Your Photo is not lucid.It is difficult to understand what colonies you want to know, colonies in the zone of inhibition may appear either due to mixed culture or due to some mutants often near the periphery of the zone.

Hi Partha,

Try to repeat it again. Please check also that the bacterial lawn of the indicator strain is not contaminated with another bacteria. Once this has been done, if you find a colony inside the inhibition zone this could be a moderately sensitive mutant. In the case that such colony dissappears I would assume that the indicator strain is sensitive.

Good luck

Jorge

I agree with the other posters - if your question is why there appear to be isolated colonies within the inhibition zone or concentric circles around your disk, these are often examples of a heteroresistant subpopulation.

In this scenario, your inoculum as a whole (~108 cfu/mL) is susceptible but a small subpopulation within that inoculum (say, ~103 cfu/mL) is resistant. You would see this as isolated colonies within the population's zone of inhibition.

You will often see this phenomenon in clinical isolates from persistent infection, high inoculum disc diffusion testing or for antibiotics with a low barrier to resistance development (i.e. Rifampin). Based on the photo, the inoculum of your tester E. coli strain may be too high since the growth is so robust (although sometimes it just grows like that - something to check, though). Good luck!

I am absolutely agree with Berti, yes they are because of heterosresistant subpopulation. Next, you can easily pick up these colonies and check their susceptibility profile to the same drug to know whether  they are resistant or tolerant variants of the parent strain. Good luck!

Check such colonies and in addition to latter answers ,they could be resistant varients

Hello Jorge Sir,

Thanks for your answer. 

If those are moderately sensitive mutanta , then it means that a clear zone would never result. 

So, further what can I do??

 Hello Andrew,

Thanks for your answer.

If there is a presence of a hetero resistant population the colonies should have grown further which is not exhibited.And it remains the same. So what possible explanation could be suggested? 

This phenomenon is due to hetero-resistant sub-population, as many researchers answered before.

 my idea that may be the heavy growth bacterial inoculum