10.22376/ijpbs.2019.10.1.p1-12 Volume 8 Issue 1 2017 (January - March) Nowadays, several pathogenic bacteria present resistance against common antibiotics, being necessary the search new drugs with activity against resistant pathogens. The capacity of microalgae to produce diverse secondary metabolites has been widely reported. Here we searched the antibacterial activity of several organic extracts from different microalgae. We used five marine diatoms and a consortium of freshwater Chlorophyceae obtained from urban wastewater treatment. Target organisms were both Gram-negative ( lessThan i greaterThan Pseudomonas fluorescens lessThan /i greaterThan , lessThan i greaterThan Serratia marcescens lessThan /i greaterThan and lessThan i greaterThan Escherichia coli lessThan /i greaterThan ) and Gram-positive ( lessThan i greaterThan Micrococcus luteus, Bacillus subtilis lessThan /i greaterThan and lessThan i greaterThan Staphylococcus epidermidis lessThan /i greaterThan ) human pathogenic bacterial strains. Antibacterial activity was evaluated by agar diffusion assays. lessThan i greaterThan P. fluorescens, S. marcescens lessThan /i greaterThan and lessThan i greaterThan S. epidermidis lessThan /i greaterThan were inhibited by many of the extracts tested, while lessThan i greaterThan E. coli, M. luteus lessThan /i greaterThan and lessThan i greaterThan B. subtilis lessThan /i greaterThan were found completely resistant. These results corroborate the potential use of microalgae extracts as a source of new antibiotic compound and open the possibility of use of wastewater cultured algae. CORONA, E., FERNANDEZ-ACERO, J. AND BARTUAL, A. Antibacterial activity; antibiogram; agar diffusion assay; microalgae; 189-194