Limitations and strengths
There are a number of limitations to this study. Although the most important limitation of this study was that the fieldwork was conducted in 2010 and 2011, we used the current resistance breakpoints published in 2017. We do not think the results would have been different if the samples had been collected more recently, since the consumption of antibiotics in our country has been stable over the last years and the main aim of the present study was to determine a possible relationship between previous antibiotic exposure and the prevalence of relevant resistant strains in the noses of healthy individuals. On the other hand, since we carried out a cross-sectional study and compared the results of the resistance patterns of the staphylococci harboured with previous antibiotic exposure, we cannot infer any causality effect between antibiotic exposure and resistance. However, we were able to compare the number of boxes of antibiotics previously dispensed and the resistance patterns of commensal staphylococci in otherwise healthy individuals.
Another limitation of the study is the fact that we considered antibiotics dispensed in the previous 4 years. Prescription of antibiotics based on reimbursement data can mimic antibiotic consumption in most European countries, but is not true in southern European countries. A landmark paper published in 2007 with the use of sales data in 2002–2005 showed a difference between prescription and consumption of up to 30% in Spain, mainly due to the over-the-counter sale of antibiotics without a medical prescription and antibiotics prescribed by the private sector [19]. A higher antibiotic consumption would very likely have been associated with greater resistance rates to antibiotics.
Another weakness is that this study monitors colonisation at a single time-point, although colonisation is known to be variable over time and it should be differentiated between persistent and intermitted carriers [20]. Only nasal swab samples were used to determine S. aureus colonization status, as the study protocol did not include swabbing of extranasal body sites. While the prevalence might be underestimated by using nasal swabs, we assume our final sample of S. aureus to be representative of all carriage. We also assume that antimicrobial resistance patterns in the commensal flora of the nose are comparable with isolates of pathogenic staphylococci. However, for purposes of empirical treatment, data on resistance of pathogenic strains would also be required.
The greatest strength of this work is that it is a study in primary care patients. We are therefore able to extrapolate these results to what we observe in primary care.