Medical Microbiology Lecture 6 PDF

Medical Microbiology: Lecture-6 3-Clostridia Clostridia are large gram-positive rods, the obligate anaerobic, blunt-ended rods. They form endospores; the position of the developing spore within the vegetative cell is useful in identifying the species. Most species are motile. These organisms also s...

Medical Microbiology: Lecture-6 3-Clostridia Clostridia are large gram-positive rods, the obligate anaerobic, blunt-ended rods. They form endospores; the position of the developing spore within the vegetative cell is useful in identifying the species. Most species are motile. These organisms also synthesize some of the most potent exotoxins. Clinically significant species of Clostridium include C. perfringens, which causes (gas gangrene) and food poisoning; C. difficile, which causes pseudomembranous colitis associated with antibiotic use; C. tetani, which causes tetanus and C. botulinum, which causes botulism. A. Clostridium perfringens It is ubiquitous in nature, with its vegetative form as part of the normal flora of the vagina and gastrointestinal (GI) tract. Its spores are found in soil. C. perfringens can cause anaerobic cellulitis, and myonecrosis (gas gangrene). Some strains cause a common form of food poisoning. Pathogenesis: C. perfringens secretes a variety of exotoxins, enterotoxins, and hydrolytic enzymes that facilitate the disease process. 1. Exotoxins: C. perfringens elaborates at least 12 exotoxins. The most important of these required for virulence in tissue, is alpha toxin. Alpha toxin is a lecithinase that degrades lecithin in mammalian cell membranes, causing lysis of endothelial cells, as well as RBC, WBC and platelets. 2. Enterotoxin: a small protein, acts in the lower portion of the small intestine, and causes loss of fluid and intracellular proteins. 3. Degradative enzymes: it is produces of hydrolytic enzymes, including proteases, DNases, hyaluronidase, and collagenases, which liquefy tissue and promote the spread of infection. Clinical significance The disease processes initiated by C. perfringens result from a combination of infection with the production of exotoxins and/or enterotoxins and degradative enzymes. 1. Myonecrosis (gas gangrene): Clostridial spores are introduced into tissue, for example, by contamination with infected soil, or by endogenous transfer from the intestinal tract. Severe and open wounds are a prime predisposing condition. Alpha toxin and other exotoxins are secreted and extensive cell killing ensues. Production of enzymes that break down ground substance facilitates the spread of infection. Fermentation of tissue carbohydrates yields gas, and an accumulation of gas bubbles in the subcutaneous spaces produces a crinkling sensation on palpation; hence, the name gas gangrene. The exudates are copious and foul smelling. As the disease progresses, increased capillary permeability leads to the exotoxins being carried by the circulation from 1  damaged tissue to other organs, resulting in systemic effects such as shock, renal failure, and intravascular hemolysis. Untreated clostridial myonecrosis is uniformly fatal within days of the initiation of gangrene. 2. Anaerobic cellulitis: This is a clostridial infection of connective tissue (Necrotizing processes). 3. Food poisoning: C. perfringens is a common cause of food poisoning. The symptoms; nausea, abdominal cramps, diarrhea occurs 8 to 18 hr. after eating contaminated food. Laboratory identification: When cultured its anaerobically on blood agar, C. perfringens grows rapidly, producing colonies with double zone of hemolysis, large gram-positive rods with endospors. Treatment and prevention: The key to both prevention and treatment of gas gangrene is immediate and thorough removal of foreign material and dead tissue, and exposure of the wound to O 2. Hyperbaric oxygen chambers increase the tissue O2 tension in the affected part and probably inhibit the pathologic process. This bacterium is sensitive to penicillin. B. Clostridium botulinum C. botulinum causes botulism, which occurs in several clinical forms. Botulism is caused by the action of a neurotoxin that is one of the most potent poisons known. It causes a flaccid paralysis. Pathogenesis: There are several types of botulinum toxin, designated A through G, but human disease is almost always caused by types A, B, or E. The botulinum serotypes and tetanus toxin constitute a homologous set of proteins whose neurotoxicity arises from proteolytic cleavage of specific synaptic vesicle peptides, causing subsequent failure of neurotransmission. In contrast to tetanus toxin, which causes constant contraction, botulinum toxins affect peripheral cholinergic synapses by blocking the neuromuscular junction and inhibiting release of the neurotransmitter, acetylcholine, preventing contraction and causing flaccid paralysis. Clinical significance: 1. Infant botulism: or a cause of floppy baby syndrome. C. botulinum colonizes the large bowel of infants 3 to 24 weeks of age; the toxin produced is slowly absorbed. Constipation, feeding problems, lethargy, and poor muscle tone are common early signs. The condition is possibly a cause of sudden infant death syndrome, but recovery is the usual outcome, following symptomatic treatment that may be prolonged. 1. Wound botulism: A rare form of botulism occurs when a wound becomes contaminated with the organism, and toxin is absorbed from that site. Laboratory identification: The organism can be cultured and identified by standard anaerobic. 