THE HOST’S PERSPECTIVE

 Once a microbe and the human host are brought into contact, the outcome of the encounter depends on what happens during each step of interaction (see Figure 1), beginning with colonization. The human host’s role in microbial colonization, defined as the persistent survival of microorganisms on a surface of the human body, is dictated by the defenses that protect vital internal tissues and organs against microbial invasion. The first defenses are the external and internal body surfaces that are in direct contact with the external environment and are the anatomical regions where the microorganisms will initially come in contact with the human host. These sur faces include:

 • Skin (including conjunctival epithelium covering the eye)

• Mucous membranes lining the mouth or oral cavity, the respiratory tract, the gastrointestinal tract, and the genitourinary tract

Fig1. General stages of microbial-host interaction. 

Because body surfaces are always present and provide protection against all microorganisms, skin and mucous membranes are considered constant and nonspecific protective mechanisms. As is discussed later in this text, other protective mechanisms are produced in response to the presence of microbial agents (inducible defenses), and some are directed specifically at particular microorganisms or (specific defense mechanisms).

Skin and Skin Structures

Skin serves as a physical and chemical barrier to microorganisms; its protective characteristics are summarized in Table 1 and Figure 2. The acellular, outermost layer of the skin, along with the tightly packed cellular layers underneath, provide an impenetrable physical barrier to all microorganisms, unless damaged. Addition ally, these layers continuously shed, thus dislodging bacteria that have attached to the outer layers. The skin is also a dry and cool environment; this is incompatible with the growth requirements of many microorganisms, which thrive in a warm, moist environment.

Table1.  Protective Characteristics of the Skin  and Skin Structures

Fig2.  Skin and skin structures. 

The follicles and glands of the skin produce various natural antibacterial substances, including sebum and sweat. However, many microorganisms can survive the conditions of the skin. These bacteria are known as skin colonizers, and they often produce substances that may be toxic and inhibit the growth of more harmful microbial agents. Beneath the outer layers of skin are various host cells that protect against organisms that breach the surface barriers. These cells, collectively known as skin associated lymphoid tissue, mediate specific and nonspecific responses directed at controlling microbial invaders.

Mucous Membranes

Because cells that line the respiratory tract, gastrointestinal tract, and genitourinary tract are involved in numerous functions besides protection, they are not covered with a hardened, acellular layer as is the skin surface. However, the cells that compose these membranes still exhibit various protective characteristics (Table 2 and Figure 3).

Table2. Protective Characteristics of Mucous Membranes

Fig3. General features of mucous membranes highlighting  protective features such as ciliated cells, mucus production, tight  intercellular junctions, and cell sloughing. 

General Protective Characteristics. Mucus is a major protective component of the membranes. This substance serves to trap bacteria before they can reach the outer surface of the cells, lubricates the cells to prevent damage that promotes microbial invasion, and contains specific chemical (i.e., antibodies) and nonspecific antibacterial substances. In addition to the chemical properties and physical movement of the mucus and trapped microorganisms mediated by ciliary action, rapid cellular shed ding and tight intercellular connections provide effective barriers to infection. As is the case with the skin, specific cell clusters, known as mucosa-associated lymphoid tissue, exist below the outer cell layer and mediate specific protective mechanisms against microbial invasion.

Specific Protective Characteristics. Besides the general protective properties of mucosal cells, the mucosal linings throughout the body have characteristics specific to each anatomic site (Figure 4).

Fig4. Protective characteristics associated with the mucosal linings of different internal body surfaces. 

The mouth, or oral cavity, is protected by the flow of saliva that physically carries microorganisms away from cell surfaces and also contains antibacterial substances, such as antibodies (IgA) and lysozyme that participate in the destruction of bacterial cells. The mouth is also heavily colonized with protective microorganisms that produce substances that hinder successful invasion by harmful organisms.

In the gastrointestinal tract, the low pH and proteolytic (protein-digesting) enzymes of the stomach prevent the growth of many microorganisms. In the small intestine, protection is provided through the presence of bile salts, which disrupt bacterial membranes, and by peristaltic movement and the fast flow of intestinal contents, which hinder microbial attachment to mucosal cells. Although the large intestine also contains bile salts, the movement of bowel contents is slower, permitting a higher concentration of microbial agents the opportunity to attach to the mucosal cells and inhabit the gastrointestinal tract. As in the oral cavity, the high concentration of normal microbial inhabitants in the large bowel also contributes significantly to protection.

