Depending on their composition or use, culture media can be categorized into several groups; these include defined, complex, selective, and enrichment medium. In a defined medium, the exact chemical composition is known. These types of media are usually composed of pure biochemicals, and are often used to study the minimal nutrient requirement of a microorganism. In contrast, the exact chemical composition of a complex medium is not known. This latter medium type often contains reagents of a biological origin, such as yeast extract and peptone, where the exact chemical composition is unknown. Complex media usually provide a large range of growth factors that assist in the cultivation of unknown and fastidious bacterial species.3
Media may also be formulated as selective or enriched. A selective medium is formulated to inhibit the growth of certain bacterial species and/or promote the growth of a specific species. These media can consist of additional selective reagents, such as high salt concentration to select for halophiles, or can be used under selective growth conditions. An enrichment medium also allows for the growth of specific bacterial species; however, enrichment media are supplemented with a reagent that permits, rather than inhibits, the growth of a particular species.3
Generally, bacterial culture media are mixtures of proteins, salts, trace elements, amino acids, and carbohydrates. The presence and volume of these components can vary significantly among bacterial species depending on the macro- and micro-nutrient requirements of each strain. The manner in which bacterial strains are cultured also varies widely. Liquid media are often used for the growth and propagation of pure batch cultures, while solid agar-based media are used for the isolation of pure cultures.
In addition to supplying nutrients, liquid medium can assist in the maintenance of pH. The pH can be sustained through one or more buffering systems such as 3-(N-morpholino)propanesulfonic acid (MOPS) or potassium phosphate. The osmotic environment can be maintained through the addition of salts, such as sodium chloride. ATCC uses numerous types of media in order to provide the optimal growth conditions for each bacterial species. The formulations for these media can be found on the ATCC website (See: NOTE 5). Several bacterial media commonly used by ATCC include the following:
Brain Heart Infusion Agarose (BHI) (ATCC medium formulation 44) is a complex, nutrient-rich, general-purpose growth medium used for culturing fastidious and nonfastidious microorganisms, including streptococci and pneumococci. It is generated from the dehydrated infusions of bovine or porcine brain and heart tissue. This medium can be supplemented with sodium chloride and disodium phosphate for osmotic and pH maintenance, respectively. ATCC currently uses both BHI agar and broth medium supplied from Becton, Dickinson and Company (BD).
Gonococcal (GC) Medium (ATCC medium formulation 814) is a growth medium used for the cultivation of Neisseria gonorrhoeae and other fastidious organisms. It can be employed as a basal medium in the preparation of Chocolate Agar, Thayer-Martin Medium, Martin-Lewis Agar, and Transgrow Agar. GC medium is prepared from a mixture of GC agar base, Dried Bovine Hemoglobin, and IsoVitaleX™ available from BD. For the selective growth of specific organisms, this medium can be supplemented with antibiotics including chloramphenicol, streptomycin, tetracycline, and ampicillin.
Haemophilus Test Medium (HTM) (ATCC medium formulation 5129) consists of a complex mixture of yeast extract, Mueller Hinton Broth, Porcine Hematin, and nicotinamide adenine dinucleotide (NAD). The main component, Mueller Hinton Agar, was originally prepared as a solid media used to test the antimicrobial susceptibility testing of common, non-fastidious, rapidly growing organisms. This initial media, however, was not suitable for fastidious organisms such as streptococci, gonococci, and Haemophilus species.5 HTM was subsequently developed for the testing of fastidious organisms.
Luria-Bertani Broth Medium (LB) (ATCC medium formulation 1065) is an all-purpose medium used by ATCC for the propagation and maintenance of Escherichia coli for molecular biology applications. It is prepared in the Miller Composition using an LB broth mixture supplied by BD, which consists of tryptone, yeast extract, and sodium chloride. LB can also be prepared as a solid medium through the addition of agar. This medium is often supplemented with various antibiotics for the maintenance of plasmid DNA.
Marine Broth (ATCC medium formulation 2) is a selective, complex growth medium used for the cultivation of heterotrophic marine bacteria. The marine environment offers a unique set of growth conditions consisting of high salinity and low temperatures. To simulate this environment, marine broth is formulated with peptone, yeast extract, and a high salt content. ATCC generates this medium using the Marine Broth 2216 preparation from BD. Marine broth may also be prepared as a solid medium through the addition of agar.
Middlebrook 7H9 Broth (ATCC medium formulation 2714) is formulated to support the growth and propagation of Mycobacterium species. This medium is prepared as a mixture of Middlebrook 7H9 broth, glycerol, sodium pyruvate, and albumin-dextrose-catalase (ADC) enrichment.
Middlebrook 7H10 Agar (ATCC medium formulation 173) is formulated for the isolation and cultivation of mycobacteria from clinical and non-clinical specimens. This medium is prepared as a mixture of Middlebrook 7H10 broth, glycerol, and oleic acid-albumin-dextrose-catalase (OADC) enrichment. It differs
from Middlebrook 7H9 broth in concentration of several salts as well as the addition of OADC enrichment rather than ADC enrichment. This medium is based on an improved formulation that was developed to promote the early growth of mycobacteria in vitro.6
Modified Chopped Meat Medium (ATCC medium formulation 1490) is a non-selective, complex medium that supports the growth of most spore-forming and non-spore-forming obligate anaerobes. This medium consists of a complex mixture of ground beef (fat-free), sodium hydroxide, trypticase peptone, yeast extract, Hemin, dipotassium phosphate, and vitamin K1 solution. Depending on the required growth conditions of each anaerobic strain, ATCC supplements this medium with various reagents such as glucose, arginine, or a combination of formate and fumarate.
Nutrient Agar/Broth (ATCC medium formulation 3) is a general-purpose medium for the cultivation of non-fastidious bacterial strains. ATCC prepares this media from a dehydrated stock provided by BD, which consists of beef extract and peptone. This medium can be further enriched by the addition of heart infusion broth. Alternatively, this medium can also be used for the selective growth of specific strains through the addition of varying concentrations of sodium chloride.
Reinforced Clostridial Medium (ATCC media formulations 1053, 2107) is used by ATCC for the cultivation and recovery of anaerobes, particularly Clostridium species, from a variety of sources. ATCC prepares this medium using dehydrated culture media from either Oxoid Limited or BD. This media consists of a complex mixture of salts, tryptose, beef extract, yeast extract, dextrose, starch, and L-Cysteine HCl.
Tryptic Soy Agar (ATCC medium formulation 18) is a medium used for the isolation and propagation of a variety of bacterial strains. ATCC often prepares this medium using a tryptic soy base, provided by BD, supplemented with 5% defibrinated sheep’s blood (ATCC medium formulation 260). The tryptic soy base mixture consists of a combination of tryptone, soytone, and sodium chloride. The addition of sheep’s blood helps to facilitate the growth of more fastidious bacteria.