Symbiosis

Symbiosis is the term that describes a close relationship between two different types of organisms in a community. Depending on the outcome of this relationship, symbiosis can be classified as mutualism, commensalism, parasitism, or amensalism (Table 9.1):

Normal Flora (Microbiota)

A newborn’s first contact with microorganisms occurs while traveling through the birth canal, where lactobacilli residing in the mother’s vagina will become the predominant organisms in the newborn’s intestine. The next exposure of the newborn to microorganisms occurs with the beginning of breathing, and this is soon followed by feeding. From then on, other orally acquired bacteria such as E. coli will begin to colonize the large intestine and will remain there throughout life. In other words, starting at birth the human body enters a state of dynamic equilibrium with microorganisms. In addition, throughout a person’s life, other microorganisms will establish residency in mucous membranes that are open to the external environment, on the skin and its derivatives (Figure 9.1). Mucous membranes open to the external environment include those of the respiratory tract (Figure 9.2), gastrointestinal tract (Figure 9.3), and urogenital tract (Figure 9.4). The microbes that establish themselves on the skin and mucous membranes usually do not cause disease and constitute the normal flora (microbiota) of the human body. This normal flora consists of resident or transient microbes:

Opportunistic Pathogens

In general, this balance between the normal flora and the human host can be maintained, but when the balance is interrupted for whatever reason, the microorganisms can cause infection and disease. They become opportunistic pathogens. An opportunistic pathogen does not cause disease in its normal habitat in a healthy person, but can cause infection under conditions of immune suppression, changes in the normal flora, or when a member of the normal flora gains access into an area of the body it normally does not inhabit.

Portal of Entry

The site where a pathogen enters the body is referred to as the portal of entry, which can be the skin, mucous membranes, the placenta, or the so-called parenteral route (other then the digestive tract route). The source of the infectious agent can be exogenous, from outside the body, or endogenous, in which case the organism is already in the body, such as in the normal flora.

Portals of entry are generally the same areas that support normal flora: the skin and the mucous membranes of the gastrointestinal, respiratory, and urogenital tracts (Figure 9.5). The majority of pathogens have their preferred portal of entry, which provides the necessary habitat for further growth and eventual spreading. Most often, if a pathogen enters the wrong portal, infection will not occur. For example, influenza virus uses the respiratory mucosa as its portal of entry, where it may successfully infect its host, but when limited to contact with the skin only, influenza virus will not cause an infection. Some infectious agents can enter via more than one portal of entry, such as the skin and mucous membranes, where the infection then can lead to various diseases. For example, Streptococcus and Staphylococcus have adapted to several portals of entry such as the skin and the urogenital and respiratory tracts.

Skin

The top layer of the skin, the epidermis, consists of a thick layer of keratinized dead cells that is the first line of defense against microbial invasion; however, some pathogens can enter the body through natural openings in the skin, such as hair follicles and sweat glands. Damage to the skin in the form of abrasions, cuts, punctures, and scrapes opens the skin to contaminants, which ultimately can cause infection. Moreover, larvae of some parasitic worms burrow through the skin to get to the underlying tissues, and some fungi use digestive enzymes to gain entry to the soft tissues.

Mucous Membranes

Mucous membranes line all the body cavities and the canals that come in contact with the outside, such as the canals of the gastrointestinal, respiratory, and urogenital tracts, as well as the conjunctiva, the thin membrane covering the eye and the underside of the eyelids. The skin and the outer layer of the mucous membranes are composed of tightly packed epithelial cells. However, in contrast to the epidermis, the mucosal lining is thin, moist, and warm, and composed of living cells.

• The gastrointestinal (GI) tract serves as the portal of entry for pathogens present in food, liquid, and other ingested substances. Microorganisms that can survive in the GI tract are adapted to survive the action of digestive enzymes and environments that undergo drastic changes in the pH. Enteric bacterial pathogens include Salmonella, Shigella, Vibrio, and certain strains of E. coli. Viruses using the GI tract as a portal of entry include poliovirus, hepatitis A virus, echovirus, and rotavirus. The most common enteric protozoans are Entamoeba histolytica and Giardia lamblia. Helminths, although not considered microbes, are infectious agents entering through the GI tract and include trematodes, cestodes, and nematodes (see Chapter 8, Eukaryotic Microorganisms).

