We have encountered a bacteria that not only is resistant, but shows increase in growth at low doses of antibiotic. However, at a higher dose of the same antibiotic there is complete suppression of the growth.
Yes, I work on the topic of biodegradation and are especially looking for that kind of bacteria. Due to acclimation I observed a better growth in presence of the antibiotic as the bacteria was able to utilize the antibiotic as nutrient to foster its growth. At a higher dose acclimation is often not sufficient any more to prevent the organisms from the effect of the antibiotic especially if incubation time is too short for a full acclimation. What kind of antibiotic you are using?
The concentrations of antibiotic correspond to how many times the MIC value? you could be observing an hormesis effect, wich is exacly what you described. One of my coordinators has some work in that field, if you have any furder doubts.
Thank you Joao for your helpful suggestion, The MIC is 100ppm of the antibiotics ampicillin. But what we notice is, the bacteria grows well between 10-60ppm range. On control plate (sans antibiotics) the colony size is small and normal. At 10 to 60 and even 80ppm doses the colony size keeps increasing.
We are checking again to see if the bacteria shows no growth at 100ppm.
Will be glad to get input from your co-ordinator too.
Murali
If the antibiotic concentration is well below the MIC and the antibiotic can permeabilise the outer membrane and/or cytoplasmic membrane, then the uptake of nutrients may be enhanced, and in turn, growth may be enhanced.
Antibiotic-dependent bacteria have been described occasionally (vancomycin-dependent enterococci, rifampicin-dependent M. tuberculosis...). These microorganisms not only are resistant, but they grow much better in the presente of the antibiotic.
Thanks Christine,
A good reasoning. Any reference to use this point?
Thank you Juan for the information.
Murali
Some plants used in bioremediation can have hormesis in the presence of antibiotics in the water in which they grow.
Thanks Ashok. Will the plants too be following the same mechanism that bacteria follow as described by Christine earlier.
Yes, Martina, I had not written about the morphological changes!! As you guessed, the bacteria changed its morphology when it grew at concentrations 10 to 60/80ppm. The colonies were thinner and spread out, almost covering 3/4th of the plate within 12hrs incubation. In the absence of the antibiotic, the colony morphology was small tight thicker circles.
Hi again, i asked my coordinator, and he pointed out that the effect observed could be simply an increase in bacterial fitness due to the presence of a stress inducing agent... do you have any data that could tell you if that effect is present in other stress agents or just the antibiotics used?
Also did you used any cell counting method to be sure, that the effect you're seeing is increased cell growth? Because as the observation goes, could be just an increase in bacterial motility by stress induced swarming effect.
Thanks Joao. We have only used antibiotics and no other stress agents. Now, the bacterial motility suggestion opens up new angle. As Martina suggested, looking under microscope would give some clue.
As soon as we look I will return with observations.
@murli: when you say you see more growth, can you be a bit more specific? Is it increased growth rate in OD measurements in bulk, more CFUs, or greater diameter of colonies per CFU? And indeed I am also curious to know what the morphology of individual bacteria is like...! You can use the standard protocol of para-formaldehyde fixation (details in: Athale C.A. & Chaudhari HC (2011) Bioinformatics. http://bioinformatics.oxfordjournals.org/content/early/2011/09/19/bioinformatics.btr501.abstract?sid=7473e166-8278-483b-909a-136f5c23cec9).
@carson: I am a bit surprised by your explanation Christine, since wouldn't permeabilization (even if its partial) also lead to loss of internal components, being non-specific?
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