sore throat preparations.

Chlorhexidine is used as a routine hand and skin disinfectant and for other topical applications. It has the ability to bind to the skin and produce a persistent antibacterial effect. It acts by altering membrane permeability of both gram-positive and gram-negative bacteria. It is cationic, and thus its action is neutralized by soaps and anionic detergents.

Glutaraldehyde and Formaldehyde

Glutaraldehyde and formaldehyde are alkylating agents highly lethal to essentially all microorganisms (Figure 3–3). Formaldehyde gas is irritative, allergenic, and unpleasant—properties that limit its use as a solution or gas. Glutaraldehyde is an effective high-level disinfecting agent for apparatus that cannot be heat-treated, such as some lensed instruments and equipment for respiratory therapy. Formaldehyde vapor, an effective environmental decontaminant under conditions of high humidity, is sometimes used to decontaminate laboratory rooms that have been accidentally and extensively contaminated with pathogenic bacteria.

Some risk of infection exists in all healthcare settings. Hospitalized patients are particularly vulnerable, and the hospital environment is complex. Infection control is the proper matching of the principles and procedures described here to general and specialized situations, together with aseptic practices to reduce these risks. “Nosocomial” is a medical term for “hospital-associated.” Nosocomial infections are complications that arise during hospitalizations. The morbidity, mortality, and costs associated with these infections are preventable to a substantial degree. The purpose of hospital infection control is prevention of nosocomial infections by application of epidemiologic concepts and methods.

History: Semmelweis and Childbed Fever

The shining example of the fundamental importance of epidemiology in detection and control of nosocomial infections is the work of Ignaz Semmelweis, which preceded the microbiologic discoveries of Pasteur and Koch by a decade. Semmelweis was assistant obstetrician at the Vienna General Hospital, where more than 7000 infants were delivered each year. Childbed fever (puerperal endometritis), which we now know is caused primarily by group A streptococci, was a major problem accounting for 600 to 800 maternal deaths per year. By careful review of hospital statistics between 1846 and 1849, Semmelweis clearly showed that the death rate in one of the two divisions of the hospital was 10 times that in the other. Division I, which had the high mortality rate, was the teaching unit in which all deliveries were by obstetricians and students. In division II, all deliveries were by midwives. No similar epidemic existed elsewhere in the city of Vienna, and the mortality rate was very low in mothers delivering at home.

Semmelweis postulated that the key difference between divisions I and II was participation of the physicians and students in autopsies. One or more cadavers were dissected daily, some from cases of childbed fever and other infections. Handwashing was perfunctory, and Semmelweis believed this allowed the transmission of “invisible cadaver particles” by direct contact between the mother and the physician’s hands during examinations and delivery. In 1847, as a countermeasure, he required handwashing with a chlorine solution until the hands were slippery and the odor of the cadaver was gone. The results were dramatic. The full effect of the chlorine handwashing can be seen by comparing mortality rates in the two divisions for 1846 and 1848 (Table 3–2). The mortality rate in division I was reduced to that of division II, and both were lower than 2%.

Unfortunately, because of his personality and failure to publish his work until 1860, Semmelweis contribution was not generally appreciated in his lifetime. As his frustration mounted over lack of acceptance of his ideas, he became abusive and irrational, eventually alienating even his early supporters. Some believe that he also suffered from Alzheimer’s disease. He died in an insane asylum in 1865, unaware that his concept of spread via direct contact would later be recognized as the most important mechanism of nosocomial infection, and that handwashing would remain the most important means of infection control in hospitals.

Infections occurring during any hospitalization could be either community acquired or nosocomial. Community-acquired infections are defined as those present or incubating at the time of hospital admission. All others are considered nosocomial. For example, a hospital case of chickenpox could be community acquired if it erupted on the fifth hospital day (incubating) or nosocomial if hospitalization was beyond the limits of the known incubation period (20 days). Infections appearing shortly after discharge (2 weeks) are considered nosocomial, although some could have been acquired at home. Infectious hazards are inherent to the hospital environment; it is there that the most seriously infected and most susceptible patients are housed and often cared for by the same staff.

