Question

Which result you may expect if TSI tube was inoculated with a colorless colony from MacConkey agar?

A. Yellow slant/yellow butt, blackening

B. Red slant/yellow butt, blackening

C. Yellow slant/ yellow butt, no blackening

D. Red slant/ yellow butt, no blackening

E. Any of the above

Answer 1

Answer:

Option E) Any of the above

Rationale:

• MacConkey agar is one of the selective and differential indicator medium.It is used to selectively isolating gram negative enteric bacteria.And also to differentiate their ability to ferment lactose on the medium.This can be identified by the indicator present in the medium.Lactose fermenters produces acid so the medium pH changes and color of neutral red turns to pink and non lactose fermenters produce colorless colonies.
• So from this the non lactose fermenting colorless colonies can be inoculated in the TSI tubes.Triple sugar test is widely used to determine whether the microorganism have the ability to ferment sugars and to produce gas( hydrogen sulfide).TSI tube has 1%lactose, 1% sucrose,0.1% glucose,sodium thiosulfate,phenol red as indicator and ferrous ammonium sulfate.
• Generally the bacteria can ferment the sugars together or separately and produce by product usually acids. Acid production will change the red color medium to a yellow color. Position of the color (top or at bottom or on both) indicates the acid production which is caused by glucose fermentation or lactose or sucrose fermentation.
• Balck precipitate is seen in butt it is because some bacteria have the ability to use sodium thiosulfate in the medium and produce H2s gas.H2s react with ferrous group and form a visible black precipitate.
• When an organism doesn't ferment lactose and ferments glucose, initially the tube is a yellow slant/yellow bottom after 8 hours it gets converted to a red slant/yellow bottom and stays the same after 24 hours this is because of alkali/acid reaction because glucose produce very low amount of acid after sometime CO2 and H2O released from organisms oxidise the slant (aerobic) so it becomes alkali and butt remains yellow because of anaerobic condition.
• If sucrose is fermented it leads to production of more acid in the medium and causes yellow slant/yellow butt,with or without H2s production based on organisms ability. If no sugars are fermented both slant and butt remains Red.For lactose fermentation both slant and butt looks yellows and cracks will be seen in the medium because of other gas production( organisms which have the ability to produce gas alone lifts the agar).

Hope this helps you:)