A Proposal Towards a Rational Classification of the Antimicrobial Activity of Acetone Tree Leaf Extracts in a Search for New Antimicrobials

Abstract

Many scientists investigate the potential of finding new antibiotics from plants, leading to more than a thousand publications per year. Many different minimum inhibitory concentrations of extracts have been proposed to decide if an extract has interesting activity that could lead to the discovery of a new antibiotic. To date, no rational explanation has been given for the selection criteria different authors have used. The cumulative percentage of plant extracts with different activities from a large experiment determining the activity of 714 acetone tree leaf extracts of 537 different South African tree species against 4 nosocomial pathogenic bacteria and 2 yeasts was calculated using a widely accepted serial dilution microplate method with p-iodonitrotetrazolium violet as indicator of growth. All the extracts were active at a concentration of 2.5 mg/mL. The formula, % of active extracts = 439 × minimum inhibitory concentration in mg/mL1.5385, describes the results for minimum inhibitory concentrations below 0.16 mg/mL, with a correlation coefficient of 0.9998. A rational approach could be to determine the minimum inhibitory concentrations of the most active 1, 3, 9, 25, 50, and > 50% of a large number of plant extracts investigated against these six important microbial pathogens. Starting with an extract concentration of 10 mg/mL, I propose the following classification based on minimum inhibitory concentrations: outstanding activity < 0.02 mg/mL, excellent activity 0.021 – 0.04 mg/mL, very good activity 041 – 0.08 mg/mL, good activity 0.081 – 0.16 mg/mL, average activity 0.161 – 0.32 mg/mL, and weak activity > 0.32 mg/mL. Higher minimum inhibitory concentrations may still be effective in ethnopharmacological studies.

LC–DAD–MS Phenolic Characterisation of Six Invasive Plant Species in Croatia and Determination of Their Antimicrobial and Cytotoxic Activity

“…The antimicrobial activity of plant extracts was classified based on the MIC values as follows: outstanding (0.02 mg/mL and below), excellent (0.021–0.04 mg/mL), very good (0.041–0.08 mg/mL), good (0.08–0.16 mg/mL), average activity (0.161–0.32 mg/mL), and weak (>0.32 mg/mL) [ 55 ].…”

Evaluation of the Phytochemical and Antimicrobial Activity of the Fractionated Methanolic Leaf Extract of Diospyros Pruessii Gurke.

“…Salmonella spp typically causes diarrhea and sometimes causes a more severe infection, typhoid fever. Staphylococcus aureus constitutes a significant public health threat, being one of the common causes of hospital-acquired and community infections [5]. The demonstration of activity against both gram-positive and gramnegative bacteria is an indication that the plants can be a source of bioactive substances that could be of a broad spectrum of activities.…”

“…In the past, humans used plants to treat common infectious diseases. Even long before humanity discovered microbes' existence, the idea that certain plants had healing potential was well accepted [5,6]. Researchers are increasingly becoming involved in screening such plants to establish their potential antimicrobial effects and identify the compounds responsible for their antimicrobial properties.…”

Antibacterial Screening, Biochemometric and Bioautographic Evaluation of the Non-Volatile Bioactive Components of Three Indigenous South African Salvia Species

“…The results indicated active extracts that could be earmarked for further investigation following a more focused approach. Suggested criteria for MIC values of plant extracts available from recently published literature [ 20 ] were considered as a guideline for activity. According to the author, for the purposes of ethnopharmacological research studies, MICs higher than 0.32 mg/mL reflect extracts that may still be effective, although 1.0 mg/mL was accepted as the upper limit of activity for this study.…”

“…According to the author, for the purposes of ethnopharmacological research studies, MICs higher than 0.32 mg/mL reflect extracts that may still be effective, although 1.0 mg/mL was accepted as the upper limit of activity for this study. Activities were further described [ 20 ] as weak (>0.32 mg/mL), average (0.16–0.32 mg/mL), good (0.081–0.16 mg/mL), very good (0.041–0.080 mg/mL), excellent (0.021–0.040 mg/mL) or outstanding (<0.02 mg/mL), and these criteria were applied to this study.…”