Prevalence of Staphylococcus aureus Isolated from Clinical Samples in a Tertiary Care Hospital: A Descriptive Cross-sectional Study

ABSTRACT Introduction: Staphylococcus aureus is one of the commonest cause of nosocomial infections. Resistant Staphylococcus aureus strain has become a matter of concern. This study was done to find out the prevalence of Staphylococcus aureus from different clinical samples. Methods: This descriptive cross-sectional study was carried out in the Clinical Microbiology laboratory from January 2019 to June 2019. Ethical approval was received from the Institutional Review Committee (Ref: 28122018010). Six hundred sixty-six sample size was calculated. Convenient sampling was done. Staphylococcus aureus was identified on the basis of its microscopy and morphological characteristics followed by catalase and coagulase test. Antibiotic sensitivity test of isolated pathogens was done using Muller Hinton Agar by Kirby-Bauer method. Statistical analysis was done by Excel 2018, point estimate at 95% confidence interval was calculated along with frequency and proportion for binary data. Results: Out of the 666 bacteria isolated from clinical specimens, 133 (19.96%) were Staphylococcus aureus at 95% confidence interval (12.91-13.60%). Seventy nine (78.95%) of which is isolated from pus and wound infections. Out of 133 Staphylococcus aureus, 94 (70.64%) were Methicillin-Resistant Staphylococcus aureus. Conclusions: This study provides valuable information regarding the high prevalence of Staphylococcus aureus from pus and wound infections. The alarming number of Methicillin-Resistant Staphylococcus aureus is worrisome finding. Antibiotics like Vancomycin and Linezolid which has not developed resistance should be cautiously used only in Methicillin-Resistant Staphylococcus aureus cases.


INTRODUCTION
Staphylococcus aureus, one of the most common causes of pyogenic infections is also a normal flora of the skin of human beings. 1 It is a coagulase-positive, Gram-positive bacterium which is frequently implicated pathogen in bloodstream infections, skin and soft tissue infections, pneumonia, device-related infections and post-operative wound infections. 2 A decrease in susceptibility of Staphylococcus aureus to beta-lactam antibiotics mainly penicillin and cephalosporin are reported worldwide. Various studies from different parts of the world show an increase in the number of Methicillin resistant Staphylococcus aureus (MRSA). According to data published by WHO in 2014, it showed greater than 80% of Staphylococcus aureus infections having MRSA. 3 MRSA is one of the commonest causes of hospital-acquired infections throughout the world. 4 The aim of the study was to find out the prevalence of Staphylococcus aureus in our setting and antibiotic resistance pattern of isolated S.aureus. where, n= required sample size p= prevalence of study (50%) q= 1-p e= margin of error, 5% Z= 2.58 at 99% CI All the clinical specimens that were appropriately collected, labelled and properly transported and processed for aerobic bacterial cultures were included in this study. Six hundred sixty six culture-positive specimen isolates obtained from blood, urine, pus, wound swab, CVP tip, Eye swab, sputum, and tracheal were included in this study. Samples received were processed according to standard microbiological procedures. 5 Suspected colonies of S. aureus were further processed. Identification of Staphylococcus aureus was done on the basis of colony character, Gram's staining, catalase test and coagulase test. 6 Further Staphylococcus aureus was characterized into Methicillin sensitive Staphylococcus aureus (MSSA) and Methicillin-resistant Staphylococcus aureus (MRSA) by using cefoxitin disc diffusion method. Isolates with a diameter of zone of inhibition (ZOI) ≥22mm were identified as MSSA and isolates with ZOI ≤21mm identified as MRSA. 7   Among Gram-positive bacteria, S.aureus is known for resistance to various commonly used antibiotics. Antimicrobial resistance is a global threat and increasing drug resistance in S.aureus is worrisome. MRSA has emerged as an important human pathogen with a wide range of antibiotic resistance. Global scenario of MRSA is not uniform and great variation in its prevalence has been observed throughout the world. Earlier reports of MRSA from Nepal reported a prevalence of 21.1%-69.1%. 11,[14][15][16] Our result showed the prevalence of MRSA as 70.64% which is alarmingly high as betalactam group of drugs are extensively used in our settings to treat bacterial infections. But this study is in accordance with data published by WHO in 2014. 3 Proportionally high MRSA in our settings might have been due to differences in the length of the study period, study site, sample size, laboratory procedure employed and infection control practices. 17 Equally likely is that the data reflects an actual increase in prevalence over the years as there is wide use of antibiotics available over the counter without specific laboratory tests in our country or it can also be transient local outbreak. 18,19 We documented a higher prevalence of resistance to antibiotics in MRSA isolates when compared to MSSA isolates. All MRSA isolates encountered in this study were completely resistant (100%) to penicillin. Similar results were noted for penicillin among MRSA strains in India. 21 A significant difference in antibiotic resistance between MRSA and MSSA was observed in case of amikacin, ciprofloxacin, gentamycin and erythromycin which correlated with other studies from Nepal. 11,[14][15][16] Antibiotics like glycopeptides, linezolid should be judiciously used only in MRSA cases. Our study did not document any resistance to these antibiotics. Many studies in the past conducted in Nepal have also not documented any resistance to glycopeptides and linezolid. 10,[14][15][16] However, the study done by Pahadi et al. showed increased vancomycin MIC among MRSA. 19 These antibiotics should be preserved for future use which will be most important in treating the MRSA cases.

METHODS
Due to the limited resources the molecular study of the S. aureus isolates was not possible. The minimum inhibitory concentration (MIC) testing was also not possible.

CONCLUSIONS
The present study establishes S. aureus as important pathogen to cause soft tissue and wound infections.
With increase in prevalence of MRSA and VRSA we are headed to the pre-antibiotic era. Irrational use of antibiotics, absence of antimicrobial stewardship program in hospitals, lack of surveillance and reporting system, failure to observe infection control practices like hand washing and barrier nursing could be some reasons for this problem. Although no isolate exhibited resistance to vancomycin and linezolid screening test and MIC determination are recommended in monitoring the response to therapy and for early detection of impending resistance among local strains and these antibiotics should only be used in MRSA cases.
Conflict of Interest: None.