A study on antimicrobial susceptibility pattern in clinical isolates of Staphylococcus aureus in Eritrea

A study on antimicrobial susceptibility pattern in clinical isolates of Staphylococcus aureus in Eritrea

Durgadas Naik^1&, Alem Teclu²

 ¹Microbiologist/Associate Professor, College of Health Sciences, Asmara, Eritrea, ²ENT Surgeon, Sembel Hospital, Ministry of Health, Eritrea

^&Corresponding author

Durgadas Govind Naik, P.O.Box-2024, University of Asmara, Asmara, Eritrea, Telephone: 00-291-7136899, Fax : 00-291-162236

Staphylococcus aureus is a major pathogen implicated in skin and soft tissue infections. S.aureus also causes abscess in deep organs, responsible for toxin mediated diseases. This organism is one of the important pathogens in hospital acquired infection. In humans, colonization of S.aureus is found in the anterior nares. In general, nasal carriage rate is higher in hospital staff and patients than in the community. Nasal carriage of these organisms in hospital staff provides a source for infection in hospitalized patients especially in pediatric and intensive care units. Further, the development of multiple drug resistance to this organism is posing serious threat to inpatients especially in pediatric and intensive care units in hospital [1, 2]. With the world-wide emergence of methicillin resistant S.aureus (MRSA), as many of the MRSA isolates are multi drug resistant, these organisms are studied with special interest. MRSA cause nosocomial infections and are associated with increased rates of illness and death [3].  

Indiscriminate use of antibiotics and prolonged hospital stay are contributing factors in the emergence of multidrug resistant strains. With the emergence of MRSA and multi drug resistance, it is important to know the trend of antimicrobial resistance in any geographic area. Although, like elsewhere in the world, S.aureus is an important pathogen in the north-east region of Africa, the antimicrobial susceptibility pattern, prevalence of MRSA strains and carriage rate in healthy hospital staff are not known. There is no published data on these aspects. This study is an attempt to find answer to these questions.

The main objectives of this work were to study the prevalence of S.aureus in clinical specimens, their antimicrobial susceptibility pattern and to assess the carriage rate of S.aureus in healthy hospital staff.

The study was conducted in the Central Health laboratory, which serves as the National Reference Laboratory for Eritrea. Of the 278 clinical isolates, 272 isolates of S.aureus were from pus and 6 were from ear discharge. The anterior nares swabs were collected from 30 healthy hospital staffs who were working in the hospitals of Asmara (the capital city of Eritrea) at the time of the study.  

Standard procedures were followed to isolate, identify and perform antimicrobial susceptibility pattern [4,5]. In brief, the specimens were collected in a culturette swab. The specimens were inoculated on Blood agar and Mannitol Salt agar (Oxoid) and were incubated at 37^o C for 18-24 hours. The suspected isolated colonies were subjected to Gram’s staining. S.aureus organisms were confirmed mainly by positive DNAse (deoxyribonuclease) and coagulase tests. Confirmed S.aureus isolates were subjected to antimicrobial sensitivity testing by standard disk diffusion method as per NCCLS standards [6].

All confirmed S.aureus isolates were subsequently tested for methicillin resistance by using oxacillin disks (1 ug). The antimicrobials used were ampicillin (10 ug), amoxicillin-c (30 ug), ciprofloxacin (5 ug), cephalexin (30 ug), chloramphenicol (30 ug), cotrimaxozole (25 ug), gentamycin (10 ug), erythromycin (15 ug), penicillin (10 U), amikasin (30 ug), and tetracycline (30ug).  American Type Culture Collection S.aureus 25923 was used as a reference strain. A total of 278 S.aureus isolates from patient specimens, and 5 isolates from 30 healthy hospital staff are included in this study.

Antimicrobial susceptibility study of S.aureus isolates revealed high resistance to ampicillin (85%), penicillin (77%) and tetracycline (78%). Low resistance was observed against amoxicillin-c (8%), amikasin (7%) and ciprofloxacin (5%). 32% of the isolates were resistant to chloramphenicol and gentamycin. About 23 % of the S.aureus isolates were resistant to erythromycin (Table 1). Although our results were similar to some of the other studies [7,8], the antimicrobial resistance to different antimicrobials varies from place to place, time to time and also depends on a number of factors like use, abuse, availability and consumption of antibiotics. In general, high resistance was observed against widely used antibiotics like penicillin. This is evident when we compared the resistance to two aminoglycoside group antibiotics gentamycin (32%) which is widely used in our setting than amikasin (7%). In one of the large studies conducted [9], 88% of the isolates were resistant to penicillin whereas only 4% were resistance to tetracycline.

