Low-dose Antibiotics Preserve Airway Bacterial Diversity, but at a Cost, Study Finds

Using lower, or suboptimal, doses of antibiotics to treat lung infections in children and young adults with cystic fibrosis (CF) leads to fewer changes in airway bacterial diversity compared to therapeutic (higher) antibiotic exposure.

The findings also suggested, however, that patients receiving therapeutic doses had greater improvements in lung function.

The research, “Changes in microbiome diversity following beta-lactam antibiotic treatment are associated with therapeutic versus subtherapeutic antibiotic exposure in cystic fibrosis,” was published in the journal Scientific Reports.

People with CF often have recurrent lung infections that gradually worsen their lung function. Treatment of these infections, known as acute pulmonary exacerbations (APEs), is typically based on antibiotics directed at pathogens such as Pseudomonas aeruginosa. However, repeated dosing with antibiotics has been suggested as the cause of decreased microbial diversity in the airways, which, in turn, could worsen lung function.

Despite current guidelines recommending higher antibiotic dosing regimens in people with CF, data from the U.S. indicate that beta-lactams — which include penicillin — were given in doses below the guidelines 38–53% of the time.

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Although CF patients often do not achieve therapeutic doses of antibiotics to clear infections, which means that their blood levels of antibiotics do not increase sufficiently for effective treatment, it remains to be determined whether short courses of subtherapeutic doses alter microbial diversity, compared to therapeutic doses.

Aiming to address this question, researchers recruited 20 patients, ages 1-21, who were treated for APEs with intravenous infusion of beta-lactam antibiotics at Washington, D.C.’s Children’s National Health System.

Four samples of respiratory fluid were collected from each patient — when they were experiencing an APE, when they were doing well, right after antibiotic treatment, and at least 30 days later. Genetic testing determined the type and relative abundance of bacteria in each sample.

Blood samples and data on lung function were also collected during antibiotic treatment. Plasma antibiotic levels and bacterial minimum inhibitory concentrations (MICs) were used to determine therapeutic versus subtherapeutic antibiotic exposure.

Of note, to achieve effective bacterial killing, the serum concentration of an antibiotic must be above the MIC of the bacteria for a certain amount of time. Subtherapeutic exposure was thereby defined as insufficient time above MIC.

A total of 31 APEs were reported over the study period, from March 2015 to August 2016, and only approximately 14 (45%) of the antibiotic courses given were considered therapeutic. Most treatment regimens (25 out of 31 antibiotic regimens administered) included a single beta-lactam antibiotic.

Patients in the therapeutic group (11 patients, median age 9) had better lung function, and were less likely to receive inhaled antibiotics at study start, compared with the nine patients (median age 14) receiving subtherapeutic doses — 45% versus 100%, respectively.

At exacerbation onset, people in the therapeutic group showed a greater trend toward having a normal flora, compared to the subtherapeutic group (43% versus 12%), as well as increased abundance of Gemella, and a decrease in unclassified Enterobacteriaceae.

There was no significant difference in the presence of Pseudomonas aeruginosa or Staphylococcus aureus between the therapeutic and subtherapeutic groups.

As for antibiotic use, participants in the therapeutic group received ceftazidime (a more narrow spectrum beta-lactam) more often (86% versus 41%), and less frequently meropenem (a broad spectrum beta-lactam; 21% versus 59%) than those in the subtherapeutic group.

Those in the therapeutic group also had a significantly shorter time from end of treatment to post-recovery — 51 versus 79 days.

At both end of treatment and post-recovery, patients in the therapeutic group had decreased bacterial diversity in their airways, which contrasted with those receiving subtherapeutic doses — who showed minimal changes or higher diversity more than one month after treatment.

Unlike participants in the therapeutic group, who showed increased or decreased relative abundance of specific bacterial genera compared to baseline, those in the subtherapeutic group showed no changes.

“With the subtherapeutic treatment group, this could represent a ‘basement effect’ where it is harder to decrease diversity when it is already low to start,” Andrea Hahn, MD, the study’s lead author, said in a press release.

Also, “patients in the subtherapeutic group had more advanced disease than those in the therapeutic group, which may influence the findings,” Hahn said.

Data further showed that receiving therapeutic antibiotics was associated with a trend toward greater improvement in lung function.

“Thus, the conclusion should not be drawn that because subtherapeutic antibiotics have less impact on changes in microbial diversity, it could be used as a strategy to prevent declining lung function,” the scientists said. “This is likely a reflection of disease severity, antibiotic exposure, and antibiotic resistance.”

The researchers believe that repeated subtherapeutic courses of antibiotics could lower microbial diversity without clearing infections, causing progressive lung dysfunction. Closer monitoring of antibiotic levels in blood to ensure that each exacerbation is treated with therapeutic-level dosing could be an effective answer, Hahn said.

“What this study shows is that levels of the antibiotics we give probably play a role in patients’ ability to recover baseline diversity,” Hahn said. “If we pay more attention to drug levels when using these types of antibiotics to ensure that dosing is sufficient, we could potentially improve patients’ clinical outcomes over time.”