Cultural Characteristics of Selected Bacteria: Colonial
Morphology
Overview
When attempting to identify unknown bacteria, it is important to note the cultural
characteristics that the organisms exhibit on and in various types of media. Proper
isolation of individual species enables the researcher to examine colonial shape and
appearance, as well as other factors such as pigmentation and smell. Some species
exhibit phenotypic expressions owing to growth at different temperatures (25o, 30o, and
37o C), so it is prudent to incubate replicate plates at more than one temperature.
Solid, agar-based media can be used to identify colonial characteristics (shape, size,
elevation, margin type), but can also serve to select for particular bacterial groups and
differentiate between two or more different species. In Microbiology 201, we use
trypticase soy agar (TSA) as an enriched medium for general bacterial isolation since
most common species and even some fastidious forms will grow on this medium. Blood
agar plates (BAP) are used both as enriched media and to differentiate between
individuals on the basis of their ability to produce hemolysins, enzymes which lyse red
blood cells. When used in combination with other forms of presumptive and
confirmatory tests, the examination of colonial characteristics provides valuable
information leading to precise identification. Click on images to see a larger view.
Trypticase Soy Agar (TSA)
Bacillus subtilis. These gram positive, sporeforming rods produce
colonies which are dry, flat, and irregular, with lobate margins.
Mycobacterium smegmatis. These acid-fast rods produce irregular
colonies with lobate margins which are dry, flat, orange-yellow, and
appear waxy due to the high concentration of lipids in the cell wall.
Staphylococcus epidermidis. Circular, pinhead colonies which are
convex with entire margins. The colonies of this gram-positive coccus
appear either the color of the agar, or whitish.
Staphylococcus aureus. Circular, pinhead colonies which are convex
with entire margins. This gram positive coccus often produces colonies
which have a golden-brown color.
Micrococcus luteus. Circular, pinhead colonies which are convex with
entire margins. This gram positive coccus produces a bright yellow, nondiffusable pigment.
Rhodospirillum rubrum. Pinpoint circular colonies which are convex
with entire margins. This gram negative spirillum produces a nondiffusable red pigment.
Serratia marcescens. These gram negative rods produce mucoid colonies
which have entire margins and umbonate elevation. Note that there are
both red and white colonies present on this plate. Some strains of S.
marcescens produce the red pigment prodigiosin in response to
incubation at 30o C, but do not do so at 37o C. This is an example of
temperature-regulated phenotypic expression.
Pseudomonas aeruginosa. This gram negative rod forms mucoid
colonies with umbonate elevation. Some strains produce a diffusable
green pigment and a distinctive fruity odor. P. aeruginosa is an
opportunistic contaminant of burn injurys, wounds such as cuts and
gunshot, and can cause bacterial pneumonia. It is often nosocomial
pathogen, easily transmitted by hands and invasive medical procedures.
Salmonella choleraesuis serovar typhimurium. This gram negative rod is
a component of the gastrointestinal tract of birds and reptiles and is an
agent of gastroenteritis in humans. It forms shiny, convex colonies with
entire margins.
Escherichia coli. This gram negative rod (coccobacillus) forms shiny,
mucoid colonies which have entire margins and are slightly raised. Older
colonies often have a darker center.
Enterobacter aerogenes. This gram negative rod is a common
contaminant of vegetable matter which forms shiny colonies with entire
margins and convex elevation.
Blood Agar Plates (BAP)
Enterococcus faecalis. This gram positive coccus forms pinpoint
colonies with convex elevation. This strain exhibits gamma ()
hemolysis; growth on the BAP, but no lysis of the red blood cells in the
agar.
Streptococcus pneumoniae. This gram positive streptococcus produces
colonies which are circular with entire margins, often raised with
depressed centers. This microbe exhibits alpha () hemolysis; a partial
breakdown of the red blood cells in the medium.
Streptococcus pyogenes. The gram positive streptococcus produces
pinpoint colonies which are circular with entire margins. It exhibits beta
() hemolysis; the complete breakdown of the red blood cells surrounding
the colonies.