In our study 26 (9%) of the S. aureus isolates were methicillin resistant (Table 2). All the methicillin resistant S. aureus were also resistant to penicillin while 96% were resistant to ampicillin. Low resistance was observed to amoxicillin-c (4%), cephalexin (8%), ciprofloxacin (8%), and amikasin (15%).  None of the 5 S.aureus isolates from healthy hospital staff was methicillin resistant.

The MRSA prevalence rate shows high regional variance. This is indicated in different studies conducted in Croatia (22%), Taiwan (75- 84%), India (31-33%), Pakistan (83%), and Malaysia (40%) [9-15]. In USA there was progressive development of resistance to methicillin from 5% (1981) to 52 % (2005). In the same study, high regional variation was found from 12.5 % to 100% [16]. Similarly, in Taiwan MRSA prevalence increased from 4.3 % (1981) to 84% (2000) [15]. A high prevalence of 83% MRSA is reported from Pakistan [9]. In contrast, the prevalence rate of MRSA was found to be low in France (6%), Ireland (5%) and United Kingdom (2%) [8].

The antimicrobial susceptibility pattern of MRSA isolates also varies with place and time. In most of the studies conducted over the years, there was a clear indication of the progressive development of antimicrobial resistance to several antibiotics. In some of the studies, high resistance to ciprofloxacin was reported in MRSA isolates ranging from 46 to 99 %, [10-12] whereas in our study this was only 8 %. In Eritrea, ciprofloxacin is not widely used, and low resistance could be due to this factor. In Western Australia, high resistance was observed against erythromycin (60%) and ciprofloxacin (26%) among the MRSA isolates. The same study reported progressive development of ciprofloxacin resistance from 11% (1998) to 26 % (2002) [13]. High resistance development against erythromycin was reported (92%) in Taiwan [14]. In this study, among MRSA isolates, 27% were resistant to erythromycin and 23 % to cotrimaxozole and high resistance (88%) was observed against tetracycline. The resistance rate to co-trimaxozole varies from 0- 66% [12,13]. In some studies resistance to cotrimaxozole was as low as 0-1% and to tetracycline 6-9% [15,17]. Our study emphasizes the need for continuous monitoring of antimicrobial resistance development in S.aureus isolates including MRSA.

S. aureus is implicated in hospital acquired infection. Further, MRSA has emerged as a serious public health problem in all regions of the world. Community acquired MRSA infections occur as skin and soft tissue infections, whereas hospital acquired MRSA is acquired in hospitals and usually causes infections in the elderly, pediatric and immuno-compromised patients. At present, MRSA infections are treatable but there is a need to prevent the spread of MRSA in community and hospital settings. The best way to prevent the spread of S.aureus and MRSA in hospital settings is to screen health care takers for the presence of these organisms. The present study made an attempt to screen hospital staff for the S. aureus organisms. Of the 30 healthy hospital staff screened, 5 were positive for S.aureus and none of these was MRSA. The important reservoirs of MRSA in hospitals are infected patients and health care workers who are carriers. The carriage rate of S.aureus in healthy hospital staff ranges from 20-76% in other studies [10,11,18-20]. This study records 17 % of nasal carriage for S.aureus.

None of the 5 isolates of S.aureus obtained from hospital staff was methicillin resistant. Similar observation was made in other studies [15,16].This is contrary to other studies where  high carriage rate of 19-38% for MRSA is reported [8,17,18]. Some of the studies indicate that the epidemiology of MRSA is changing and hospitalization is no longer necessarily a risk factor [21, 22]. Although five isolates from 30 healthy hospital staff is very limited in numbers to draw a definite conclusion, our study also suggests this as far as MRSA is concerned.

In conclusion, our study emphasizes the need for continuous monitoring of the antimicrobial susceptibility pattern of S.aureus isolates including MRSA for the selection of appropriate therapy. When compared to the prevalence rate of MRSA in clinical isolates in other countries, it appears that in Eritrea, the spread of MRSA in community and hospital settings is limited. However, further molecular studies are recommended to study and monitor the epidemiology of multiple drug resistant S.aureus and MRSA.

The authors declared they have no competing interests.

All the authors have equally contributed to sample collection, data analysis and drafting of the manuscript. All authors have read and approved the final version of the manuscript.

Table 1: Antimicrobial susceptibility pattern of S.aureus isolates

Table 2: Resistance pattern of Methicillin Resistant S.aureus isolates

Authors thank the technical staff of the department of Microbiology for their assistance during the study.

