Identification and potential application of quorum quenching bacterial endophytes from South African medicinal plants

Abstract

Medicinal plants harbour diverse endophytic bacteria, which produce potentially therapeutic secondary metabolites. This study aimed to identify bacterial endophytes from South African medicinal plants that may disrupt pathogenic bacterial biofilms through quorum quenching (QQ) mechanism. Bacterial endophytes were isolated from six medicinal plants: Artemisia afra, Alpinia galangal, Aloe vera, Bulbine alooides, Mondia whitei and Tulbaghia violacea. Isolated endophytes were screened for quorum quenching activity. Those without QQ activity were screened for quorum sensing (QS) activity. QQ active isolates were then identified through Matrix Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF) and 16S rRNA. The identified isolates were then assessed for their antibiofilm properties. A total of 34 endophytic bacteria were isolated from medicinal plants. Analyses by MALDI-TOF mass spectrometry identified Bacillus (33.3%) as the dominant genus, followed by Pseudomonas (29.2%), Acidovorax (12.5%), Micrococcus (4.2%), Raoultella (4.2%) and 12.5% unidentified species. Analyses by the 16S rRNA sequence revealed a 70.83% similarity to MALDI-TOF results. Of the twenty-four isolates screened for QQ activity, fourteen isolates (58.3%) showed potent AHL signal molecule inhibition. Isolation and amplification of the gene (aiiA) showed that the occurrence of AHL lactonase in cell-free lysate and sequence alignment demonstrated that AiiA constitutes ''HXHXDH'' zinc-binding motif preserved in a few groups of metalloenzymes. Effects of QQ isolates against biofilm-forming pathogens: Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus, Serratia marcescens, and Bacillus subtilis depicted different effects where whole-cell isolates displayed biofilm inhibition range of 10% - 84%. Whereas, cell-free lysate isolates resulted in the reduction of cell attachment and restraint of biofilm arrangement with a 0.074% -38% and 1.63% - 64% decrease respectively. Visualization of biofilm under Confocal Laser Scanning Microscopy (CLMS) indicated a reduction in complete biomass development in treated pathogens. This study shows that South African medicinal plants harbour bacterial endophytes, which may serve as potential QQ candidates for the design of treatment against bacterial infections through evidence-based approaches.