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/mL^1.5385, 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.

Key words

antimicrobial - minimum inhibitory concentration - tree leaves - classification of activity

^# Dedicated to Professor Arnold Vlietinck on the occasion of his 80th birthday.

Publication History

Received: 12 November 2020

Accepted after revision: 13 April 2021

Article published online:
06 May 2021

• References

• 1 Walsh C. Where will new antibiotics come from?. Nat Rev Microbiol 2003; 1: 65-70
• 2 Martens E, Demain A. The antibiotic resistance crisis, with a focus on the United States. J Antibiot (Tokyo) 2017; 70: 520-526
• 3 Årdal C, Balasegaram M, Laxminarayan R, McAdams D, Outterson K, Rex JH, Sumpradit N. Antibiotic development – economic, regulatory and societal challenges. Nat Rev Microbiol 2020; 18: 267-274
• 4 Van Wyk C, Botha FS, Vleggaar R, Eloff JN. Obliquumol, a novel compound, isolated from the leaves of sneeze wood (Ptaeroxylon obliquum) has excellent activity against Candida albicans and may present a new antifungal scaffold molecule. S Afr J Sci 2018; 37: 1-5
• 5 Holford P, Burne J. Food is better Medicine than Drugs. London: Piatkus Books Ltd.; 2016
• 6 Farnsworth NR, Morris RW. Higher plants – the sleeping giant of drug development. Am J Pharm Sci Support Public Health 1976; 148: 46-52
• 7 Cowan MM. Plant products as antimicrobial agents. Clin Microbiol Rev 1999; 12: 564-582
• 8 Hostettmann K, Marston A, Ndjoko K, Wolfender JL. The potential of African plants as a source of drugs. Curr Org Chem 2000; 4: 973-1010
• 9 Abdallah EM. Plants: An alternative source for antimicrobials. J Appl Pharm Sci 2011; 01: 16-20
• 10 Eloff JN. Avoiding pitfalls in determining antimicrobial activity of plant extracts and publishing the results. BMC Complement Altern Med 2019; 19: 106
• 11 Gibbons S. Anti-staphylococcal plant natural products. Nat Prod Rep 2004; 21: 263-277
• 12 Cos P, Maes L, Sindambiwe JB, Vlietinck AJ, Berghe VD. Bioassays for antibacterial and antifungal Activities. Biological Screening of Plant Constituents. Training Manual. Trieste, Italy: UNIDO-ICS; 2006: 19-28
• 13 Van Vuuren SF. Antimicrobial activity of South African medicinal plants. J Ethnopharmacol 2008; 119: 462-472
• 14 Kuete V. Potential of Cameroonian plants and derived products against microbial infections: a review. Planta Med 2010; 76: 1479-1491
• 15 Van Vuuren SF, Holl D. Antimicrobial natural product research: A review from a South African perspective for the years 2009–2016. J Ethnopharmacol 2017; 208: 236-252
• 16 Mendoza N, Silva EME. Introduction to phytochemicals: secondary metabolites from plants with active principles for pharmacological importance.
• 17 Fabry W, Okemo PO, Ansorg R. Antibacterial activity of eastern African medicinal plants. J Ethnopharmacol 1998; 60: 79-84
• 18 Aligiannis N, Kalpotzakis E, Mitaku S, Chinou IB. Composition and antimicrobial activity of the essential oils of two Origanum species. J Agric Food Chem 2001; 40: 4168-4170
• 19 Holetz FB, Pessini GL, Sanches NR, Cortez DAG, Nakamura CV, Filho BPD. Screening of some plants used in the Brazilian folk medicine for the treatment of infectious diseases. Mem Inst Oswaldo Cruz 2002; 97: 1027-1031
• 20 Eloff JN. Quantifying the bioactivity of plant extracts during screening and bioassay-guided fractionation. Phytomedicine 2004; 11: 370-371
• 21 Rios JL, Recio MC. Medicinal plants and antimicrobial activity. J Ethnopharmacol 2005; 100: 80-84
• 22 Roersch CMFB. Commentary: A classification system for antimicrobial activity based on MIC-values: fake or reality?. J Ethnopharmacol 2012; 139: 678
• 23 Pauw E, Eloff JN. Which tree orders in Southern Africa have the highest antimicrobial activity and selectivity against bacterial and fungal pathogens of animals?. BMC Complement Altern Med 2014; 14: 317
• 24 Vlietinck AJ, Van Hoof L, Totte J, Lasure A, Vanden Berghe D, Rwangabo PC, Mvukiyumwami J. Screening of hundred Rwandese medicinal plants for antimicrobial and antiviral properties. J Ethnopharmacol 1995; 46: 31-47
• 25 Eloff JN. A sensitive and quick microplate method to determine the minimal inhibitory concentration of plant extracts for bacteria. Planta Med 1998; 64: 711-714
• 26 Eloff JN. Which extractant should be used for the screening and isolation of antimicrobial components from plants?. J Ethnopharmacol 1998; 60: 1-8
• 27 Eloff JN, Famakin JO, Katerere DRP. Combretum woodii (Combretaceae) leaf extracts have high activity against Gram-negative and Gram-positive bacteria. Afr J Biotechnol 2005; 4: 1161-1166
• 28 Kotze M, Eloff JN. Extraction of antibacterial compounds from Combretum microphyllum (Combretaceae). S Afr J Bot 2002; 68: 62-67
• 29 Netshiluvhi TR, Eloff JN. Effect of water stress on antimicrobial activity of selected medicinal plant species. S Afr J Bot 2016; 102: 202-207
• 30 Netshiluvhi TR, Eloff JN. Temperature stress does not affect antimicrobial activity of some South African medicinal plants. S Afr J Bot 2019; 123: 93-97
• 31 Gertsch J. How scientific is the science in ethnopharmacology? Historical perspectives and epistemological problems. J Ethnopharmacol 2009; 122: 177-183