In a a dose response study for a an antibiotic, bacterial growth inhibition decreases as the concentration of the antibiotic decrease and show a zero?

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Asst.Prof.Muthanna Hadi Hussain

There may be something wrong in the calculation.

Michael J. Benedik

Please double check what you wrote to be sure it is what you meant. You wrote that INHIBITION decreases as antibiotic concentration decreases. This is as expected. You get the most inhibition at higher antibiotic concentrations and minimal or no inhibition at low concentration.

Now if you meant growth instead of inhibition, then yes things are backwards.

Ghadeer A R Y Suaifan

Thanks Dr. Benedik,

A direct relationship between Drug concentration and bacterial growth inhibition was observed. So inhibition at the highest concentration was 100 percent and this inhibition decreased as the concentration decreased and we got zero growth inhibition at 0.015 uM. But, then at lower concentration (0.0078 and .0039 uM) the inhibition retrieved and was 10 and 22 percentage respectively. Experiment was repeated and calculation rechecked. Full growth, sterility control and standard antibiotic was tested. the standards antibiotic gives no inhibition at all low concentrations tested.

Have you repeated this entire dilution series independently a second time to rule out just an error in the procedure. Because it just doesn't make sense otherwise.

Salah A. Alshehade

There are a few potential explanations for the non-monotonic dose response you observed in your antibiotic study:

Experimental artifact: At very low concentrations, there may be more variability/error in the measurements, leading to aberrant results. The retrieval of inhibition at lower concentrations could be due to experimental noise rather than a real effect. You may want to repeat the lowest concentration measurements to see if the effect replicates.

Hormesis : Some compounds can induce a biphasic, hormetic dose response, where a low dose stimulates a response and higher doses become inhibitory. The low dose stimulation is thought to act via a different mechanism than the inhibition at higher doses. This type of dose-response has been reported for some antibiotics.

Active metabolites: The parent compound you are testing may get metabolized into an active form at lower concentrations, leading to increased efficacy. As the concentration increases, metabolism becomes saturated and the inhibition decreases. You could explore the potential for active metabolites.

Solubility : There may be issues with the solubility or aggregation state of the antibiotic at higher concentrations that make it less available and effective. The dose-response could reflect changes in solubility or particle formation.

Kem Phillips

Could this be a sample size problem? If there were too few observations, noise may be obscuring your results at several dose levels.

Sanaa Bardaweel

If we rule out any possible experimental error, this may be due to penetration issue. Do we know if the compound of interest dimerizes or forms any complexes in a concentration dependent manner?

Also, does your compound have color that may interfere with the detection method? absorbance I guess?

Mahmoud A. Al-Sha'er

There may be a threshold concentration below which the antibiotic is not effective in inhibiting bacterial growth. This threshold might be around 0.015 uM, explaining the zero inhibition observed at that concentration. At lower concentrations (0.0078 and 0.0039 uM), the antibiotic may start to exhibit hormesis, a phenomenon where low doses have a stimulatory effect. In this case, the low concentrations might stimulate bacterial growth inhibition rather than inhibit it.