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.
Article Summary: MacConkey's Agar is a specialized bacterial growth medium that is selective for Gram-negative bacteria and can differentiate those Gram- bacteria that are able to ferment lactose.
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VIDEO: MacConkey's Agar (MAC) Quiz and Review
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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). This is the selective aspect of this specialized medium.
Why Is the Color of MAC Colonies Important?
If the bacterial colonies growing on MacConkey's are pink, they are Gram- lactose-fermenting bacteria. This is the differential aspect of this specialized media. These pink colonies are typically coliform bacteria in the family Enterobacteriaceae, including, but not limited to, the genera Escherichia, Klebsiella, Enterobacter, Hafnia and Citrobacter. The presence of coliform bacteria is commonly used as an indicator of unsanitary food and water.
Non-lactose fermenting, non-coliform bacteria growing on MAC are not pink, and include members of Enterboacteriaceae such as the genera Proteus, Morganella, Providencia, Edwardsiella, Salmonella, Shigella and Yersenia (plague bacteria).
Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2014) Microbiology with Diseases by Taxonomy, 4th ed., Pearson Benjamin Cummings.
PHOTOS OF MACCONKEY'S AGAR: 1. Sterile plate of MacConkey's agar; 2. Kitchen sink sample plated in TSY (front) and MAC (back, note very pink Gram-negative lactose fermenters); 3. E. coli, happy to be plated on MAC; 4. Salmonella (G -, lac -) plated on MAC; 5. E. coli (G -, lac +) plated on MAC. Click here for more photos.
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.
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).
VIDEO: How to Interpret MacConkey's Agar (MAC) Bacterial Growth Medium
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).