Comparative study of the antibacterial properties of aqueous extract of combined brown macroalgae and some antibiotics against certain pathogenic bacteria in marine aquatic animals

In the past decade, there has been a significant increase in the resistance of pathogenic bacteria to antibacterial agents, which has direct consequences on human pathogenicity and mortality. This study aimed to comparatively examine the antibacterial effects of the aqueous extract of a premix of brown macroalgae (Sargassum ilicifolium, Nizimuddinia zanardini, Cystoseira indica, and Padina australis) and antibiotics (oxytetracycline, cephalexin, erythromycin, and nitrofurantoin) against pathogenic bacteria (Vibrio parahemolyticus, V. harveyi, V. alginolyticus, and Acute hepatopancreatic necrosis disease (AHPND+) and the probiotic bacterium Lactobacillus plantarum in marine aquaculture. After sampling and transferring the samples to the laboratory, the brown macroalgae samples (S. ilicifolium, N. zanardini, C. indica, and P. australis) were washed, dried, mixed in equal proportions (1:1:1:1), and ground. An aqueous solvent was used for extraction. To investigate the antibacterial effect of the aqueous extract of the macroalgae premix on 4 species of gram-negative bacteria (V. parahemolyticus, V. harveyi, V. alginolyticus, and AHPND+) and the probiotic bacterium L. plantarum, the agar disk diffusion test was employed. The results showed that the aqueous extract of the brown macroalgae premix had an inhibitory effect on the growth of gram-negative bacteria (V. parahemolyticus, V. harveyi, V. alginolyticus, and AHPND+). Among the mentioned gram-negative bacteria, the inhibitory effect of the aqueous extract of the brown macroalgae premix on the growth of V. parahemolyticus, V. harveyi, and AHPND+ bacteria was significantly greater than on V. alginolyticus (P > 0.05). The extract of the brown macroalgae premix showed no inhibitory effect on the growth of the Gram-positive bacterium L. plantarum, and no zone of inhibition was observed. However, the bacterium V. alginolyticus exhibited greater sensitivity to the brown macroalgae premix extract, with a zone of inhibition measuring 8.43 ± 0.06 mm, compared to cephalexin, which showed no zone of inhibition (P > 0.05). Overall, although according to the results, the antibacterial property of the aqueous extract of the brown macroalgae premix studied was found to be lower than that of standard antibiotics, the brown macroalgae premix extract can be introduced as a potential source of new natural antimicrobial compounds and, given their abundance, can be utilized for pharmaceutical applications and dietary supplements.