2 Treatment and prevention: Antitoxin, which neutralizes unbound botulinum toxin, C. Clostridium tetani The introduction of tetani spores into even small wounds via contaminated soil is probably a common occurrence. But a combination of the extreme O2 sensitivity of vegetative C. tetani and widespread immunization against its exotoxin, make the resulting disease, tetanus, rare in developed countries. Growth of C. tetani is completely local, but it produces a powerful neurotoxin that is transported to the central nervous system, where it causes spastic paralysis. Pathogenesis: Tetanus toxin, called tetanospasmin, is an extremely potent toxin. It is transported from an infected locus by retrograde neuronal flow or blood. The heavy fragment (B) mediates binding to neurons and cell penetration of the light fragment (A). The A fragment blocks neurotransmitter release at inhibitory synapses, and thus causes severe, prolonged muscle spasms. The A fragment has been shown to be a protease; it cleaves a small synaptic vesicle protein (synaptobrevin) and abolishes the flow of inhibitory neurotransmitters. Clinical significance: Tetanus has an incubation period varying from four days to several weeks. A shorter period is usually associated with more severe disease and wounds closer to the brain. Tetanus presents as a spastic paralysis, in which muscle spasms often first involve the site of infection. In the early stages of the disease, the jaw muscles are affected, so that the mouth cannot open (trismus). Gradually, other voluntary muscles become involved, and any external stimulus (for example, noise or bright light) precipitates a painful spasm, and sometimes convulsions. Death, which occurs in fifteen to sixty percent of cases, is usually the result of paralysis of chest muscles leading to respiratory failure. Laboratory identification: is rod with terminal spore (drum stick), and characteristic swarming growth on anaerobic blood agar. Treatment: Treatment with human hyperimmune globulin (tetanus immune globulin). The organism is sensitive to penicillin. Prevention: Active immunization with tetanus toxoid prevents tetanus. It is usually administered to children as a triple vaccine with diphtheria toxoid and pertussis antigens (DPT). 3 4-Corynebacterium diphtheriae Corynebacteria are small, slender, pleomorphic, gram-positive rods& Chinese letters. They are nonmotile, unencapsulated, and do not form spores, containing chromatin granules called volutin granules or Babes- Ernest granules present in cytoplasm and aggregated in the poles of the cells. Pathogenesis: Diphtheria is caused by the local and systemic effects of a single exotoxin that inhibits eukaryotic protein synthesis. Diphtheria, caused by C. diphtheriae, is an acute respiratory or cutaneous disease and may be a life-threatening illness, diphtheria is a serious disease throughout the world, particularly in those countries where the population has not been immunized. Clinical significance Infection may result in clinical disease which has two forms respiratory and cutaneous or in an asymptomatic carrier state. 1. Upper respiratory tract infection: Diphtheria consists of a strictly local infection, usually of the throat. The infection produces a distinctive thick, grayish, adherent exudate (pseudomembrane) that is composed of cell debris from the mucosa and inflammatory products 2. Cutaneous diphtheria: A puncture wound or cut in the skin can result in introduction of C. diphtheriae into the subcutaneous tissue, leading to a chronic, non healing ulcer with a gray membrane. Laboratory identification: gram-positive rods& Chinese letters. They are containing chromatin granules called volutin granules or present in cytoplasm and aggregated in the poles of the cells (staining by Albert stain). Most species are facultative anaerobes; grow aerobically on standard laboratory media such as Chocolate & blood agar. Prevention: The cornerstone of diphtheria prevention is immunization with toxoid, usually administered in the DPT triple vaccine, together with tetanus toxoid and pertussis antigens. The initial series of injections should be started in infancy. Booster injections of diphtheria toxoid (with tetanus toxoid) should be given at approximately ten-year intervals throughout life. The control of an epidemic outbreak of diphtheria involves rigorous immunization and a search for healthy carriers among patient contacts. 5-Bacillus anthracis 4 It is gram-positive rods, nonmotile, encapsulated, spores forming, and facultative aerobes. Pathogenesis: B. anthracis possesses a capsule that is antiphagocytic and is essential for full virulence. The organism also produces three exotoxins; edema factor is responsible for the severe edema usually seen in B. anthracis infections; lethal toxin is responsible for tissue necrosis; protective antigen mediates cell entry of edema factor and lethal toxin. Clinical significance 1. Cutaneous anthrax: About 95% of human cases of anthrax are cutaneous. Upon introduction of organisms or spores that germinate, a papule develops. It rapidly evolves into a painless, black, severely swollen malignant pustule, which eventually crusts over. The organisms may invade regional lymph nodes and then the general circulation, leading to fatal septicemia. Although some cases remain localized and heal, the overall mortality in untreated cutaneous anthrax is 20%. 2. Pulmonary anthrax (woolsorter's disease) is caused by inhalation of spores. It is characterized by progressive hemorrhagic lymphadenitis (inflammation of the lymph nodes), and has a mortality rate approaching 100 percent if left untreated. Laboratory identification: B. anthracis is easily recovered from clinical materials. Microscopically, the organisms appear as blunt-ended bacilli that occur singly, in pairs, or frequently in long chains. They do not sporulate often in clinical samples, but do so in culture. The spores are oval and centrally located. On blood agar, the colonies are large, grayish, and nonhemolytic, with an irregular border. Treatment: Cutaneous anthrax responds to doxcycline, ciprofloxacin, or erythromycin. Multidrug therapy (ciprofloxacin + rifampin + vancomycin) is recommended for inhalation anthrax. 5