MRSA – too many hurdles to overcome: a study from Central India

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) are commonly associated with nosocomial infections and are usually resistant to many antibiotics. This study describes the prevalence of MRSA strains and their antibiogram in a tertiary care hospital in Central India. The detection of MRSA was done by a cefoxitin (30 µg) disc diffusion test. Antibiotic sensitivity tests were done as per the Clinical and Laboratory Standards Institute guidelines 2006.

Of the 280 S. aureus strains studied: 145 (51.8%) strains were MRSA; 51 (35.2%) MRSA strains were inducible clindamycin resistant; and all (100%) MRSA strains were resistant to penicillin and sensitive to vancomycin and linezolid.

In order to detect the MRSA strains, cefoxitin disc diffusion tests should be used routinely in any microbiology laboratory to enable prompt treatment for the patient.

Introduction

Staphylococcus aureus is one of the most versatile nosocomial pathogens worldwide and can cause skin infections to life-threatening systemic illness.

Methicillin-resistant S. aureus (MRSA) was first observed in 1961 after methicillin was introduced into clinical use in 1960. The prevalence of MRSA strains has increased worldwide. Presently, MRSA are among the most important causes of hospital infections. The mechanism of resistance to methicillin was uncovered in 1981. Methicillin resistance occurs due to the presence of the altered penicillin-binding protein PBP2a which is encoded by the mecA gene. MRSA strains with intermediate resistance to vancomycin (VISA) and resistant to vancomycin have been reported.^1,2 Prompt and accurate detection of MRSA strains is necessary for the optimal treatment of patients.

The present study was undertaken in order to detect the prevalence of MRSA strains isolated from different clinical specialties and their antibiotic susceptibility pattern.

Material and methods

A total number of 280 S. aureus strains were isolated from different clinical specimens such as pus and wound swab, blood, sputum, body fluids, endotracheal tube aspirate, urine, etc., and were identified as per conventional methods.³ The specimens were collected from different clinical specialties of a tertiary care hospital in a rural setup from July 2007 to June 2009.

Methicillin resistance was detected by cefoxitin (30 µg) disc diffusion test and the antibiotic susceptibility test was performed according to Clinical and Laboratory Standards Institute (CLSI) guidelines.⁴

Inducible clindamycin resistance was detected by D-zone test as per National Committee for Clinical Laboratory Standards guidelines 2004.⁵ In order to confirm VISA the E-test for vancomycin (AB Biodisk, Solna, Sweden) was used.

Observations

The prevalence of MRSA and methicillin-sensitive S. aureus (MSSA) was 145 (51.8%) and 135 (48.2%), respectively. Of 156 S. aureus strains isolated from pus and wound swab 89 (57.1%), and of the 36 blood cultures positive for S. aureus, 23 (63.9%), were found to be MRSA. Of seven (2.5%) strains isolated from endotracheal tube aspirate, five (71.4%) were MRSA. From an intravenous cannula tip and a Foley's catheter tip all (100%) strains isolated were characterized as MRSA. Eighty-nine (61.4%) MRSA strains were isolated from pus and wound swabs and 23 (15.9%) were isolated from blood. Of the 135 MSSA strains, 67 (49.6%) were isolated from pus and wound swabs and 22 (16.3%) from blood.

We isolated 240 (85.7%) S. aureus strains from the inpatient department (IPD), 33 (11.8%) from the intensive care unit (ICU), operation theatre (OT) and medical and paediatric/neonatal departments, and seven (2.5%) from the outpatient department (OPD). One hundred and twenty-three (84.8%) MRSA strains were isolated from the IPD and 13.8% (20/145) were isolated from the ICU. Two (1.4%) MRSA strains were isolated from postoperative patients attending surgery at the OPD. Thirty-seven (25.5%) MRSA strains were isolated from the surgical ward and 34 (23.4%) from the orthopaedic ward (Table 1). Of the eight MRSA strains isolated from the OT and the medical ICU, two (25%) were from the OT-ICU and six (75%) were from the medical ICU. Of the 135 MSSA strains, 40 (29.6%) were isolated from the surgical ward and 20 (14.8%) were from the orthopaedic ward.

