Antimicrobial and cytotoxic activities of secondary metabolites from bacteria associated with marine algae of the Kenya coast

Abstract

The marine environment is a source of biologically active metabolites with great potential for the development of pharmaceuticals. Due to the rapidly increasing number of pathogenic microbes and tumorous cells that possess resistance towards established therapies causing a threat to public health, lead structures for the development of new drugs are on high demand. This study aimed to isolate bacteria associated with marine algae of the Kenyan coast, in an attempt to evaluate their antimicrobial and cytotoxic activity and identify the compounds responsible for bioactivity. Bacterial endophytes and epiphytes were isolated from 44 seaweed species of red, green and brown algal division, collected at three sites (Mkomani, Mtwapa, and Kibuyuni) along the Kenya coast. The obtained isolates were tested for their antimicrobial activity against eight human pathogenic strains of Gram-positive bacteria (Staphylococcus aureus (ATCC 25922) and Methicillin Resistant Staphylococcus aureus (MRSA), Gram-negative bacteria (Escherichia coli (ATCC 25923) and Salmonella typhi), fungi (Candida albicans (ATCC 90028), Cryptococcus neoformans, Trichophyton metagrophyte and Microsporum gypseum). Isolates that had a broad spectrum of inhibitory activity were further investigated for anticancer activity against human larynx Hep-2 cells. The active isolates were identified using the 16S ribosomal DNA gene sequence. Culture fermentation and bioassay guided fractionation was carried out on the active isolates. The compounds present in active factions were identified by GC-MS analysis. The study obtained 3493 bacterial isolates with bacterial epiphytes being the most abundant (54%) compared to bacterial endophytes (46%). Initial antimicrobial screening results revealed that 695 isolates (20%) inhibited the growth of at least one test organism, while further screening showed that 69 isolates (10%) had antimicrobial activity against three or more test pathogens. The results also showed that there was significant difference (p=0.001) in the mean susceptibility patterns of the Gram-negative and Gram-positive test strains, with Gram-positive (16.64±9.81) being more susceptible compared to Gram negative (12.37±6.94). The study showed there was a significant difference in the inhibitory activities among the three sampling sites, suggesting that the geographical location influences the production and bioactivity of secondary metabolites. A total of 33 isolates (48%) showed cytotoxity against Hep-2 cell line. The Phylogenetic analysis of 16S rDNA gene sequences showed they belong to the phyla Firmicutes (79%), Proteobacteria (12%) and Actinobacteria (9%). The active marine bacteria were assigned to the genera Bacillus, Geobacillus, Desulfovibrio, Massilia and Streptomyces. In addition, the metabolites produced significant cytotoxic activity against the tumorous Hep-2 cells compared to the normal cells (p<0.05). Cytotoxic profiles ranged from low IC50 value of 0.24mg/ml-1 to a high of 50.01 mg/ ml-1. Identification of the active metabolites showed the presence of several compounds such as phenolics, fatty acids, alkaloids, esters, indoles, alcohols, ketones, alkenes, alkanes, amines, nitriles, furan and azoles derivatives in the bioactive metabolites. These diverse ranges of compounds are known to have antimicrobial and cytotoxic activities.