1. Lowy FD. Staphylococcus aureus infections. N Engl J med. 1998; 339:520-32. This article on PubMed

2. Kaplan SL. Implications of methiciliin-resistant Staphylococcus aureus as a community acquired pathogen in pediatric patients. Infect Dis Clin North Am. 2005;19:747-57. This article on PubMed

3. Cosgrove SE, Sakoulas G, Perenceich EN, Schwaber MJ, Karchmer AW, Carmeli Y. Comparison of mortality associated with methicillin-resistant and methicillin susceptible Staphylococcus aureus bacteremia: a meta-analysis. Clin Infect Dis. 2003;36:53-9 . This article on PubMed

4. Cheesbrough M, Medical laboratory Manual for Tropical countries Vol II, London, Butterworth-Heinemann. 1985.

5. Betty AF, Daniel FS, Alice SW. Staphylococcus, Micrococcus and similar organisms, In:Baily & Scott´s Diagnostic Microbiology, 11 th edn, Mosby 2002.

6. NCCLS. Performance standards for antimicrobial disk susceptibility 157 tests: Approved Standards - Seventh Edition. NCCLS document M2-A7. 2000, USA

7. Uwaezuoke JC, Aririatu LE. A Survey of Antibiotic Resistant Staphylococcus aureus Strains from clinical Sources in Owerri. J Appl Sci Environ Mgt. 2004;8:67-9

8. Denton M, Connel BO, Bernard P, Jarlier V, Williams Z, Santerre HA. The EPISA study: antimicrobial susceptibility of Staphylococcus aureus causing primary or secondary skin and soft tissue infections in the community in France, the UK and Ireland. J Antimicrob Chemother. 2008;61:586-8. This article on PubMed

9. Mehta AP, Rodrigues, C, Sheth K, Jani, S, Hakimiyan A, Fazalbhoy N. Control of methicillin resistant Staphylococcus aureus in a tertiary care centre - A five year study. J Med Microbial. 1998;16:31-4

10. Rajaduraipandi K, Mani KR, Paneerselvam K, Mani M, Bhaskar M, Manikandan P. Prevalence and antimicrobial susceptibility pattern of methicillin resistant Staphylococcus aureus : A multicentre study. Indian J Med Microbiol. 2006;24:34-8. This article on PubMed

11. Qureshi AH, Rafi S, Qureshi SM, Ali AM. The current susceptibility patterns of methicillin resistant Staphylococcus aureus to conventional antimicrobials at Rawalpindi. Pak J Med Sci. 2004;20:361-4

12. Pulimood TB, Lalitha MK, Jesudasan MV, Pandian R, Selwyn JJ. The spectrum of antimicrobial resistance among methicillin resistant Staphylococcus aureus in a tertiary care in India. Indian J Med Res. 1996;103:212-5. This article on PubMed

13. Lynne D, Geoffry WC, Frances GO, John WP, Keryn C, Warren BG, Thomas VR. Methicillin-resistant Staphylococcus aureus , Western Australia. Emerging Infect Dis. 2005;11:1584-7. This article on PubMed

14. Wang CC, Lo WT, Chu ML, Siu LK. Epidemiological typing 178 of community acquired methicillin-resistant Staphylococcus aureus isolates from children in Taiwan. Clin Infect Dis. 2004;39:481-5. This article on PubMed

15. Fang YH, Hsueh PR, Hu JJ, Lee PI, Chen JM, Lee et al. community acquired methicillin-resistant Staphylococcus aureus isolates from children in Northern Taiwan. J Microbiol Immunol Infect. 2004;37:29-32. This article on PubMed

16. Debra AG, Michael JD. Prevalence and regional variation in meticillin resistant Staphylococcus aureus (MRSA) in the USA and comparative in vitro activity of tigecycline, a glycylcycline antimicrobial. J Med Microbiol. 2007;56:1189-95. This article on PubMed

17. Jessica MB, Kathryn CS, Kathleen HH, Richard ND, David JB, Anita G, Ruth L. Community associated methicillin-resistant Staphylococcus aureus, Minnesota, 2000-2003. Emerg Infect Dis. 2005;11:1532-5. This article on PubMed

18. Soon TH, Fong Ny, Jamal F. Antibiotic susceptibility of community acquired Staphylococcus aureus. Med J Malaysia. 1986;41:24-9. This article on PubMed

19. Chong SC, Chow SY, Afra AB, Zamberi S, Nyi NN, Farida J, Norlijah O. Nasal carriage of Staphylococcus aureus among healthy adults. J Microbiol Immunol Infect. 2006;39:458-64. This article on PubMed

20. Enas A, Salwa B, Asem A. Antibiotic Resistance Patterns of mecA-Positive Staphylococcus aureus Isolates from Clinical Specimens and Nasal Carriage, Microbial Drug Resistance, 2004;10:321-4. This article on PubMed

21. Herold BC, Immergluck LC, Maranan MC. Community-acquired methicillin resistant Staphylococcus aureus in children with no identifiable predisposing risk. JAMA. 1998;279:593-8. This article on PubMed

22. Chambers HF. The changing epidemiology of Staphylococcus aureus. Emerg Infect Dis. 2001;7:178-87. This article on PubMed