Abstract
Marine invertebrates, as holobionts, contain symbiotic bacteria that coevolve and develop antimicrobial substances. These symbiotic bacteria are an underexplored source of new bioactive molecules to face the emerging antibiotic resistance in pathogens. Here, we explored the antimicrobial activity of bacteria retrieved from the microbiota of two sea anemones (Anemonia sulcata, Actinia equina) and two holothurians (Holothuria tubulosa, Holothuria forskali). We tested the antimicrobial activity of the isolated bacteria against pathogens with interest for human health, agriculture and aquaculture. We isolated 27 strains with antibacterial activity and 12 of these isolates also showed antifungal activity. We taxonomically identified these strains being Bacillus and Vibrio species the most representative producers of antimicrobial substances. Microbiome species composition of the two sea anemones was similar between them but differed substantially of seawater bacteria. In contrast, microbiome species composition of the two holothurian species was different between them and in comparison with the bacteria in holothurian feces and seawater. In all the holobiont microbiomes Bacteroidetes was the predominant phylum. For each microbiome, we determined diversity and the rank-abundance dominance using five fitted models (null, pre-emption, log-Normal, Zipf and Zipf-Mandelbrot). The models with less evenness (i.e. Zipf and Zipf-Mandelblot) showed the best fits in all the microbiomes. Finally, we tracked (using the V4 hypervariable region of 16S rRNA gene) the relative abundance of these 27 isolates with antibacterial activity in the total pool of sequences obtained for the microbiome of each holobiont. Coincidences, although with extremely low frequencies, were detected only in the microbiome of H. forskali. This fact suggests that these isolated bacteria belong to the long tail of rare symbiotic bacteria. Therefore, more and more sophisticated culture techniques are necessary to explore this apparently vast pool of rare symbiontic bacteria and to determine their biotechnological potentiality.
Highlights
Worldwide, microbial pathogens, including bacteria and fungi, are responsible for multiple diseases
We performed a well diffusion agar-plate assay. This assay consisted in placing stainless steel cylinders (Oxford towers) of the 193 selected strains in the presence of the pathogens: Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 15692, Erwinia amylovora ATCC 49946, Agrobacterium tumefaciens ATCC 33970, Vibrio brasiliensis LMG 20546, V. anguillarum ATCC 68554, V. mediterranei ATCC 43341, and V. coralliilyticus ATCC BAA450
In the holobionts Anemonia sulcata, Actinia equina, Holothuria forskali, and H. tubulosa we found symbiotic bacteria with antibacterial activity against human, plants or aquaculture pathogens (Table 1) and antifungal activity against plant pathogens (Table 2)
Summary
Microbial pathogens, including bacteria and fungi, are responsible for multiple diseases. These infections affect humans and crops, livestock, and aquaculture generating the need of new antimicrobial agents, in part, due to the increase in the antibiotic resistance of some pathogens [1]. To control potential bacterial diseases, antibiotics have been routinely used as daily prophylactic doses [5] This uncontrolled and frequent use of antibiotics has resulted in the development of resistance in several pathogens. Irrespectively of its origin, new antimicrobial products are necessary to control pathogenic bacteria in this particular food sector and to face the antibiotic resistance problem in general [8,9]
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