culture characteristics of common organisms

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COLONY MORPHOLOGY

A) While studying colonies on solid media, the following features are noted:

Size: diameter in millimeters

Shape: circular, irregular, pin-point(punctiform), filamentous, spindle

shaped etc.

Surface: smooth, wavy, rough, papillate, glistening, dull etc.

Edge: entire, undulate, crenated, fimbriate, or curled.

Elevation: flat, raised, convex, umbonate.

Emulsifiability: whether the colony is emulsifiable or not.

Color: white, yellow, black, pink etc.

Consistency: butyrous, viscid, membranous, brittle, friable etc.

Change in medium: change in the color of the medium, liquefaction of

medium etc.

Density: opaque, translucent, transparent etc.

B) While studying growth in liquid media, the following features are noted:

Degree of growth.

Presence of turbidity and its nature.

Presence of deposit and its character.

Nature of the surface growth such as pellicle.

Pigment production if any.

C) Colony characteristics used in bacterial identification.

1) Hemolysis on Blood Agar

a) Alpha: Partial clearing of blood around colonies with green discoloration

of the medium; outline of red blood cells intact. e.g.: Streptococcus

pneumoniae, Viridans group of Streptococci

b) Beta: Zone of complete clearing of blood around colonies owing to lysis of

the red blood cells. e.g.: Staphylococcus aureus, Streptococcus pyogenes.

c) Gamma: No change in the medium around the colony; no lysis or

discoloration of the red blood cells. e.g.: Enterococcus species.

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2) Pigment production in agar medium

a) Water-soluble pigments discoloring the medium e.g.: Pseudomonas

aeruginosa

b) Non-diffusible pigments confined to the colonies e.g.: Staphylococcus

aureus, Serratia marcescens.

3) Changes in Differential Media suggestive of fermentation reaction

Various dyes, pH indicators are included in differential plating media to

serve as indicators of enzymatic activities and aid in identifying bacterial

isolates.

Medium

Sugar

pH Indicator

Fermentation +ve

Fermentation ve

MacConkey

Agar

Lactose

Neutral red

Pink e.g.:

Escherichia coli,

Klebsiella

pneumonia

Pale e.g.: Proteus

spp, Shigella spp,

Salmonella spp.

Deoxycholate

Citrate Agar

Lactose

Neutral red

Should be pink.

Usually Lactose

fermenting

bacteria are

inhibited in this

medium due to

the high content

of Deoxycholate

Pale e.g.: Proteus

spp, Shigella spp,

Salmonella spp.

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Xylose

Lysine

Deoxycholate

Agar

Xylose,

Lactose,

Sucrose

Phenol Red

Yellow e.g.:

Escherichia coli

(ferment Lactose

& Sucrose. Acid

produced cannot

be neutralized by

the amines

released from the

decarboxylation of

lysine );

Citrobacter &

Proteus (ferment

Xylose only. The

acid produced is

not neutralized as

they do not

decarboxylate

lysine).

Red e.g.: Shigella

spp (do not

ferment any of

the sugars),

Salmonella spp.

(ferment Xylose

only. But the

acid produced is

neutralized as it

decarboxylates

lysine)

Thiosulphate

Citrate Bile

Sucrose

(TCBS) Agar

Sucrose

Bromothymol

Blue

Yellow e.g.: Vibrio

cholerae

Green e.g.: Vibrio

parahaemolyticus

4) H2S production:

Certain organisms produce H2S which imparts blackening of the medium.

The media which help in detecting H2S contains certain indicators such as

ferric ammonium citrate/ferric citrate/ferrous sulphate which detect H2S by

imparting black color to the colony.eg: Salmonella typhi, Proteus spp.

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D) Characteristic Colony Morphology of Commonly Isolated Bacteria.

Bacteria Media Colony characteristics

Staphylococcus aureus Blood Agar Convex, entire edge, 2-3 mm,

golden/ creamy glistening

colonies with zone of beta

hemolysis.

Streptococcus pyogenes Blood Agar Convex or pulvinate,

translucent, circular,

pinpoint colonies with wide

zone of beta hemolysis.

Streptococcus pneumoniae Blood Agar Umbilicate or flat, translucent,

butyrous or mucoid colonies

with zone of alpha hemolysis

Escherichia coli MacConkey Agar

Blood Agar

Large, pink, circular, smooth

to rough, moist to dry, flat

colonies.

Large, circular, greyish white,

moist, smooth opaque or

translucent colonies.

Klebsiella pneumoniae MacConkey Agar

Blood Agar

Large, pink, circular, smooth,

mucoid, convex colonies.

Large, circular, greyish white,

mucoid, smooth, opaque

colonies.

Salmonella spp. MacConkey Agar

XLD Agar

Small, pale, circular, convex,

smooth colonies.

Small, red, convex colonies

with black centers due to H2S

production.

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Shigella spp. MacConkey Agar

XLD Agar

Small, pale, circular, convex,

smooth colonies.

Small, red, circular, convex,

smooth colonies without black

centers.

Proteus spp. Blood Agar

MacConkey Agar

Flat, gray, irregular,

spreading as thin film

(swarming growth) over agar

surface, fishy smell.

Flat, irregular, pale, non-

spreading (no swarming)

colonies.

Pseudomonas aeruginosa Nutrient Agar

MacConkey Agar

Flat, greenish colonies having

effuse margins, with pigment

diffusing into the medium

Large, flat, irregular, pale

colonies with effuse margins

Vibrio cholerae TCBS Agar

Large, circular, convex, moist,

smooth, yellow colonies.

culture characteristics of common organisms

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