Macrolide resistance determinants are prevalent and readily selected for in viridans group streptococci among healthy Norwegian adults

Abstract number: p1811

Littauer  P., Haldorsen  B., Krämer  M., Aasnæs  B., Simonsen  G.S., Sundsfjord  A.


Norway has a low prevalence of antimicrobial-resistant bacteria including macrolide resistant (MR) respiratory tract pathogens. We have observed an increase in macrolide consumption in Norway and there is a lack of knowledge on the reservoir of macrolide resistance determinants among viridans group of streptococci (VGS) in the pharyngeal flora.


Examine the occurrence, selection and persistence of macrolide resistance determinants in VGS pharyngeal flora in healthy Norwegian adults before and after treatment with azithromycin.


Throat samples were collected before (day 1), after treatment (day 7) and after 3 months (day 90) from 20 healthy volunteers. The samples were plated directly as a lawn on PDMII agar plates with 5% defibrinated blood with an erythromycin Etest strip. Photos were used as quantitative comparisons. Up to 10 morphological different colonies with erythromycin Etest MIC >=1mg/ml from each specimen were collected; day1 (n = 59), day 7 (n = 157) and day 90 (n = 76). In total 86 representatives MR, VGS-isolates were selected for further studies: (i) MICs of erythromycin, tetracycline and penicillin were determined by Etest. (ii) PCR's for erm(B), erm(TR), and mef(A/E), and subsequent sequence-typing of mef. Species identification was performed by sodA sequencing.


A total of 17/20 persons carried a low number (<5) of MR VGS in day1 specimens, while 20/20 had a significant higher number (>100) of MR strains in day2 specimens. In day90 specimens, 20/20 carried a low number of MR, resembling day1. Reduced susceptibility to penicillin was observed in 32/86 (34%) isolates. Tetracycline resistance was found in 43/86 (57%), and mainly in erm(B)-positive strains. mef(A/E)-positive dominated day1 (53%) and erm(B) day2 specimens 52%. Sequence typing revealed mef(E) (n = 41) and mef(A) (n = 3). SodA sequence; S. mitis (n = 42), S. oralis (n = 9), S. parasanguinis (n = 12), S. salivarius (n = 22), and S. sanguinis (n = 1).


There is a pool of VGS carrying macrolide resistance determinants in the normal pharyngeal flora of healthy adults that are readily selected for during azithromycin exposure. The mef(E) and erm(B) were the most prevalent resistance genes and co-resistance to tetracycline was frequently observed, resembling the findings in Norwegian clinical isolates of S. pneumoniae. These VGS may provide a pool of resistant bacteria that may transfer resistance determinants to more pathogenic organisms.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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