Antibiotic resistance in strains of Streptococcus pneumoniae has been rising since 2013 because of changing susceptibility profiles, based on data from 1,201 isolates collected from 448 children in primary care settings.
“New strains expressing capsular serotypes not included in the 13-valent pneumococcal conjugate vaccine are emerging to cause disease, and strains that acquire antibiotic resistance are increasing in frequency due to their survival of the fittest advantage,” wrote, of Rochester (N.Y.) General Hospital Research Institute, and colleagues.
Similar Darwinian principles occurred after the introduction of PCV-7, the study authors added.
In a prospective cohort study published in, the researchers reviewed 1,201 isolates collected from the nasopharynx during healthy periods, and from the nasopharynx and middle ear fluid (MEF) during episodes of acute otitis media, in children aged 6-36 months who were seen in primary care settings.
The isolates were collected during 2006-2016 to reflect the pre- and post-PCV13 era. Children received PCV-7 from 2006 until April 2010, and received PCV-13 after April 2010.
Overall, the number of acute otitis media (AOM) cases caused by S. pneumoniae was not significantly different between the PCV-7 and PCV-13 eras, nor was the frequency of pneumococci identified in the nasopharynx during healthy visits and visits at the start of an AOM infection.
The researchers examined susceptibility using minimum inhibitory concentrations (MIC). During healthy visits, the MIC50 of isolated pneumococci was low (no greater than 0.06 mcg/mL) for all four beta-lactam drugs tested. And it didn’t change significantly over the study years.
In contrast, among the nasopharyngeal and MEF isolates during AOM, the MIC50 to penicillin, amoxicillin, ceftriaxone, and meropenem during 2013-2016 rose significantly, the investigators said.
A change in antibiotic susceptibility within a subtype also contributed to the development of PCV-13 resistance.
The study authors identified three serotypes that affected the changes in susceptibility in their study population. Serotypes 35B and 35F increased their beta-lactam resistance during 2013-2016, and serotype 11A had a higher MIC to quinolones and became more prevalent during 2013-2016. Those three serotypes accounted for most of the change in antibiotic susceptibility, the researchers said.
In addition, “the frequency of strains resistant to penicillin and amoxicillin decreased with the introduction of PCV-13, but rebounded to levels similar to those before PCV-13 introduction by 2015-2016,” the investigators noted.
The study findings were limited by several factors, including the homogeneous study population and potential lack of generalizability to other settings. In addition, the researchers did not study antibiotic consumption or antibiotic treatment failure, and they could not account for potential AOM cases that may have been treated in settings other than primary care.
However, the investigators said the results support the need for additional studies and attention to the development of the next generation of PCVs, the PCV-15 and PCV-20. Both include serotypes 22F and 33F, but neither includes 35B or 35F. The PCV-20 also includes 11A and 15B.
The study was supported in part by the National Institutes of Health and Sanofi Pasteur. Some isolates collected during the 2010-2013 time period were part of a study supported by Pfizer. The researchers had no relevant financial conflicts to disclose.
SOURCE: Kaur R et al. Clin Inf Dis. 2020 Feb 18. .