South African Searsia and Bauhinia species with activity against diarrhoeagenic pathogens and isolation of compounds from S. batophylla

Abstract

Diarrhoea is the incessant movement of the bowel due to an alteration of the gastrointestinal tract's normal physiological and morphological functioning. Diarrhoea is a symptom of an infection in the bowel of man and animals, which can be caused by several species of bacteria, viruses, and parasites. The emergence of antimicrobial resistance has reduced the effectiveness of conventional antibiotics, leading to difficulty treating infections, and even death, especially in young animals and children under five. It is, therefore, necessary to search for alternative diarrhoeal remedies and determine their mode of activity. This study evaluated the antibacterial activity, cytotoxicity, antioxidant activity, anti-inflammatory activity, antibiofilm and quorum sensing inhibition of some selected South African medicinal plant species used to treat infectious diarrhoea. Extracts of the leaves of Bauhinia bowkeri, B. galpinii, B. variegata, Searsia batophylla, S. lancea, S. leptodictya, S. pendulina and S. guienzi, as well as Brachylaena transvaalensis were screened for their antibacterial potency against a panel of bacteria using standard methods. Six plant acetone extracts of B. bowkeri, B. galpinii, S. batophylla, S. lancea, S. leptodictya and S. pendulina had good to outstanding antibacterial activity against the tested bacterial strains. B. bowkeri had the best minimum inhibitory concentration (MIC) of 0.01 mg/mL against Salmonella Enteritidis. Similar antibacterial activity was obtained against Escherichia coli isolates by B. bowkeri. The six selected plant acetone extracts exhibited notably less cytotoxicity in terms of higher LC50 values against Vero kidney cells compared to doxorubicin, the positive control, with values ranging from 0.03 to >1 mg/mL. The selectivity index (SI) values against the pathogens tested ranged from 0.09 to 50.75. The SI values greater than 10 are promising because the ratio of therapeutic concentration to toxic concentration is highly favourable. Phytochemical profiling of acetone leaf extracts was evaluated using thin layer chromatography eluted with different solvent systems of varying polarity. Total polyphenol and total flavonoid contents, antioxidant and anti-inflammatory activities were determined on the antibacterially active extracts of the plant extracts a using microtube dilution method. The phytochemical study revealed that the plant extracts contain a broad spectrum of compounds. S. lancea and S. batophylla had the highest level of flavonoids (270.55 ± 30.80 mg/QE/g) and phenolic content (443.36± 24.65 gallic acid equivalent/g), respectively. In addition, the extract of S. batophylla exhibited the most significant free radical scavenging activity in the ABTS assay (IC50 = 0.19 ± 0.02 μg/mL), while B. bowkeri had the best activity in the DPPH assay (IC50 = 14.31 ± 0.92 μg/mL). In the anti-inflammatory assays, S. batophylla acetone leaf extract was the most active in terms of inhibiting lipoxygenase as well as nitric oxide production. Against at least one tested bacterial species, all the plant extracts reduced the formation of bacterial biofilms by more than 50%. Furthermore, only the fully-grown biofilm of B. cereus was sensitive to the tested acetone extracts, with S. pendulina acetone leaf extract at 1 mg/mL eradicating 98.22% of preformed biofilm. S. lancea exhibited the highest efficacy, with the extracts' minimal quorum sensing inhibitory concentrations ranging from 0.08 to 0.32 mg/mL. The optimum half maximum violacein production inhibitory value was 0.17 mg/mL for the S. lancea extract indicating promising ability to inhibit quorum sensing. Bioassay-guided fractionation of the chloroform fraction of the acetone leaf extract of S. batophylla yielded three compounds, namely dodecanamide, 7,9-di-tert-butyl-1-oxaspiro (4,5) deca-6,9-diene-2,8-dione and 3-oxo-olean-18-en-28-oic. This is the first report of compounds isolated from the plant. However, individual compounds had poor antibacterial activity compared to the fraction from which they were isolated, which implies that their antimicrobial activity may be synergistic. All the compounds had low cytotoxicity to mammalian Vero cells. This study reveals that the selected indigenous plant species based on ethnobotanical use and preliminary antibacterial efficacy are worth considering in developing alternative antimicrobial therapy in prevention and management of diarrhoea.