PS9-16. Effect of Red Algae (Asparagopsis Taxiformis) Supplementation on Rumen Antimicrobial Resistance Gene Abundances in Goats Fed Two Forage Diets.

Abstract Red algae (Asparagopsis taxiformis) supplementation in ruminant diets has gained attention due to its ability to reduce methane production and alter rumen microbial communities. However, its influence on antimicrobial resistance within the rumen microbiome remains unclear. Therefore, this study evaluated the effect of red algae supplementation on antimicrobial resistance gene (ARG) abundances in the rumen of mature Spanish goats. Four treatments were evaluated: two forage types (alfalfa hay or Bermuda grass hay) without or with red algae supplementation (0 and 4 g/day), with eight goats per treatment. Ruminal fluid was collected after 21 days of feeding, and metagenomic DNA was extracted for Illumina shotgun sequencing (150 bp; ∼60 million reads per sample). Sequence reads were analyzed using the Resistance Gene Identifier in BWT mode against the Comprehensive Antibiotic Resistance Database. Average ARG abundance was 0.31 ± 0.02% of total reads (range: 0.27–0.36%), with 138 ± 15.3 (mean ± standard deviation) ARGs identified per sample and 58 core ARGs shared across samples. The most abundant ARGs included tet(Q) and tet(W), along with 16S rRNA mutations associated with resistance in Mycobacteroides abscessus (gentamicin), Mycolicibacterium smegmatis (neomycin), and Salmonella enterica (spectinomycin), and 23S rRNA mutations in Clostridioides difficile (erythromycin and clindamycin), Escherichia coli (clindamycin), Streptococcus pneumoniae (macrolides), and Chlamydia trachomatis (macrolides). Overall, ARGs were most frequently associated with resistance to tetracycline, macrolide, lincosamide, and aminoglycoside antibiotics. Principal component analysis with pairwise PERMANOVA indicated that ARG profiles differed among treatment groups (P < 0.05). Bermuda grass hay feeding increased (P < 0.10) resistance to aminoglycoside, rifamycin, streptogramin–streptogramin A–pleuromutilin, peptide, and phenicol classes compared with alfalfa hay. In alfalfa hay diet, red algae supplementation decreased (P < 0.10) the abundance of seven ARGs, including tet(44), Mycobacterium tuberculosis 16S rRNA mutation (amikacin resistance), and Staphylococcus aureus rpoC (kirromycin resistance). However, it increased (P < 0.10) ARGs such as Escherichia coli 16S rRNA (spectinomycin) and 23S rRNA (clindamycin) mutations and lsaB and by drug class, increased (P < 0.10) antibiotic resistance associated with peptide, streptogramin–streptogramin A–pleuromutilin, and cephalosporin–penicillin β-lactam classes. In the Bermuda grass hay–based diet, red algae supplementation affected (P < 0.10) 46 ARGs, including reductions in 23 ARGs, and by drug class decreased (P < 0.10) resistance to phenicol, tetracycline, pactamycin-like antibiotics, and macrolides-streptogramins, while increasing resistance to peptide, aminocoumarin, and cycloserine-like antibiotics. In conclusion, Bermuda grass hay feeding may increase resistance to several antibiotic classes compared with alfalfa hay, whereas red algae supplementation may reduce resistance to some antibiotic classes in the rumen microbiome of goats.