Temporary changes seen in the gut bacteria of horses and other animals during therapeutic courses of antibiotics appear to be inescapable, according to the authors of a just-published review.
Antibiotic use disturbs the gastrointestinal microbiota by killing bacteria beneficial for animal health and favoring the emergence of potential pathogens. Furthermore, antibiotics favor the emergence of resistant bacteria.
Tony Rochegüe and his fellow scientists, writing in the journal Animals, describe the intestines as a reactor of antibiotic resistance emergence, and the presence of antibiotics promotes the selection of resistant bacteria that can spread in the environment and propagate to further hosts.
Rochegüe and his colleagues set out to analyze the effect of antibiotics on the intestinal microbiota and antibiotic resistance in animals, focusing on the main food-producing and companion animals.
They found that, irrespective of antibiotic classes and animal hosts, therapeutic doses of antibiotics decreased species diversity and richness, favoring the bloom of potential gut-related pathogens and the selection of antibiotic resistance. The time necessary for a return to normal was variable among studies.
These negative effects seem inescapable, they said, but often were reduced when an antibiotic was administered orally.
Sub-therapeutic dosages caused an increase in bacteria involved in sugar metabolism, suggesting a link with weight gain. “This result should not be interpreted positively, considering that parallel information on antibiotic resistance selection was rarely reported and selection of antibiotic resistance is known to occur also at low antibiotic concentration.”
They noted that several studies had investigated the use of antibiotics as growth promoters. “This knowledge could inspire alternative strategies to antibiotics, such as probiotics, for improving animal performance.”
The review team observed that horses are considered both food-producing and companion animals, making them a potential reservoir of antimicrobial resistance for humans by direct contact.
They noted that, in Australia, a high occurrence of tetracycline resistance genes had been reported in a study, while in Europe the prevalence of third and fourth-generation cephalosporin resistance is high in E. coli from healthy horses’ feces, with medication recognized as risk factors for its occurrence.
The review team traversed other horse studies indicating the development of antimicrobial resistance to drugs.
In horses, bacteria residing in the gut that help break down plant cellulose seem to be most affected by antibiotic treatments, they said, undergoing a decrease in abundance.
“This decrease could negatively affect horses’ health, considering that these bacteria are crucial to digest fibers, which are the main components of horses’ diet.”
The authors said further studies are necessary to clarify the effects of antibiotics in animals and to improve therapy duration, dosages, and administration routes for reducing the negative effects of treatment.
Current knowledge on animals’ intestinal microbiota and the effects of antibiotics is blurred by the various dosage levels, administration routes, and methods for analysis.
Major knowledge gaps need to be filled to improve antibiotic use and reduce the negative effects of these drugs, they said.
“Based on the current knowledge, it is difficult to choose an antibiotic, or an antibiotic class, that could have less negative effects compared to others and in the meantime serve as successful treatment.
“Comparison among studies is hindered by variations in the experimental design including drug concentration, antibiotic combination, therapeutic regimen, and duration of the treatment.”
Harmonization of experimental procedures is crucial to enable better comparisons between research results, they added.
A better understanding of the gut microbiota composition and function in animals could open up strategies for its modulation to improve animal health and performance, and to minimize the negative impact of antibiotics.
The study team comprised Rochegüe, Marisa Haenni, Stanislas Mondot, Chloé Astruc, Géraldine Cazeau, Tristan Ferry, Jean-Yves Madec and Agnese Lupo, from a range of French institutions, including Unité Antibiorésistance et Virulence Bactériennes – the Antibiotic resistance and Bacterial Virulence Unit, part of the University of Lyon.
Rochegüe, T.; Haenni, M.; Mondot, S.; Astruc, C.; Cazeau, G.; Ferry, T.; Madec, J.-Y.; Lupo, A. Impact of Antibiotic Therapies on Resistance Genes Dynamic and Composition of the Animal Gut Microbiota. Animals 2021, 11, 3280. https://doi.org/10.3390/ani11113280