Question

Explain the different biochemical tests of bacteria that rely on the fermentation of carbohydrates?

Answer 1

Answer:

There is a vast array of biochemical tests used for identifying the bacteria. One of the important component in these tests is ability of bacteria to ferment certain sugars and others which can not. These tests give better understanding of bacterial metabolism and it also help to differentiate one group from other.

Most of these tests for carbohydrate fermentation use a liquid nutrient broth which contains the carbohydrate being tested as the only source of Carbon and energy. An indicator is used to see change in pH that accompanies fermentation of sugar due to production of acid. Some bacteria also produce gas which can be seen as bubbles collected in inverted Durham's tube in the liquid broth or as creaked solid media if so used.

Among the many tests that had been applied for fermentation of carbohydrate a bunch of these are explained in brief hereafter:

Glucose -It tests the ability of the bacteria to ferment glucose as well as its ability to convert the end products of glycolysis to gas. It is commonly used to differentiate between gram negative bacilli of Enterobacteriaceae depending on their ability to produce acid and gas.  

Lactose - This is the major test differentiating common bacteria into lactose fermenters and lactose non-fermenters. This test is commonly used as part of growth/ isolation media like MacConkey's agar.

Sucrose - Not many bacteria can use sucrose making it a taste of choice to differentiate between the 2 groups. Among Enterobacteriaceae group Klebsiella is found to be sucrose positive.

Arabinose - Most of the Lactose fermenters also ferment arabinose with or without production of gas but most of the Lactose non-fermenters do not ferment arabinose. For example - Salmonella, Shigella, Proteus, etc.

Maltose - A disaccharide of glucose , not utilized by many organisms helps in species identification. Proteus vulgaris ferments maltose but Proteus mirabilis cannot.

Mannose - Almost following similar reactions like arabinose , organisms Salmonella, Shigella, Proteus do not ferment mannose. Ability of Staphylococcus aureus to ferment mannose is used as an identification tool.

Raffinose, Ribose and Xylose are other carbohydrates fermented differentially by common bacteria.

TSI - Triple Sugar Iron agar is a very good differentiating tool to identify bacteria that use 3 sugars present in the medium. ( Glucose, Lactose, Sucrose) under aerobic and anaerobic conditions. It combines polypeptides in the media to know which bacteria (Pseudomonas) will not ferment Lactose and Sucrose and rather use the polypeptides as source of Carbon. It also differentiates bacteria as H2S producers and non producers

KIA - Kliger's Iron agar is a differential medium to identify bacteria that ferment Glucose and Lactose to form Acid an gas and also identify those who produce H2S.