• The respiratory tract is the most frequently used portal of entry. Pathogens are able to enter the mouth and nose by air, via dust particles, moisture, and respiratory droplets from an infected person. Mucous membranes that line the upper respiratory tract are continuous with the membranes of the sinuses and of the auditory tubes, and pathogens can be transferred from one site to the other. Examples of bacteria using this portal of entry include the causative agents of sore throat, meningitis, diphtheria, and whooping cough. Examples of viral agents include those causing the common cold, influenza, measles, meningitis, mumps, rubella, and chickenpox (see Chapter 11, Infections of the Respiratory System).

• The urogenital tract is a portal of entry for pathogens that are generally contracted by sexual contact (see Chapter 17, Sexually Transmitted Infections/Diseases). However, girls and women who are not sexually active are also susceptible to lower urinary tract infections because of the close proximity of the anus to the female urethra. Therefore urinary tract infections caused by E. coli, as opportunistic organisms, are more common in women than men. Vaginal yeast infections are common in women who are under stress; taking birth control pills, antibiotics, and/or steroids; who are pregnant; or because of other factors. Vaginal yeast infections are usually due to an overgrowth of Candida albicans.

• The conjunctiva is generally a good barrier against infectious agents, but some bacteria such as Haemophilus aegyptius (pinkeye), Chlamydia trachomatis, and Neisseria gonorrhoeae can easily attach to this membrane.

Placenta

The placenta, an organ formed by maternal and fetal tissues, is usually an effective barrier against microorganisms that may be present in the maternal circulation. However, some microbes can cross the placenta and infect the embryo or fetus, sometimes causing spontaneous abortions, birth defects, or premature births.

HEALTHCARE APPLICATION
Pathogens That Cross the Placenta

Microbe	Pathogen	Condition	Effect on Unborn
RNA viruses	Lentivirus (HIV)	AIDS	Immunosuppression (AIDS)
	Rubivirus	German measles	Severe birth defects or death
DNA viruses	Cytomegalovirus	Usually asymptomatic	Deafness, microcephaly, mental retardation
	Parvovirus B-19	Erythema infectiosum	Abortion
Bacteria	Treponema pallidum	Syphilis	Abortion, multiorgan birth defects, syphilis
	Listeria monocytogenes	Listeriosis	Granulomatosis infantiseptica, death
Protozoans	Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, encephalitis, microcephaly, mental retardation, blindness, anemia, jaundice, rash, pneumonia, diarrhea, hypothermia, deafness

Parenteral Route

Technically the parenteral route is not a portal of entry, but a means of bypassing other portals of entry. In this manner, pathogens are introduced directly into the subcutaneous tissue, as happens with punctures by a nail, thorn, or contaminated needles. This way of pathogen entry can also be achieved by cuts, bites, stab wounds, deep abrasions, or surgery.

Virulence and Pathogenicity

After pathogens have gained entry into the body they must undergo adhesion and several other steps to be able to multiply, thereby causing an infection and possible disease.

Virulence refers to the degree of pathogenicity or disease-evoking power of a specific microbe. Virulence therefore is the degree of pathogenicity of a microbe and is based on virulence factors. A pathogen is a microorganism that is capable of causing disease. The ability of a microorganism to cause disease is directly related to the number of infecting organisms, the portal of entry, the host defense mechanisms (see Chapter 20, The Immune System), and the intrinsic characteristics of the bacteria and their virulence factors. These include the following:

Toxins

An organism that produces toxins is called toxigenic. This is an underlying mechanism by which many microorganisms, especially bacteria, produce disease.

Chemically, bacterial toxins are either lipopolysaccharides associated with the cell wall of gram-negative bacteria, or proteins released from the bacterial cell. Toxins associated with the cell wall are called endotoxins, and proteins released from a bacterium are referred to as exotoxins (Figure 9.6). Bacterial toxins are among the most powerful toxins known to humankind.

Exotoxins

Exotoxins are typically soluble proteins secreted by bacterial cells during their exponential growth phase. They can act at sites other than the location of the infection and bacterial growth. Often the toxins produced are species specific and associated with a particular disease (Table 9.5). For example, Clostridium tetani is the only organism to produce the tetanus toxin, and Corynebacterium diphtheriae makes diphtheria toxin. As with other proteins, these enzymes can be denatured by heat, acid, and proteolytic enzymes (enzymes that break down proteins).

