CORYNEBACTERIUM DIPHTHERIAE - PowerPoint Presentation

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CORYNEBACTERIUM DIPHTHERIAE

Prepared by

Assit.Prof.Dr

.

Najdat

B.Mahdi

Corynebacteria

are 0.5–1

μm

in diameter and several micrometers

long. Characteristically, they possess irregular swellings at one end that give them the

Irregularly distributed within the rod (often

near the poles) are granules staining deeply with aniline dyes(metachromatic- granules) that give the rod a beaded appearance .

Corynebacteria

are small, slender, pleomorphic, gram-positive rods

of distinctive morphology that tend to stain unevenly. They are

nonmotile

, and they do not form spores.

Corynebacterium

is a large genus of diverse habitat. Most species are

facultative anaerobes, and those associated with humans, including

the pathogen C.

diphtheriae

, grow aerobically on standard laboratory

media such as blood agar.

Corynebacterium diphtheriae

Diphtheria, caused by C.

diphtheriae

, is an acute respiratory or cutaneous

disease and may be life threatening. The development of

effective vaccination protocols and

wide spread immunization beginning

in early childhood has made the disease rare in developed

countries. However, diphtheria is a serious disease

throughout the world, particularly in those countries where the

has not been immunized

population

Epidemiology:

C.

diphtheriae

is found in the throat and

naso

-pharynx of carriers and in patients with diphtheria. This disease Is a local infection.

, usually of the throat, and the organism is

primarily spread by respiratory droplets, usually by convalescent or asymptomatic carriers. It is less frequently

spread by direct contact

with an infected individual or a contaminated fomite.

Pathogenesis

Diphtheria is caused by the local and systemic effects of a single exotoxin that inhibits eukaryotic protein synthesis.

The toxin molecule is a heat-labile polypeptide that is

composed of two fragments, A and B. Fragment B binds to susceptible

cell membranes and mediates the delivery of fragment

A to its target. Inside the cell, fragment A separates from fragment

B and catalyzes a reaction between nicotine adenine dinucleotide

(NAD+) and the eukaryotic polypeptide chain elongation

factor, EF-2 The toxin is encoded on a β-

coryne

-

phage and only those strains in which the phage is integrated

into the

C.

diphtheriae

chromosome produce toxin. Toxin gene

expression is also regulated by environmental conditions. Low

iron conditions induce toxin expression, whereas high iron

condtions

repress toxin production .

Clinical significance

Upper respiratory tract infection

: Diphtheria is a strictly localized

infection, usually of the throat. The infection produces distinctive thick, grayish, adherent exudate (

pseudomembrane

) that is composed of cell debris from the mucosa and inflammatory products. It coats the throat

and may extend into the nasal passages or downward in the

respiratory tract, where the exudate sometimes obstructs the.

airways, even leading to suffocation., generalized symptoms occur caused by production and absorption of toxin . Although all human cells are sensitive to diphtheria toxin, the major clinical effects

involve the heart and peripheral nerves. Cardiac conduction

defects and myocarditis may lead to congestive heart failure

and permanent heart damage.

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,

nonhealing

ulcer with a gray membrane. Rarely, exotoxin production leads to

tissuem

degeneration and death.

Diagnostic Laboratory Tests

Dacron swabs from the nose, throat, or other suspected

lesions must be obtained before antimicrobial drugs are

administered. Swabs should be collected from beneath any

visible membrane. The swab should then be placed in semisolid

transport media such as

Amies

. Smears stained with

alkaline methylene blue or Gram stain show beaded rods in

typical arrangement.

Specimens should be inoculated to a blood agar plate (to

rule out hemolytic streptococci) and a selective medium such

as a

tellurite

plate (

eg

,

cystine-tellurite

blood agar [CTBA] or

modified

Tinsdale’s

medium) and incubated at 37°C in 5% CO2.

Plates should be examined in 18–24 hours. In 36–48 hours, the

colonies on

tellurite

medium are sufficiently definite for recognition

of C

diphtheriae

. On

cystine

tellurite

agar, the colonies

are black with a brown halo.

A presumptive C diphtheriae isolate should be subjected

to testing for

toxigenicity

1. Modified

Elek

immunoprecipitation

method described

by the World Health Organization

.

2. Polymerase chain reaction (PCR)–based methods have

been described for detection of the diphtheria

toxin gene .

3. Enzyme-linked

immunosorbent

assays can be used

to detect diphtheria toxin

4. An

immunochromatographic

strip assay allows detection

of diphtheria toxin in a matter of hours

..

The latter two assays are not widely available.

Immunity: Diphtheria toxin is antigenic and stimulates the production of antibodies that

(

neutralize the toxin’s activity.(Note: Formalin

treatment of the toxin produces a toxoid that retains

the antigenicity but not the toxicity of the molecule.

Prevention:

The corner stone of diphtheria prevention is immunization with toxoid, usually administered in the

DTaP

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 10-year intervals throughout life. The control of an epidemic outbreak of diphtheria involves rigorous immunization and a search for healthy carriers among patient contacts.

REFERENCES

Medical

Microbiology-

Jawetz

,

Melnick

, &

Adelberg

’

2016)

)

Twenty-Seventh Edition

Lippincott’sIllustrated

ReviewsMicrobiologThird

Edition