The sputum specimen of a 70 year old male was cultured on a 5% sheep blood agar. The culture showed the presence of a-haemolytic colonies next day. The further processing of this organism is most likely to yield.
Ans. is 'b' i.e., Gram positive cocci in pairs, catalase negative and bile solubleStreptococci can be divided into three group based on their specific hemolytic ability.Classification of streptococci based on their ability to hemolyze Red blood cells:Beta hemolytic r They completely lyse the RBC's leaving a clear zone of hemolysis around the colonyAlpha hemolytic - They only partially lyze the RBC's leaving a greenish discoloration of the culture medium surrounding the colony.This discolored area contains analysed RBC's and a green coloured metabolite of hemoglobinGamma hemolytic - These streptococci are unables to hemolyze the RBc's and therefore we should really not use the word hemolytic in this situation (the term non hemolytic streptococci is often used to avoid confusion)In the question the culture of the sputums yields a hemolysis colonies. So we have to look for streptococci causing a hemolysis.Streptococci causing a hemolysis are :Streptococcus pneumoniaViridans streptococciEnterococcus fecalis and other enterococci (group D)Now we move in the reverse direction i.e. we take each options one by one and see whether they fit in any of the above streptococci.Option 'd'It can be easily ruled out as streptococci are gram positive Option 'c'This one also falls one of favour as streptococci are catalase negative (staphylococcic are catalase positive).Now we are left with options 'a' and 'b'The characteristics given in option 'a' matches with those of streptococci viridans and option 'b' matches with those of streptococcus pneumonia. Here is a comparison of Streptococcus pneumonia and Streptococcus viridans Streptococus pneumoniaeStreptococcus viridansMorphologyCapsulated, lanceolate diplococciNon capsulated, oval or round cells in chainsHemolysisa hemolytica hemolyticQuellung testPositiveNegativeColoniesInitially dome, later draughtsman coloniesDome - shapedGrowth in liquid mediaUniform turbidityGranular turbidity, powdery depositBile solubilityInvariably positiveInvariably negativeOptochin sensitivityPositiveNegativeIntraperitoneal inoculation inFatal infectionNonpathogenic in miceBut the point which favours streptococcus pneumonia is, presence of respiratory tract infection (probably pneumonia) in the patient, Streptococcus viridans rarely causes respiratory infection.
Gram positive cocci in short chains, catalase negative and bile resistant
Gram positive cocci in pairs, catalase negative and bile soluble
Gram positive cocci in clusters, catalase positive and coagulase positive
Gram negative coccobacilli catalase positive and oxidase positive
Bacteria