Lab 14
Msc. Shaimaa obead
Corynebacterium
Kingdom: Bacteria
Phylum: Actinobacteria
Family: Corynebacteriaceae
Genus: Corynebacterium
Species: C. diphtheriae
C. pseudotuberculosis
Rhodococcus equi
Corynebacterium is a genus of Gram-positive, catalase positive, nonspore-forming, non-motile, rod-shaped bacteria that are straight or
slightly curved.
Metachromatic granules are usually present representing stored phosphate
regions. These granules give (Beaded) shape and seen by Albert's stain.
The bacteria group together in a characteristic way, which has been
described as the form of a V or L shapes or like "Chinese letters".
Others can cause human disease. C. diphtheriae, for example, is the
pathogen responsible for diphtheria.
 Culture
Corynebacterium grows slowly, even on enriched media. The bacteria
grow on Loeffler's serum medium and/or on blood tellurite agar
(BTA) or Potassium tellurite agar (selective medium) aerobically at
37C for 24 hrs.
The colonies of C. diphtheriae are small, granular, grey, smooth, and
creamy with irregular edges.
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On BT Agar C. diphtheriae present in 3 biotypes: Gravis, mitis and
intermedius biotypes. The most severe disease is associated with the
gravis biotype.
Colony of gravis biotype is large, non-hemolytic & grey.
Colonies of mitis biotype are small, hemolytic and black
Colonies of intemedius biotype are intermediate in size, non-hemolytic
with black center & grey margin.
 Biochemical test:
1. Catalase: All Corynebacterium species are catalase positive
2. Carbohydrate Fermentation Test
Each species of corynebacteria has its specific carbohydrate
fermentation pattern
C.diphtheriae can be differentiated from other Corynebacterium
species by fermentation of glucose and maltose but not sucrose with
production of acid only.
 Elek’s Test:
Principle: It is toxin/antitoxin reaction. Toxin production by
C.diphtheriae can be demonstrated by a precipitation between
exotoxin and diphtheria antitoxin.
2
Procedure:
A strip of filter paper impregnated with diphtheria antitoxin is placed
on the surface of serum agar. The organism is streaked at right angels
to the filter paper. Incubate the plate at 37C for 24 hrs
Results:
After 48 hrs incubation, the antitoxin diffusing from filter paper strip
and the toxigenic strains produce exotoxin, which diffuses and
resulted in lines four precipitation lines radiating from intersection of
the strip and the growth of organism.
precipitation lines
strip of diphtheria antitoxin
C. diphtheriae
Elek’s Test
C. pseudotuberculosis
Short rods; no granules; clubs rare.
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Rhodococcus equi
Rhodococcus equi is a Gram-positive coccobacillus bacterium. Closely
related to Mycobacteria and Corynebacteria. Colonies may be rough,
smooth or mucoid and are colourless, yellow, orange or red. The
organism is commonly found in dry and dusty soil and can be important
for diseases of domesticated animals (horses and goats).
 Biochemical
test
which
differentiated
between
Corynebacterium spp.:
Corynebacterium spp.
Blood
Catalase Glucose Maltose Sucrose
haemolysis
C. diphtheriae
+
+
+
+
_
C. pseudotuberculosis
_
+
_
_
_
Rhodococcus equi
_
_
_
_
 Diseases which cause by Corynebacterium spp.:
Corynebacterium spp.
Host
Disease
C. diphtheriae
Human
Diphtheria
C. pseudotuberculosis
Animals (sheep)
Caseous lymphadenitis
Rhodococcus equi
Animals(horse)
Suppurative bronchopneumonia
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