Dear All,
What caused the increase in antibacterial activity of extract as its concentration decreased?
We ran a MIC analysis to specify the lowest concentration of plant crude extract (on a water base) that inhibits bacteria.
We observed that as the concentration decreases, the antibacterial activity increases. The extract, along with bioactive compounds, also contains natural sugars, acids, macro and microelements and pectic compounds.
Is high sugar or pectin content a reason for the encapsulation of bacteria?
I would appreciate any thoughts regarding this observation.
Many thanks!
Usually one compares INCREASING concentrations of any substance/extract etc. on any process to see dose-response curve, starting from 0. In any case, there need to be logical changes (stable level, increase or decrease) in the process if a suitable range of concentrations/intensities etc. has been chosen. And of course, biological extracts can contain different types of molecules with different type of effects on the same process. If response levels are just fluctuating in a chaotic manner with changes in a dose then something is wrong with the test system, more likely. And yes, sugars can be used by microorganisms for growth promotion if the medium itself is not optimal for growth, but they can act also as osmoticum and inhibit growth.
Dear Dr. Ievinsh!
Thank you very much for your comment and explanation. Referring to the test system used to evaluate microorganism susceptibility to plant extract, I am sure everything is correct since we triple-checked it, and repeatability is sufficient.
As a positive control, we used antibiotics, which worked in a dose-dependent manner.
However, the extracts show the opposite effect, which I guess is due to homeopathy. Less side compounds in the well result in better solubility of bioactives, fewer interactions between high- and low-molecular-weight compounds, more volume for bioactives to take action, and a better inhibitory effect.
For some extracts, we also observed no antimicrobial effect, and this is due to the relatively high content of growth-promoting substances such as the amino acids proline and alanine. I also checked the content of saccharides, and it seems these compounds could not be treated as osmolytes. The concentration is below 2.5%, which is way too low.
To me, this points to necessity for a wider concentration range to be tested at the left end of concentrations. I cannot imagine the curve as there needs to be a concentration range where activity increases, then, after reaching the highest point, it decreases? Even below control values?
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