Fatty acid profile and antimicrobial activity of chloroform extracts from the red microalga Porphyridium purpureum against pathogenic intestinal bacteria

Introduction Microalgae such as Porphyridium purpureum have been investigated due to their unique biochemical composition and potential biological activities. Antimicrobial resistance is a major public health concern, particularly affecting enteric bacteria of the intestinal microbiota. Methods Antimicrobial activity was evaluated by determining the minimum inhibitory concentration (MIC) against pathogenic intestinal bacteria, including Escherichia coli , Enterococcus faecalis , and Salmonella typhimurium . Fatty acid profile was performed using gas chromatography–mass spectrometry (GC–MS). Cytotoxicity of the lipid extract of P. purpureum was assessed in Vero cells, and the selectivity index (SI) was calculated as the ratio between cytotoxicity and antibacterial activity. Results The isolated lipid extract and exopolysaccharides from P. purpureum showed no direct antimicrobial activity, however, the lipid extract exhibited a synergistic effect when combined with ciprofloxacin against S. typhimurium and E. coli . Fatty acid analysis revealed the presence of major compounds such as methyl palmitate, methyl arachidonate, and methyl linoleate. The extract presented an IC 50 = 1149.21 μg/mL in Vero cells and a selectivity index >10, indicating low cytotoxicity. Discussion The observed synergistic activity was restricted to the ciprofloxacin combinations evaluated against Gram-negative bacteria. The fatty acids identified in the extract, such as methyl palmitate, methyl arachidonate, and methyl linoleate, may contribute to this differential activity by targeting membrane integrity. Conclusion The lipid extract of P. purpureum did not exhibit antibacterial activity when tested alone but demonstrated a potential modulatory effect in combination with ciprofloxacin against Gram-negative bacteria. These findings suggest that the extract may serve as a source of compounds with antibiotic adjuvant potential and warrant further studies to identify the active constituents and elucidate the mechanisms underlying the observed synergistic effect.