Gram positive bacilli

Non-spore forming

- Corynebacterium

- Listeria

Spore- forming

- Bacillus

- Clostridium

Non-spore formingCorynebacterium

Corynebacterium

Some species are part of normal flora of skin and mm.

Medically important species isCorynebacterium 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 agar2. Loeffler’s serum:

Best morphology3. 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 angina3- C. diphtheriae

.

Diagnosis Mainly clinicalLaboratory 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)

DiphtheroidsCorynebacteria 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

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 K13. Listeria monocytogenes

CaseA 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 Ab. N. meningitidis, group Cc. Listeria monocytogenesd. 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 testb- Coagulasec- Catalase testd- Culture on blood telluritee- ELISA test for antibody detection

4- C. diphtheriae has the following morphology:a- Gram negative cocci arranged in pairsb- Gram positive cocci arranged in chainsc- Gram positive club-shaped bacillid- Gram positive cocci in clusterse- 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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