Risk factors assessment for nasal colonization of Staphylococcus aureus and its methicillin resistant strains among pre-clinical medical students of Nepal

Staphylococcus aureus (S. aureus) is a normal flora of moist squamous epithelium of the anterior nares. Majority of the populations (60 %) are intermittent carriers while 20 % of the population is always colonized with S. aureus and 20 % of populations never carry this organism [1]. The evidence suggests that the populations harboring S. aureus and its methicillin resistant (MRSA) strains are at higher risk for developing invasive infection [2–4].

A range of minor as well as life threatening conditions like skin infections (pimples, impetigo, boils, cellulitis, folliculitis, carbuncles, scalded skin syndrome, abscesses), pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome (TSS), and septicemia can be caused by S. aureus [5]. Coagulase negative Staphylococcus species (CoNS) act as the most common causative agents of nosocomial bacteremia [6].

Nasal carriage of MRSA contributes as a major risk factor for subsequent infection and transmission of this pathogen [7, 8]. Prolonged hospitalization, antibiotics exposure, and the presence of other patients with MRSA colonization or infection in the hospital are the major risk factors for acquiring MRSA infections. MRSA causes life threatening infections and greater mortality than that from methicillin-sensitive S. aureus (MSSA) infections. Vancomycin, a glycopeptide antibiotic is the drug of choice to treat MRSA infections. However, the continuous use of vancomycin may promote growth of vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA) [9]. Therefore, this study was conducted to assess the nasal carriage rate of S. aureus, MRSA, its antimicrobial susceptibility profile and the associated risk factors in the medical students prior to their clinical exposure.

This simple and cross-sectional study was carried out at Department of Microbiology, Chitwan Medical College (having a 600 bed teaching hospital) in the city of Bharatpur, Chitwan District, Narayani Zone, Nepal in March, 2014. A total 200 medical students who were studying in their first year of their medical education and were not exposed to their clinical posting were enrolled in this study.

Now-a-days, the rate of urbanization is increasing in search of better life style, jobs, medical facilities and better education facilities. The result of our study also indicates that majority of the participants (80.5 %) belonged to urban area and 19.5 % of the participants belonged to family having health care professions. As pets are being taken as close friends to humans, 60 (30 %) participants of our study were having pets (mostly dog). Common cold, a minor form of URTIs, is common in Nepal and thus, 44 (22 %) of the participants were suffering from upper respiratory tract infections (URTI) during the study period.

Although, large proportion of S. aureus carriage is through the anterior nares of the nasal passages, it can be found on the skin of the host [1]. The combination of defective host immunity and the bacterial ability to evade host innate immunity results in the ability of the nasal passages to harbor S. aureus [14]. Approximately 20 % of individuals act as persistent carriers and almost always carry one type of strain [15]. In this study, we detected 15 % nasal carriage of S. aureus in medical students, similar to the rates detected from Malaysia [16, 17] and Iraq [18].

In this study, a non-significant association was observed between the S. aureus colonization and type of residence. The present analysis found that 73.3 % of the students positive for nasal colonization of S. aureus were from cities and 83.3 % of participants used public vehicle for travelling. The changing pattern of life style in urban area like frequent visit of shopping malls and theatres, attending parties and travelling via public vehicles bring peoples to be in close contact and easily can transmit the pathogens to others. Moreover, contaminated door handles of public vehicles also act as source infections.

Although, non-significant association was observed between nasal carriage of S. aureus and clinical factors such as medication and antibiotic use in last 3 months, there was significant association (P = 0.035) with upper respiratory tract (nasal) infections in our study. Studies from other settings have also reported increased spread of S. aureus during an episode of URTIs [19]. In a study conducted from Malaysia, 22.5 % cases of nasal carriage were associated with URTIs and 9.9 % cases were associated with recent antibiotic use [17].

The reports of recent study have documented that S. aureus can survive on dogs and cats [20, 21]. Some authors believe that health-care workers’ dogs should be considered a significant source of antibiotic-resistant S. aureus, especially during outbreaks [20]. In our study, we found that 30 % of the nasal carriers of S. aureus have had contact with pet (mostly dog), the findings being much lower than the result of 77 % from Virginia [22]. The lower rate of our study may be due to the rather still uncommon practice of domesticating pets in Nepal than others.

Although, the overall resistance rates to commonly prescribed antibiotics in isolates were below 50 %, as expected the rate was high (73 %) with penicillin because only a small proportion of the S. aureus lineages do not produce beta-lactamases [23–26]. Similarly, a higher rate of resistance (92 %) to ampicillin was also reported in a study conducted in Brazil [27].

