Broths are liquid media produced by mixing dry or powder components with water.
Agar is a complex carbohydrate, derived from red algaes such as Chondrus (Irish moss) or Gelidium. Agar is supplied in a powdered form and must be melted to mix with water. It melts at 100 oC and solidifies at 40 oC, but will not re-liquefy at room temperature. Most bacteria cannot hydrolyze agar, so when dissolved in broths in concentrations between 0.5% and 1.5% it can serve as a solid base for the culturing of bacteria.
A defined (synthetic) medium is one in which the exact chemical composition is known.
Chemically undefined media such as brain-heart infusion agar and blood agar contain ingrediants that have not been measured. These media can be useful in the culturing of fastidious microbes that need additional nutrient content to survive.
Complex media are those produced through the addition of nutients from the digestion of beef, soybeans, milk or blood. These can be a combination of both defined and undefined types, such as McConkey's agar and Columbia CNA agar.
Selective media inhibit one group of bacteria while allowing others to grow. Examples include mannitol salt agar (MSA), which contains 7.5% salt, allowing halotolerant staphylococci to grow while inhibiting others, McConkey's and Hektoen agars that contain bile salts to inhibit most gram-positive cells while encouraging the growth of gram-negative enteric rods.
Differential media enable groups to be separated by their ability to utilize nutrients or by their form of growth. Blood agar can be used to separate microbes on the basis of their ability to lyse red blood cells and utilize hemoglobin. a-hemolytic organisms (center, above) such as Streptococcus pneumoniae partially break red blood cells (RBCs) down, producing an olive green color around the colonies. b-hemolytic organisms such as Streptococcus pyogenes and Staphylococcus aureus (left, above) completely break RBCs down, producing a clearing of the agar. g-hemolytic organisms such as Enterococcus faecalis and Staphylococcus epidermidis do not destroy RBCs (right, above) but grow on the surface of the agar.
Carbohydrate broths can also serve as differential media with the addition of pH indicators such as phenol red and Durham tubes to trap gas by-products of fermentation.
Media such as McConkey's (above, plates b and c), Hektoen, Columbia CNA and MSA are both selective and differential, so can be used to perform more than one test.
Obligate anaerobes can be culture in special reducing media such as sodium thioglycollate or in anaerobe chambers and handled in anaerobe hoods.
Transport media containing
low nutrients or special buffers are used to transport microbes from one
place to another and keep them isolated from the outside environment.