Erythromycin susceptibility of viridans streptococci from the normal throat flora of patients treated with azithromycin or clarithromycin.
OBJECTIVE
To study the emergence of macrolide resistance in throat flora following treatment with clarithromycin or azithromycin.


METHODS
Throat samples were collected before and after treatment and plated as a lawn on Columbia blood agar with an erythromycin E test strip. Minimum inhibitory concentrations (MICs) of erythromycin, clarithromycin and azithromycin were determined against isolates of distinct morphology with erythromycin E test MIC results equal to or greater than 2 mg/L. Polymerase chain reaction techniques were used to determine the genetic mechanisms of resistance.


RESULTS
There were 749 resistant isolates of which 474 (63%) were streptococci. Only a quarter of the patients had no resistant streptococci before treatment started. There were increases in the numbers of resistant isolates and in the number of patients carrying a resistant flora during and after treatment. The most common genes identified were mefA/E in isolates with low-level resistance and ermA/M in isolates with high-level resistance.


CONCLUSIONS
There is a pool of streptococci carrying genes associated with macrolide resistance in the normal respiratory flora of generally healthy adults. Differences between the patients treated with clarithromycin and those treated with azithromycin were difficult to assess because of the large number of patients in each group with macrolide-resistant streptococci before treatment. Although there were some differences these were not statistically significant.
King
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A
Department of Infection, GKT School of Medicine, St Thomas' Hospital, London, UK. anna.king@kcl.ac.uk
Bathgate
T
T
Phillips
I
I
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D017963:Azithromycin / Q000627:therapeutic use*; D017291:Clarithromycin / Q000627:therapeutic use*; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D004917:Erythromycin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D006801:Humans; D008826:Microbial Sensitivity Tests; D010614:Pharynx / Q000382:microbiology; D016133:Polymerase Chain Reaction; D013290:Streptococcal Infections / Q000188:drug therapy* / Q000382:microbiology*; D013291:Streptococcus / Q000187:drug effects* / Q000235:genetics
D000900:Anti-Bacterial Agents; C058417:erythromycin E; D004917:Erythromycin; D017963:Azithromycin; D017291:Clarithromycin