Use of selective and differential media to identify bacteria

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Introduction

MacConkey agar is generally used in the isolation of gram-negative bacteria and the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria. MacConkey agar is also common to differentiate bacteria by their characteristics to ferment sugars other than lactose such as lactose is being replaced in the medium by another sugar. These modified media are widely used to differentiate gram-negative bacteria or to differentiate between phenotypes with mutations (Vasanthakumari, 2009).

MacConkey can be either selective or differential media to be used in the isolation and differentiation of gram negative rods. The inclusion of crystal violet and bile salts in the media prevent the growth of gram-positive bacteria and fastidious gram-negative bacteria . Some gram negative bacteria are able to tolerate bile due to their relatively bile-resistant outer membrane which protects the bile-sensitive cytoplasmic membrane. Gram negative bacteria growing on the media are differentiated by their characteristic to ferment sugar lactose. Bacteria that can ferment lactose causes the pH of the MacConkey media to drop and the change in pH is detected to be red in colour. As the pH continues to drop in the media, the neutral red is then absorbed by the bacteria which appears as bright