Colony Morphology Flashcards
Primary plating
-refers to the inoculation of the clinical specimen onto the initial media
-allows for visualization of growth and colony morphology
-observe colony morphology in 18-24 hours post culture
-age of culture can affect the size and characteristics of colonies
Colony Morphology: Role and Importance
-provides a presumptive identification to the physician in times of critical need
-enhances the quality of patient care through rapid results and cost-effectiveness
-plays a significant role in quality control
*helps maintain the accuracy of automated systems
*helps to determine mixed cultures or errors in commercial systems so you can troubleshoot
List of Selective Media
- Blood agar: grows most organisms
- chocolate agar: most organisms and fastidious organisms
- MacConkey’s agar: gram-negative organisms, particularly enterics
-Columbia CNA: gram-positive organisms
Differential media
Macconkey agar: selectively grows gram-negative and shows lactose fermenters
* pink colonies (lactose fermenters), colorless (non-lactose)
ex: Escherichia coli/ Citrobacter: dry pink colonies
ex: klebsiella/ enterobacter: large mucoid pink colonies
Hemolysis
-hemolysis of blood agar: observations in the media immediately surrounding or underneath the colony
-caused by enzymatic or toxin activity of bacteria
-most important in the presumptive identification of streptococci and enterococci
-use transillumination (light source to see)
Two types of hemolysis
Alpha hemolysis and beta hemolysis, and non-hemolytic colonies sometimes referred to as gamma hemolytic
Alpha hemolysis
-partial clearing of blood that results in a green discoloration (or brown) of the medium
ex: streptococcus pneumoniae, certain viridans strep
Beta hemolysis
-complete clearing of blood cells around the colonies
ex: S pyogenes (GAS), S agalactiae (GBS), listeria monocytogenes
Form or Margin
-refers to the edge of colonies
-smooth filamentous, rough, or irregular
ex: bacillus anthracis is filamentous, diphtheroid have rough edges
-swarming: a hazy blanket of growth on the surface
ex: proteus
-dry appearance: diphtheroid (gram-positive bacilli)
-yeast: creamy, white to yellowish with feet or pedicules
size
-colonies may be described as being large, small, medium, or pinpoint
-gram-positive is generally smaller than gram-negative.
-staphylococcus is usually larger than Streptococcus
Elevation
-determined by tilting the culture plate and looking at the side of the colony
-raised, convex, flat, umbilicate, or umbonate
raised: raised flat top
convex: dome shape
flat: not raised
umbilicate: convex with depressed center (pitting) ex: S pneumoniae (if no capsule)
-umbonate: convex with protruding nipple
Density
-colony density may be described as transparent, opaque, translucent
ex: Beta hemolytic strep: translucent (except group B)
ex: group B strep: semi-opaque (bull’s eye colony)
ex: most staph and gram-negative rods are opaque
color
-used to describe a particular colony genus in general
-white, gray, yellow, buff
ex: coagulase negative staph are white
ex: enterococcus and most gram-neg are gray
ex: micrococcus and Neisseria are yellow or off white
ex: diphtheroid are buff
consistency
-determined by touching a colony with a sterile loop
-wet, creamy, dry, waxy, or sticky (colony comes off plate)
ex: S. aureus: creamy
ex: Neisseria: sticky
ex: Norcardia: brittle, crumbly
ex: streptococci: dry except mucoid variety
ex: Diphtheroid: dry and waxy
Pigment
-inherent characteristics of specific organisms
ex: pseudomonas aeruginosa: green or green with metallic sheen
ex: Serratia marcescens: brick red
ex: chromobacterium violaceum: purple
ex: prevotella melaninogenica: brown black anaerobe
