are a challenge to all health care systems worldwide . numbers of infections due to methicillin - resistant staphylococcus aureus ( mrsa ) are consistently high and remain a major risk to patients , especially when hospitalized . furthermore , a growing prevalence of infections caused by multi - drug resistant ( mdr ) gram - negative bacteria is reported . testing for nasal colonization with clinical relevant bacteria is an uncomplicated approach for estimating the extent of bacterial resistance - pattern in hospital settings . therefore , analysis of nose colonizing bacteria serves not only for monitoring circulating pathogenic bacteria and their transmission pathways , but may also allow estimation of the compliance of infection control precautions among health care professionals . therefore , the aim of this study was to characterize the effects of clinical exposure and evaluate the possible role of medical students in the transmission of pathogenic bacteria at a 2,200 bed tertiary care university teaching hospital . in total , 86 medical students voluntarily presented themselves to perform a nasal swab after a written announce of the study . as the medical school is integrated into the vienna general hospital , students are daily exposed to the hospital environment . furthermore , the sample collection was performed directly after the summer period of compulsory internships , thus guaranteeing additional exposure . a special focus was put on epidemiologically significant bacteria such as mrsa , methicillin - resistant staphylococcus epidermidis ( mrse ) , streptococcus pneumoniae and gram - negative mdr - bacteria ( e.g. extended spectrum beta - lactamase producing enterobacteriaceae ) . furthermore , we determined the expression of panton - valentine leucocidin ( pvl ) in order to characterize the pathogenicity of yielded s. aureus isolates . the study protocol was approved by the ethic committee of the medical university of vienna ( ethic committee no . : of 86 participants , 79 fulfilled the inclusion criteria , which consisted in i ) no severe respiratory infection ii ) no hospitalization iii ) no antibiotic treatment in the last six months . these criteria were established in order to ensure nasal carriage associated to health care environment . nasal swabs were taken from both nostrils and transferred on blood agar ( ba ) , chocolate agar ( choc ) and macconkey agar ( mck ) plates . verification of staphylococci was performed by i ) characteristic phenotypically growth on ba plate ii ) gram stain iii ) positive katalase reaction . colonies meeting these criteria were incubated on mannitol salt agar ( msa ) plates . isolates displaying yellow growth in msa plates were deemed as staphylococcus aureus ( s. aureus ) and subsequently verified by analytical profile index ( api ) and pcr . the pcr for identification of s. aureus and pvl identified isolates of s. aureus were screened for oxacillin - resistance by 24 h incubation on oxacillin screen agar ( mueller hinton agar , oxacillin 6 g / ml , 4% nacl ) . additionally , oxacillin minimum inhibition concentrations ( mics ) of staphylococcus aureus isolates were determined by microtiter dilution method according to clsi guidelines . staphylococci isolates not displaying yellow growth on msa plates were deemed as coagulase - negative staphylococci ( cons ) . further identification by api was only executed in case of positive oxacillin - resistance screening . s. pneumoniae was identified by alpha - haemolysis on choc plates and sensitivity to optochin . bacterial colonies growing on mck plates , indicating gram - negative bacteria , were subsequently analysed by gram - stain , oxidase reaction and api . resistance - testing for all isolates was performed on mueller hinton ii agar using disk diffusion susceptibility testing following eucast criteria 1 ) . all 20 s. aureus isolates showed no growth on oxacillin - screen agar and displayed a mic range of 0.250.5 mg / l with a mic50 of 0.25 mg / l and a mic90 of 0.5 mg / l ( table 2 ( tab . two isolates identified as staphylococcus epidermidis showed growth on oxacillin - screen agar and displayed mics of 8 and 4 mg / l , respectively . these were sensitive to amoxicillin / clavulanic acid , cefuroxime , cefepime , ciprofloxacin , fosfomycin and imipenem . the results of this control sample indicate a very low prevalence of multidrug resistant bacteria among medical students at the medical university of vienna . moreover , the s. aureus isolates did not express pvl , thus indicating low pathogenicity . colonisation with s. aureus did not significantly correlate with clinical exposure , sex or smoking habits , the latter finding contrary to previous studies   pertaining to methicillin - sensitive s. aureus , our findings are consistent with two other studies among medical students in europe , . however , the low prevalence of mrsa found among viennese medical students diverged from findings in gdnsk , poland , which found a prevalence of 4.5% among clinically exposed students . nevertheless , pvl expression was not detected as well . the other study , performed in budapest , hungary , came to similar results concerning mrsa with 0.67% . healthy non - medical students at texas state university showed similar s. aureus colonisations , whereas the mrsa prevalence was remarkably high with 7.4% . another study among healthcare professionals detected 43.8% s. aureus colonisation and 6.6% mrsa prevalence among the study population . a study conducted in malaysia among non - medical individuals found a prevalence of 23.4% concerning s. aureus and 0.28% mrsa . overall , this data indicates strong geographical differences among nasal colonization by mrsa , with low prevalence rates in central europe . surprisingly , 10.1% of our study population were positive on gram - negative bacterial colonisation . however , the bacteria found did not show resistance against tested antibiotics , indicating low pathogenic potential . considering our data we conclude that clinical exposure does not contribute to nasal carriage of multidrug - resistant bacteria among medical students in vienna . this might be explained by well - established hygienic precautions or comparably low circulation of resistant bacteria or the small sample size of investigated individuals , which might be the limiting factor of this study .
background : multi - drug resistant bacteria are increasing and remain a major public health challenge worldwide . in order to understand the potential role of medical students as a reservoir for circulating pathogenic bacteria and their transmission , we analysed the nasal colonisation among 86 clinically exposed medical students of the medical university of vienna , which is integrated into general hospital of vienna.methods : nasal swabs obtained from 79 students were eligible for further analysis . 
 nasal swabs were analysed for gram - positive and gram - negative bacteria with special emphasis on methicillin - resistant staphylococcus aureus.results : 25.3% of participants were positive for staphylococcus aureus colonization ; none of the isolates showed methicillin - resistance or expression of pantoin - valentine - leukocidin . 
 however , 2.5% were positive for methicillin - resistant staphylococcus epidermidis . 
 no participant showed streptococcus pneumoniae colonisation . furthermore , 
 10.1% of the samples displayed growth of gram - negative bacteria , yet none showed any relevant drug-resistance.conclusion : in conclusion , our investigation did not reveal any clinically relevant multi - drug resistant bacterial colonisation among clinically exposed medical students in vienna . 
 this might be explained by well - established hygienic precautions or comparably low circulation of resistant bacteria .