Bacteriology Lecture 4 2024-2025 PDF

BACTERIOLOGY Lecture 4 Clinical Microbiology Gram positive organisms: A. Gram positive cocci B. Gram positive rods  I. Gram-positive organisms A. Gram-positive Cocci There are 2 medically important genera: Staphylococcus and Streptococcus. They are non-moti...

BACTERIOLOGY Lecture 4 Clinical Microbiology Gram positive organisms: A. Gram positive cocci B. Gram positive rods  I. Gram-positive organisms A. Gram-positive Cocci There are 2 medically important genera: Staphylococcus and Streptococcus. They are non-motile They do not form spores Clinical Microbiology Gram positive organisms: A. Gram positive cocci B. Gram positive rods Clinical Microbiology B. Gram positive rods  B. Spore forming Gram positive rods There are 4 medically important genera of gram-positive rods: Bacillus, Clostridium, Corynebacterium, and Listeria, Bacillus and Clostridium form spores, whereas Corynebacterium and Listeria do not. Members of the genus Bacillus are aerobic, whereas those of the genus Clostridium are anaerobic.  B. Spore forming Gram positive rods I. Spore-Forming Gram-Positive Rods BACILLUS : There are 2 medically important Bacillus species: Bacillus anthracis and Bacillus cereues.  B. Spore forming Gram positive rods 1. Bacillus anthracis Disease B anthracis causes anthrax, which is common in animals but rare in humans.  B. Spore forming Gram positive rods Important Properties: B anthracis is a large gram-positive rod with square ends, frequently found in chains. Its antiphagocytic capsule is composed of D-glutamate. (unique-capsules). It is non-motile, whereas other members of the genus are motile.  B. Spore forming Gram positive rods Transmission: Spores of the organism persist in soil for years. Humans are infected by spores on animal products such as hides, bristles, and wool or by contact with sick animals. The portals of entry are skin, mucous membranes, and respiratory tract.  B. Spore forming Gram positive rods Pathogenesis: B anthracis invades the host and produces anthrax toxin, which has 3 components: protective antigen, lethal factor, and edema factor.  B. Spore forming Gram positive rods Edema factor, an exotoxin, is an adenylate cyclase dependent on protective antigen for its binding and entry into the cell. Lethal factor in the presence of protective antigen is rapidly fatal for mice. The mode of action of lethal factor is unknown.  B. Spore forming Gram positive rods Clinical Findings: The typical lesion is a painless ulcer with a black, necrotic eschar. Local edema is striking. The lesion is called a “malignant pustule”. Untreated cases progress to bacteremia and death. Diseases 1. Cutaneous Anthrax Painless necrotic black-based ulcerating papule May complicate by septicemia " septic shock” Most common Caused by Bacillus anthracis infection of the skin 2. Gastrointestinal Anthrax Gastrointestinal necrosis and hemorrhage, abdominal pain, vomiting and bloody diarrhea may complicate by septicemia "septic shock 50% mortality if untreated. Caused by ingestion of Bacillus anthracis 3. Pulmonary Anthrax may complicate by septicemia "septic shock 100% mortality if untreated (!) caused by inhalation of Bacillus anthracis "Bacillus anthracis infection of the lungs. “Wool-sorter’s disease” (pulmonary anthrax) is a life- threatening pneumonia caused by inhalation of spores.  B. Spore forming Gram positive rods Laboratory Diagnosis 1- Gram Staining 2- Culture: Nutrient Agar 3- Biochemical tests A- Casienase B- Amylase C- Carbohydrate fermentation  B. Spore forming Gram positive rods Treatment: Penicillin is the most effective treatment. No resistant strains have been isolated clinically.  B. Spore forming Gram positive rods 2. Bacillus cereus: Disease. B cereus causes food poisoning.  B. Spore forming Gram positive rods Transmission: Spores on grains such as rice survive steaming and rapid frying. The spores germinate when rice is kept warm (e.g., reheated fried rice). The portal of entry is the gastrointestinal tract.  B. Spore forming Gram positive rods Pathogenesis: B cerues produces 2 enterotoxins. Their mode of action is unclear.  B. Spore forming Gram positive rods Clinical Findings: There are 2 syndromes: (1) “Emetic form“ involves a short incubation period (4 hours) with nausea and vomiting and is similar to staphylococcal food poisoning; (2) “Diarrheal form” involves a long incubation period (18 hours) with diarrhea and resembles clostridial gastroenteritis.  