Published on March 10, 2014
Gram positive bacilli Non-spore forming - Corynebacterium - Listeria Spore- forming - Bacillus - Clostridium
Non-spore forming Corynebacterium
Corynebacterium Some species are part of normal flora of skin and mm. Medically important species is Corynebacterium diphtheriae
Morphology Gram-positive bacilli Club-shaped Arranged at acute angles or parallel to each other (Chinese letters). Meta-chromatic granules. Non-spore forming
Methylene blue stain: Beaded appearance
Culture Characters Aerobic. Growth on: 1. Blood agar 2. Loeffler’s serum: Best morphology 3. Blood tellurite agar: Selective & differential Grey to black colonies
Virulence factors Diphtheria ExotoxinDiphtheria Exotoxin Exotoxin is dependent on: 1. Lysogenic prophage. 2. Low extracellular iron concentration.
Disease: Diphtheria Upper respiratory tract infection. Transmitted by droplets. Characterized by: 1- Local pseudomembrane. 2- Toxemia. Complications: Airway obstruction Toxic myocarditis and heart failure Nerve paralysis
Clinical Manifestations - Cervical lymphadenitis (Bull neck) - Toxaemia with low grade fever D.D. of sore throat: 1- S. pyogenes 2- Vincent’s angina 3- C. diphtheriae
Diagnosis Mainly clinical Laboratory confirmation: A- Specimen: Throat swab from the pseudomembrane.
B- Direct Detection: Microscopic examination (Gram stain): • Gram-positive bacilli • Chinese letters appearance
B- Direct Detection: Microscopic examination (Methylene Blue stain): Meta-chromatic granules
1- Loeffler’s serum: Best morphology 2- Blood tellurite agar: grey/black colonies 3- Blood agar to exclude S. pyogenes C- Cultivation:
D- Identification: Microscopic examination: 1- Gram stained smear: Gram-positive club-shaped bacilli (Chinese letters). 2- MB stained smear: showing meta-chromatic granules.
The isolated organism is Corynebacterium diphtheriae Is it Toxigenic or Not?
E- Toxigenicity Tests: a) Elek’s test: most common assay. b) PCR: detection of toxin gene. c) ELISA: detection of toxin from culture.
Elek’s test: An antigen-antibody reaction in which the Ag is soluble “Precipitation”.
Diagnosis of carriers Throat or nasal swabs are subjected to the same procedures: Isolation + Toxigenicity tests
What treatment is prescribed? Treatment should be IMMEDIATELY started if diphtheria is clinically suspected. Diphtheria antitoxin and antibiotics. Treatment of symptoms & complications e.g. respiratory support.
How can we prevent this disease? By Vaccination Diphtheria toxoid + pertussis vaccine + tetanus toxoid in a trivalent vaccine: DPT For close contacts of a case: (booster of diphtheria toxoid + antibiotic chemoprophylaxis)
Diphtheroids Corynebacteria that resemble C.diphtheriae in morphology. They are mainly commensals.
Case A 4-year-old male child presented with fever of 38°C. Physical examination revealed clear chest, exudative pharyngitis and bilaterally enlarged cervical lymph nodes. A throat culture was taken and a course of penicillin was started.
Case (cont.) The child’s course worsened, he became increasingly lethargic, developed respiratory distress and was hospitalized. On admission, he had a fever of 38°C and an exudate in the posterior pharynx described as a yellowish, thick membrane which bled when scraped and removed. The patient’s medical history revealed that he had received no immunizations.
Listeria monocytogenes Gram-positive rods (coccobacilli) Microscopic examination:
Listeria resembles Corynebacteria in morphology but is MOTILE.
Diseases Abortion, premature delivery or sepsis during the peripartum period. Neonatal meningitis Septicaemia and meningitis (in immunocompromised adults). Food poisoning (dairy products or undercooked meat)
Neonatal meningitis Meningitis caused by Listeria is almost always seen in neonates. Causes of Neonatal Meningitis: 1. Group B Streptococci 2. E. coli K1 3. Listeria monocytogenes
Case A one month old girl was admitted to hospital with acute meningitis. The Gram stain of CSF revealed Gram-positive short rods. What is the cause of neonatal meningitis? a. N. meningitidis, group A b. N. meningitidis, group C c. Listeria monocytogenes d. S. pneumoniae
How did the mother contract it? Listeriosis is a food-borne infection. Listeria resists drying, heating and freezing without forming spores. Commonly contaminated food items: 1. Dairy products (esp. unpasteurized milk and soft cheeses). 2. Undercooked meat (chicken, hot-dogs). 3. Refrigerated food.
1- C.diphtheriae is cultured on: a- Nutrient agar. b- Chocolate agar. c- Loffler’s serum. d- Lowenstein-Jensen medium. e- MacConkey’s agar.
2- Blood tellurite agar is a(n): a- Enriched medium b- Enrichment medium c- Simple medium d- Selective and differential medium e- Indicator medium only
3- Which of the following is a toxigenicity test for C.diphtheriae ? a- Elek’s test b- Coagulase c- Catalase test d- Culture on blood tellurite e- ELISA test for antibody detection
4- C. diphtheriae has the following morphology: a- Gram negative cocci arranged in pairs b- Gram positive cocci arranged in chains c- Gram positive club-shaped bacilli d- Gram positive cocci in clusters e- Gram positive capsulated diplococci
5- The toxin of C. diphtheriae is only produced by those strains that are: a- Encapsulated. b- Glucose fermenters. c- Sucrose fermenters. d- Lysogenic e- Endotoxin producers.
6- A 1-week old newborn develops meningitis. Short gram-positive rods are isolated. The mother had eaten unpasteurized cheese during pregnancy. What is the most likely etiological diagnosis? a- C. diphtheriae. b- S. pyogenes. c- L. monocytogenes. d- S. pneumoniae. e- S. agalactiae
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