TABLE 9.5

Examples of Bacterial Exotoxins

Organism	Toxin	Activity	Pathology
Bacillus anthracis	Anthrax toxin (EF)	Edema factor (EF) causes increased levels of intracellular cAMP in phagocytes and the formation of ion-permeable pores in membranes	Hemolysis
Bordetella pertussis	Adenylate cyclase (AC) toxin	Causes increased levels of cAMP in phagocytes and the formation of ion-permeable pores in membranes	Hemolysis; whooping cough
Escherichia coli	Heat-labile (LT) toxin	Increases the level of cAMP in cells of the GI tract, causing secretion of water and electrolytes	Severe abdominal cramps; bloody diarrhea; high fever
	Heat-stable (ST) toxin	Stimulates guanylate cyclase, promotes secretion of water and electrolytes from intestinal epithelium	Severe abdominal cramps; bloody diarrhea; high fever
Pseudomonas aeruginosa	Exotoxin A	Inhibits protein synthesis	Various
Vibrio cholerae	Cholera enterotoxin	Increases the level of cAMP in cells of the GI tract, causing secretion of water and electrolytes	Cholera
Clostridium botulinum	Botulinum toxin	Disrupts secretion of acetylcholine at neuromuscular synapse, preventing muscular contraction	Botulism
Clostridium tetani	Tetanus toxin	Prevents the breakdown of acetylcholine by enzymes at the neuro-muscular synapse, causing constant muscle contraction	Tetanus

cAMP, Cyclic adenosine monophosphate; GI, gastrointestinal.

Like enzymes, many bacterial exotoxins are substrate specific and generally the site of damage caused by the toxin indicates the location of the substrate for that particular toxin. This has led to naming these toxins according to their target, such as enterotoxin, neurotoxin, leukocidin, or hemolysin to name a few.

Endotoxins

Endotoxins are lipopolysaccharides and structural components of gram-negative bacterial cell walls (e.g., Shiga toxin). Endotoxins are released from the cell walls during lysis initiated by an effective host defense or by the action of antibiotics. After release, these toxins can also act on sites remote from the original site of infection and growth. In contrast to exotoxins, endotoxins are less potent and less specific in their actions. Because these toxins are lipopolysaccharides and not proteins, they are heat stable and even boiling for 30 minutes does not destabilize them. However, certain oxidizing agents such as superoxide, peroxide, and hypochlorite have been reported to be effective in neutralizing them. A general comparison between bacterial exotoxins and endotoxins is presented in Table 9.6.

TABLE 9.6

Comparison of Bacterial Exotoxins and Endotoxins

Feature	Exotoxin	Endotoxin
Chemical nature	Protein	Lipopolysaccharides
Location	Extracellular	Part of cell wall
Specificity	High degree	Low degree
Potency	Relatively high	Relatively low
Denatured by heat	Yes	No
Antigenic	Yes	Yes
Toxoid*	Yes	No

^*A toxoid is an endotoxin modified so that it is no longer toxic, but still causes the host to generate antitoxin.

MEDICAL HIGHLIGHTS

Microbial Enzymes versus Blood Clots and Bronchitis

Interesting discoveries have been made in the treatment of blood clots and bronchitis. The treatments were discovered in an unusual place: in the proteolytic enzymes produced by a bacterium and a fungus. For centuries the Japanese have used a fermented food product called natto, which was known to contribute to healthy skin and cardiovascular health, and to help prevent osteoporosis. Studies have shown that the enzyme produced by the fermenting bacterium Bacillus natto is effective at dissolving blood clots, potentially preventing heart attacks and strokes. This enzyme, called nattokinase, can dissolve clots even faster than plasmin, the main clot-dissolving enzyme produced by the body. Furthermore, nattokinase is longer acting and also has been shown to stimulate the body to produce more of its own clot-dissolving enzymes such as plasmin and urokinase.

In the case of bronchitis, an enzyme from the fungus Aspergillus melleus has been shown to reduce the painful inflammation and mucus associated with bronchitis. According to studies, the enzyme breaks down proteins in the mucus, making it less viscous and “sticky” and thus reducing mucus buildup and inflammation of the affected tissue.

