Question

All of the following are characteristic of Staphylococcus aureus EXCEPT
1- produces coagulase 2- ferments mannitol 3- grows on an MSA plate (i.e., halotolerant) 4- grows in chains

You have isolated gram-positive cocci from a throat culture that you cannot identify as staphylococci or streptococci. A test for one enzyme can be used to distinguish these bacteria quickly. What is the enzyme?
1- dogalase 2- catalase 3- lactamase

A candle jar is used to culture Streptococci because they
1- grow best at reduced oxygen levels 2- are obligate anaerobes 3- enjoy the romance a candle can provide

Bacteria that produce clear zones on a blood agar plate, are secreting a type of
1- sigma-hemolysin 2- alpha-hemolysin 3- beta-hemolysin 4- no hemolysin

Blood agar is
1- a differential medium 2- a selective medium

Answer 1

Q1) Which is not the characteristic features of Staphylococcus aureus-

Answer:- 4- grows in chains.

Explanation:- Staphylococcus aureus are Gram positive, round shaped bacterium. S.aureus is differentiated from other staphylococci by coagulase test, they ferment mannitol or are mannitol positive, Mannitol salt agar or MSA plate is the selective and differential medium for S.aureus, they can tolerate high salt concentration of MSA plate, as S.aureus can ferment mannitol, they can turn the phenol red in the agar to yellow.

But Staphylococcus grows in clusters not in chains, so option -4 is the answer,

Streptococcus bacteria grows in chains.

Q2) The most important test to differentiate between staphylococcus or streptococcus is-

Answer:- 2- catalase test.

Explanation:-As both Staphylococcus and Streptococcus are Gram positive cocci, to differentiate between them the test done is catalase test, it is positive in presence of catalase enzyme in Staphylococcus, and negative for Streptococcus. So it is an important test to determine whether the gram +ve cocci is Staphylococcus or Streptococcus. Catalase is an enzyme which can convert hydrogen peroxide into water and oxygen gas. In this test the bacteria is mixed with H2O2 if bubbles appear, due to production of oxygen gas than the bacteria is catalase positive that is Staphylococcus found. But if no bubbles appear than it is catalase negative that is Streptococcus is present.

If the bacteria is Staphylococcus than coagulase test is done to distinguish between S.aureus (coagulase positive) and S.epidermis (coagulase negative).

Q3) candle jar is used to grow Streptococci because –

Answer- They grow best in reduced oxygen level.

Explanation:- Most of the streptococci are facultative anaerobes they grow in presence as well as in absence of oxygen some streptococci are obligate anaerobes they grow in absence of oxygen, so when a candle is burned in a jar the level of oxygen get reduced in the jar which helps in the growth of streptococcus bacteria. Some Streptococci have some components which are very sensitive to oxygen so they better grow in a candle jar where co2 level is more and O2 is less.

Q4) Bacteria which produces a clear zone on blood agar plate are secreting a type of –

Answer- 3- Beta haemolysin.

Explanation- in case of beta haemeolysis , a clear zone is produced because RBC is haemolyzed completely. so option -3 is correct.

In case of alpha haemolysis a green zone is produced around the bacterial growth due to partial haemolysis of RBC.

In case of gamma haemolysis there is no change around the bacterial growth, because blood is not haemolysed.

Q5) Blood agar is considered as a-

Answer- Differential media

Explanation- Differential media are the bacterial growth medium used to identify and differentiate the closely related microorganisms, by the presence of some specific dyes or chemicals the organisms produce characteristic changes or growth patterns, which helps in differentiating the micro-organisms. Blood agar is a differential media, because it helps in detection of haemolysis by cytolytic toxins produced by some bacteria like some strains of bacillus, Streptococcus, enterococcus etc.

It is usually used to detect presence of Strepotococcus pyogenes.

in selective media it allows growth of a certain bacteria but do not allow the growth of other organisms.