EX. 14 | Fermentation

Fermentation Tubes

Detects acid and gas production from carbohydrates

---

Durham Tube

• An inverted tube inside the fermentation tubes that traps gas

---

Phenol red indicator

• Acid-base indicator in the fermentation medium

MRVP broth

Glucose-supplemented nutrient broth used for both MR and VP tests

MR Test (Methyl red)

Detects large acid production from glucose

Results

• Red = Positive = Lots of acid produced

• Yellow = Negative = Little to no acid

VP Test (Voges Proskauer) Test

Detects neutral product acetoin

Results

• Pink/Red = Positive = Acetoin produced

• Light Brown = Negative = No acetoin

---

Acetoin

• End product detected in VP test

Citrate Test (Bromothymol Blue Indicator)

Purpose

• To determine whether a bacterium can survive using citrate as its only carbon source

Citrate lyase — An enzyme that breaks down citrate into pyruvate

Simmons citrate agar —Medium used to determine citrate utilization

Bromothymol blue — pH indicator in citrate agar

Results

• Blue = Positive = Citrate used

• Green = Negative = Citrate not used

  • A positive result would change from green to blue

Fermentation

Purpose

• To detect whether bacteria can ferment specific carbohydrates (glucose, lactose, sucrose) and produce acid and or gas
  
  

Important Notes

• Occurs with or without oxygen

• After 24 hours, bacteria may grow oxidatively on peptone after exhausting carbohydrate, producing ammonia → turning the indicator red again (can cause false negatives) — (which is why recording observations at 24 and 48 hours is beneficial
  
  

Results

• Positive result — Acid production - Medium turns yellow

• Negative result — No acid production — Medium remains red or neutral due to ammonia from peptone use

Color Indicator (Phenol red indicator)

• Yellow = Positive result = gas/acid production

• Red = Negative result = no gas or acid production

  • A positive result would change from red to yellow