Mucous Membrane Protective Activity Structures

Mucosal cells Rapid sloughing for bacterial removal Tight intercellular junctions prevent bacterial penetration Goblet cells Mucus production:

Protective lubrication of cells Bacterial trapping Contains specific antibodies with specific activity against bacteria Provision of antibacterial substances to mucosal surface; Lysozyme: degrades bacterial cell wall

Lactoferrin: competes for bacterial iron supply Lactoperoxidase; production of substances toxic to bacteria Mucosa-associated Mediates specific responses against lymphoid tissue bacteria that penetrate outer layer found on the skin. However, the cells that compose these membranes still exhibit various protective characteristics (Box 3-2 and Figure 3-4).

General Protective Characteristics. A major protective component of mucous membranes is the mucus itself. This substance serves to trap bacteria before they can reach the outer surface of the cells, lubricates the cells to prevent damage that may promote microbial invasion, and contains numerous specific (i.e„ antibodies) and nonspecific antibacterial substances. In addition to mucous activity and flow mediated by cilia action, rapid cellular shedding and tight intercellular connections provide effective barriers. As is the case with 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 linings of the different body tracts have other characteristics specific to each anatomic site (Figure 3-5).

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 lysozyme, which destroys bacterial cell walls, and antibodies. The mouth is also heavily colonized with microorganisms that contribute to protection by producing substances that hinder successful invasion by harmful agents.

In the gastrointestinal tract, the low pH and proteolytic (protein-destroying) enzymes of the stomach help keep the numbers of microorganisms low. In the small intestine, protection is provided by the presence of bile salts that disrupt bacterial membranes and by the fast flow of intestinal contents that hinders microbial attachment to mucosal cells. Although the large intestine also produces bile salts, the movement of bowel contents is slower so that a higher concentration of microbial agents are able to attach to the mucosal cells and inhabit this portion of 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 also significantly contributes to the removal of potentially infective agents. The cells lining the trachea contain cilia (hairlike cellular projections) that move mucus and the microorganisms trapped within upward and away from delicate cells of the lungs (see Figure 3-4). These barriers are so effective that only, inhalation of particles smaller than 2 to 3 |*m have a chance of reaching the lungs.

In the female urogenital tract, the vaginal lining and the ectocervix are protected by heavy colonization with normal microbial inhabitants and a low pH, A thick mucous 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

Mouth-

Sloughing cells Flow of saliva Lysozyme Resident microflora

Stomach-

LowpH

Proteolytic enzymes

Lungs -

Macrophages

Nasopharynx

Resident microflora Secretions (lysozyme, phagocytes) Ciliated cells

Vagina LowpH Resident microflora

Colon

Slow flow

Mucus, sloughing cells Abundant resident microflora Bile salts

Figure 3-5 Protective characteristics associated with the mucosal linings of different internal body surfaces.

Urethra -

Urine flow

Bladder

Flushing action of urine LowpH

Physical barrier of urethra

Mouth-

Sloughing cells Flow of saliva Lysozyme Resident microflora

Nasopharynx

Resident microflora Secretions (lysozyme, phagocytes) Ciliated cells

Lungs -

Macrophages

Small intestine Fast flow Mucus

Sloughing cells Bile salts

Colon

Slow flow

Mucus, sloughing cells Abundant resident microflora Bile salts

Vagina LowpH Resident microflora

Bladder

Flushing action of urine LowpH

Physical barrier of urethra

Urethra -

Urine flow

Figure 3-5 Protective characteristics associated with the mucosal linings of different internal body surfaces.

Stomach-

LowpH

Proteolytic enzymes

□ High concentration of resident microflora urethra of males and females is naturally colonized with microorganisms, but stricture at the urethral opening provides a physical barrier that, combined with 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 3-5) are referred to as colonizers, or normal flora. Some are transient colonizers because they merely 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 permanent.

The normal flora of body surfaces vary considerably with anatomic location. For example, environmental conditions, such as temperature and oxygen availability, vary considerably between the nasal cavity and small bowel. Only microorganisms with the metabolic capacity to survive under the circumstances that each anatomic location offers will be 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 that are isolated from patient specimens. Normal flora organisms frequently are found in clinical specimens. This is a result of contamination of normally sterile specimens during collection 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.ev commensal), or harmless, relationship between a colonizer and the human host. Alternatively, colonization may be the first step in the process of developing infection and disease. Whether colonization results in a harmless or harmful situation depends on thé host and microorganism characteristics. In either case, successful initial colonization depends on the microorganism's ability to survive the conditions first encountered on the host surface (Box 3-3).

BOX 3-3 Microbial Activities Contributing to Colonization of Host Surfaces

SURVIVAL AGAINST ENVIRONMENTAL CONDrTIONS: Localization in moist areas Protection within Ingested or inhaled debris Expression of specific metabolic characteristics (e.g., salt tolerance)

ATTACHMENT AND ADHERENCE TO HOST CELL SURFACES: Pili

Adherence proteins

Biofilms

Various protein adhesins MOTILITY

PRODUCTION OF SUBSTANCES THAT COMPEIE WITH HOST FOR ACQUISITION OF ESSENTIAL NUTRIENTS (E.G., SIDEOPHORES FOR CAPTURE OF IRON)

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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