• The normal microbial flora serves both as a defense mechanism against infection, and is also a source of potential pathogens. The total normal flora of the body probably contains more than 100 distinct species of microorganisms.
• Beneficial effects of normal flora:
• (1) immunologic priming
• (2) protective or exclusionary effects on potential pathogens
• (3) production of essential nutrients (Vit K., Vit. B.)
• Definitions:
• Carrier State – organism present in host without causing any apparent effects. Temporary or "permanent".
• Infection – multiplication of an organism in host with the recognition of some resulting change in the host, without necessarily any clinically apparent disease (ex: positive TB skin test without symptoms).
• Disease – interaction between organism and host resulting in change or damage to the host, as manifested by symptoms, lab abnormalities or gross or histologic damage.
• Interactions amongst normal flora:
• Neutral
• Synergistic
• Antagonistic

Anaerobic Infections – all arise endogenously except for tetanus and botulism

• General Description: Anaerobic bacteria are part of the normal flora of the skin and are prevalent on all mucous membranes. The body’s normal Eh (+120 mV) defends against infection by anaerobes. Lowered redox potential permits anaerobe multiplication and prevents efficient function of granulocytes. The lowering results from ischemia, tissue necrosis, and growth of facultative bacteria in a wound. Therefore vascular disease, cold, shock, edema, trauma, etc., predispose to anaerobic infection. Since most anaerobes in body reside on mucosal surfaces infections also usually arise at or near mucosal surfaces.
• What makes normal flora go bad? Flora becomes pathogenic when their balance is upset (Ý stress, nutritional probs, etc.).
• Features of anaerobic infections: foul-smelling discharge, infection proximal to mucosal surface, gas in tissues, extensive tissue necrosis, abscess formation, absence of growth in routine culture, unique morphology in gram stain.
• Diagnosis: is difficult due to anaerobic nature of organisms. Drainage of abscesses and excision of necrotic tissue are important therapeutic considerations.
• Treatment: includes use of antimicrobial agents (clindamycin, chloramphenicol, cephalosporins, and metronidazole). Surgical removal of infection is vital. Antitoxin administration (if toxin release is involved). Steroids. Immunosuppressives.
• Disease Examples:
• Ludwig’s Phlegmon -- polymicrobial infection in mandible. Surgical drainage of pus necessary. (dental infection)
• Lung Abscess – organisms are normal flora which get aspired into lung. Cause inflammation. (aspiration pneumonia)
• Diabetic Foot Ulcer – mixed infection result in ulcer. See gas bubbles on xray. (gas gangrene)
• Bacteroides fragilis – most prevalent anaerobe recovered in mixed infections
• Disease: Sepsis, peritonitis, and abdominal abscess.
• Characteristics: Anaerobic, gram-negative rods.
• Habitat/Transmission: Predominant anaerobe in human colon. Transmission via spread from colon rupture to blood or peritoneum.
• Pathogenesis: Polysaccharide capsule is capable of inducing abscess formation and is antiphagocytic. Superoxide dismutase allows bug to survive O₂ exposure until the environment becomes hospitable to anaerobic infection. Predisposing factors to infection include bowel surgery and penetrating abdominal wounds.
• Prevention: Cefoxitin administration before bowel surgery reduces postoperative infections. No vaccine available.
• Treatment: Metronidazole; only drug which is bacteriocidal for this bug.

• These are infections by more than one independent pathogen = Synergistic Infection.
• Examples:
• Bacteroides melaninogenicus (oral flora) and an a -hemolytic streptococcus result in a spreading, necrotizing abscess. No abscess seen when present alone.
• Mixture of fusobacteria, spirochetes, streptococci, and vibrios (oral anaerobes) cause angina, trench mouth, and putrid lung abscess.
• Bacteroides asaccharolyticus in presence of succinate producing Klebsiella pneumoniae result in infection.
• Bacteroides fragilis which produce penicillinase in presence of Fusobacterium necrophorum are penicillin resistant.
• Staph. Aureus and microaerophilic streptococcus cause Meleney’s synergistic gangrene of skin.