Ciprofloxacin became the most widely used quinolone antibiotic after its introduction into clinical use in the late 1980s and early 1990s [28, 29]. In recent years, resistance has been developed in many bacteria, making it significantly less effective [30, 31]. We identified ciprofloxacin resistance rate of 36.7 % in this study much higher than the result (8.8 %) from Brazil [27]. This high rate of resistance in our study may be because of its indiscriminate use in our setting as a consequence of low cost and easy availability. Once effective against staphylococcal infections, the aminoglycoside antibiotics such as kanamycin, gentamicin, streptomycin etc., have been found less effective because of the development of mechanisms to inhibit the action, which occurs via protonated amine and/or hydroxyl interactions with the ribosomal RNA of the bacterial 30S ribosomal subunit [32]. As a consequence of low cost and easy availability, there has been indiscriminate use of this antibiotic similarly as with ciprofloxacin in our context. We observed the rate of gentamicin resistance as 33.3 % and amikacin resistance as 10 %. Similar rate of gentamicin resistance (25 %) was also detected by Sharma et al. [33] whereas lower rate of amikacin resistance (4 %) was also reported from Brazil [27].

Today, the therapeutic roles of erythromycin and trimethoprim–sulfamethoxazole (co-trimoxazole) are increasingly limited due to its extensive use for the treatment of both minor and serious staphylococcal infections. One third (33.3 %) of our isolates were resistant to erythromycin and 20 % isolates were resistant to co-trimoxazole, the results are in agreement with the reported finding from Iran [34].

Glycopeptides like vancomycin and teicoplanin could be reserved for the management of MRSA infections because of its high efficacy in virtually all isolates of S. aureus [35, 36]. The result of susceptibility in the current study to vancomycin and teicoplanin is comparable to that of other studies conducted worldwide [18, 27, 33]. The promising efficacy of glycopeptides is probably due to high cost and low usage of these regimens in Nepal. However, increased resistance to teicoplanin has been reported overseas [37–40]. Thus, glycopeptides especially vancomycin can be used empirically for serious staphylococcal infections while waiting for susceptibility testing results to come through [41].

Methicillin, the first antibiotic of β-lactamase-resistant penicillins (methicillin, oxacillin, cloxacillin, and flucloxacillin) was detected to be ineffective only after two years of its introduction (introduced in 1959) in England and it was the first case of MRSA [42]. Then until 1990s, when there was an explosion in MRSA prevalence in hospitals, MRSA generally remained an uncommon finding, [43]. In present study, two different methods, the cefoxitin disk and oxacillin disk methods were employed for the detection of MRSA. According to CLSI guidelines, the mecA mediated resistance to oxacillin can be detected by using cefoxitin disk or oxacillin disk method but cefoxitin disk method is more preferred because it is easier to read and also cefoxitin acts as an inducer of the mecA gene [13]. Results of the present study indicated that, 4 % (8/200) students harbored the MRSA in their nasal cavity. Similarly, 6 % MRSA carriage rate in medical students before clinical exposure was also detected from India [44].

Macrolide-resistant isolates of S. aureus may be resistant to only macrolides mediated by efflux mechanism encoded by the msrA gene or may have constitutive or inducible resistance to clindamycin mediated by methylation of the 23S rRNA encoded by the erm gene [13]. Clindamycin is considered as one of the drugs of choice in S. aureus infections but an erm gene mediated inducible resistance may result in treatment failure [45]. In our study, we identified 4 isolates (13.3 %) as D-zone test positive, indicating inducible resistance to clindamycin. Similar result of inducible resistance to clindamycin was also reported from Nepal [10].

Spread of S. aureus (including MRSA) is generally through human-to-human contact, although recently some veterinarians have documented that the infection can also spread through pets [46]. In a case control study, 73 % of MRSA were recovered from pets (cat, dog and other rodents) [22]. Similarly, in our study we observed that 75 % of MRSA carriers have had contact with pet especially dog. High proportion of MRSA (75 %) colonization was also significantly associated with URTIs (P = 0.009) and visit to public amusement places (P = 0.018). Although the resident type was not associated with the colonization rate of S. aureus, it did the colonization of MRSA in our study (P = 0.007).

Our study had a few limitations, we selected a limited pool of antibiotics for susceptibility testing and molecular studies could not performed to confirm MRSA isolates due to financial constraints.

The result of this study highlights the nasal carriage of S. aureus and their methicillin resistant counterparts in the medical students. This study indicates that carriage of this organism has no significant association with socio-demographic and habitual risk factors. However, URTIs can enhance the carriage of S. aureus as well as their MRSA strains. Recent visit to public amusement places, contact with dog (pet) and rural residence were also documented as the significant risk factors contributing the MRSA colonization. The colonization of S. aureus and MRSA can play a key role in the epidemiology and pathogenicity of community as well as hospital associated infections. From this study, the longitudinal surveillance of nasal carriage of S. aureus should be made an essential protocol to minimize the transmission of this organism from community to hospital and vice versa.