Lactobacillus acidophilus is a gram-positive bacterium commonly used in yogurt p

Question

Lactobacillus acidophilus is a gram-positive bacterium commonly used in yogurt production. It is an obligate homofermentative bacterium, which means that it is only capable of carrying out fermentation metabolism to produce lactic acid. It is also an aerotolerant anaerobe, which means it is able to tolerate atmospheric oxygen concentrations but does not use oxygen in its metabolic processes. Describe the results you would expect to obtain if you tested this bacterium using the blood agar test. Explain the reason you would expect to observe that result. Only use the information given in the question.

Explanation / Answer

Blood agar is an enriched media containing 5% sheep blood with other general nutrients and is used for cultivating fastidious microorganisms. Certain microorganisms like Streptococcus pneumoniae, Streptococcus pyogenes and Listeria monocytogenes grow in this media and produce exozymes, commonly reffered as hemolysins which lyse red blood cells (hemolysis) and degrade hemoglobin. Thus, partial or complete hemolysis around and under the growing colonies is observed. There are mainly three type of hemolysins, namely alpha hemolysin, beta hemolysin and gamma hemolysin. Bacterial species, for example Streptococcus pyogenes producing beta hemolysin are capable of complete destruction of red blood cells and hemoglobin. Such results are called as beta-hemolysis where a clear zone around the bacterial growth colonies is observed. On the other hand, Streptococcus pneumoniae cause partial hemolysis, mostly under aerobic condition, leaving a greenish tint around its colonies which is a result of by-product of hemoglobin oxidation. This is known as alpha hemolysis. If the microorganisms do not induce hemolysis, then the colonies show neither of the alpha nor beta hemolysis characters. There may be slight discolouration of the medium. This is called gamma hemolysis.

Lactobacillus acidophilus do not posses any hemolytic character as they do produce . They will form characteristic rough, small, convex, opaque colonies but not show any characristic hemolytic pattern. If the blood agar media plate is kept for longer time then slight discolouration around and at the bottom of the colonies will be obsereved.  

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