Colony characteristics are observed after the primary isolation bacteria in solid culture medium. The appearance of bacterial colony on culture medium is usually characteristic and helps in preliminary identification. It is described under following headings:
Size: diameter of the colony is measured in millimeters
Shape: Shape refers to the general form of the colony which might be punctiform, circular, filamentous , irregular, rhizoid or spindle like.
Elevation: It describes the side view of the colony. It may be flat, raised, convex, pulvinate, umbonate or umblicate.
Color: Colonies may be colored due to the properties of some media or due to pigment production by the bacteria. In some differential medium and indicator medium, some bacteria produce colored colonies due to change in pH or enzymatic activity. For example, Escherichia coli produce pink colonies in MacConkey agar, black colonies of Corynebacterium diphtheriae on tellurite blood agar.
Pigment produced by certain bacteria may also color the colonies. For example, blue, green, brown or red pigments (pyocyanin, pyoverdin, pyomelanim, pyorubrin etc.) produced by Pseudomonas aeruginosa or golden yellow colonies of Staphylococcus aureus or red colonies of Serratia marcescens etc.
Consistency: It may be mucoid, friable, firm, butyrous or brittle
Surface: It may be glistening or dull
Density: It may be opaque, translucent or transparent
Hemolysis on blood agar: Some bacteria produce an enzyme (hemolysin) that lyses the red blood cells surrounding the colonies on blood agar, forming a zone of hemolysis. It may be
Alpha hemolysis: This type of lysis is identified when we observe greenish discoloration around the colonies on blood agar due to partial digestion of hemoglobin. Outine of the RBC is intact. e.g. Streptococcus pneumoniae
Beta hemolysis: This type of lysis is identified when we observe zone of complete clearing of blood around the colonies due to complete lysis of RBCs. e.g. Staphylococcus aureus
Gamma hemolysis: This type of lysis is identified when we observe that there is no lysis on blood agar by the colonies. e.g. Enterococcus spp
Alpha prime hemolysis: This type of lysis is identified when we observe halo of incomplete lysis immediately surrounding the colonies with a second zone of complete lysis at the periphery.
Each distinct colony represents an individual bacterial clone. If a culture medium consist isolates with similar single colony morphology, then it should be identified as pure culture. If there are colonies with more than one colony morphology, then it should be identified as mixed infection or specimen contamination. If there is isolates with more than one colony morphology in, pus, urine from patients with complicated UTI, etc it might be significant, all colony morphotypes should be processed for identification and antimicrobial susceptibility testing.
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