Algicidal activity of Aeromonas hydrophila sdPS-7 isolate against toxic marine dinoflagellate Alexandrium minutum KB- 5

Aims: Several harmful dinoflagellate species are widely found in Malaysian marine waters. These microalgae are capable of producing a variety of toxins that can intoxicate humans through consumption of contaminated seafood. Therefore, the present work aimed to investigate the potential of a marine bacterium sdPS-7 isolate as a biological control agent against Alexandrium minutum KB-5. Methodology and results: The isolate sdPS-7 was obtained from marine sediments in Malaysia and examined for its algicidal activity toward the toxic dinoflagellate A. minutum KB-5 in laboratory culture. Based on morphological and molecular characterization, this isolate was identified as Aeromonas hydrophila sdPS-7. Alexandrium minutum KB-5 cultures were exposed to cell free bacteria culture filtrate and the effect on dinoflagelate growth was evaluated based on direct cell counts in the treated samples compared to non-treated control cultures. The bacterial filtrate was treated at different temperatures, salinities and exposed to repeated freezing and thawing five times to study its algicidal activity stability thereafter. The results showed a potent inhibition of the growth of A. minutum KB-5. The strongest effect was observed when the bacterium culture filtrate was mixed with A. minutum KB-5 cultures that were in lag phase, resulting in 99% or higher mortality. The cell-free filtrate proved to be heat- stable when exposed to temperatures of 2 °C to 100 °C for one hour each. There was also no substantial salinity as well as (freezing – thawing) effect on the filtrate algicidal activity. Conclusion, significance and impact of study: This study illustrated the potential use of the marine bacterial Aeromonas hydrophila sdPS-7 filtrates in controlling the growth of the toxic dinoflagellate A. minutum KB-5.

Anti-biofilm activity of the marine bacterium Pseudoalteromonas ruthenica KLPp3 against Serratia marcescens and Vibrio alginolyticus

Aims: Pseudoalteromonas ruthenica KLPp3 is the marine Gram-negative strain isolated from the surface of mud crab at Pulau Perhentian Malaysia. In this work, the anti-biofilm activity of P. ruthenica supernatant was examined on Serratia marcescen and Vibrio alginolyticus. Methodology and results: The crude extract of P. ruthenica KLPp3 was obtained using ethyl acetate. The subminimum inhibitory concentration (MIC) of the crude extract was determined using the minimum inhibitory test. The subMIC crude extract was tested against two of the S. marcescen virulence factors, which are the swarming ability and production of prodigiosin. The crystal violet assay was used to test the anti-biofilm activity of the sub-MIC crude extract against S. marcescen and V. alginolyticus. The productions of prodigiosin were reduced by 72%. The swarming area was reduced by 56.06%. It inhibits 26.9% and 48.5% of biofilm production in S. marcescens and V. alginolyticus respectively. The crude extract was heat stable. Conclusion, significance and impact of study: Besides combating the S. marcescens virulence factor, P. ruthenica KLPp3 crude extract in sub-MIC reduces the formation of biofilm of S. marcescens and V. alginolyticus, which may find applications in biofilm inhibition and prevention.

Isolation and characterization of pigmented bacteria showing antimicrobial activity from Malaysian marine environment

Aims: Natural products play a prominent role in the discovery of leads for the development of drugs in the treatment of human diseases. Much of nature remains to be explored, especially marine and microbial environments. Methodology and results: Fifty-five pigmented marine bacteria were isolated from sponges, seawater, mangrove sediment, sea cucumber and mussel from different coastal area of Malaysia. The antimicrobial activities of these bacteria were investigated by disk diffusion method against pathogenic bacteria. Out of 55 isolates, 18 isolates exhibited antimicrobial activity, which based on morphological characterization, 53% of them were Gram positive and 47% were Gram negative. All active isolates were able to tolerate more than 4% NaCl in the nutrient agar medium that indicated they were autochthonous to marine environment and moderate salt tolerant in nature. Molecular identification of isolates by the strong antimicrobial activities indicates that isolates WPRA3 (JX020764) and SM11-3j belong to genus Serratia and isolate SDPM1 (JQ083392) belongs to genus Zooshikella. Conclusion, significance and impact of study: The results of present study revealed that the active isolates are potential producer of antimicrobial secondary metabolites and might be utilized as drug candidate