Physiological Characteristics: Oxidative and Fermentation Tests

Physiological Characteristics: Oxidative and Fermentation Tests

Introduction

Understanding the metabolic capabilities of microorganisms is essential in microbiology. This set of laboratory notes focuses on differentiating bacteria based on their ability to carry out oxidative (respiratory) or fermentative metabolism, using a variety of biochemical tests. These tests are fundamental for bacterial identification and classification.

O/F Glucose Test

Purpose and Principle

• Purpose: To determine whether a microorganism metabolizes glucose oxidatively (using oxygen) or fermentatively (without oxygen).

• Principle: The O/F (Oxidative/Fermentative) Glucose test uses a differential medium containing glucose, peptone, and bromothymol blue (a pH indicator that turns yellow at acidic pH).

Procedure

• Inoculate two tubes of O/F glucose agar with the test organism by stabbing the agar.

• Overlay one tube with mineral oil to create anaerobic conditions; leave the other exposed to air (aerobic).

• Repeat the procedure with control organisms: Escherichia coli (facultative anaerobe) and Pseudomonas (strict aerobe).

Interpretation of Results

• A color change from green to yellow indicates acid production (positive test).

• Yellow only in the aerobic tube suggests oxidative metabolism.

• Yellow in both tubes suggests fermentative metabolism.

Organism	Aerobic Condition	Anaerobic Condition	Interpretation
Unknown?
Pseudomonas	yellow	green	Strict aerobe
E. coli	yellow	yellow	Facultative anaerobe

Fermentative Tests

Overview

Fermentative tests are used to determine the ability of bacteria to ferment specific sugars and to identify the end products of fermentation. These tests are crucial for distinguishing between bacterial species.

• Specific sugar fermentation (e.g., glucose, mannitol, lactose)

• Mixed acid fermentation (MR-VP test)

• 2,3-Butanediol fermentation

• Citrate fermentation

Specific Sugar Fermentation Reactions

• Use of sugar broth containing a Durham tube to trap gas produced during fermentation.

• Phenol red is used as a pH indicator; it turns yellow in acidic conditions, indicating fermentation.

• Test tubes are inoculated with the unknown organism and controls (e.g., E. coli in glucose broth).

Mixed Acid Fermentation (MR-VP Test)

• Certain bacteria (e.g., Escherichia, Proteus, Salmonella, Aeromonas) ferment glucose to produce a mixture of acids (lactic, succinic, acetic, formic acids) and gases (CO2, H2).

• The MR-VP medium contains glucose, peptone, and dipotassium phosphate.

• The Methyl Red (MR) test detects mixed acid production by a red color change at low pH.

• The Voges-Proskauer (VP) test detects neutral end products (e.g., 2,3-butanediol) using Barritt's reagent.

2,3-Butanediol Fermentation

• Bacteria such as Enterobacter and Serratia ferment glucose to produce 2,3-butanediol, a neutral end product.

• Usually, bacteria are positive for either mixed acid or 2,3-butanediol fermentation, not both.

• This distinction helps in identifying Gram-negative bacteria.

Citrate Fermentation

• Some bacteria can use citrate as a sole carbon source, breaking it down to oxaloacetate and pyruvate, which are further fermented to formate, lactate, acetate, acetoin, and CO2.

• Simmons citrate agar contains citrate and ammonium salts. Utilization of citrate increases pH, changing the medium from green to Prussian blue.

• Positive result: blue color; Negative result: green color.

Result	Interpretation
Blue	Positive for citrate utilization
Green	Negative for citrate utilization

Oxidative Tests

Overview

Oxidative tests are used to detect the presence of enzymes involved in aerobic respiration, such as cytochrome oxidase and catalase, and the ability to reduce nitrate.

Cytochrome C Oxidase Test

• Detects the presence of cytochrome c oxidase, an enzyme that transfers electrons from cytochrome c to oxygen.

• Tested using an artificial electron acceptor, N,N,N',N'-tetramethyl-p-phenylenediamine, which changes color if the enzyme is present.

• Commonly positive in aerobic organisms like Pseudomonas aeruginosa.

Catalase Test

• Detects the enzyme catalase, which breaks down hydrogen peroxide (a toxic byproduct of aerobic respiration) into water and oxygen.

• Positive result: bubbling upon addition of hydrogen peroxide.

• Control: Staphylococcus aureus (positive control).

Nitrate Reduction Test

• Some bacteria use nitrate as a terminal electron acceptor in anaerobic respiration.

• Bacteria may reduce nitrate to nitrite, or further to nitrogen gas (N2) or nitrous oxide (N2O).

• Test uses beef extract medium with potassium nitrate and a Durham tube to detect gas production.

• Detection of nitrite is performed using Nitrate reagents A and B, which produce a deep red color if nitrite is present.

Summary Table: Key Biochemical Tests

Test	Purpose	Positive Result	Example Organism
O/F Glucose	Oxidative vs. fermentative metabolism	Yellow color (acid production)	E. coli, Pseudomonas
MR-VP	Mixed acid or 2,3-butanediol fermentation	Red (MR), Pink (VP)	E. coli, Enterobacter
Citrate	Citrate utilization	Blue color	Enterobacter aerogenes
Cytochrome Oxidase	Presence of cytochrome c oxidase	Color change (dark purple/blue)	Pseudomonas aeruginosa
Catalase	Presence of catalase	Bubbling	Staphylococcus aureus
Nitrate Reduction	Reduction of nitrate to nitrite/gas	Red color/gas in Durham tube	E. coli, Bacillus

Key Equations

• Catalase reaction: $2H_2O_2 \rightarrow 2H_2O + O_2$

• Nitrate reduction: $NO_3^- \rightarrow NO_2^- \rightarrow N_2O \rightarrow N_2$

Additional info:

• These tests are part of the standard biochemical identification of bacteria in clinical and research laboratories.

• Interpretation of results often requires comparison with known control organisms.