Microbiology Lab Exam

The correct answer is "Salt and Mannitol." MSA (Mannitol Salt Agar) is a selective and differential medium used for the isolation and differentiation of Staphylococcus species. The high concentration of salt (7.5%) in MSA inhibits the growth of most bacteria except for staphylococci, which can tolerate high salt concentrations. Mannitol, a sugar alcohol, is included in MSA as a differential component. Staphylococcus aureus, a pathogenic species, ferments mannitol, causing a change in the color of the agar from red to yellow. This allows for the differentiation of S. aureus from other non-fermenting staphylococci.

The black precipitate in a TSIA tube is caused by the reaction between sulphide and iron salt, which forms ferrous sulphide. This reaction occurs when sulphide ions react with iron ions in the presence of a suitable salt. The formation of ferrous sulphide results in the black precipitate observed in the TSIA tube.

E.coli does turn red/pink on MAC agar, giving a positive reaction. MAC agar is a selective and differential medium used for the isolation of Gram-negative bacteria, such as E.coli. It contains crystal violet and bile salts which inhibit the growth of Gram-positive bacteria, allowing only Gram-negative bacteria like E.coli to grow. Additionally, MAC agar contains lactose and a pH indicator called neutral red. E.coli can ferment lactose, producing acid, which causes the pH indicator to turn red/pink. Therefore, the positive reaction indicates the presence of E.coli on MAC agar.

Enterobacter is lactose positive, meaning it can ferment lactose and produce acid. This acid production causes the MAC agar to turn red or pink. Proteus and Shigella, on the other hand, are lactose negative and do not ferment lactose, so they would not cause the agar to change color.

If the bacteria cannot ferment lactose and/or sucrose, it means that they are unable to break down these sugars for energy. In the TSIA tube, the presence of lactose and sucrose is indicated by the production of acid, which turns the tube yellow. However, since the bacteria cannot ferment these sugars, there will be no acid production, and therefore the tube will remain pink.

If there is growth in both tubes of an O/F test, it indicates that the bacteria can grow in both aerobic and anaerobic conditions. This suggests that the bacteria is a facultative anaerobe, meaning it can switch between aerobic and anaerobic metabolism depending on the availability of oxygen.

Shigella is lactose negative, meaning it does not ferment lactose. This can be observed on MAC agar, where lactose fermenting bacteria produce pink colonies while lactose negative bacteria produce brown colonies. Therefore, Shigella turns brown on MAC agar.

The correct combination for E.coli is Lac +, MR +, indole +. This means that E.coli can ferment lactose (Lac +), produces acid from glucose (MR +), and can produce indole.

The correct answer is yellow because when glucose is fermented, it produces acid, which causes the pH of the medium to become acidic. In an O/F tube, a pH indicator called bromthymol blue is used. It turns yellow in an acidic environment, indicating that fermentation of glucose has occurred. Pink and purple are not the correct answers because they do not correspond to the color change caused by fermentation.

E. coli is the only Indole positive bacteria among the given options. Indole is a compound that is produced by certain bacteria when they break down the amino acid tryptophan. E. coli is known to produce indole, while Enterobacter and Salmonella are not. Therefore, E. coli is the correct answer.

Proteus and Salmonella are both ONPG negative, meaning that they do not produce the enzyme necessary to metabolize ONPG (o-nitrophenyl-beta-D-galactopyranoside). This enzyme is typically produced by some bacteria to break down ONPG into a yellow compound, indicating a positive reaction. Since both Proteus and Salmonella are negative for this reaction, the statement is true.

Salmonella does not ferment lactose, which is why it turns light brown on MAC agar. E. coli and Klebsiella, on the other hand, do ferment lactose and would appear pink or red on MAC agar.

The slope of a TSIA tube turns yellow when the bacteria can utilize lactose and/or sucrose. This indicates that the bacteria are fermenting the sugars present in the medium, producing acid and causing a change in color. Pink or purple colors would indicate different metabolic reactions or the absence of sugar fermentation.

Klebsiella is a lactose positive bacteria because it has the ability to ferment lactose, a sugar found in milk. This can be determined by observing the bacteria's ability to produce acid and gas when grown on a lactose-containing medium. Salmonella and Proteus, on the other hand, are lactose negative bacteria as they do not possess the necessary enzymes to break down lactose.

If the VP test comes back positive, it means that the MR result is negative.

Shigella and Klebsiella are both non-motile bacteria. Shigella is a genus of bacteria that causes severe diarrhea and is incapable of motility. Klebsiella, on the other hand, is a genus of bacteria that can be motile, but it is more commonly non-motile. Therefore, the statement that Shigella and Klebsiella are both motile is false.

