gram stain??

[~eryna~]

i put this in homework help but i got no reply so i will try here

 

I have this question for micro220. It has me stumped

 

A Gram-stain on a bacterial suspension showed that the organism was Gram-positive. The suspension was left overnight and a Gram-stain showed the presence of a mixture of purple and pink staining cells. Explain

 

I’m sure there is a really simple explanation for this, I just can think. Any ideas would be appreciated

[ecoli]

you know what gram-positive and gram-negative bacteria are, right?

[claytonm]

I have two thoughts on this one.

 

1. The bacterial suspension could contain two strains of bacteria. The first is a Gram + (purple) that is at a very high optical density and has a fast growth rate. The second is a Gram - (pink) that has a slow growth rate and was at a very low optical density. The Gram - may not have been differentiable when the first Gram stain was performed since it was at such low numbers. When the suspension was left to grow overnight, the Gram - grew to a density that was viewable for the second Gram stain. This would explain the mixture of Gram + and Gram - bacteria present

 

2. The first gram stain could have been performed incorrectly. The smear prep on the slide could have been too thick, thereby not allowing the Decolorizing 95% Ethanol to be effective, or the Alcohol may not have been left on the slide long enough to be effective.

 

I think that choice 2 is the most likely explanation...I have seen this very thing happen in an intro micro lab when students are asked to make Gram stains. Choice 1 is less likely because even though the Gram - may have been at very low numbers, it still should have been detected in the initial Gram stain. I think your professor/teacher was trying to throw you off by saying that the culture was left to grow overnight. This is just my opinion so see what others have to say about it.

[~eryna~]

lol, yes i know what a gram stain is. i have done about 20 of them in the last 4 days

 

thankyou for your reply claytonm

 

i was thinking the same thing, but i was not completely sure. it seems like a very open question. i thought maybe the conditions were more favourable for the gram -ve then the positive and they out numbered the +ve over a period of time

[NPK]

Some bacteria are Gram variable and exhibit staining properties in different parts of their growth cycle. eg. a bacterium I have been working with, Thermoanaerobacter ethanolicus, stains Gram positive during early logarythmic growth, and stain Gram negative from a growing culture.

[frostyburn]

I would go with what claytonm said in part 1. of his post, but having a gram variable bacteria as NPK stated is also a possibility.

 

Another possibility could be that as the bacteria continued to grow in the culutre, the build up of toxic metabolites began to cause the cells to become "unhealthy" and retain less of the crystal violet; gram stains are usually done with fresh cultures for this reason. Gram stain a fresh culture (48 hr) of a known gram positive, then stain an old (1 week+) culture of the same bacteria and you will see the difference.

[tejaswini]

if the culture of the bacteria is old then they show differences in their gram nature that is a gram positive organism may stain negetive.i think this could be the probable explanation.