Nasal Carriage by Staphylococcus aureus among Healthcare Workers and Students Attending a University Hospital in Southern Brazil: Prevalence, Phenotypic, and Molecular Characteristics

Abstract

Background: Staphylococcus aureus can asymptomatically colonize the human anterior nares and skin, and nasal colonization by this bacterium represents a potential risk for development of invasive infections. The aim of this study was to determine the prevalence of S. aureus nasal carriage among healthcare workers and students attending a university hospital and to characterize the isolates phenotypically and molecularly.

Methods: A cross-sectional study was performed with 324 volunteers. Cultures from nasal samples were obtained and S. aureus isolates were characterized according to their antimicrobial susceptibility profile and four virulence factors-encoding genes. MRSA isolates were characterized regarding their oxacillin/cefoxitin susceptibility, SCCmec, and REP-PCR types. Potential risks for S. aureus and MRSA carriage were analyzed.

Results: Of 324 nasal samples, 42.9% were identified as S. aureus, of which 28.8% were MRSA. S. aureus carriers were significantly higher in males and students (OR = 2.898, 95%CI 1.553-5.410); however, no variables were associated with MRSA carriage. All isolates were susceptible to vancomycin and the highest rate of resistance was observed for penicillin (90.6%). All isolates harbored the coa gene, and 97.8%, the icaA gene; 15.8% and 6.5% were positive for tst and lukS-PV/lukF-PV genes, respectively. Among MRSA isolates, 45% carried the mecA gene but were phenotypically susceptible to oxacillin/cefoxitin; two harbored the tst and none had lukS-PV/lukF-PV genes. All MRSAs were distributed into six SCCmec types and type I (62.5%) was the most frequent. REP-PCR typing identified four main clusters among MRSA isolates.

Conclusion: High prevalence of healthcare workers and students were identified as nasal carriers of S. aureus exhibiting different antimicrobial resistance profiles, including mecA-positive oxacillin-susceptible S. aureus (OS-MRSA) and the presence of virulence-encoding genes. Both cohorts may represent potential sources for the emergence of a successful S. aureus strain highly adapted to the hospital environment.

Figure 1

Flowchart of the procedures and the results for nasal swabs obtained from healthcare workers and students attending the University Hospital of Londrina from December 2017 to May 2018. PCR: polymerase-chain reaction; MSSA: methicillin-susceptible Staphylococcus aureus; MRSA: methicillin-resistant S. aureus; MIC: minimal inhibitory concentration; OS-MRSA: mecA-positive oxacillin-susceptible S. aureus; FOX: cefoxitin; OXA: oxacillin.

Figure 2

Phenotypic and molecular characteristics of 40 methicillin-resistant Staphylococcus aureus (MRSA, mecA positive) and descriptive variables of MRSA carriers. The UPGMA banding pattern obtained by REP-PCR based on Dice similarity coefficient showing the genetic relatedness of MRSAs. Vertical dashed line marks the position of similarity coefficient value of 0.85. REP-PCR: repetitive element sequence based-polymerase chain reaction; MIC: minimal inhibitory concentration; R resistant; S susceptible; SCCmec: staphylococcal cassette chromosome mec; NT: nontypeable; HW: healthcare worker; US: undergraduate student; PS: postgraduate student; F: female; M: male; N: no; Y: yes.