Effects of Menthol Supplementation in Feedlot Cattle Diets on the Fecal Prevalence of Antimicrobial-Resistant <i>Escherichia coli</i>

<div><p>The pool of antimicrobial resistance determinants in the environment and in the gut flora of cattle is a serious public health concern. In addition to being a source of human exposure, these bacteria can transfer antibiotic resistance determinants to pathogenic bacteria and endanger the future of antimicrobial therapy. The occurrence of antimicrobial resistance genes on mobile genetic elements, such as plasmids, facilitates spread of resistance. Recent work has shown <i>in vitro</i> anti-plasmid activity of menthol, a plant-based compound with the potential to be used as a feed additive to beneficially alter ruminal fermentation. The present study aimed to determine if menthol supplementation in diets of feedlot cattle decreases the prevalence of multidrug-resistant bacteria in feces. Menthol was included in diets of steers at 0.3% of diet dry matter. Fecal samples were collected weekly for 4 weeks and analyzed for total coliforms counts, antimicrobial susceptibilities, and the prevalence of <i>tet</i> genes in <i>E</i>. <i>coli</i> isolates. Results revealed no effect of menthol supplementation on total coliforms counts or prevalence of <i>E</i>. <i>coli</i> resistant to amoxicillin, ampicillin, azithromycin, cefoxitin, ceftiofur, ceftriaxone, chloramphenicol, ciprofloxacin, gentamicin, kanamycin, nalidixic acid, streptomycin, sulfisoxazole, and sulfamethoxazole; however, 30 days of menthol addition to steer diets increased the prevalence of tetracycline-resistant <i>E</i>. <i>coli</i> (<i>P</i> < 0.02). Although the mechanism by which menthol exerts its effects remains unclear, results of our study suggest that menthol may have an impact on antimicrobial resistance in gut bacteria.</p></div

Prevalence of <i>tet</i>A or <i>tet</i>B in fecal <i>Escherichia coli</i> phenotypically resistant to tetracycline in steers fed diets supplemented with or without 0.3% menthol.

<p>Prevalence of <i>tet</i>A or <i>tet</i>B in fecal <i>Escherichia coli</i> phenotypically resistant to tetracycline in steers fed diets supplemented with or without 0.3% menthol.</p

Prevalence of tetracycline resistant isolates that do not carry <i>tet</i>A or <i>tet</i>B genes in fecal <i>Escherichia coli</i> in steers fed diets supplemented with or without 0.3% menthol.

<p>Prevalence of tetracycline resistant isolates that do not carry <i>tet</i>A or <i>tet</i>B genes in fecal <i>Escherichia coli</i> in steers fed diets supplemented with or without 0.3% menthol.</p

Prevalence of antimicrobial-resistant fecal <i>Escherichia coli</i> isolates from cattle fed diets with or without 0.3% menthol.

<p>Prevalence of antimicrobial-resistant fecal <i>Escherichia coli</i> isolates from cattle fed diets with or without 0.3% menthol.</p

Multidrug resistance (MDR; ≥ 5 antimicrobials) prevalence (%) in fecal <i>Escherichia coli</i> from steers fed diets supplemented with or without 0.3% menthol.

<p>Multidrug resistance (MDR; ≥ 5 antimicrobials) prevalence (%) in fecal <i>Escherichia coli</i> from steers fed diets supplemented with or without 0.3% menthol.</p