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Use of selective and differential media to identify bacteria

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Introduction

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 pink to red colonies on the agar (Pattison et al., 2008). ...read more.

Middle

shape -Small sized colony -Yellow colony with smooth margin -Medium sized colony - White colony with smooth margin MacConkey 1 There were no growth visible - Red colony with smooth margin -Medium sized -Red colony with smooth margin - Beige colony with smooth margin and slightly raised MacConkey 3 There were no growth visible - There were no growth visible - Red colony with smooth margin - Beige colony with smooth margin and slightly raised Gram Stain Positive ( rod shaped ) Positive ( cocci shaped) Negative ( Rod Shaped ) Negative ( rod shaped ) Based on the results obtained in table 1, it can be concluded that of the four bacterium tested for Gram stain, two of the bacterium which is Bacillus megaterium and Enterococcus faecalis were tested gram positive and the arrangement were chain like and pairing respectively. Moreover the other two bacterium which consist of Esherichia coli and Proteus vulgaris both tested Gram negative and both also are observed to be rod- shaped. All four bacterium were also observed to grow in the nutrient agar plate. But only Bacillus megaterium did grow when placed in the media MacConkey agar plate 1. Besides, both Bacillus megaterium and Enterococcus faecalis did not grow when placed in the media MacConkey agar plate 3. ...read more.

Conclusion

. Non-fastidious are organisms produce their own nutrients do not need special agar to grow. Thus, based on this experiment, nutrient agar is a non-selective medium that allows all four organisms to grow which is Bacillus megaterium ,Enterococcus faecalis, Esherichia coli and Proteus vulgaris. As MacConkey agar plate 1 contains bile salt 2, which inhibits the growth of most Gram-positive bacteria except the enterococci. MacConkey agar plate 3 contains bile salt 3 that allows only the Gram negative organism to grow. This can be shown on the table above as when the mixed broth culture is inoculated to the Mac 3 plate, only Esherichia coli and Proteus vulgaris can be seen growing in the media (Vasanthakumari, 2009) . Conclusion Based on the experiment, it can be concluded that Bacillus megaterium are rod-shaped and arranged in a chain-like sequence. Besides, Enterococcus faecalis is spherical in shape and usually grouped in pairs. Esherichia coli are rod- shaped and arranged in clusters. Lastly Proteus vulgaris are rod shaped and usually randomly arranged with no particular arrangement. Only the Gram negative bacteria are able to grow on the MacConkey 3 plate . MacConkey agar plate 1 inhibits the growth of most Gram positive bacteria which is why Enterococcus faecalis can be seen growing in the media. ...read more.

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