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Determination of the pattern of resistance to antibiotics among strains of Staphylococcus aureus isolated from the nose or pharynx
* 1 , 2 , 3 , 2
1  Doctorado en Ciencias Biológicas y de la Salud, Universidad Autónoma Metropolitana
2  Departamento de Atención a la Salud, UAM-X
3  Instituto Nacional de Perinatología, CDMX
Academic Editor: Marc Maresca


Introduction. The pathophysiology of Staphylococcus aureus in nasal carriers has been extensively studied, however, it must be admitted that the clinical relevance of S. aureus carriers in the pharynx has not been extensively investigated. This omission appears to be justified, since the nose is mentioned as the primary site of S. aureus colonization. From there, other regions are colonized by manual spread In the general adult population, S. aureus can be commonly found in other body sites such as the axillae (8%), chest/abdomen (15%), perineum (22 %), intestine (17-31%), vagina (5%) (Sollid J et al; 2014) and from 4 to 64% in the pharynx. Some studies mention a higher rate of carriers in the pharynx than in the nose when samples are taken in parallel. The objective of the work was to determine if there are differences in the pattern of resistance to antibiotics of strains isolated from the nose and pharynx.

Methods. Pharyngeal and nasal exudates were performed on 98 university students once a month for three months. The exudates were incubated in Tripticasein Soy Broth at 37 °C for 24 h, followed by seeding in Salt and Mannitol Agar Petri dishes using the cross streak method and re-seeding to obtain isolated colonies. All strains that were coagulase-positive mannitol fermenters were identified as S. aureus. If a person presented three isolates of S. aureus, they were considered persistent carriers, if they presented one or two isolates in a row, they were considered intermittent carriers, and if the bacteria were never isolated, they were considered non-carriers. All strains of S. aureus underwent antibiogram against: ciprofloxacin, fosfomycin, trimethoprim-sulfamethoxazole, penicillin, vancomycin, tetracycline, erythromycin, oxacillin, clindamycin, gentamicin and cafalothin by the Kirby-Bauer method and minimum inhibitory concentration for oxacillin, following the indications of the CLSI.

Results. A total of 81 (± 9.1) strains of S. aureus were isolated from the pharynx and 43 (± 6.6) strains from the nose of the students during the three samples taken. In the case of the pharyngeal strains, 81.4% (± 4.7) were resistant to penicillin, 12.5% (± 4.1) to clindamycin, 8.6% (± 2.8) to erythromycin, 2.78% (± 1.19) they are resistant to tetracycline and oxacillin. For ciprofloxacin, fosfomycin, vancomycin, gentamicin and cephalothin, the percentage of resistant strains was less than 1%.

In the case of the strains isolated from the nose, it was found that 84.3% (± 4.6) are resistant to penicillin, 18.2% (± 4.8) to erythromycin, 12.4% (± 4.4) to clindamycin, 4.49% (± 3.8) to tetracycline, 3.6% (±0.9) were resistant to oxacillin. For ciprofloxacin, fosfomycin, trimethoprim-sulfamethoxazole, gentamicin, and cephalothin, the percentage of resistant strains was less than 1%.

Discussion. More carriers of S. aureus were found in the pharynx (75%) than in the nose (40%) in the three samples taken, which coincides with some results published in the literature (Mertz D, 2007, Hamdan et al 2018). In the case of the percentage of resistant strains, it is very similar regardless of the isolation site, and the variation in resistance during the three intakes is similar.

Conclusions. More strains of S. aureus were isolated from the pharynx than from the nose. No differences were found in resistance to antibiotics, nor changes in the percentage of resistant strains in the pharynx and nose.

Keywords: Staphylococcus aureus; nose; pharynx; resistant