The infectious agents responsible for nosocomial infections arise from various sources, including patients’ own microbiota. In addition to any immunocompromising disease or therapy, the hospital may impose additional risks by treatments that breach the normal defense barriers. Surgery, urinary or intravenous catheters, and invasive diagnostic procedures all may provide opportunistic microbes with access to usually sterile sites. Infections in which the source of organisms is the hospital rather than the patient include those derived from hospital personnel, the environment, and medical equipment.

Hospital Personnel

Physicians, nurses, students, therapists, and any others who come in contact with the patient may transmit infection. Transmission from one patient to another is called cross-infection. The vehicle of transmission is most often the inadequately washed hands of a medical attendant. Another source is the infected medical attendant. Many hospital outbreaks have been traced to hospital personnel, particularly physicians, who continue to care for patients despite an overt infection. Transmission is usually by direct contact, although airborne transmission is also possible. A third source is the person who is not ill, but is carrying a virulent strain. For Staphylococcus aureus and group A streptococci, nasal carriage is most important, but sites such as the perineum have also been involved in outbreaks. An occult carrier is less often the source of nosocomial infection than a physician covering up a boil or a nurse minimizing “the flu.”

Environment

The hospital air, walls, floors, linens, and the like are not sterile and, thus, could serve as a source of organisms causing nosocomial infections, but the importance of this route has generally been exaggerated. With the exception of the immediate vicinity of an infected individual or a carrier, transmission through the air or on fomites is much less important than that caused by personnel or equipment. Notable exceptions are when the environment becomes contaminated with Mycobacterium tuberculosis from a patient or Legionella pneumophila in the water supply. These events are most likely to result in disease when the organisms are numerous or the patient is particularly vulnerable (eg, after heart surgery or bone marrow transplantation).

Medical Devices

Much of the success of modern medicine is related to medical devices that support or monitor basic body functions. By their very nature, devices such as catheters, implants, and respirators carry a risk of nosocomial infection because they bypass normal defense barriers, providing microorganisms access to normally sterile fluids and tissues. Most of the recognized causes are bacterial or fungal. The risk of infection is related to the degree of debilitation of the patient and various factors concerning the design and management of the device. Any device that crosses the skin or a mucosal barrier allows microbes in the patient or environment to gain access to deeper sites around the outside surface. Possible access inside the device (eg, in the lumen) adds another and, sometimes, greater risk. In some devices, such as urinary catheters, contamination is avoidable; in others, such as respirators, complete sterility is either impossible or impractical to achieve.

The risk of contamination leading to infection is increased if organisms that gain access can multiply within the system. The availability of water, nutrients, and a suitable temperature largely determine which organism will survive and multiply. Many of the gram-negative rods such as Pseudomonas, Acinetobacter, and members of Enterobacteriaceae can multiply in an environment containing water and little else. Gram-positive bacteria generally require more physiologic conditions.

Even with proper growth conditions, many hours are required before contaminating organisms multiply to numbers sufficient to cause disease. Detailed studies of catheters and similar devices show that the risk of infection begins to increase after 24 to 48 hours and is cumulative even if the device is changed or disinfected at intervals. It is thus important to discontinue transcutaneous procedures as soon as medically indicated. The medical devices most frequently associated with nosocomial infections are listed in the following text. The infectious risk of others can be estimated from the principles discussed previously. New devices are constantly being introduced into medical care, occasionally, without adequate consideration of their potential to cause nosocomial infection.

Urinary Catheters

Urinary tract infection (UTI) accounts for 40% to 50% of all nosocomial infections, and at least 80% of these are associated with catheterization. The infectious risk of a single urinary catheterization has been estimated at 1%, and indwelling catheters carry a risk that may be as high as 10%. The major preventive measure is maintenance of a completely closed system through the use of valves and aspiration ports designed to prevent bacterial access to the inside of the catheter or collecting bag. Unfortunately, breaks in closed systems eventually occur when the system is in place for more than 30 days. The urine itself serves as an excellent culture medium once bacteria gain access.

