Diphtheria - Prac. Microbiology

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Information about Diphtheria - Prac. Microbiology

Published on March 10, 2014

Author: cud2018

Source: slideshare.net

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”.

Elek’s Test

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.

Review Questions

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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