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Abstract
Despite azithromycin being used in some countries to treat infections caused by Gram-negative pathogens, no resistance breakpoint for Escherichia coli exists. The aim of this study was to analyse the levels and mechanisms of azithromycin resistance in E. coli. The presence of chromosomal (rplD, rplV and 23S rRNA) mutations, 10 macrolide resistance genes (MRGs) and efflux pump overexpression was determined in 343 E. coli isolates. Overall, 89 (25.9%) isolates had MICs ≥ 32 mg/L to azithromycin, decreasing to 42 (12.2%) when assayed in the presence of Phe-Arg-β-Napthylamide, with 35 of these 42 possessing at least one MRG. Efflux pumps played a role in azithromycin resistance affecting the Minimal Inhibitory Concentration (MIC) levels of 91.2% isolates whereas chromosomal alterations seem to have a minimal role. At least one MRG was found in 22.7% of the isolates with mph(A) being the most commonly found gene. The mph(A) gene plays the main role in the development of azithromycin resistance and 93% of the mph(A)-carrying isolates showed a MIC of 32 mg/L. In the absence of a specific resistance breakpoint our results suggest a MIC of 32 mg/L to be considered in order to detect isolates carrying mechanisms able to confer azithromycin resistance.
Highlights
Despite azithromycin being used in some countries to treat infections caused by Gram-negative pathogens, no resistance breakpoint for Escherichia coli exists
Regarding the feasibility to considered azithromycin as an alternative to treat diarrhoeagenic E. coli in the studied areas, the present study showed moderate azithromycin resistance levels highlighting some concerns about its usefulness as treatment in the absence of antibiotic susceptibility data, especially when EAEC or DAEC isolates are present
The presence of a series of EAEC isolates in which no PAβN-effect was observed opens the door to different options, including the presence of alterations in the outer membrane composition which results in a possible azithromycin impaired permeability leading to an increase in the basal azithromycin resistance levels, combined with lesser efflux pump activity, at least in regard to PAβN-inhibitible efflux pumps
Summary
Despite azithromycin being used in some countries to treat infections caused by Gram-negative pathogens, no resistance breakpoint for Escherichia coli exists. Efflux pumps played a role in azithromycin resistance affecting the Minimal Inhibitory Concentration (MIC) levels of 91.2% isolates whereas chromosomal alterations seem to have a minimal role. While low permeability prevents the action of most of macrolide agents against Enterobacteriaceae[9], this basic character confers to azithromycin a true role in the treatment of diarrhoeal infections related to different Enterobacteriaceae[11,12]. Despite ranking amongst the most frequent etiological causes of diarrhoea[15,16], and the association of some specific diarrhoeagenic pathotypes with high levels of children mortality[16], at present no clinical breakpoint for resistance in E. coli has been established.
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