Medias for Bacterial Growth

1. Medias for Bacterial Growth

2. Objectives • Distinguish between the different types of media used to grow bacteria. • Contrast selective and differential medias. • Identify bacteria from the growth on agar plates.

3. Growth media- • Growth media (singular: medium) are used to cultivate bacteria. Media are mixtures of nutrients that the microbes need to live, and the necessary moisture and pH to support microbial growth

4. Culture Media • 1. Chemically Defined • the exact chemical composition is known • used to grow fastidious organisms • 2. Complex Media • exact chemical composition is not known • most bacteria and fungi are grown with this

5. Agar is… • a seaweed extract that becomes a solid at room temperature. • Many normal flora and clinically important microbes can be grown either in liquid medium (sometimes called broth), or on Petri dishes when adding agar to the liquid to make it solid.

6. Selective Media • Inhibits the growth of some bacteria while selecting for the growth of others • Example: • Brilliant Green Agar • dyes inhibit the growth of Gram (+) bacteria • selects for Gram (-) bacteria • Most G.I. Tract infections are caused by Gram (-) bacteria

7. Selective Media • EMB (Eosin Methylene Blue) • dyes inhibit Gram (+) bacteria • selects for Gram (-) bacteria • G.I. Tract infections caused by Gram (-) bacteria

8. Differential Media • Differentiates between different organisms growing on the same plate • When certain microbes grow on that medium, it exhibits a color change that gives us information about the type of microbe growing there. • Example: • Blood Agar Plates (TSA with 5% sheep blood) • used to differentiate different types of Streptococci

9. Blood Agar (BAP) • is a differential bacterial growth medium that can distinguish normal from pathogenic bacteria based on the interaction of sheep's blood and bacterial hemolytic enzymes. • Alpha hemolysis (α-hemolysis) - bacterial enzymes partially break down the blood cells. Results in a yellowish/greenish/brownish discoloration (like a bruise) around the colony, indicating incomplete hemolysis. – strep throat • Gamma hemolysis is no hemolysis at all. The bacteria have no effect on the red blood cells, and no change to the color of the medium. – normal flora/ strep on skin

10. Hemolysis Bacteria lysis of the RBCs – pathgenic strep

11. Selective and Differential Media • Mannitol Salt Agar (MSA) • used to identify Staphylococcus aureus • Mannitol Salt Agar • High salt conc. (7.5%) inhibits most bacteria • sugar Mannitol • pH Indicator (Turns Yellow when acid)

13. Nutrient Agar • TSY (Tryptic Soy Agar) • The medium that we use most often in lab is called "TSY" (Tryptic Soy Agar), a complex nutrient medium which supports the growth of a wide variety of microbes.

14. Selective and Differential Media • MacConkey’s Agar • used to identify Salmonella • MacConkey’s Agar • Bile salts and crystal violet (inhibits Gram (+) bacteria) • lactose • pH Indicator Many Gram (-) enteric non-pathogenic bacteria can ferment lactose, Salmonella can not

15. Bacteria, known as “lactose fermenters”, eat the lactose and in the process create an acidic end product that causes the pH indicator, neutral red, to turn red.  It is the actual colonies of lactose fermenting bacteria that appear pink. Non-lactose fermenting bacteria will be colorless (or, if they have any color, it will be a color other than pink). 

16. What Does it Mean if I Find Bacterial Colonies Growing on MacConkey's?  • Whenever bacterial colonies are growing on MacConkey’s, you know that they are Gram-negative bacteria (since Gram+ do not grow on this type of medium). If the colonies are pink, you know that they are Gram- lactose fermenting bacteria.