In its simplest form, a bacterial growth medium is designed to support the growth of bacteria. Depending on which bacteria you want to culture, you may have a range of different media to choose from, each containing a rather unique blend of sometimes surprising (and odd!) components!
In this article, I will take you through the ABCs of bacterial soup and give you an overview about many of the funky things you may encounter as you strive to feed your bacteria and keep them happy! This article will also cover many components you might never meet but are worth knowing about as you progress in your career in microbiology!
A. Agar – Usually of seaweed origin and used as a setting agent in bacterial growth media and to prevent oxygen getting to the bottom of bacterial slant cultures.
Antibiotics – Various antibiotics are added to agar cultures to prevent the growth of undesired bacteria or as part of a selection strategy in cloning experiments. Examples include ampicillin, kanamycin, streptomycin, metronidazole, and tetracycline.
B. Beef extract – One of several undefined components that appear in complex bacteriological medias. This component is a source of vitamins and growth factors to aid the growth of fastidious bacteria.
Bile salts – Often added to agars to prevent the growth of most gram-positive bacteria. It is a component of MacConkey agar, a selective media for differentiating gram-negative gut bacteria.
C. Carbon source – Could be any carbon source, including: glucose, sucrose, lactose, or galactose. Carbon is generally used as the main energy source for bacteria. Depending on the bacteria in question, specific carbon sources are required. Sometimes the carbon source is carefully selected to aid in isolation and identification of specific bacterial species.
Crystal Violet dye – Also found in MacConkey agar (and others) and is used to prevent the growth of certain gram-positive bacteria.
D. DMSO – Although not strictly for bacterial growth, DMSO may be added to media as a cryoprotectant, to prevent the formation of ice crystals upon freezing of bacterial cells for storage and later use.
E. Eosin Y – A dye used in combination with Methylene Blue in Eosin methylene blue agar. This is a selective agar for the isolation of fecal coliforms. In the presence of fecal coliforms, sucrose and lactose are fermented, lowering the pH of the medium. The two dyes combine to form a dark purple precipitate at low pH.
F. Fructose – A sugar that is fermented by some bacteria and is added to culture media in the presence of a pH indicator (often phenol red) that changes color if/when the fructose is fermented.
G. Gelatin – This is used in nutrient gelatin medium to test the ability of certain bacteria to produce an exoenzyme, gelatinase, that hydrolyzes gelatin. It is useful in distinguishing between Staphylococcus aureus and Staphylococcus epidermidis, and it is also used in distinguishing certain enteric bacteria.
H. Horse blood – Horse blood is used at a concentration of approximately 5 % in blood agar plates. The presence of blood greatly enriches media, facilitating the growth of fastidious bacteria, while simultaneously monitoring hemolytic activity (ability to rupture red blood cells) in the cultured cells. Sheep blood is also used in this way.
I. Iodine – This is sometimes added to bacterial agar cultures containing starch. Iodine changes to a blue/black color in the presence of starch, so observing a colorless zone after addition of iodine indicates that your cultured bacteria hydrolyzes starch.
J. Junlon – Addition of this anionic polymer at low concentrations is useful in aiding dispersed growth of filamentous bacteria (e.g., Streptomyces spp.).
K. K Vitamin – Often required as a supplement to support the growth of anaerobic bacteria.
L. Lecithin – Added to bacterial media to detect and investigate phospholipase activity in certain pathogenic bacteria.
M. Mannitol – Used in Mannitol Salt Agar at a concentration of approximately 7.5 % to select for Staphylococci bacteria, because they are able to withstand high levels of salt.
Major elements – These reflect the basic composition of a bacterial cell and are similar to the major elements required by mammalian cells (e.g., carbon, hydrogen, nitrogen, phosphorus, oxygen, sulphur, potassium, magnesium, calcium and iron). These are provided through the addition of water, small molecules, macromolecules, and inorganic ions.
N. Neutral Red Dye – This dye, when present in agar, changes to a pink color during lactose fermentation. It is one of the main components of MacConkey agar.
O. Oxygen – Unless you are controlling your setup to ensure an anaerobic growth environment, you can be 100% certain that oxygen will be present in your soup!
P. Peptones – Peptones come from proteolytic digests of animal milk or meat and are used in a number of different nutrient media for the growth of bacteria (also used in fungal growth media).
R. Ribose – A sugar that is fermented by some bacteria and is added to culture media in the presence of a pH indicator (often phenol red) that changes color if/when the ribose is fermented.
S. Soytone – An enzymatic digest of soybean meal that is used as a general and non-selective source of nutrition for bacteria and other organisms.
T. Trace elements – Metal ions required in such small amounts that they most often cannot be measured and aren’t added specifically to growth media. Nevertheless, they are important for bacterial growth and make their way into the soup as water contaminants. The main trace elements for bacteria are manganese, cobalt, zinc, and copper.
U. Urea – Sometimes used in combination with a pH indicator to aid in the identification and discrimination of enteric bacteria. Only bacteria that produce the exoenzyme, urease, hydrolyze urea to ammonia and carbon dioxide, which increases the pH and turns the media pink. Certain bacteria, such as members the Proteus genus, rapidly degrade urea. This test is essential in discriminating Proteus spp. from Salmonella spp.
V. Vitamins – Some bacteria are remarkably capable of synthesizing all the vitamins they need, while others need supplements as coenzymes for essential metabolic function. The vitamins requirements of bacteria are very similar to our own needs!
W. Water – Hopefully, this component needs no explanation!
X. Xylose – Along with lysine, the sugar xylose is an important component of lysine deoxycholate agar. This is a selective medium used to isolate Salmonella and Shigella species from clinical and food samples. This medium has a pH of around 7.4 and is bright pink/red in color because of the presence of phenol red. Gut bacteria, such as Salmonella, ferment xylose and produce acid that changes the color to yellow. Shigella colonies cannot do this and, therefore, remain red.
Y. Yeast extract – An undefined mixture of yeast origin that supports the growth of many bacteria and provides a source of B-vitamins, amino acids, and nitrogen.
Z. Zwitterion compounds – These, as well as phosphates, citrates, acetates, and certain amino-acids, are all examples of buffering agents that are used to maintain the pH of culture media.
So, that’s all for the bacterial alphabet soup for now! The list presented here is by no means exhaustive, so please feel free to add to it by writing to us in the comments section!