Cultural Characteristics of Selected Bacteria: Colonial Morphology



    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, non-diffusable pigment.

Rhodospirillum rubrum.  Pinpoint circular colonies which are convex with entire margins.  This gram negative spirillum produces a non-diffusable 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 (g) 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 (a) 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 (b) hemolysis; the complete breakdown of the red blood cells surrounding the colonies.

Growth on Slants and in Broths