Use of an electrochemical biotesting technique for comparing the antibiotic properties of plant extracts obtained using liquefied CO₂

We present a biotesting technique that allows periodic (every 2 hours) recording of changes in the pH, redox potential and electrical conductivity of a liquid nutrient medium incubated in the presence or absence of viable test microorganisms and test samples. This technique was used to compare the pro- and antibiotic activity of various concentrations of subcritical whole extracts obtained from 10 plant species using liquefied CO₂ against Lactobacillus acidophilus and other test microorganisms, such as Chlorella vulgaris and Rhodotorula glutinis. It was established that the proposed technique provides for a quicker, more objective and informative, as well as less material-consuming and labour-intensive, microbiological testing compared to conventional visual methods aimed at assessing effects of pharmaceutical, food and other products on the activity of test microorganisms. Among the studied plant varieties, extracts from celandine roots (Chelidonium majus) and calendula officinalis flowers (Calendula officinalis) at a concentration of 3% vol. and above demonstrated the most active prolonged antibiotic properties. Extracts from mistletoe shoots (Viscum album) and walnut leaves (Juglans regia) at a concentration of 0.2% vol. showed the most active prolonged probiotic properties. The initial pro- and antibiotic activity of the tested extracts was in most cases greater than their prolonged activity. At the same time, the medium-term (with respect to the period of interaction of the extracts and microorganisms) pro- and antibiotic activity of the tested extracts showed intermediate values between their initial and prolonged activity. It is noteworthy that a decrease in the concentration of the tested extracts in the test environment led to a steady decrease in their antibiotic activity, at the same time as increasing their probiotic activity. Thus, it is clear that the pro- and antibiotic activity of pharmaceutical, food and other products, including various plant extracts, is determined not only by the choice of raw materials and the method for extracting biologically active substances, but also by the concentration of extracts, the time of their interaction with microbiota and other living organisms, as well as by a variety of other factors. The exact nature of these dependencies can only be established through multiple tests, which can conveniently be carried out using the methodology presented in this work.

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