Microbiology Lab Practical

Photoreactivation

• UV light causes pyrimidine dimers in DNA, which can lead to mutations in DNA sequence

• Photolyase is an enzyme activated by visible light to perform DNA repair of pyrimidine dimers

MacConkey Broth

Identify E. coli and coliform bacteria

• Purple to yellow = lactose fermentation

• Contains lactose which can be fermented to lactic acid

• Acidity of lactic acid causes indicator to turn yellow

What is MacConkey broth differential for

lactose fermentation

What is MacConkey broth selective for

Gram negative bacteria

Mannitol Salt Agar (MSA)

High concentration of salt

What is MSA selective for

• Selective against most Gram - bacterial

• Selective form some Gram + bacteria (halophiles)

What is MSA differential for

Differential for organisms that can ferment mannitol

• byproduct is acidic and turns the phenol red to a yellow color

Sheep’s Blood Agar

Tests for the presence of hemolysis

Hemolysis = lysis of red blood cells

Alpha Hemolysis

• Brown

• Slight halo

• Partial

Beta Hemolysis

• Yellow

• Distinct halo

• Complete

Gamma Hemolysis

• No halo or color change

• No hemolysis

Pure Culture

A population of microbes in which all organisms are descendants of the same organism (single progenitor)

KOH test

• G+ or G-

• KOH dissolves thin layer of PG of G- cells

• Causes cell lysis, resulting in viscous solutionwith mucoid string

• G+ cells are unaffected by KOH, therefore they do not lyse

KOH Steps

•add KOH to slide first

•Scoop a relatively large amount of bacteria

•Mix bacteria at least 20-30 seconds

•Lift loop and look for stringiness

Catalase Test

• Bubbles = Catalase +

• No bubbles = Catalase -

• First, add bacteria to slide

  -then add hydrogen peroxide and look for bubbles