Although there has been some movement toward more judicious use of antibiotics, there are still people who take the drugs thinking it will help the common cold, and dentists who regularly prescribe antibiotics for patients prior to some common oral surgeries like third molar extraction.
These practices are baffling to scientists like Egija Zaura, PhD, an associate professor in oral microbial ecology at the Academic Centre for Dentistry Amsterdam (ACTA), who is concerned about the increasing rise in antibiotic resistance.
Even a single course of antibiotics can disrupt the diversity of microorganisms in the gut for as long as a year, according to a study by Zaura and colleagues reported this week in mBio. Among 66 healthy adults prescribed different antibiotics, the drugs enriched genes associated with antibiotic resistance and severely affected microbial diversity in the gut for months after exposure. By contrast, microorganisms in saliva showed signs of recovery in as little as few weeks.
“Certainly we cannot live or survive without antibiotics; that’s out of the question,” Zaura says. “But there are situations when we should not use them, like when there are no evidence-based reasons.”
In the study participants’ fecal samples, researchers saw a decline in the abundance of health-associated species that produce butyrate, a substance that inhibits inflammation, cancer formation and stress in the gut. It’s not clear why the oral cavity returns to normal sooner than the gut, Zaura says, but it could be because the gut is exposed to a longer period of antibiotics. Another possibility, she says, is that the oral cavity is intrinsically more resilient toward stress because it is exposed to different stressors every day.
“My message would be that antibiotics should only be used when really, really necessary,” she says. “Even a single antibiotic treatment in healthy individuals contributes to the risk of resistance development and leads to long-lasting detrimental shifts in the gut microbiome.”
In the double-blind study, Zaura and colleagues looked at healthy adult volunteers from the United Kingdom and Sweden, who were randomly assigned to receive a full course of one of four antibiotics (ciprofloxacin, clindamycin, amoxicillin or minocycline) or a placebo. Investigators collected fecal and saliva samples from participants at baseline; immediately after taking the study drugs; and one, two, four and 12 months after finishing the medications. They performed 16S rRNA gene amplicon sequencing to identify the presence of bacteria on 389 fecal and 391 saliva samples. Then, they performed metagenomic shotgun sequencing on samples where researchers saw the largest differences before and after antibiotic usage, to study the emergence of antibiotic resistance.
Participants from the United Kingdom started the study with more antibiotic resistance than did the participants from Sweden, which could result from cultural differences. There has been a significant decline in antibiotic use in Sweden over the last two decades, Zaura said.
Fecal microbiome diversity was significantly reduced for up to four months in participants taking clindamycin and up to 12 months in those taking ciprofloxacin, though those drugs only altered the oral cavity microbiome up to one week after drug exposure. Exposure to amoxicillin had no significant effect on microbiome diversity in either the gut or oral cavity but was associated with the greatest number of antibiotic-resistant genes.
Further study to understand the mechanisms behind the oral microbiome’s resilience might prove useful in combating microbial imbalances in the gut, Zaura says.
-- Karen Blum