Microbiology Lab Review: Chapter 13

Lab Review

Chapter 13: Water Testing 1 with Selective and Differential Media

1. [CH-13-01] Define term “selective media”. Give a specific example of a selective medium and explain how it works –

Media tailored to specific applications by addition of selective agents that inhibit certain bacteria, allowing other types to grow.

Ex: Thayer-Martin agar: Chocolate agar mix with antimicrobials to promote growth of fastidious bacteria like Neisseria spp.

2. [CH-13-02] Define the term “differential media”. Give a specific example of a differential medium and explain how it works –

Media containing certain enzymes substrates and/or indicators that allow differentiation of growing bacteria into groups.

Ex: Blood agar differentiates by type of hemolysis. Alpha (green halo) = partial lysis of RBC, Beta (clear halo) = complete lysis of RBC, and Gamma (no halo) = no lysis of RBC at all.

3. [CH-13-03] What makes mannitol salt agar a selective medium? What makes Mannitol salt agar a differential medium? –

Selective medium: salt concentration (~7.5%)

Differential medium: Mannitol (monosaccharide) and Phenol red (pH indicator)

4. [CH-13-04] What components of MacConkey agar make it a selective medium? What components of MacConkey agar make it a differential medium? –
   Selective medium: Bile salts, crystal violet
   Differential medium: Lactose (sugar), neutral red (pH indicator)

5. [CH-13-05] What components of EMB agar make it a selective medium? What components of EMB agar make it a differential medium? –

Selective medium: Eosin and methylene blue dyes
Differential medium: Lactose (sugar), Methylene blue (color indicator)

6. [CH-13-06] Which medium is used to tell apart Staphylococcus aureus from Staphylococcus epidermidis? Explain how it works –

Mannitol Salt Agar (MSA), selects for obligate and facultative halophiles (Staphylococcus spp. Are facultative halophiles). Then the ability of the bacteria to ferment mannitol is indicated by phenol red. Staphylococcus aureus ferments mannitol forming an acid that makes it colored yellow. Staphylococcus epidermidis does NOT ferment mannitol making it colored pink.

7. [CH-13-07] Which medium is used to identify Escherichia coli? Explain how it works –

Eosin and Methylene Blue Agar (EMB), uses eosin to prevent the growth of gram-positive bacteria and lactose to determine fermenting capabilities of bacteria. Escherichia coli as a gram-negative bacteria with strong lactose fermenting capabilities appears as a special metallic green sheen on these plates allowing for easy recognition.

8. [CH-13-08] How can blood agar be used as a differential medium? –

Based on the ability of the bacteria to perform hemolysis on the red blood cells. Alpha (green halo), Beta (clear halo), and gamma (no halo).

9. [CH-13-09] You have observed the formation of dark colonies with the green metallic sheen on EMB agar. What does it tell you?
   The bacteria are gram-negative, lactose fermenters with strong acid production. A large amount of acid will cause precipitation of the dye resulting in the green metallic sheen.
   
   
10. [CH-13-10] Assuming that all organisms cultivated on blood agar came from the throats of healthy students, why is it important to cover and seal the plates with a tape?
    Blood agar is a nutrient-rich plate capable of hosting numerous types of pathogenic bacteria and environmental bacteria. Thus, care must be made to prevent possible outside contaminants from getting into the plate. Parafilm helps by making sure that the lid and base of petri dish do not accidentally come apart during transport.