Faced with the significant disturbances, mainly of anthropogenic origin, which affect the Mediterranean coastal ecosystem, it is necessary to set up rapid diagnostic tools for these disturbances. Posidonia oceanica, which is one of the pillars of this ecosystem, has often been used to assess the state of health of the Mediterranean coastal environment. Despite environmental changes, this plant can accumulate reserves which allow it, during relatively long periods, to keep a stationary phenology. It may also represent important mechanisms for the dissemination of antibiotic resistance genes among pathogenic bacterial populations. The present study aims to determine the multi-drug resistance patterns among isolated and identified epi-endophytic bacterial strains in Posidonia oceanica seagrass collected from Mahdia coastal seawater (Tunisia). 43 isolates were obtained, seven of them were selected and identified. These isolates belonged to the genus Bacillus. Susceptibility patterns of these strains were studied toward commonly used antibiotics in Tunisia. All identified isolates were completely resistant to Aztreonam, Ceftazidime, Trimethoprim, Amoxicillin and Rifampicin.