culture media

There are many different culture media depending on which organism or group of organisms you wish to cultivate. We will be making Tryptic Soy Agar (TSA) a general purpose culture medium. General purpose because it is a rich medium, providing a variety of nutrients so it is appropriate for many organisms. Culturing organisms with specific nutritional (or even environmental) needs requires special purpose media. A reference we will use often in class is the Difco Manual. We have a hard copy in the lab - I find the online version extremely helpful. The picture above shows some of the types of culture media you will use this semester (the Appendix at the end of your lab manual lists the formulations for all media we are using this semester). There are liquid growth media (TSB, trypticase soy broth is an example - this link is a page from the Difco Manual), many solid media (TSA, tryptic soy agar is an example of a solid medium) as well as several semi-solid media (fluid thioglycollate medium is an example of a semi-solid growth medium). There are selective media, those media selecting for the growth of one organism over another. MacConkey agar is an example of a selective growth medium; it contains bile salts and crystal violet, both compounds inhibit the growth of Gram positive organisms making it a good medium for growing Gram negative bacteria such as the enteric organism Escherichia coli. An antibiotic such as ampicillin may be added to MacConkey to select, for example, Escherichia coli cells resistant to this antibiotic. There are also differential media. Selective media may be differential as well. Differential media allow you to "tell the difference" between organisms with different traits that may be growing on the medium. For example, colonies of an organism that can use one of the ingredients in the medium as a carbon source may turn one color while those that use a different component of the medium as a carbon source might turn another color. This week you will be making a general purpose growth medium. As the name implies many different organisms with a wide variety of traits would be able to grow on the medium.
slant and Petri plates
Cultures may be grown in tubes, flasks, on Petri plates or any convenient form. The picture on the left compares two very common formats for growing microorganisms; "slants" and Petri plates. Slants are named because the growth surface is slanted. This is done after autoclaving (sterilization) while the medium is still liquid. Slanting increases the surface area available for growth. Petri plates have an even greater growth surface. Petri plates are appropriate for many applications. Because Petri plates have a relatively thinner layer of medium below the growing surface they tend to dry out rather quickly. A slant, because the medium is thicker, lasts longer and it is a good way to keep a culture collection or to save organisms for more than a week or so. Slants will last for a month or possibly more.