Medical Bacteriology MBIO 460 Lecture 8 Dr. Turki Dawoud 2 nd Semester 1436/1437 H

Normal Microbial Flora of Other Body Regions  Each individual mucous membrane supports the growth of a specialized group of microorganisms.  These organisms are part of the normal local environment and are characteristic of healthy tissue.  In many cases, pathogenic microorganisms cannot colonize mucous membranes because of the competitive effects of the normal flora.  Here we discuss two mucosal environments and their residential microorganisms.

Respiratory Tract  The anatomy of the respiratory tract is shown in Figure In the upper respiratory tract (nasopharynx, oral cavity, larynx, and pharynx), microorganisms live in areas bathed with the secretions of the mucous membranes.  Bacteria continually enter the upper respiratory tract from the air during breathing, but most are trapped in the nasal passages and expelled with the nasal secretions.  A restricted group of microorganisms, however, colonizes respiratory mucosal surfaces in all individuals. Figure The respiratory tract. In healthy individuals the upper respiratory tract has a large variety and number of microorganisms. By contrast, the lower respiratory tract in a healthy person has few if any microorganisms

 The microorganisms most commonly found are staphylococci, streptococci, diphtheroid bacilli, and gram-negative cocci.  Even potential pathogens such as Staphylococcus aureus and Streptococcus pneumoniae are often part of the normal flora of the nasopharynx of healthy individuals (Table 28.1).  These individuals are carriers of the pathogens but do not normally develop disease, presumably because the other resident microorganisms compete successfully for resources and limit pathogen activities.  The innate immune system and components of the adaptive immune system such as IgA are particularly active at mucosal surfaces and may also inhibit the growth of pathogens.

 The lower respiratory tract (trachea, bronchi, and lungs) has no resident microflora in healthy adults, despite the large number of organisms potentially able to reach this region during breathing.  Dust particles, which are fairly large, settle out in the upper respiratory tract.  As the air passes into the lower respiratory tract, the flow rate decreases markedly, and organisms settle onto the walls of the passages.  The walls of the entire respiratory tract are lined with ciliated epithelium, and the cilia, beating upward, push bacteria and other particulate matter toward the upper respiratory tract where they are then expelled in the saliva and nasal secretions.  Only particles smaller than about 10 µm in diameter reach the lungs. Nevertheless, some pathogens can reach these locations and cause disease.

Urogenital Tract  In the male and female urogenital tracts (Figure 28.11), the bladder itself is typically sterile, but the epithelial cells lining the downstream urethra are colonized by facultatively aerobic gram- negative rods and cocci (Table 28.1).  Potential pathogens such as Escherichia coli and Proteus mirabilis, normally present in small numbers in the body or local environment, can multiply in the urethra and become pathogenic under altered conditions such as changes in pH. Such organisms are a frequent cause of urinary tract infections, especially in women Figure Microbial growth in the genitourinary tract. (a) The genitourinary tracts of the human female and male, showing regions (red) where microorganisms often grow. The upper regions of the genitourinary tracts of both males and females are sterile in healthy individuals. (b) Gram stain of Lactobacillus acidophilus, the predominant organism in the vagina of women between the onset of puberty and the end of menopause. Individual rod-shaped cells are 3–4 µm long.

 The vagina of the adult female is weakly acidic and contains significant amounts of glycogen. Lactobacillus acidophilus, a resident organism in the vagina, ferments the polysaccharide glycogen, producing lactic acid that maintains a local acidic environment (Figure 28.11b).  Other organisms, such as yeasts (Torulopsis and Candida species), streptococci, and E. coli, may also be present.  Before teens, the female vagina is alkaline and does not produce glycogen, L. aci-dophilus is absent, and the flora consists predominantly of staphylococci, streptococci, diphtheroids, and E. coli.  After menopause, glycogen production come to an end, the pH rises, and the flora again resembles that found before youth.

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