The present invention provides crude extracts from the red seaweeds: Polysiphonia lanosa (PL), Polysiphonia urceloata (PU), Cystoclonium purpureum (CP) and Devaleraea ramentacia (DR), method of preparation and their use for inhibiting the growth of microbial cells, particularly bacteria causing acne, such as Propionibacterium acnes, or causing nosocomial infections such as MRSA in humans or MRSP in dogs.