Question

I am always found in pairs, sometimes the pair appear as short chains. I am protected by a coat that can swell in the presence of specific antiserum. I am not exactly spherical but more lanceolate with sides facing each other being more rounded. I can't survive on simple nutrition and presence of blood boosts my growth. I can lyse blood but only partially. I grow better in a candle jar. As I grow old, I become suicidal. My colonies may have central depression and may appear like carom coin. I don't froth in H2O2. Sometimes people carry me in their throat and I often infect their lower respiratory tract. I can infect lungs, middle ear, and even brain.

which organism i am.

which

Answer 1

The organism mentioned has features similar to that of Streptococcus pneumoniae. These are Gram positive bacteria which are lancet shaped and usually present in diplococcus form. They are found in the throat and cause blood infections, pneumonia(lungs), meningitis(brain) and also middle ear infections in children. It is a fast growing bacterium which needs optimal conditions like 35-37°C with ~5% CO2 and can grow in candle jar as well. As the growth curve of this species reaches its stationary phase (old) they begin to die or become suicidal. This phenomena is dependent on the secretion of hydrogen peroxide by the bacteria due to the aerobic metabolism by pyruvate oxidase. Moreover,these bacterias do not express the enzyme catalase which is required for the lysis of hydrogen peroxide, so for their growth they require blood agar as a medium to neutralize the large amount of H2O2 produced. The bacteria partially lyse the RBC cells to produce a green coloured hemoglobin molecule (alpha hemolysis) which results in the formation of depression like appearance.

So for the diffrentiation and identification of this bacteria from the others a catalase test is run. For this test , if catalase enzyme is produced by the cell then the will be broken down into H2O and O2. The oxygen produced gives off as bubbles in the liquid which result in froth formation. S.pneumoniae are catalase negative so they do not produce any froth.