B. Spore forming Gram positive rods Laboratory Diagnosis: Confirmation of B. cereus as causative agent in a foodborne outbreak requires: (1) Isolation of strains of the same serotype from the suspect food and feces or vomitus of the patient. TTT Only symptomatic treatment is given. Prevention: Rice should not be reheated  B. Spore forming Gram positive rods CLOSTRIDIUM There are 4 medically important Clostridium species: C tetani, C botulinum, C perfringens and C difficile. All clostridia are anaerobic, spore-forming, gram-positive rods.  B. Spore forming Gram positive rods CLOSTRIDIUM 1. Clostridium tetani: Disease : C tetani causes tetanus (lockjaw).  B. Spore forming Gram positive rods Transmission: Spores are widespread in soil. The portal of entry is usually a wound site, eg, were a nail penetrates the foot, but the spores also can be introduced during “skin- popping.” A technique used by drug addicts to inject drugs into the skin.  B. Spore forming Gram positive rods Germination of spores is favored by necrotic tissue and poor blood supply in the wound. Pathogenesis: Tetanus toxin (tetanospasmin) is an exotoxin produced by vegetative cells at wound site.  B. Spore forming Gram positive rods The toxin binds to ganglioside receptors and blocks release of inhibitory mediators (eg, glycine) at spinal synapses. Tetanus toxin and botulinum toxin are among the most toxic substances known. They are proteases that cleave the proteins involved in mediator release.  B. Spore forming Gram positive rods Clinical Findings : Violent muscle spasms; lockjaw (trismus) due to rigid contraction of the jaw muscles, which prevents the mouth from opening; a characterisitic grimace known as “risus sardonicus”; and exaggerated reflexes occur. Respiratory failure with a high mortality rate Opisthotnos position  B. Spore forming Gram positive rods Laboratory Diagnosis: There is no microbiologic or serologic diagnosis. Organisms are rarely isolated from the wound site C tetani produces a terminal spore, i.e., a spore at the end of the rod. This gives the organism the characteristic appearance of a “tennis racket”.  B. Spore forming Gram positive rods Treatment : Tetanus immune globulin is used. Penicillin or metronidazole is probably useful. An adequate air way must be maintained and respiratory support given. Benzodiazepines, e.g., valium, should be given to prevent spasms.  B. Spore forming Gram positive rods Prevention: Tetanus is prevented by immunization with tetanus toxoid (HCHO-treated toxin) in childhood and every 10 years thereafter.  B. Spore forming Gram positive rods CLOSTRIDIUM 2. Clostridium botulinum Disease C botulinum causes botulism.  B. Spore forming Gram positive rods Transmisson: Spores, widespread in soil, contaminate vegetables and meats. When these foods are canned or vacuum packed without, adequate sterilization, spores survive and germinate in the anaerobic environment.  B. Spore forming Gram positive rods Toxin is produced within the canned food and ingested preformed. The highest-risk foods are (1) alkaline vegetables such as green beans, peppers and mushrooms and (2) smoked fish.  B. Spore forming Gram positive rods This toxin is relatively heat-labile, it is inactivated by boiling for several minutes. Thus, disease can be prevented by sufficient cooking.  B. Spore forming Gram positive rods Pathogenesis: Botulinum toxin is absorbed from the gut and carried via the blood to peripheral nerve synapses, where it blocks release of acetylcholine.  B. Spore forming Gram positive rods It is a protease that cleaves the proteins involved in actylcholine release. Along with tetanus toxin, it is among the most toxic substances known,  B. Spore forming Gram positive rods Clinical Findings: Descending weakness and paralysis including respiratory muscle failure No fever is preset.  B. Spore forming Gram positive rods Two special clinical forms occur: (1) wound botulism, in which spores contaminate a wound, germinate, and produce toxin at the site; and  B. Spore forming Gram positive rods Two special clinical forms occur: (2) infant botulism, in which the organisms grow in the gut and produce toxins. Ingestion of honey containing the organism is implicated in transmission of infant botulism. Affected infants develop weakness or paralysis and may need respiratory support but usually recover spontaneously.  B. Spore forming Gram positive rods Laboratory Diagnosis: The organism is usually not cultured. Botulinum toxin is demonstrable in uneaten food and the patient’s serum by mouse protection tests.  