Chapter 8: Colony Morphology Flashcards
Primary plating:
inoculation of clinical specimen onto laboratory culture media
Media used: selective, differential, grow fastidious organisms
Plate reading:
Observe colony morphology and compare
- usually 18-24 hours post culture
Blood agar:
morphology, grows most organism
Chocolate agar:
most organism and fastidious organisms
MacConkey Agar:
gram-negative, particular enterics
Lactose fermenters are:
pink
Escherichia coli/Citrobacter: dry pink colonies
Klebsiella/Enterobacter-like organisms: large mucoid pink colonies
Nonlactose fermenters are:
colorless
Colony characteristics:
- Hemolysis
- Size
- Form or Margin
- Elevation
- Density
- Color
- Consistency
- Pigment
- Odor
Blood agar is most important in presumptive identification of
streptococci and enterococci
- must use transillumination to determine
beta hemolysis:
complete clearing of blood cells around the colonies
Examples: Streptococcus pyogenes, Streptococcus
agalactiae, Listeria monocytogenes
alpha hemolysis:
partial clearing of blood that results in a green discoloration of the medium
Examples: Streptococcus pneumoniae, certain viridans strep
Non-hemolytic colonies are sometimes referred to as
gamma hemolytic
Colony Size:
Large, medium, small, or pinpoint
– Helps suggest what you might find on Gram stain
– Gram-positive is generally smaller than gram- negative
– Staphylococcus is generally larger than Streptococcus
Form or Margin:
Swarming: hazy blanket of growth on surface (ex. proteus spp.), shows movement of bacteria
Bacillus anthracis is filamentous; diphtheroid colonies have rough edges
Elevation:
Umbilicate example
S. pneumoniae (if no capsule)
Umbonate example:
diphtheroids
Density:
Transparent, translucent, opaque
– Most β-hemolytic strep translucent (except group B)
– Group B strep: semiopaque (bull’s-eye colony)
– Opaque includes most staph and gram-negative
rods
Color:
White, gray, yellow, buff
– Coagulase negative staph are white.
– Enterococcus and most gram-negative rods are gray.
– Micrococcus and Neisseria are yellow or off-white.
– Diphtheroids are buff
Consistency:
Determine by touching a colony with a loop
Brittle (splinters), creamy, dry, waxy, or sticky
(entire colony comes off the plate)
– S. aureus: creamy
– Neisseria: sticky
– Nocardia: brittle
– Streptococci: dry
– Diphtheroid: dry and waxy
Pseudomonas aeruginosa pigment:
green or green metallic sheen
Serratia marcescens pigment:
brick red
Kluyvera pigment:
blue
Chromobacterium violaceum pigment:
purple
