In the study, a total of 104 pre-clinical and 138 clinical students were enrolled. No nasal MRSA carriage was detected in the pre-clinical group. However, one MRSA isolate was obtained from a female student in the clinical group (Table 1.). When examining hand fingerprints, a lower number of MSSA isolates were compared to a nasal carriage. No MRSA isolates were detected from the hand fingerprints of the students in either group.

Positive-to-total, percentage in parentheses: The first number are the positive samples, the second are total samples, and the percentages are the positive-to-total ratio in parentheses.

Among both groups of students and in both types of samples, the most frequently detected genes encoding enterotoxins were seg and sei. In the pre-clinical student group, seg occurred in 19.1% of nasal isolates and 27.3% of fingerprint isolates. Similarly, sei was found in 21.3% of nasal swab isolates and 27.3% of fingerprint isolates. In the clinical student group, the prevalence of seg and sei in nasal swab isolates was even higher, reaching 40.0%. For fingerprint isolates, sei was detected in 42.7% of cases, while seg was present in 20.0% of cases. Furthermore, in the clinical student group, additional genes encoding enterotoxin production, namely sea and seb were found. These genes were not detected in the pre-clinical student group.

Isolates from both groups of students showed positive results for the presence of eta and etb genes. In the pre-clinical student group, the genes were found exclusively in nasal swab isolates, with eta being present in 6.4% and etb in 8.5% of the isolates. Among the nasal swab isolates of clinical students, eta was detected in 5.1% of the isolates, and etb was present in 8.5%. Interestingly, the etb gene was only detected in the fingerprints of clinical students, accounting for 5.7% of the isolates. Furthermore, the tst gene was identified in isolates from the nose and fingerprints of clinical students, with a prevalence of 8.5% and 17.1% respectively. However, no isolate carrying the pvl gene was confirmed in either group.

All the MSSA and MRSA isolates obtained were subjected for antibiotic resistance screening. The results revealed that the most frequent resistance was observed against both erythromycin and clindamycin. Among the nasal isolates, approximately 34.0% of isolates from pre-clinical students and 23.7% of isolates from clinical students exhibited resistance to erythromycin. Similarly, 23.7% of nasal isolates from pre-clinical students and 22.0% of nasal isolates from clinical students showed resistance to clindamycin.

Notably, among the nasal isolates from pre-clinical students, a significant proportion (29.8%) exhibited inducible clindamycin resistance (iMLS). This trend was also observed in the nasal isolates from clinical students, with 22.0% of isolates showing the iMLS type when resistance to erythromycin and clindamycin was detected. Furthermore, a smaller percentage of isolates displayed constitutive resistance (cMLS) to erythromycin and clindamycin. Specifically, two (4.2%) isolates from nose swabs of pre-clinical students and one (1.7%) isolate from the nose swabs of clinical students exhibited the cMLS type of resistance.

Among the clinical students, the most frequently observed resistance was to ciprofloxacin, with 22.0% of isolates from the nose and 20.0% of isolates from fingerprints showing resistance to this antibiotic. In contrast, resistance to ciprofloxacin was present in only 2 isolates (4.3%) obtained from nasal swabs of pre-clinical students, resistance to chloramphenicol was found in 3 isolates (1.2%) obtained from all students, and resistance to tetracycline was observed specifically from the clinical students.

A multidrug resistance, defined as resistance to three or more antibiotic groups, was recorded in two isolates. It was found only in the group of clinical students. Both two cases involved resistance to erythromycin and clindamycin, in combination with chloramphenicol. Regarding the MRSA isolate, resistance to cefoxitin was detected, confirming its resistance to methicillin. Resistance to fusidic acid was observed, while resistance to mupirocin was not detected. More detailed information on virulence and antibiotic-resistance genes is presented in Supplementary Table 1.

Concurrent capture of MSSA from both the nose and fingerprints was observed in 10 out of 104 pre-clinical students (9.6%) and in 28 out of 138 clinical students (20.3%). In 60% of pre-clinical students, the same spa type was identified in isolates from both the nose and fingerprints. Among clinical students, this was observed in 32.1% of cases.

An interesting finding is that spa type t10060 was frequent (12.1%) in the preclinical group, whereas it has been reported once (0.7%) in the clinical group. Other more frequently represented spa types among the pre-clinical students were t084 (8.6%), t085 (6.9%), t078 (5.2%), and t267 (5.2%). The spectrum of spa types varied between both student groups. For the clinical group, the five most common spa types were t20949 (5.3%), t491 (5.3%), t078 (4.3%), t1451 (4.3%), and t179 (4.3%). More information on the frequency of each spa type is provided in Supplementary Table 1. The MRSA isolate belonged to spa type t051. A total of 15 (14.2%) nasal swab isolates and 12 (26.1%) fingerprint isolates could not be assigned to the spa type, these sequences are not found in the available database.

Molecular typing of the MRSA isolate obtained from a female student in the clinical student group revealed it to be assigned to sequence type ST2149, belonging to the clonal complex CC8.

The phylogenetic tree of all recovered spa types is shown in Fig. 1. A total of 58 different spa types were detected among the obtained isolates. The spa types were differentiated into 6 clusters, the larger clusters including the most frequently occurring spa types were labeled I., II., and III. Eighteen different spa types were obtained from a group of pre-clinical students (red), and 27 were obtained from clinical students (blue). Common spa types for both groups were recorded 13 (green). The MRSA isolate is highlighted by the red frame.

The evolutionary history was inferred using the UPGMA method (Tamura et al. 2004). The optimal tree is shown. The evolutionary distances were computed using the Maximum Composite Likelihood method and are in the units of the number of base substitutions per site. This analysis involved 55 nucleotide sequences. All ambiguous positions were removed for each sequence pair (pairwise deletion option). There was a total of 584 positions in the final dataset. Evolutionary analyses were conducted in MEGA11 (Tamura et al. 2021). Red pre-clinical students, blue clinical students, Green—both groups