Mannitol Salt Agar

The cell wall of a bacterial cell has many functions. Not only does it protect the interior of the cell from changes in its environment but it also helps give the cell shape. In bacterial cells, the cell wall helps prevent cytolysis due to an imbalance of water pressure between the inside and the outside of a cell. The cell wall also contributes to the pathogenicity of a species. As explained by our textbook, “the chemical composition of the cell wall helps to differentiate major types of bacteria” (Tortora, Funke, and Case 84). The two main types of bacteria are gram positive and gram negative. Their cell walls are composed of peptidoglycan. The peptidoglycan is a chain of repeating disaccharide held together by polypeptides. The disaccharide …show more content…
The first test involved the Mannitol Salt Agar (MSA). MSA is an example of both a selective and a differential media. It selects for halotolerant bacteria and with the aid of phenol-red differentiates for the ability of a microbe to ferment mannitol or not. If a microbe has the ability to ferment mannitol, it produces acidic waste products which react with the phenol-red to yield a yellow color. If the microbe is unable to ferment mannitol, it will consume the proteins present and will produce a pink/red color. The DNase agar is a differential media that differentiates between microbes who have the ability to produce the DNase enzyme or not. The agar contains DNA as well as methyl green and peptides. If a microbe can produce the enzyme, the agar will appear colorless. If a microbe cannot produce DNase, the agar remains green. Another differential test that was done was the coagulase test. This test differentiates between microbes that can convert fibrinogen into fibrin(clotting). If clotting is present, then the microbe is coagulase positive; otherwise the microbe is coagulase negative. The catalase test differentiates between microbes than can produce the enzyme catalase or not. If a microbe has the catalase enzyme, when hydrogen peroxide is added, bubbles appear making it catalase positive. If no gas or bubbles are observed, the microbe is catalase negative. A differential agar used for this …show more content…
Based on the data collected, we were able to truly see how different each of these closely related microorganisms really are. Each microorganism has their own specific chemical makeups. In our biochemical analysis of gram negative and gram positive bacteria, we were able to determine several characteristics of our bacteria samples. In our biochemical analysis of gram negative bacteria, we were able to determine that out of our 8 samples, Proteus vulgaris, Pseudomonas aeruginosa, and Salmonella enteritidis are unable to ferment lactose. Klebsiella pneumonia and Enterobacter aerogenes can ferment lactose to a certain extent where as Citrobacter freundii and Escherichia coli excessively ferment lactose. In our indole test, only Escherichia coli and Proteus vulgaris are able to convert tryptophan to indole. We were able to learn a lot from the TSI agars. The Citrobacter freundii, Proteus vulgaris, and Salmonella enteritidis all utilized sulfur and thus produced H2S leaving behind a black color at the bottom of the test tube. Based on the red color we know that Pseudomonas aeruginosa has exhausted the glucose present in the TSI agar. Serratia marcescens utilizes the sugars very slowly. This can be accredited to the fact that the slant is pinkish but the bottom of the test tube is still yellow. In our biochemical analysis of gram positive bacteria, we learned that only Staphylococcus aureus