MLAB 2434 –
MICROBIOLOGY
KERI BROPHY-MARTINEZ
Use of Colony Morphology for
Presumptive ID of Microorganism
Importance of Colonial
Morphology as a Diagnostic Tool



Provide a presumptive diagnosis in times of critical need
 Best guess giving type of specimen and what is seen
before confirmatory tests
 Help jump-start therapy
Enhance quality of patient care through rapid results and cost
effectiveness
 Prevents time-consuming testing that yields few results
Play a significant role in quality control
 Helps maintain accuracy of commercial or automated
systems
 Troubleshoot mixed cultures or errors in commercial
systems
Colony Morphology (cont’d)

Initial Observation &
Interpretation of Cultures
“Read” routine cultures after 18 to
24 hours of incubation
 If cultures cannot be read after 24
hours, refrigerate to prevent
overgrowth and drying

Colony Morphology (cont’d)
Compare growth on different media
 Examples:

• If good growth on BAP but no growth
on MAC, the bacteria is probably a
gram positive organism
• If MAC growth is pink, the organism is
a lactose fermenter
Fermenting vs. NonFermenting
Colony Morphology (cont’d)

Gross Colony Characteristics

Hemolysis on BAP (Chocolate agar
does not show true hemolysis)
• α hemolysis
• green discoloration on the media
• Due to partial lysing of the RBC’s
Colony Morphology (cont’d)
β
hemolysis –
complete
clearing of
blood around
colonies on
BAP
Colony Morphology (cont’d)

Gamma
hemolysis
 Absence of
hemolysis,
nonhemolytic
Colony Morphology (cont’d)
Size ( large, medium, small, pinpoint)
 Margin or edge (smooth,
filamentous)
 Elevation ( raised, convex, flat)
 Density( transparent, translucent,
opaque)
 Consistency (brittle, creamy, dry)
 Pigment (production of color)
 Odor (fruity, old sock)

Colony Morphology (cont’d)
Shape, edge and elevation of bacterial
colonies
Liquid Media Environment
Growth of Organisms in Liquid
Media


Streamers or vines
are associated with
certain streps
Puff balls associated
with streps
Growth of Organisms in
Liquid Media

Turbidity


Cloudiness of the
medium
Often see gas
References
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
Engelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory
Diagnosis of Infectious Diseases: Essentials of Diagnostic
Microbiology . Baltimore, MD: Lippincott Williams and
Wilkins.
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook
of Diagnostic Microbiology (4th ed.). Maryland Heights, MO:
Saunders.