Experiment 7: Comparison of Colony Appearance of Coliforms and Other Bacteria Grown on EMB and Endo Agars

What do coliform bacteria look like?

• rod shaped

• Gram-negative

• non-spore forming

• motile or non-motile

• ferment lactose with the production of acid and gas

What is nutrient agar?

• general purpose

• nutrient rich medium

• used to grow non-fastidious organisms

• contains peptone (for Nitrogen)

• Beef or yeast extract (vitamin, carbs, N, salt)

• Salts/NaCl

• Neutral pH around 7.5

• no dyes or compounds for differentiation

• positive control

What is EMB agar?

• identifies Gram negative enteric bacteria

  • enterobacteria

• selection: partially inhibits Gram positive bacteria

• differential: eosin Y + methylene blue differentiate among organisms

  • E.coli = purple colnies with metallic green sheen bc strong fermenters

  • Enterobacter aerogenes = pink colonies so weak fermenters

  • other colonies are clear/transluscent, dont ferment lactose

What is Endo-broth media?

• sodium desoxycholate and sodium lauryl sulfate inhibit gram positive bacteria

• has lactose + other nutrients for coliforms to ferment

• lactose breakdown releases aldehydes

  • Basic fuchsin stain reacts w aldehydes

• Shiny green coating.

Why do we use Endo-broth media?

need selective media bc raw water has many microorganisms unrelated to contamination

What are the 6 bacteria used in this experiment?

• E.coli

• Salmonella typhimurium

• Pseudomonas aeruginosa

• Citrobacter freundii

• Bacillus subtilis

• Staphylococcus aureus

Why can’t S. aureus grow on EMB or Endo agar?

Bc S.aureus is gram positive and those media are selective for gram negative

What grows on EMB agar?

What grows on Endo agar?

What grows on nutrient agar?