The Oxidation-Fermentation (OF) Test
The Oxidation-Fermentation (OF) Test, often referred to simply as the OF test or the Hugh and Leifson test, is a fundamental biochemical technique in microbiology. Developed by Hugh and Leifson in 1953, its primary purpose is to differentiate bacteria, particularly Gram-negative rods, based on their metabolic pathways for carbohydrate utilization, typically glucose. Bacteria can catabolize carbohydrates via two distinct primary routes: oxidative metabolism (aerobic respiration), which requires oxygen, and fermentative metabolism, which can occur independently of oxygen. The OF test provides a clear visual distinction between organisms that are strictly oxidative, those that are fermentative (utilizing both aerobic and anaerobic pathways), and those that are non-saccharolytic (unable to utilize the test carbohydrate at all).
Principle of the OF Test
The principle of the OF test revolves around detecting the production of acid from a test carbohydrate, most commonly glucose, under two different environmental conditions: one aerobic (open to air) and one anaerobic (sealed from air). The test medium, known as Hugh and Leifson’s medium, is a semi-solid agar that contains a specific concentration of the carbohydrate, a small amount of peptone, and the pH indicator bromothymol blue.
During metabolism, the end products of the carbohydrate catabolism determine the color change. Fermentative organisms rapidly metabolize the carbohydrate in the anaerobic environment, producing a relatively high concentration of strong organic acids (such as lactic acid and acetic acid). This substantial acid production dramatically lowers the medium’s pH, causing the bromothymol blue indicator to turn from its original green color (neutral pH) to bright yellow (acidic pH).
In contrast, organisms that are strictly oxidative metabolize the carbohydrate through aerobic respiration, which generates much smaller quantities of weaker organic acids during pathways like glycolysis and the Krebs cycle. For this reason, the OF medium is formulated to be more sensitive than traditional fermentation media. The increased concentration of glucose (1.0% compared to 0.5% in some other media) and a decreased concentration of peptone (protein source) are crucial. The higher glucose ensures enough substrate is available for detectable acid production, while the lower peptone minimizes the production of alkaline amine products that could mask the weak acid color change. The color change to yellow in the open tube signifies acid production, confirming carbohydrate utilization.
Composition and Preparation of OF Medium
The specialized composition of Hugh and Leifson’s OF basal medium is what provides its distinct functional advantage and high sensitivity. The key components typically include: glucose (or another carbohydrate, such as xylose, mannitol, lactose, sucrose, or maltose) at a high concentration; peptone at a low concentration; sodium chloride and dipotassium phosphate to maintain osmotic balance and buffering capacity, respectively; the pH indicator bromothymol blue; and agar at a reduced concentration (2.5-3.0g/L).
The indicator, bromothymol blue, is green at a neutral pH of 7.1. As the pH drops due to acid production (typically below pH 6.0), the color shifts to yellow. If the bacteria break down the small amount of peptone rather than the carbohydrate, alkaline amines are produced. This raises the pH (typically above pH 7.6) and causes the medium to turn a deep blue, which signifies a non-saccharolytic (non-carbohydrate-using) result.
The semi-solid consistency, achieved by the low agar concentration, allows for easy stab-inoculation and promotes uniform color diffusion throughout the medium, which is important for observation. The low agar content also assists in determining motility of the organism, although that is not the primary purpose of the test. To ensure truly anaerobic conditions in the covered tube, the media is often boiled to remove dissolved oxygen and allowed to cool immediately before inoculation.
Test Procedure and Anaerobic Conditions
The OF test is performed in duplicate for each test organism to establish the two essential environmental conditions. The standard procedure is as follows:
Firstly, two separate tubes of OF medium are inoculated with a pure, isolated colony of the test organism. Inoculation is performed using a sterile inoculating wire by stabbing the semi-solid agar vertically deep into the medium (around 2.5 to 3 cm depth). Care must be taken not to introduce air bubbles, which can complicate the interpretation.
Secondly, one of the two inoculated tubes is overlaid immediately with a layer of sterile mineral oil, typically about 1 cm thick. This oil layer acts as a physical barrier, preventing the diffusion of atmospheric oxygen into the medium, thereby creating the necessary anaerobic environment for the fermentation-only reaction. The cap of this tube (the covered or sealed tube) is tightened.
The second tube is left open to the air, allowing for aerobic respiration to occur, establishing the oxidative condition. The cap of this tube (the open or unsealed tube) is left loose to ensure free gas exchange and oxygen availability.
Finally, both tubes are incubated aerobically at an appropriate temperature, usually 35 ± 2°C, for 24 to 48 hours. While results are often visible quickly, organisms that are slow-growing or ‘late fermenters’ may require observation for up to 4, or even 14 days, before a definitive color change can be recorded. Prolonged incubation is sometimes necessary to prevent a false-negative interpretation.
Interpretation of OF Test Results
The result of the OF test is determined by observing the color change of the bromothymol blue indicator in the open and covered tubes after incubation. The three main possible outcomes are:
Fermentative Organism
The result is Fermentative if the organism is capable of acid production under both aerobic and anaerobic conditions. Both the open tube (where both oxidation and fermentation can occur) and the covered tube (where only fermentation can occur) will show a color change to yellow throughout the medium. This indicates the strong acid production characteristic of fermenters. A classic example of a fermentative organism is *Escherichia coli*, which belongs to the *Enterobacteriaceae* family.
Oxidative Organism
The result is Oxidative if the organism can only metabolize the carbohydrate via aerobic respiration. In this case, the open tube will turn yellow, typically starting near the top surface where oxygen is readily available, while the covered tube will remain green or occasionally turn blue. The lack of a color change in the anaerobic tube confirms the organism is unable to perform fermentation. This reaction pattern is common among non-fermenting Gram-negative bacteria such as *Pseudomonas aeruginosa*.
Non-saccharolytic Organism
The result is Non-saccharolytic (or negative) if the organism is unable to metabolize the test carbohydrate by either oxidation or fermentation. Neither tube will turn yellow. Both tubes will remain green (no change in pH). However, the open tube may sometimes turn blue at the surface due to the organism utilizing the peptone in the medium, which liberates alkaline amine by-products that raise the pH. An example of a non-saccharolytic organism is *Alcaligenes faecalis*.
Significance and Applications
The OF test is a critical early step in the identification schema of various bacteria in the clinical and diagnostic microbiology laboratory. Its primary utility lies in rapidly and reliably classifying unknown bacterial isolates into metabolic groups:
– **Differentiation of Gram-Negative Rods:** It is indispensable for distinguishing the fermentative organisms (e.g., *Enterobacteriaceae*) from the non-fermentative Gram-negative rods (e.g., *Pseudomonas* and *Alcaligenes* species). This initial biochemical categorization significantly narrows the number of potential organisms, guiding the selection of subsequent identification tests.
– **Classification of Gram-Positive Cocci:** A modification of the medium can be used to distinguish between the fermentative *Staphylococcus* genus and the oxidative *Micrococcus* genus, which have similar Gram-stain characteristics.
– **Study of Carbohydrate Metabolism:** The test provides concrete evidence regarding the carbohydrate-utilizing ability of a bacterium under defined aerobic and anaerobic conditions, which is a key characteristic for species differentiation.
Despite its utility, the OF test is a phenotypic assay and must be complemented by other biochemical, immunological, and molecular methods for complete and accurate bacterial identification. Furthermore, the possibility of false results due to the acidity of mineral oil, prolonged incubation needs for slow-growers, or the inability of some fastidious organisms to grow in the medium necessitates careful technique and interpretation.