Vascular Catheters

Needles and plastic catheters placed in veins for fluid administration, monitoring vital functions, or diagnostic procedures are a leading cause of nosocomial bacteremia. These sites should always be suspected as a source of organisms whenever blood cultures are positive with no apparent primary site for the bacteremia. Contamination at the insertion site is generally staphylococcal, with continued growth in the catheter tip. Organisms may gain access somewhere in the lines, valves, bags, or bottles of intravenous solutions proximal to the insertion site. The latter circumstance usually involves gram-negative rods. Preventive measures include aseptic insertion technique and appropriate care of the lines, including changes at regular intervals.

Respirators

Machines that assist or control respiration by pumping air directly into the trachea have a great potential for causing nosocomial pneumonia if the aerosol they deliver becomes contaminated. Bacterial growth is significant only in the parts of the system that contain water; in systems using nebulizers, bacteria can be suspended in water droplets small enough to reach the alveoli. The organisms involved include Pseudomonas, Enterobacteriaceae, and a wide variety of environmental bacteria such as Acinetobacter. The primary control measure is periodic changing and disinfection of the tubing, reservoirs, and nebulizer jets.

Blood and Blood Products

Infections related to contact with blood and blood products are generally a risk for healthcare workers rather than patients. Manipulations ranging from phlebotomy and hemodialysis to surgery carry varying risk of blood containing an infectious agent reaching mucous membranes or skin of the healthcare worker. The major agents transmitted in this manner are hepatitis B, hepatitis C, and human immunodeficiency virus (HIV). Control requires meticulous attention to procedures that prevent direct contact with blood, such as the use of gloves, eyewear, and gowns. Cuts and needle sticks among healthcare workers carry a risk approaching 2%. Identification of hepatitis virus and HIV carriers is a part of a protective process that must be balanced by patient privacy considerations. Healthcare facilities all have established policies concerning serologic surveillance of patients and the procedures to follow (eg, testing, prophylaxis) when blood-related accidents occur. Similarly, products for transfusion undergo extensive screening to protect the recipient.

Infection control is the sum of all the means used to prevent nosocomial infections. Historically, such methods have been developed as an integral part of the study of infectious diseases, often serving as key elements in the proof of infectious etiology. Semmelweis’ handwashing is the first example. Later in the 19th century, Joseph Lister achieved a dramatic reduction in surgical wound infections by infusion of a phenolic antiseptic into wounds. This local destruction of organisms was known as antisepsis, and it sometimes included liberal applications of disinfectants, including sprays to the environment. As it became recognized that contamination of wounds was not inevitable, the emphasis gradually shifted to preventing contact between microorganisms and susceptible sites—a concept called asepsis. Asepsis, which combines containment with the methods of sterilization and disinfection previously discussed, is the central approach of infection control. The measures taken to achieve asepsis vary, depending on whether the circumstances and environment are most similar to the operating room, hospital ward, or outpatient clinic.

Asepsis

Operating Room

The surgical suite and operating room represent the most controlled and rigid application of aseptic principles. The procedure begins with the use of an antiseptic scrub of the skin over the operative site and the hands and forearms of all who will have contact with the patient. The use of sterile drapes, gowns, and instruments serves to prevent spread through direct contact, and caps and face masks reduce airborne spread from personnel to the wound. The level of bacteria in the air is generally related more to the number of persons and amount of movement in the operating room than to incoming air. The net effect of these procedures is to draw a sterile curtain around the operative site, thus minimizing contact with microorganisms. Surgical asepsis is also used in other areas where invasive special procedures such as cardiac catheterization are carried out.

