The antibiofilm potential and therapeutic adjuvant properties of Salvia aurea and Sphedamnocarpus pruriens, against Mycobacterium smegmatis

Abstract

Despite its discovery over a century ago, Mycobacterium tuberculosis (Mtb) is a rapidly advancing pathogen which has placed an immense disease burden on global populations and to this day, the fallout from this disease continues to pose one of the most significant challenges to the epidemiological and biopsychosocial management of tuberculosis (TB). A complex four-drug regimen coupled with months of therapeutic intervention as well as the ever-evolving development of antimicrobial resistance have plagued conventional TB therapy. Reclusive and sheltered from the harmful effects of the external environment, mycobacterial cells are known to envelop themselves within a complex three-dimensional biofilm matrix. Antimicrobial and immunological resistance are provided by the resilient extracellular polymeric substance which constitutes the primary structural element of biofilm integrity. Mycobacterial biofilms have become entwined within the concept of persistence and TB infectious continuity. The aim of this study was therefore to determine whether an ethanolic extract of Salvia aurea L. and/or Sphedamnocarpus pruriens (A.Juss.) Szyszyl. is capable of the consistent inhibition of Mycobacterium smegmatis (a mycobacterial model organism) biofilm growth throughout the various stages of biofilm development, from formation to dispersal. In addition, this study aimed to determine the adjuvant potential of both extracts by examining the cytotoxic potential of the extracts as well as their radical scavenging, quorum sensing and enzymatic inhibition activity. No antimycobacterial activity was present within the concentration range tested for both extracts, with minimum inhibitory concentration (MIC) values greater than 1000 µg/mL. This, however, was favourable due to the attractiveness towards a selective antibiofilm effect. With biofilm selectivity indices (BSI) of 5.84 and 4.68, respectively, both Sa. aurea and Sp. pruriens were more than two times greater than the BSI of the positive, control, ciprofloxacin (BSI – 2.24). The antibiofilm maturation and elimination of pre-formed biofilm assays revealed a progressive decline in activity of both extracts with minimum biofilm inhibitory concentration MBIC values increasing to above 1000 µg/mL. Salvia aurea proved to be a statistically significant reducer of percentage viability in MRC5 lung fibroblast cells whilst Sp. pruriens stimulated cellular proliferation at a concentration of 200 µg/mL and 50 µg/mL, respectively. A profound interaction between the plant samples and the autoinducer-II reagent prevented the accurate determination of autoinducer-II inhibitory activity. Sphedamnocarpus pruriens exhibited a radical scavenging activity of 89.42 % at a concentration of 100 µg/mL, which was not only greater than the 70.11 % exhibited by Sa. aurea but was also greater than the positive control – ascorbic acid (Vitamin C) with a percentage scavenging activity of 84.21 %. Exceeding 100 % enzyme activity, both Sa. aurea and Sp. pruriens displayed notable stimulation of the CYP1A2 isoform at all concentrations tested. On the contrary, greater than 90 % of CYP3A4 activity was reduced by Sa. aurea at a concentration of 50 µg/mL. This study highlighted that ethnobotanically selected, Southern African medicinal plants contain immense potential as antibiofilm leads whilst exhibiting low to moderate toxicity and at the same time, providing a diverse array of supportive, supplementary activities which can be attributed to the richness and phytochemical abundance present within the extracts.