In the upper respiratory tract, nasal hairs keep out large airborne particles that may contain microorganisms. The cough-sneeze reflex significantly contributes to the removal of potentially infective agents. The cells lining the trachea contain cilia (hairlike cellular projections) that move microorganisms trapped in mucus upward and away from the delicate cells of the lungs (see Figure 3); this is referred to as the mucociliary escalator. These barriers are so effective that only inhalation of particles smaller than 2 to 3 µm have a chance of reaching the lungs.

In the female urogenital tract, the vaginal lining and the cervix are protected by heavy colonization with normal microbial inhabitants and a low pH. A thick mucus plug in the cervical opening is a substantial barrier that keeps microorganisms from ascending and invading the more delicate tissues of the uterus, fallopian tubes, and ovaries. The anterior urethra of males and females is naturally colonized with microorganisms, and a stricture at the urethral opening provides a physical barrier that, combined with a low urine pH and the flushing action of urination, protects against bacterial invasion of the bladder, ureters, and kidneys.

THE MICROORGANISM’S PERSPECTIVE

 As previously discussed, microorganisms that inhabit many surfaces of the human body (see Figure 4) are referred to as colonizers, or normal flora (also referred to as normal microbiota). Some are transient colonizers, because they are able to survive, but do not multiply, on the surface and are frequently shed with the host cells. Others, called resident flora, not only survive but also thrive and multiply; their presence is more persistent.

The body’s normal flora varies considerably with anatomic location. For example, environmental conditions, such as temperature and oxygen availability, differ considerably between the nasal cavity and the small bowel. Only microorganisms with the metabolic capability to survive under the physiologic conditions of the anatomic location are inhabitants of those particular body surfaces.

Knowledge of the normal flora of the human body is extremely important in diagnostic microbiology, especially for determining the clinical significance of microorganisms isolated from patient specimens. Organ isms considered normal flora are frequently found in clinical specimens. This may be a result of contamination of normally sterile specimens during the collection process or because the colonizing organism is actually involved in the infection. Microorganisms considered as normal colonizers of the human body and the anatomic locations they colonize are addressed in Part VII.

Microbial Colonization

Colonization may be the last step in the establishment of a long-lasting, mutually beneficial (i.e., commensal), or harmless, relationship between a colonizer and the human host. Alternatively, colonization may be the first step in the process for the development of infection and disease. Whether colonization results in a harmless or damaging infection depends on the characteristics of the host and the microorganism. In either case, successful initial colonization depends on the microorganism’s ability to survive the conditions first encountered on the host surface (Box 1).

Box1. Microbial Factors Contributing to Colonization  of Host Surfaces

To avoid the dryness of the skin, organisms often seek moist areas of the body, including hair follicles, sebaceous (oil, referred to as sebum) and sweat glands, skin folds, underarms, the genitals or anus, the face, the scalp, and areas around the mouth. Microbial penetration of mucosal surfaces is mediated by the organism becoming embedded in food particles to survive oral and gastrointestinal conditions or contained within airborne particles to aid survival in the respiratory tract. Microorganisms also exhibit metabolic capabilities that assist in their survival. For example, the ability of staphylococci to thrive in relatively high salt concentrations enhances their survival in and among the sweat glands of the skin.

Besides surviving the host’s physical and chemical conditions, colonization also requires that microorganisms attach and adhere to host surfaces (see Box 1). This can be particularly challenging in places such as the mouth and bowel, in which the surfaces are frequently washed with passing fluids. Pili, the rodlike projections of bacterial envelopes, various molecules (e.g., adherence proteins and adhesins), and biochemical complexes (e.g., biofilm) work together to enhance attachment of microorganisms to the host cell surface.

In addition, microbial motility with flagella allows organisms to move around and actively seek optimum conditions. Finally, because no single microbial species is a lone colonizer, successful colonization also requires that a microorganism be able to coexist with other microorganisms.

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مواضيع ذات صلة

Microorganism Entry, Invasion, and Dissemination: The Host’s Perspective

The Encounter Between Host and Microorganism

Systemic Approach to Anemia

Comparison of Etiologies of Anemia in Adults and Children

Mechanisms of Anemia

Peptic Ulcer Disease

Nosology of Hemoglobinopathies

Infections of the Urinary Tract: Types of Infection and Their Clinical Manifestations

Toxic Goitre

Infections of the Urinary Tract : Pathogenesis

Infections of the Urinary Tract : Etiologic Agents

Infections of the Sinuses