Bacteriacin sensitivity is a method used to differentiate Beta Haemolytic bacteria. Bacteriacins are proteinaceous toxins produced by certain bacteria to inhibit the growth of other bacteria. By testing the sensitivity of the bacteria to bacteriacins, it can be determined whether they are Beta Haemolytic or not. If the bacteria are sensitive to bacteriacins, it indicates that they are Beta Haemolytic.

Shigella does not ferment glucose to acid and gas. Shigella is a type of bacteria that causes gastrointestinal infections, specifically shigellosis. Unlike some other bacteria, such as Escherichia coli, Shigella does not produce gas during fermentation. Instead, it ferments glucose to acid only. Therefore, the correct answer is False.

TBA stands for Tellurite Blood Agar, which is a selective and differential medium for Corynebacterium. The presence of tellurite in the agar inhibits the growth of most bacteria except for Corynebacterium, making it a selective medium. Additionally, the medium contains blood, which allows for the differentiation of Corynebacterium species based on their ability to hemolyze red blood cells. Therefore, TBA is both selective and differential for Corynebacterium.

Egg yolk contains lecithin, a type of phospholipid. Lecithin can be broken down into fatty acids by the enzyme lecithinase.

The control LIA tube tests for glucose fermentation. This test is used to determine if an organism can ferment glucose as a source of energy. In the LIA (Lysine Iron Agar) test, the presence of glucose fermentation is indicated by a yellow color change in the medium. The absence of glucose fermentation results in a purple color. Therefore, the control LIA tube is used as a comparison to determine if the organism being tested can ferment glucose.

Enterobacter is VP positive because it produces acetoin from glucose fermentation. The Voges-Proskauer (VP) test is used to determine the ability of an organism to produce acetoin, which is detected by the addition of alpha-naphthol and KOH to the culture. If the organism is VP positive, a red color develops, indicating the presence of acetoin.

MAC agar is a selective and differential medium used for the isolation and differentiation of Gram-negative bacteria, particularly members of the Enterobacteriaceae family. The agar contains bile salts, which inhibit the growth of Gram-positive bacteria, and the sugar mannitol, which allows the differentiation of lactose-fermenting and non-fermenting bacteria. Lactose-fermenting bacteria produce acid, causing the agar to turn yellow, while non-fermenting bacteria do not produce acid and the agar remains pink. Therefore, the presence of salt and mannitol in MAC agar allows for the selective and differential identification of specific bacterial species.

Proteus and Salmonella are both known to produce H2S. H2S is a gas that has a distinct rotten egg smell and is produced by certain bacteria during the breakdown of sulfur-containing compounds. E. coli, Enterobacter, Shigella, and Klebsiella are not typically associated with H2S production.

The correct answer is Protease and Klebsiella. Urease is an enzyme that breaks down urea into ammonia and carbon dioxide. E. coli and Salmonella are both negative for urease, meaning they do not produce the enzyme. However, Protease and Klebsiella are positive for urease, indicating that they can break down urea.

The test used to differentiate between alpha haemolytic bacteria is optochin sensitivity and bile solubility. Optochin sensitivity test helps in identifying Streptococcus pneumoniae, which is sensitive to optochin, while other alpha haemolytic bacteria are resistant. Bile solubility test is used to differentiate between S. pneumoniae and other alpha haemolytic bacteria. S. pneumoniae is bile soluble, whereas other alpha haemolytic bacteria are not.

E. faecalis (gamma haemolysis) is a type of bacteria that can grow on media containing NB + 6.5% NaCl and MAC agar. However, it cannot grow on TSIA media. Gamma haemolysis refers to the lack of hemolysis or destruction of red blood cells. E. faecalis is able to tolerate high salt concentrations, which is why it can grow on NB + 6.5% NaCl media. MAC agar is a selective and differential medium that allows for the growth of gram-negative bacteria while inhibiting the growth of gram-positive bacteria. Therefore, E. faecalis can grow on MAC agar as well.

E. coli and Shigella are citrate negative and do not use citrate as a carbon source. Citrate negative bacteria are unable to utilize citrate as a sole source of carbon for growth. This can be determined by performing a citrate utilization test, where the bacteria are grown on a medium containing citrate. If the bacteria are unable to use citrate, the medium remains green. E. coli and Shigella are both members of the Enterobacteriaceae family and belong to the same genus, which explains their similar metabolic characteristics.

E. coli, Enterobacter, Salmonella, and Klebsiella are all capable of decarboxylating lysine in LIA tubes.

The purpose of the ELEK test is to differentiate C. diphtheriae from other normal flora organisms known as diphtheroids. This means that the test is used to distinguish the specific bacterium C. diphtheriae from other bacteria that are similar in appearance and may be present in the normal flora of individuals.