Portal of Exit

To infect others, pathogens must leave the infected patient by a portal of exit. Oftentimes this is the same as the portal of entry, such as the entrance and exit of infectious particles in and out of the respiratory system. However, pathogens can also leave the host by excretion/secretion in various parts of the body, by defecation, or via blood. Secretory substances such as nasal secretions, saliva, sputum, respiratory droplets, tears, and earwax, are also ways by which pathogens can exit a host and infect another, usually by casual contact.

Etiology of Infectious Diseases

As mentioned earlier, not all infections cause disease and it is essential to differentiate between an infection and a disease. An infection is the colonization of a host by an infectious agent. When an infectious agent causes pathological changes and interferes with the normal functioning of the body it is termed infectious disease. The study of the cause of disease is called etiology.

Patterns of Infection

Infections have varied patterns and are named accordingly as local infections, focal infections, systemic infections, mixed infections, acute and chronic infections, and primary and secondary infections.

• Local infections are the simplest ones; the organism enters the body and remains confined to a specific tissue. This is the case with boils, fungal skin infections, and warts.

• Focal infections occur when the pathogen spreads from a local infection to other tissues.

• Systemic infections occur when an infection spreads to several sites and tissue fluids, usually by way of the circulatory system.

• Septicemia is a systemic infection caused by the multiplication of pathogens in the blood. This condition is a common example of sepsis, a toxic inflammatory condition arising from the spread of microbes.

• Bacteremia represents the presence of bacteria in the blood.

• Toxemia refers to the presence of toxins in the blood.

• Viremia is the term used to describe viruses in the blood.

• Mixed infections occur when several infectious agents concurrently establish themselves at the same site. Some mixed infections are synergistic infections in which microbes cooperate in breaking down the tissue.

• Acute infections are infections that appear rapidly, with pronounced (severe) symptoms, but then rapidly vanish.

• Chronic infections have usually less severe symptoms than acute infections, but they persist for long periods of time.

• A primary infection is an initial infection, which is followed by complications due to another microbe.

• A secondary infection follows a primary infection. In general, secondary infections are caused by a different microbe, which gains access to the body through a lesion of either the skin or the mucous membrane damaged by the first pathogen, or they can be microbes of the normal flora that become opportunistic because of a compromised immune system.

• A subclinical infection is one that does not cause any apparent symptoms, and can be carried over long periods of time without causing symptoms of illness in the individual.

Koch’s Postulates

The germ theory of disease was proposed by Louis Pasteur and Robert Koch (see Chapter 1, Scope of Microbiology). Although Louis Pasteur was convinced that microbes caused disease in humans he was not able to link a specific microbe with a particular disease. After many years of experimentation, Koch was able to establish that specific microorganisms cause specific diseases. Koch developed a series of conditions—postulates—that must be met to identify a particular microbe as pathogenic and the cause of a particular disease. Using his postulates he proved that Bacillus anthracis is the causative agent for anthrax and that Mycobacterium tuberculosis causes tuberculosis. This concept is now known as Koch’s postulates (Figure 9.7), and can be summarized as follows:

• The suspected pathogen must be present in every case of the disease, and absent in healthy individuals.

• The pathogen must be isolated from the diseased host and grown in pure culture outside the host.

• The pathogen from the pure culture must cause the same disease when inoculated into a healthy, susceptible experimental host.

• The same pathogen must be isolated from the experimental host and must be shown to be identical to the original pathogen.

Exceptions to Koch’s Postulates

Although Koch’s postulates provide the basics of the etiology of infectious disease, they are not always appropriate to use, for the following reasons:

• Some pathogens cannot be cultured in the laboratory.

• Many infectious diseases can be caused by several different pathogens.

• A particular pathogen can cause different diseases, depending on the portal of entry.

• Some diseases are caused by a combination of pathogens, such as liver infections caused by both the hepatitis B virus and the hepatitis D virusoid. The latter cannot cause infection without the hepatitis B virus. Other exceptions are polymicrobial diseases caused by biofilms.

• Some pathogens are exclusively human pathogens and ethical considerations prevent human inoculation to prove Koch’s postulates. These exceptions include diseases caused by viruses or animal parasites. Also, Rickettsia and Chlamydia are obligate intracellular parasites without an animal model.

