Introduction

Common bacteriological culture media (tryptic soy agar, chocolate agar, Thayer-Martin agar, MacConkey agar, eosin-methylene blue agar, hektoen agar, mannitol salt agar, and sheep blood agar) are shown uninoculated and inoculated with one or more of the following bacteria: Escherichia coli, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, Salmonella enteritidis, Shigella sp., Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, and Streptococcus pyogenes. 

Isolation of bacteria is accomplished by growing ("culturing") them on the surface of solid nutrient media. Bacteriological media normally consists of a mixture of protein digests (e.g., peptone, tryptone) and inorganic salts, hardened by the addition of 1.5% agar. Examples of standard general purpose media that will support the growth of a wide variety of bacteria include nutrient agar, tryptic soy agar, and brain heart infusion agar. A medium may be enriched by the addition of blood or serum. Examples of enriched media include sheep blood agar and chocolate (heated blood) agar.

Selective media contain ingredients that inhibit the growth of some organisms but allow others to grow. For example, mannitol salt agar contains a high concentration of sodium chloride that inhibits the growth of most organisms but permits staphylococci to grow.

Differential media contain compounds that allow groups of microorganisms to be visually distinguished by the appearance of the colony or the surrounding media, usually on the basis of a biochemical difference. Blood agar is one type of differential medium, allowing bacteria to be distinguished by the type of hemolysis produced. Many differential media are also selective.  For example, most of the standard enteric agars, such as MacConkey and eosin-methylene blue, are selective for gram-negative coliforms and are also used to differentiate lactose-fermenting and non-lactose-fermenting bacteria.

Several examples of commonly used bacteriological media are shown in the images. 

For each media, there is an image of an uninoculated plate, as well as plates with one or more types of bacteria cultured on them. Examination of the plates demonstrates the colony morphology, changes in the media pigmentation, and patterns of growth (or no growth) that occur with different types of bacteria. This information provides the first step towards the identification of pathogens.

Discussion

The information that can be gleaned from the results of culturing an unidentified bacterial isolate on a variety of selective and/or differential media can prove valuable to the identification of the microbe. These images are organized and labeled for comparisons and provide a visual reinforcement of the differences in bacterial colonies on different culture media.The concepts of bacterial selection and differentiation are illustrated with these images, with the advantage that they can be accessed for review purposes outside of the wet lab.

Clicking on "Examples" will expand example thumbnails of more plates. Clicking on an image will enlarge it to approximately the size of the browser and provide information in the "notes" section under each image. Clicking on the enlarged image will reduce it to its original thumbnail size. More than one enlarged image may be viewed concurrently.

Methods

This presentation was created using HTML/CSS, JPEG images, and Javacript. 

Photographs were taken using a standard digital camera and the plates were cropped out, cleaned up, and enhanced using Adobe Photoshop.  The longer side of each image is approximately 800 pixels. 

The thumbnail zoom-in effect uses a modification of the Javacript library Highslide JS by Torstein Hønsi (http://highslide.com/), under Creative Commons Attribution-NonCommercial 2.5 license.

The red arrow icons for expanding the example lists came from RedCons icon set by Ken Saunders (http://www.mouserunner.com/), used under Creative Commons license Attribution-NonCommercial-ShareAlike 2.5.

References

1.  Konman, E. W., S. D. Allen, W. M. Janda, P. C. Schreckenberger, and W. C. Winn, Jr.  1997.  Color atlas and textbook of diagnostic microbiology, 5th ed.  Lippincott-Raven, Philadelphia, PA.
2.  Loboffe, M. J., and B. E. Pierce.  2005.  A photographic atlas for the microbiology laboratory, 3rd ed.  Morton Publishing Co.,  Englewood, CO.