Table 1

Incidence of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) strains isolated from different clinical specialties

Clinical specialties	S. aureus No.	MRSA		MSSA
Clinical specialties	S. aureus No.	No.	(%)	No.	(%)
Surgery ward	77	37	25.5	40	29.6
Medicine ward	33	17	11.7	16	11.9
Gynaecology and obstetrics ward	28	14	9.7	14	10.4
Orthopaedic ward	54	34	23.4	20	14.8
Paediatric ward	23	8	5.5	15	11.1
Dermatology ward	11	6	4.1	5	3.7
Chest ward	11	6	4.1	5	3.7
Semi paying ward	3	1	0.7	2	1.5
Paediatric and neonatal intensive care unit	21	12	8.3	9	6.7
Occupational and medical intensive care unit	12	8	5.5	4	3
Outpatient department	7	2	1.4	5	3.7
TOTAL	280	145	51.8	135	48.2

Inducible clindamycin resistance (iMLS_B phenotype) was detected in 18.2% (51/280) S. aureus strains and 35.2% (51/145) of MRSA strains. All 145 (100%) MRSA strains were resistant to penicillin. Resistance to ciprofloxacin, erythromycin, pristinamycin, mupirocin and rifampicin was 122 (84%), 108 (74.5%), 96 (66.2%), 16 (11%) and 24 (16.6%), respectively, among MRSA isolates. All 145 (100%) MRSA and 135 (100%) MSSA strains were sensitive to vancomycin and linezolid. The zone diameter with a vancomycin disc in two MRSA strains was 9 mm and 8 mm in another. However, the minimum inhibitory concentration of vancomycin found by the E-test method was 3 mg/L for those three MRSA strains. Hence, the three strains were characterized as vancomycin-sensitive. There were 134 (99.3%), 74 (54.8%), 131 (97%) and 59 (43.7%) MSSA strains that were sensitive to clindamycin, ciprofloxacin, gatifloxacin and penicillin, respectively.

Discussion

Our hospital is a tertiary care hospital situated in a rural setup attended by patients from the villages of Vidarbha and the adjoining states. A lack of awareness and the indiscriminate use of antibiotics by the patients before coming to the hospital might have been contributory factors to the high prevalence of MRSA (51.8%) in our study.

In India, the incidence of MRSA shows a large variation, from 6.9% to 81%.^6,7 This is largely due to the different detection methods used and the different S. aureus strains studied by various workers. The incidences of MRSA strains are also increasing worldwide. Basak and Deshpande in 1997⁸ reported the incidence of MRSA in our hospital as 30.6%, whereas in the present study the incidence of MRSA was 51.8%.

The isolation of MRSA strains was quite high compared to MSSA from the orthopaedic ward, OT, medical ICU and paediatrics/neonatal ICU. The isolation of MSSA from the paediatric ward patients (65.2%) was more than MRSA (34.8%) and from the surgical ward 52% MSSA strains were isolated compared to 48.1% MRSA.

In our study, 18.2% of strains were iMLS_B phenotype (i.e., inducible clindamycin resistant). In another study from our hospital, Mallick et al. reported that 18.6%⁹ of their S. aureus strains were iMLS_B phenotype. As clindamycin is the most commonly used antibiotic for the treatment of skin and bone infections caused by MRSA strains, the increasing resistance to clindamycin limits its therapeutic use.

In our study, 11% and 16.6% of MRSA strains were resistant to mupirocin and rifampicin, respectively, whereas Mehta et al. reported 22% resistance to Mupirocin.¹⁰

MRSA strains were more resistant to all antibiotics than MSSA strains except for vancomycin and linezolid (Figure 1). Of the MRSA strains, 22% were resistant to five commonly used antibiotics (e.g. penicillin, erythromycin, clindamycin, ciprofloxacin and tetracycline).

Figure 1

Comparison of antibiotic resistance pattern among methicillin-resistant Staphylococcus aureus (MRSA; n = 145) and methicillin-sensitive S. aureus (MSSA; n = 135)

MRSA strains are resistant to a wide range of antibiotics, sometimes even to vancomycin. Hence MRSA strains should be routinely tested using the cefoxitin disc diffusion test as per the CLSI guidelines for prompt patient treatment and also for controlling the transmission of MRSA strains in any health-care setup.

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