In vitro antimicrobial activities of Plocamium rigidum and Plocamium cornutum from the Namibian coast line

The aim of this study was to determine suitable extraction solvents for antimicrobial compounds and the antimicrobial activity of two algal species, Plocamium cornutum and Plocamium rigidum collected from the coastline of Namibia. Samples were collected at low tide from the intertidal area of the coastline at Luderitz and Henties Bay. The samples were collected about 5 to 10 cm under water by hand and placed in a sealable polythene bag and refrigerated at -20°C. Dried algae extracts were reconstituted in distilled water, hexane, dichloromethane, ethanol, methanol and chloroform, respectively and tested in vitro for antimicrobial activity using the Kirby Bauer disc diffusion method against 12 pathogens (Escherichia coli, Staphylococcus aureus, Staphylococcus saprophyticus, Pseudomonas aeruginosa, Streptococcus pyogenes, Proteus mirabilis, Listeria monocytogenes, Shigella sonnei, Salmonella species, Enterococcus faecalis, Candida albicans and Staphylococcus epidermidis). Screening confirmed that water extracts showed no activity against all the pathogens as the extracts were insoluble in water. The Plocamium extracts in the remaining solvents showed varying degrees of antimicrobial activity. Both dichloromethane and methanol extracts reconstituted in chloroform showed the greatest activity amongst the five different solvents that were used. Ampicillin (10 µg/ml) showed no antimicrobial activity against S. epidermidis whilst a zone of inhibition of 6.26±0.07 mm was recorded for 10 µg/ml of P. cornutum extract reconstituted in chloroform. An ethanolic extract of P. rigidum showed a zone of inhibition of 6.35±0.25 mm against L. monocytogenes while the standard ampicillin had no activity. Extracts of P. rigidum in ethanol and P. cornutum in chloroform are evidently potential lead candidate antibiotics in vitro against L. monocytogenes and S. epidermidis, respectively. Key words: Antimicrobial activity, Plocamium cornutum, Plocamium rigidum, Listeria monocytogenes.