Although Koch’s postulates do not always seem to apply, scientists have found other means to reach the desired outcome of the postulates by following other routes. Furthermore, Koch’s postulates are at work in the food industry, where microbes need to be carefully monitored, and the desired microbe must be isolated in the desired end product. Despite the challenges Koch’s postulates remain useful for confirming microbial roles in infection, disease, and other processes. The concepts used to prove cause have been employed as modules of proof in other scientific disciplines. For example, Eccles used a similar pattern to show that acetylcholine (ACh) was a chemical transmitter in the central nervous system, which established mammalian neurotransmission as being chemical, when it had previously been promoted as electrical.

Diseases in the Population

Diseases in a population can be reported as the prevalence of the disease, which is the total number of existing cases in the entire population, or the disease can be measured by incidence, which is the number of new cases over a certain period of time compared with the general healthy population. The equations used to determine these rates are as follows:

< ?xml:namespace prefix = "mml" />Prevalence=(total number of cases in populationtotal number of persons in population)×100=%

Incidence=(number of new casestotal number of susceptible persons)(usually reported per 100,000 persons)

Changes in prevalence and incidence of diseases are generally monitored every season, annually, and on a long-term basis to predict trends. These monitoring statistics make it possible to define the frequency of a disease in a population. The frequency of a disease and the geographic location of the occurrence allow for classification of the disease into endemic, sporadic, epidemic, and pandemic disease categories.

Reservoirs

For an infectious disease to continue to exist, its causative agent(s) must have a permanent place to reside. Sites where pathogens are maintained as a source of infection are called reservoirs of infection. These can be in the form of animal, human, or nonliving reservoirs.

Animal Reservoirs

Many of the pathogens that normally infect animals, either domesticated or wild, also can affect humans. Diseases that occur primarily in animals and can be transmitted to humans are referred to as zoonoses (Table 9.7). Humans can be infected by zoonoses by several routes: direct contact with the infected animal; contact with animal waste, or with waste-contaminated food or water; contact with dust from contaminated hides, fur, or feathers; consumption of infected animal products; or through the medium of insect vectors. Humans often are the definitive (final) host for zoonotic pathogens and usually are not a significant reservoir for reinfection of animal hosts. However, zoonotic diseases transmitted by blood-sucking arthropods are likely to be transmitted back to the species of the animal host.

TABLE 9.7

Examples of Zoonoses

Disease	Infectious Agent	Animal Reservoir	Mode of Transmission
Viral
Rabies	Lyssavirus spp.	Bats, skunks, raccoons, foxes, dogs	Direct contact via bite or scratch of infected animal
Hantavirus pulmonary syndrome	Hantavirus spp.	Rodents, primarily deer mice	Inhalation of viruses from dried feces and urine or direct contact with rodent saliva, feces, or urine
Yellow fever	Flavivirus spp.	Monkeys	Bite of Aedes mosquito
Influenza (some)	Influenza virus	Swine, birds	Direct contact
Bacterial
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact with infected animals, inhalation
Brucellosis	Brucella spp.	Domestic livestock	Direct contact with contaminated meat, milk, or animal
Lyme disease	Borrelia burgdorferi	Deer	Tick bites
Rocky Mountain spotted fever	Rickettsia rickettsii	Rodents	Tick bites
Typhus	Rickettsia prowazeki Rickettsia typhi

Rodents Louse bites Bubonic plague Yersinia pestis Rodents Flea bites Leptospirosis Leptospira spp. Wild animals, domestic dogs and cats Direct contact with urine, soil, water Psittacosis (parrot fever) Chlamydia psittaci Birds, especially parrots Direct contact, bird droppings Salmonellosis Salmonella spp. Poultry, rats, reptiles Ingestion of contaminated food and water Fungal Ringworm

Trichophyton spp.

Microsporum spp.

Epidermophyton spp.

Domestic animals Direct contact Protozoan Malaria Plasmodium spp. Monkeys Bite of Anopheles mosquito Toxoplasmosis Toxoplasma gondii Cats and other animals Ingestion of contaminated meat, inhalation of pathogen, direct contact Helminthic Tapeworm infestation Dipylidium caninum Dogs Ingestion of larvae transmitted in dog saliva Fasciola infestation Fasciola hepatica Sheep, cattle Ingestion of contaminated meat Trichinellosis Trichinella spiralis Pigs, bears Ingestion of undercooked contaminated meat

Contact Transmission

In contact transmission the pathogen is spread from one host to another either by direct contact, indirect contact, or respiratory droplets.