Hospital Ward

Although theoretically desirable, strict aseptic procedures as used in the operating room are impractical in the ward setting. Asepsis is practiced by the use of sterile needles, medications, dressings, and other items that could serve as transmission vehicles if contaminated. A “no touch” technique for examining wounds and changing dressings eliminates direct contact with any nonsterile item. Invasive procedures such as catheter insertion and lumbar punctures are carried out under aseptic precautions similar to those used in the operating room. In all circumstances, handwashing between patient contacts is the single most important aseptic precaution.

Outpatient Clinic

The general aseptic practices used in the hospital ward are also appropriate to the outpatient situation as preventive measures. The potential for cross-infection in the clinic or waiting room is obvious, but it has been little studied regarding preventive measures. Patients who may be infected should be segregated whenever possible using techniques similar to those of hospital ward isolation. The examining room may be used in a manner analogous to the private rooms on a hospital ward. Although this approach is difficult because of patient turnover, it should be attempted for infections that would require strict or respiratory isolation in the hospital.

Isolation Procedures

Patients with infections pose special problems because they may transmit their infections to other patients either directly or by contact with a staff member. This additional risk is managed by the techniques of isolation, which place barriers between the infected patient and others on the ward. Because not every infected patient presents with suspect signs and/or symptoms, some precaution should be taken with all patients. In the system recommended by the Centers for Disease Control and Prevention, these are called standard precautions and include the use of gowns and gloves when in contact with patient blood or secretions. These are particularly directed at protecting healthcare workers from HIV and hepatitis infection. For those with suspected or proven infection, additional precautions are taken, the nature of which is determined by the known mode of transmission of the organism. These transmission-based precautions are divided into those directed at airborne, droplet, and contact routes. The airborne transmission precautions are for infections known to be transmitted by extremely small (< 5 μm) particles suspended in the air. This requires that the room air circulation be maintained with negative pressure relative to the surrounding area and be exhausted to the outside. Those entering the room must wear surgical masks, and in the case of tuberculosis, specially designed respirators. Droplet precautions are for infections in which the organisms are suspended in larger droplets, which may be airborne, but generally do not travel more than 3 ft from the patient who generates them. These can be contained by the use of gowns, gloves, and masks when working close to the patient. Contact precautions are used for infections that require direct contact with organisms on or that pass in secretions of the patient. Diarrheal infections are of special concern because of the extent to which they contaminate the environment. Details of the precautions and examples of the typical infectious agents are summarized in Table 3–3.

Organization

Modern hospitals are required to have formal infection control programs that include an infection control committee, epidemiology service, and educational activities. The infection control committee comprises representatives of various medical, administrative, nursing, housekeeping, and support services. The committee establishes the institution’s infection control procedures and regularly reviews information on the status of nosocomial infections in the hospital. When epidemiologic circumstances warrant it, the committee must be empowered to take drastic action such as closing a hospital unit or suspending a physician’s privileges.

The epidemiology service is the working arm of the infection control committee. Its functions are conducted by one or more epidemiologists who usually have a nursing background. This work requires familiarity with clinical microbiology, epidemiology, infectious disease, and hospital procedures, as well as immense tact. The main activities are surveillance and outbreak investigation. Surveillance is the collection of data documenting the frequency and nature of nosocomial infections in the hospital to detect deviations from the institutional or national norms. Although routine microbiologic sampling of the hospital environment is of no value, programs to sample some of the medical devices known to be nosocomial hazards can be useful. On-the-spot investigation of potential outbreaks allows early implementation of preventive measures. This activity is probably the single most important function of the epidemiology service. Suspicion of an increased number of infections leads to an investigation to verify the facts, establish basic epidemiologic associations, and relate them to preventive measures. The primary concern is cross-infection, in which a virulent organism is being transmitted from patient to patient.

Prevention

The prevention of nosocomial infections is contingent on basic and applied knowledge drawn from all parts of this book. Applied with common sense, these principles can both prevent disease and reduce the costs of medical care.