Key Media and Concepts that a Physician has to Know

 Bacteria have particular requirements for growth.  Media = mixtures of nutrients that the microbes need to live.  This media can be liquid or solid.  In the case of solid media (the focus of this presentation), this provides a surface (agar=jello=seaweed) and the necessary moisture, nutrients, and pH to support microbial growth

 Current technology allows us to cultivate only 10% of the bacterial species that inhabit our bodies.  Types of media: ◦ Non-selective: Does not discriminate between what types of bacteria are grown. ◦ Selective: Selective media isolates one bacteria over another from a polymicrobic speciment (e.g., sputum). ◦ Differential: On a single agar plate, one type of organism can be differentiated from another.

 Tryptic Soy Agar (TSA—no, not airport security) ◦ A complex mixture of proteolized caesin and soybeans  A basic medium used for culturing many kinds of microorganisms. This agar is mainly used as an initial growth medium for the purposes of: observing colony morphology, developing a pure culture, achieving sufficient growth for further biochemical testing, and culture storage.  Used for organisms that live in the environment and in the body

 Contains blood cells from an animal (e.g. a sheep); most bacteria (if they can be propagated outside of the body) will grow on this medium. ◦ Used for organisms that are typically confined to vertebrate hosts. ◦ For the Streptococci, this can also be a differential media; differentiating alpha-, beta- and gamma-hemolytic streptococci.

 This is blood agar in which the red blood cells have been lysed.  The oxidation of the hemoglobin turns the medium brown.  Releases NAD and Heme from the red blood cells.  Allows organisms such as Haemophilus to grow because the do not have to lyse the red blood cells in order to access NAD and Heme.

 Acidic media used to isolate yeast.  Inhibits growth of bacterial species.  Think yeast/fungi if you hear growth on sabouroub agar

 CNA agar is a selective medium designed to propagate gram- positive cocci  This is basically blood agar supplemented with colistin (C) and naladyxic acid (NA)—get it?  CNA agar is used to isolate gram positive cocci from complex flora from oral specimens, among others  Think gram positive cocci when you see this agar.

 Thayer-Martin agar (or Thayer- Martin medium) (aka, Martin Lewis medium) is chocolate agar supplemented with antibiotics. It is used for culturing and primarily isolating pathogenic Neisseria.  Contains vancomycin, which is able to kill most gram-positive organisms; colistin is added to Kill most gram-negative organisms except Neisseria, and nystatin which suppresses fungal growth.  Think pathogenic Neisseria when you hear this medium!

 This is agar designed to select for gram-negative rods. (exceptions, Pseudomonas spp.)  The selective agent is bile salts (same as secreted by the gall bladder).  This is also a differential agar; it contains lactose and a pH indicator. If the gram-negative rod ferments lactose, and turns the colonies red– like those in the upper right section of the lower figure.  The upper figure demonstrates growth of bacteria that do not turn the colonies pink (lactose negative).  This media differentiates E. coli (lactose positive) from Salmonella and Shigella (lactose negative).