Vehicle Transmission

Any transmission of pathogens that require some sort of medium is considered a vehicle transmission. Modes of transmission in this category include airborne, waterborne, and foodborne transmissions, as well as transmission through bodily fluids and intravenous fluids. Foodborne, waterborne, and airborne diseases are also shown in the Healthcare Applications tables of Chapter 1 (Scope of Microbiology).

Vector Transmission

Vectors are animals, usually arthropods, that carry pathogens from one host to another. Vectors can transmit the pathogens either biologically or mechanically (Box 9.2).

Types of Nosocomial Infections

Healthcare facilities, by their very nature, are filled with sick people capable of shedding a variety of pathogens from different portals of exit. Furthermore, people with infections and/or diseases have a stressed, weakened, and often a compromised/suppressed immune system, making them even more susceptible to any infectious or opportunistic agents. Healthcare-associated infections can be categorized as exogenous, endogenous, and iatrogenic infections.

Nosocomial infections can be caused by a variety of organisms (Table 9.8), some of which cause specific infections and disease; others have the capacity to cause a variety of infections, depending on their portal of entry. Bacteria that gain entrance to the bloodstream have the chance to settle at a site different from the site of entry if not treated promptly. Septicemia is less common, but can occur when surgery is performed on an infected area or an area where bacteria normally grow (i.e., intestine).

Transmission

Because of the large number and variety of pathogens in the healthcare environment and the compromised state of the patients, the routes and chain of transmission constitute an ongoing concern for healthcare providers and their patients. The principal routes of concern are as follows:

Therefore certain hospital areas are reserved for specialized care, including burn, hemodialysis, recovery, intensive care, and oncology units. Unfortunately, these environments also provide a risk for an epidemic spread of nosocomial infections from patient to patient.

Diagnostic and certain therapeutic procedures provide a possible route of transmission through fomites. For example, the urinary catheter, commonly used to drain the urinary bladder, is a fomite in many nosocomial infections. Intravenous catheters, used for the delivery of fluids, nutrients, or medication, need to penetrate the skin and can be a potential fomite for infection. Other potential fomites are the various components of respiratory equipment, needles, and surgical dressings. All these potential ways of transmission are the reason for the use of many control measures aimed at the prevention of nosocomial infections in the healthcare environment.

Antimicrobial Resistance in Healthcare Settings

Drug-resistant pathogens are becoming an increasing threat to the population (see Chapter 18, Emerging Infectious Diseases), particularly in healthcare settings. Of all patients with nosocomial infections in the United States, 99,000 annually die as a result of their infection. More than 70% of the bacteria causing these infections are resistant to at least one of the drugs commonly used to treat them. Persons infected with drug-resistant organisms generally have longer hospital stays and require second- or third-choice drugs that may be less effective and more toxic.

Control and Prevention

Each healthcare facility has a safety program, policies, and procedures (see Chapter 5, Safety Issues) that will include procedures and precautions designed to reduce the incidence of HAIs. These procedures are aimed at preventing the transmission of pathogens to patients and healthcare personnel. In general, these measures include disinfection, medical asepsis (good housekeeping, hand washing, bathing, sanitary food handling, proper hygiene, measures to prevent the spread of pathogens between patients), surgical asepsis, and isolation of contagious and particularly susceptible patients.

Any source of agents that can cause nosocomial infections must be kept clean and disinfected, including items such as humidifiers, which provide both a suitable environment for bacteria to grow and a method of airborne transmission. Furthermore, according to the CDC hand washing is the single most important means of preventing the spread of infection. It has been shown in several studies that effective hand washing by all medical and support staff significantly reduced (by 50%) deaths from nosocomial infections when the hospital personnel followed strict guidelines about washing their hands frequently. Unfortunately, it has been reported by the CDC that on average healthcare workers wash their hands only 40% of the time before interacting with patients.

All accredited hospitals have an infection control committee and most hospitals have at least one infection control nurse, epidemiologist, or specialized microbiologist. The role of these individuals is to identify problem sources such as improper adherence to hospital guidelines for disease prevention and control, and the occurrence of antibiotic-resistant strains of bacteria.

The CDC has published guidelines and recommendations for the prevention of healthcare-associated infections, including guidelines for the protection of patients, guidelines for the protection of healthcare workers, and other guidelines and recommendations for other topics dealing with infection control. All these guidelines are available at the CDC website.