Your peptide is inhibiting the growth of the sensitive strains. The resistant strains grow at a faster rate because they are resistant to the effects of the peptide.

No, I include a positive control, that contains no peptide or any antibiotic. I can monitor the growth using this control. Thus there is no inhibition here from the peptide. The growth is due to some other factor.

Probably your peptide is no active against resistant strains and probably the mechanism of action or target is similar than the drug resistant. Maybe your peptide should be used as substrate for the resistant strains and because this these strains are growth faster. However, are just possibilities.

I am not sure people understand the question. I know that there is a difference between sensitive and resistant bugs. But, there is faster growth between the different strains. This is monitored by using a drug free control. Also, the sensitive strains on,y change the color of alamar blue, pink, on day 7, whereas the drug resistant strain changes it as early as five days.

If you are using a consistent approach to making your MacFarlands then you should not be seeing that drastic of a difference in growth rates. You could try plating to check your CFUs, but as you know that is quite difficult with MTB.

Are you using more than one resistant strain? If you are using more than one, is this effect consistent across resistant strains? If you are only using one strain, the faster growth rate may just be a characteristic of the single resistant strain you are using and it might not be the best choice for your experiments. The best choice can often be a strain that has been serially passaged on media containing the peptide until it becomes resistant (if at all possible) so then you are working in two similar genetic backgrounds.

Thanks Sara, I am only working with one MDR strain. I guess it's fast growing characteristic is something unique to it. Strangely enough, one my colleagues, doing similar experiments and working with cultures from MGIT has experienced the same problem. Some MDR strains really grow considerably more faster than some other strains. So we are unsure how to rectify this. I though about using a lower starting CFU/ml.

i stand with the sara answer

Use some other mdr strain also to check the growth profile

Determine CFU at some specific stage by same dilution methods for all (positive control, research strain and resistant strain)

Can you determine the lag time of your strain if other strain is not possible

As you suspect, this could be due to a higher growth rate or higher inoculum. For reference, it would be worthwhile to determine each of these as they will be important for other types of experiments. If it is higher inoculum as a consequence of altered OD/CFU ratio you can make the appropriate correction. If the growth rate of the resistant strain is increased, you could stagger your inoculations in order to read all strains at the same time or decrease your inoculum of the resistant strain (I would guess ~4x to account for the two days difference). I do not see a problem of comparing MICs of strains with different growth rates, in fact MICs for different species are routinely compared.

I could not understand the mechanism of colour change of almara blue? is it due to the conc. of CO2 or something else. Secondly you are talking about the McFerland standard, i do not think there is much of difference in CFU/ml in sensitive and resistant strains but resistant strains must change the colour more rapidly in drug containing media and there is possibility of rapid growth in resistant starains, there must be genetic variation which make the resistant strain rapid grower? I have many questions keeping your query in my mind.

first you do sensitive as well as resistant strain growth curve then find out mic values generally m.tb takes 8 days ( in 96 well plate ) to grow. for finding of mic values you can use XTT- menadione assay of Upasana sing which is established in our lab. we use this protocol for high throughput screening against H37 Ra .

Another idea hit in my mind if it could help you. I think you are using H37Ra sensitive to all drugs? if you can try any control strain which is MDR or resistant to few drugs and use that strain as positive control and compare your results and growth rates?

All MIC assays are simply an aggregate of growth rate, time, sensitivity to the given compound. If you're in the same range of CFU, you should have reproducible results. This is a no-brainer; if you use sub-inhibitory concentrations, you'll not only see a longer time to emergence of the drug-sensitive strain but also need to extend the assay to detect small differences in MIC.

http://www.ncbi.nlm.nih.gov/pubmed/20516272

Breda while trying Sheda proposal, you seem to be right in thinking that the mdr strain may be multiplying faster. as this may be one of the ways mtb deploys its resistance pattern. to ascertain this determine the lag time as suggested ismail.