When bacterial cultures are grown in a laboratory, they are growing in a captive environment, somewhat like a tiny microbial zoo. These captive-bred bacteria are totally dependent on people to provide the proper environment for their survival and growth. A nutrient-rich media is required in order to grow bacteria in the lab.
What is Bacterial Growth Media?
A growth medium (plural: media) is a mixture of nutrients, moisture and other chemicals that bacteria need for growth. Media are used to grow bacterial colonies (millions of bacteria having arisen through the binary fission of a single progenitor).
Using Media to Identify Bacteria
Like the differential staining of bacteria, special types of media can be used to provide clues about a microbe’s identity. There are many types of media that are specific about what they grow, or that provide information about the type of microbes present.
Differential and selective media are special types of agar that can can exclude certain types of bacteria and even test for certain bacterial metabolic capabilities. MacConkey’s (MAC), Blood agar (BAP) and Mannitol Salt (MSA) are three examples of these specialized types of media.
Selective and Differential Media
If a bacterial growth medium is selective, that means that it grows only certain types of microbes while inhibiting the growth of others. Agar is considered a differential growth medium if, when specific microbes are present, the medium or bacterial colonies themselves exhibit a color change that provides information about their identity.
MacConkey's Agar Is Selective
MacConkey's is a selective medium that inhibits the growth of Gram-positive bacteria due to the presence of crystal violet and bile salts. Gram-negative bacteria grow well on MAC.
MacConkey's Agar Is Differential
MAC is also a differential, meaning that it differentiates or distinguishes between groups of bacteria on the basis of a color change reaction. MacConkey’s contains two additives that make it differential; neutral red (a pH indicator) and lactose (a disaccharide).
Bacteria, known as “lactose fermenters”, eat the media’s lactose, and, in the process, create an acidic end product that causes the pH indicator, neutral red, to turn pink. With MacConkey’s, it is not the media that changes color, but rather the actual colonies of lactose fermenting bacteria that appear pink. Non-lactose fermenting bacteria will be colorless (or, if they have any color, will be their natural color rather than pink).
What Does it Mean if Bacterial Colonies Grow on MacConkey's?
Whenever bacterial colonies are growing on MacConkey’s Agar, they are Gram-negative bacteria (since Gram+ do not grow on this type of medium). If the colonies are pink, they are Gram- lactose-fermenting bacteria. These pink colonies are typically coliform bacteria in the family Enterobacteriaceae, inlcuding the genera Escherichia, Klebsiella, Enterobacter, Hafnia and Citrobacter. Non-lactose fermenting, non-coliform members of Enterboacteriaceae include the genera Proteus, Morganella, Providencia, Edwardsiella, Salmonella, Shigella and Yersenia (plague bacteria).
About the Main Image
The photos associated with this article display bacterial colonies growing on MacConkey’s Agar. The colonies that appear pink, are Gram-negative lactose fermenters. Other colonies that do not appear pink are Gram- bacteria that do not eat (ferment) the sugar lactose.
For more information on microbiology, see the Virtual Microbiology Classroom of the science education website Science Prof Online or Todar's Textbook of Bacteriology.
Sources
Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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