Results

Demographic characteristics
A total of 4,001 nasal swabs were collected, and 3,969 were tested for identification. Finally, 776 (19.6%) were S. aureus positive, of which 765 samples (19.3%) (95% CI, 18.1–20.5) were tested for resistance. The mean age of evaluated carriers was 43.2 years (SD: 23.1). Carriage was higher in children less than 14 (136 cases; 35.6%) than in individuals aged 15–64 (459 cases; 19.4%) and people more than 65 years old (170 positive cases; 14%). Among the 765 evaluated subjects colonised with S. aureus, 370 (48.2%) were male.

Antimicrobial susceptibility
Based on the antimicrobial susceptibility reports among the 765 evaluated S. aureus isolates, 10 (1.3%; 95% CI, 0.7%–2.4%) were classified as MRSA. The highest resistance rates were observed to penicillin, followed by azithromycin, erythromycin and clindamycin, with resistance percentages of 87.1%, 11.6%, 11.2%, and 9.7%, respectively. A total of 1.3% strains were resistant to oxacillin (Table 1); 83 strains did not exhibit any resistance to the antibiotics tested (10.8%).

Association with previous antibiotic exposure
Of the 765 individuals harbouring S. aureus strains, 208 were found not to have taken antibiotics in the previous 4 years (27.1%). Of the subjects, 18.1% had taken one box, 13.0% two boxes, 11.5% three boxes of antibiotics, and 30.3% more than three boxes. The mean number of boxes of antibiotics consumed in the previous 4 years was 3.3 (SD: 5.2 and range 0–66), with penicillin being the most frequent antibiotic taken (mean of 1.5 packages; SD: 2.3), followed by ophthalmological presentations including antibacterial drugs (mean of 0.5 containers; SD: 2.9) and topical antibiotics (0.3 boxes; SD: 0.8).
Although not statistically significant, a trend was shown in which patients who had more than one antibiotic dispensed in the previous 4 years tended to harbour more staphylococcal strains resistant to any antibiotic compared to those without using antibiotics (OR, 1.07; 95% CI, 0.99–1.16; P = 0.55). Isolation of penicillin-resistant staphylococci was significantly associated with the previous number of packages of penicillin (OR, 1.18; 95% CI, 1.04–1.35) but not with other antibiotics packages. No association was observed between the number of boxes among individuals harbouring S. aureus resistant to oxacillin, clindamycin or azithromycin. However, we did observe a statistically significant association between harbouring MRSA and the previous prescription of any antibiotics (OR, 1.6; 95% CI, 1.16–2.20) (Table 2).
Table 2.  Association of the previous number of antibiotics boxes dispensed and the staphylococcal resistance. A multivariate logistic regression analysis model.
Variable  95% CI  OR  P value
Staphylococcus aureus resistance
 Age (years)  0.98–1.00  0.99  0.24
 Sex (women)  0.45–1.14  0.72  0.16
 Any antibioticsa  0.99–1.16  1.07  0.55
Penicillin-resistant Staphylococcus aureus
 Age (years)  0.99–1.01  1.00  0.76
 Sex (women)  0.46–1.01  0.70  0.11
 Penicillin packages (number)  1.04–1.35  1.18  0.01
Methicillin-resistant Staphylococcus aureus (MRSA)
 Age (years)  0.99–1.05  1.02  0.22
 Sex (women)  0.15–2.43  0.61  0.48
 Any antibioticsa  1.16–2.20  1.60  <0.01
aHave taken one or more antibiotic boxes.