For centuries, infectious diseases had a huge toll on the global mortality and morbidity rates. Antibiotics emergence was fundamental for the process of controlling, preventing and fighting infectious diseases, regardless of their degree of fatality. However, the antibiotic resistance problem is developing and increasing every day, endangering the efficacy of the available antibiotics and threatening the effective treatment of infectious diseases. Antibiotic resistance is considered among the greatest threats to global health in the 21st century. The development and discovery of new antibiotics to combat antibiotic resistance strains is a challenging task today as difficulties in clinical development, scientific bottlenecks, regulatory and economic issues have all hindered the field from gaining enough pace. Considerable interest in the potential of marine microalgae to produce various bioactive compounds have brought these single cellular organisms into focus. Several methods for assessing the antimicrobial potential of microalgae exist, and there are advantages and disadvantages associated with all of them. Throughout working with this master thesis, four chosen strains of microalgae, namely Isochrysis galbana, Dunaliella tertiolecta Nannochloropsis oculate and Tetraselmis suecica were cultured and assessed in terms of potential for antimicrobial compounds, using Vibrio alginolyticus as a bacterial test organism. Two methods were used for the mentioned purpose; the co-culturing and the mechanical disruption algal cells methods. Cultures of the microalgae strains I. galbana and D. tertiolecta showed antibacterial potential against V. alginolyticus, as they successfully reduced the bacterial growth when microalgae and bacteria were co-cultivated. Temperature affected antimicrobial activity against V. alginolyticus, as these microalgae displayed higher potential for antimicrobial activity when co-cultivated at a temperature of 25 °C than at 20 °C. The co-cultivation experiments also demonstrated that the antimicrobial potential may differ, depending on the algae growth phase. I. galbana reduced the growth of V. alginolyticus during algal exponential and stationary growth phases, while it increased bacterial growth, during algal death phase. In further experiments, disrupted algal material from all four microalgae was added to cultures of V. alginolyticus. The results from these investigations did not demonstrate any significant antimicrobial activity, as the disrupted algae material seemed to stimulate bacterial growth, rather than reducing it. However, a small inhibition zone was observed using the disc diffusion method to test cell-free supernatant from the algal material of T. suecica. The obtained results show that the used methods, the co-culturing and the mechanical disruption algal cells methods, could prove efficient if optimized and adjusted in a correct manner.