Microbiology of the Mouth and Respiratory Tract

Mouth and Respiratory Tract

• Viridans streptococci are implicated in the parthenogenesis of dental carries
• Gingival crevices and tonsillar crypts are primarily inhabited by anaerobic flora (e.g., Bacteroides and Spirochetes)
• Normal flora of the pharynx and trachea are similar to those found in the oral cavity
• Transient carriage of potentially pathogenic organisms can occur in the pharynx, including Haemophilus influenza, Streptococcus pneumonia, Neisseria meningitides, and Mycoplasma
• Initial colonization by pathogenic organisms may occur in the upper respiratory tract, including Neisseria meningitides, Corynebacterium diphtheria, and Bordetella pertussis

Conjunctiva

• Normal flora in the conjunctiva are very scanty due to the flow of tears containing lysozyme
• Transient colonization of the conjunctiva can occur by Neisseria, Moraxella, and Corynebacterium
• Staphylococci and Streptococci may also be present

Digestive Tract

• Helicobacter pylori is the most common cause of duodenal ulcers
• The small intestine has scanty flora due to constant peristaltic movement
• Most bacteria cultured in the small intestine are transient, including Streptococci, Lactobacilli, and Bacteroides
• The colon is inhabited predominantly by anaerobes (95%-99%), including Bacteroides flagilis, Bifidobacterium/Lactobacillus bifidum, Eubacterium, Peptostreptococcus, and Clostridium
• In bottle-fed infants, the predominant intestinal flora is Lactobacillus acidophilus
• 1%-4% of the flora of the colon are facultative aerobes, predominantly Escherichia coli and Enterobacteriaceae

Functions of Intestinal Flora

• Synthesis of Vitamin B complex and vitamin K
• Conversion of bile into bile acids
• Competition with transient flora for nutrients
• Prevention of colonization of the intestines by transient flora
• Production of potentially pathogenic by-products of metabolism that are toxic to transient flora

Genitourinary Tract

• The urinary system is considered sterile at about 1 cm of the urethra
• In the anterior urethra, flora include S. epidermidis, enterococci, and diptheroids
• Mycobacterium smegmatis may be found as normal commensals in male and female secretions
• Gardnerella vaginalis, Bacteroides, and alpha streptococci may be found in the penile urethra
• The female urethra is sterile or contains Staphylococcus epidermidis
• Vaginal flora varies depending on age, hormonal levels, and vaginal pH
• Lactobacillus spp. are the predominant flora in female infants
• From 1 month to puberty, microorganisms that may inhabit the vagina include Staphylococcus epidermidis, Streptococci, diphtheroids, and Escherichia coli
• Onset of puberty, predominant flora include Lactobacillus acidophilus, Corynebacterial, Peptostreptococci, Streptococci, Bacteroides, and Staphylococci
• Lactobacillus plays a crucial role in preventing gonococcal infection by producing lactic acid
• Fungi such as Torulopsis and Candida may also be found (10-30%)
• Conditions that allow the overgrowth of these fungi can lead to vaginal infections such as vaginitis

Resident and Transient Flora

• Resident flora: relatively fixed types of organisms that are regularly found in a given area of the body at a given age, contributing to overall health
• Transient flora: temporarily inhabiting the skin and mucous membrane for hours, days, or weeks, derived from the environment

Other Beneficial Effects of Normal Flora

• Preventing pathogenic organisms from attaching to and penetrating the skin and other tissues by producing mucin
• Aiding in the digestion of food by producing enzymes such as cellulose, galactosidase, and glucosidase in the intestines
• Helping in the metabolism of steroids