Bacterial Identification: Simplified
Notes
INTRODUCTION
Bacterial identification determines the species of bacteria using
physical, biochemical, or genetic methods. It’s critical for diagnosing
infections and guiding treatment.
KEY METHODS
1.Traditional Phenotypic Methods
G RAM S TAINING :
Differentiates bacteria into Gram-positive(purple) and Gram-
negative (pink) based on cell wall structure.
BACTERIUM CATALA Coagula Hemoly Key test
SE se sis
Staphylococcus + + Beta Golden
aureus colonies
Staphylococcus + - Gamma Novobiocin
epidermidis sensitive
Streptococcus - N/A Beta Bacitracin
pyogenes sensitive
Streptococcus - N/A Alpha Optochin
pneumoniae sensitive
Enterococcus faecalis - N/A Gamma Bile-esculin+,
PYR+
Bacillus anthracis + N/A Gamma Non-motile,
spores
Listeria + N/A Beta Tumbling
monocytogenes motility
, Colony Morphology:
Observing shape, size, color, and texture of colonies on agar.
FEATURE TERM Description Example Bact
Size Pin Point <1 mm (tiny dots) Streptococcus
Small 1–2 mm Haemophilus
Medium 3–4 mm E. coli
Large >5 mm Bacillus subtilis
Shape Circular Perfect circle Staphylococcus aureus
Irregular Jagged, uneven Bacillus anthracis
Rhizoid Root-like branches Bacillus anthracis
Filamentou Thread-like, spreading Nocardia
s
Margin Entire Smooth, sharp edge Klebsiella
(○○○)
Undulate Wavy (≈≈≈) Lactobacillus
Lobate Lobed/flower-like (✿✿✿) Pseudomonas
Filamentou Fuzzy/hairy edge Actinomyces
s
Elevation Flat No elevation (—) Proteus vulgaris
Raised Slightly thick (◡) Salmonella
Convex Dome-shaped (◠) Staph. aureus
Umbonate Button-like (raised Yersinia pestis
center) (⦿)
Surface Smooth Glossy, even E. coli (non-mucoid)
Rough Dry, granular Mycobacterium
Mucoid Slimy, wet (capsule) Klebsiella pneumoniae
Wrinkled Folded/crinkled Bacillus subtilis* (older)
Colour White/ Most common Staphylococcus
cream epidermis
Yellow Bright or pale Micrococcus luteus
Golden Metallic Yellow Staphylococcus aureus
Red/Pink Diffused or colony Serratia marcescens
specific
Notes
INTRODUCTION
Bacterial identification determines the species of bacteria using
physical, biochemical, or genetic methods. It’s critical for diagnosing
infections and guiding treatment.
KEY METHODS
1.Traditional Phenotypic Methods
G RAM S TAINING :
Differentiates bacteria into Gram-positive(purple) and Gram-
negative (pink) based on cell wall structure.
BACTERIUM CATALA Coagula Hemoly Key test
SE se sis
Staphylococcus + + Beta Golden
aureus colonies
Staphylococcus + - Gamma Novobiocin
epidermidis sensitive
Streptococcus - N/A Beta Bacitracin
pyogenes sensitive
Streptococcus - N/A Alpha Optochin
pneumoniae sensitive
Enterococcus faecalis - N/A Gamma Bile-esculin+,
PYR+
Bacillus anthracis + N/A Gamma Non-motile,
spores
Listeria + N/A Beta Tumbling
monocytogenes motility
, Colony Morphology:
Observing shape, size, color, and texture of colonies on agar.
FEATURE TERM Description Example Bact
Size Pin Point <1 mm (tiny dots) Streptococcus
Small 1–2 mm Haemophilus
Medium 3–4 mm E. coli
Large >5 mm Bacillus subtilis
Shape Circular Perfect circle Staphylococcus aureus
Irregular Jagged, uneven Bacillus anthracis
Rhizoid Root-like branches Bacillus anthracis
Filamentou Thread-like, spreading Nocardia
s
Margin Entire Smooth, sharp edge Klebsiella
(○○○)
Undulate Wavy (≈≈≈) Lactobacillus
Lobate Lobed/flower-like (✿✿✿) Pseudomonas
Filamentou Fuzzy/hairy edge Actinomyces
s
Elevation Flat No elevation (—) Proteus vulgaris
Raised Slightly thick (◡) Salmonella
Convex Dome-shaped (◠) Staph. aureus
Umbonate Button-like (raised Yersinia pestis
center) (⦿)
Surface Smooth Glossy, even E. coli (non-mucoid)
Rough Dry, granular Mycobacterium
Mucoid Slimy, wet (capsule) Klebsiella pneumoniae
Wrinkled Folded/crinkled Bacillus subtilis* (older)
Colour White/ Most common Staphylococcus
cream epidermis
Yellow Bright or pale Micrococcus luteus
Golden Metallic Yellow Staphylococcus aureus
Red/Pink Diffused or colony Serratia marcescens
specific