B. Spore forming Gram positive rods Mice are inoculated with a sample of the clinical specimen and will die unless protected by antitoxin. Treatment: Trivalent antitoxin (types A. B, and E) is given, along with respiratory support.  B. Spore forming Gram positive rods Prevention: Proper sterilization of all canned and vacuum-packed foods is essential. Food must be adequately cooked to inactivate the toxin. Swollen cans must be discarded (clostridial proteolytic enzymes cause gas formation that swells cans).  B. Spore forming Gram positive rods 3. Clostridium perfringens: C perfringens causes 2 distinct diseases: gas gangrene and food poisoning.  B. Spore forming Gram positive rods 3. Clostridium perfringens: A. Gas Gangrene. Transmission: Spores are located in the soil; vegetative cells are members of the normal flora of the colon and vagina. Gas gangrene is associated with war wounds, automobile and motorcycle accidents.  B. Spore forming Gram positive rods Pathogenesis: Organisms grow in traumatized tissue (especially muscle) and produce a variety of toxins. The most important is alpha toxin (lecithinase), which damages cell membranes, including those of erythrocytes resulting in hemolysis.  B. Spore forming Gram positive rods Clinical Findings: Pain, edema, and cellulitis occur in the wound area. Crepitation indicates the presence of gas in tissues. Hemolysis and jaundice are common, as are blood- tinged exudates. Shock and death can ensue. Mortality rates are high.  B. Spore forming Gram positive rods Treatment: Penicillin is the antibiotic of choice. Prevention: Wounds should be cleansed and debrided. Penicillin may be given for prophylaxis.  B. Spore forming Gram positive rods 3. Clostridium perfringens: B. Food Poisoning: Transmission: Spores are located in soil and on food. The heat-resistant spores survive cooking, germinate, and the organisms grow to large numbers in reheated foods, especially meat dishes.  B. Spore forming Gram positive rods Pathogenesis: During sporulation (spore formation) in the gastrointestinal tract, an enterotoxin is produced. The mode of action of the enterotoxin is unknown.  B. Spore forming Gram positive rods Clinical Findings: The disease has an 8- to 16-hour incubation period and is characterized by watery diarrhea with cramps and little vomiting. It resolves in 24 hours.  B. Spore forming Gram positive rods Treatment: Symptomatic treatment is given; no antimicrobial drugs are administered. Prevention: There are no specific preventive measures. Food should be adequately cooked to kill the organism.  B. Spore forming Gram positive rods 4. Clostridium difficile: Disease: C difficile causes antibiotic-associated pseudomembranous colitis.  B. Spore forming Gram positive rods Transmission: The organism is part of the flora of the gastrointestinal tract in approximately 3% of the general population. Up to 30% of hospitalized patients become colonized. It is transmitted by the fecal-oral route. The hands of hospital personnel are important intermediaries.  B. Spore forming Gram positive rods Pathogenesis: Antibiotics suppress drug-sensitive normal flora, allowing C difficile to multiply and produce exotoxins A and B. Exotoxin A is an enterotoxin that causes an outpouring of fluid, resulting in watery diarrhea. Exotoxin B is a cytotoxin that causes damage to the colonic mucosa, leading to pseudomembrane formation. Clindamycin and ampicillin are 2 of many antibiotics that cause this disease. C difficile rarely invades the intestinal mucosa.  B. Spore forming Gram positive rods Clinical Findings: C difficile causes diarrhea, associated with pseudomembranes (yellow-white plaques) on the colonic mucosa. The diarrhea is usually not bloody and neutrophils are found in the stool in about half of the cases. Pseudomembranous colitis  B. Spore forming Gram positive rods Laboratory Diagnosis: Exotoxin B is detected in filtrates of stool samples by its cytotoxic effect on cultured cells. It is identified by inhibition of cytotoxicity by specific antibody. Anaerobic culture is usually not done. An ELISA that detects both exotoxins A and B is available.  B. Spore forming Gram positive rods Treatment: The causative antibiotic should be withdrawn. Oral metronidazole or vancomycin should be given and fluids replaced. Metronidazole is preferred because using vancomycin may select for vancomycin-resistant enterococci.