|Year : 2016 | Volume
| Issue : 2 | Page : 111-114
Inducible clindamycin resistance among the clinical isolates of Staphylococcus aureus in a tertiary care hospital
Taruna Singh1, Arvind B Deshmukh2, Vikrant Chitnis3, Trupti Bajpai4
1 Department of Pharmacology, SS Medical College, Rewa, Madhya Pradesh, India
2 Department of Microbiology, JIIU's n Institute of Medical Science and Research, Medical College and Noor Hospital, Jalna, Maharashtra, India
3 Department of Microbiology, CHL Apollo Hospital, Indore, Madhya Pradesh, India
4 Department of Microbiology, Sri Aurobindo Institute of Medical Sciences Medical College and PG Institute, Indore, Madhya Pradesh, India
|Date of Web Publication||14-Apr-2016|
Department of Microbiology, Sri Aurobindo Institute of Medical Sciences Medical College and PG Institute, MR.10 Crossing, Indore-Ujjain Highway, Indore, Madhya Pradesh
Source of Support: None, Conflict of Interest: None
Introduction: Clindamycin is commonly used to treat skin and soft tissue infections caused by Staphylococcus aureus particularly in methicillin-resistant S. aureus (MRSA) infections. Inducible clindamycin resistance (inducible macrolide–lincosamide–streptogramin B resistance [MLSB]) is a critical factor in antimicrobial susceptibility testing. This study was aimed to detect MLSB phenotypes and inducible clindamycin resistant phenotypes among S. aureus. Materials and Methods: A total of 649 S. aureus isolates from different clinical samples were evaluated, and methicillin-resistance was determined using the cefoxitin (30 μg) disc. Inducible resistance to clindamycin was detected by D-zone test as per Clinical and Laboratory Standards Institute guidelines. Results: Of the 649 isolates, 404 (62.2%) were methicillin-sensitive S. aureus (MSSA) and 245 (37.8%) were MRSA. Ninety-six (14.8%) isolates showed inducible clindamycin resistance, 175 (27%) showed constitutive resistance, and 55 (8.5%) isolates showed MS phenotype. Inducible and constitutive resistance phenotypes were found to be higher in MRSA (25% and 64.8%) as compared to MSSA (8.7% and 4.7%), respectively. Conclusions: Considering the higher prevalence of clindamycin resistance in MRSA isolates as compared MSSA isolates, routine D-test of MRSA isolates is strongly recommended to prevent treatment failure. Therefore, inducible clindamycin detection should be the part of S. aureus sensitivity testing in all the microbiology laboratories.
Keywords: Constitutive macrolide–lincosamide–streptogramin B phenotype, D-test, inducible clindamycin resistance, inducible macrolide–lincosamide–streptogramin B phenotype, MS phenotype, Staphylococcus aureus
|How to cite this article:|
Singh T, Deshmukh AB, Chitnis V, Bajpai T. Inducible clindamycin resistance among the clinical isolates of Staphylococcus aureus in a tertiary care hospital. Int J Health Allied Sci 2016;5:111-4
|How to cite this URL:|
Singh T, Deshmukh AB, Chitnis V, Bajpai T. Inducible clindamycin resistance among the clinical isolates of Staphylococcus aureus in a tertiary care hospital. Int J Health Allied Sci [serial online] 2016 [cited 2019 Sep 16];5:111-4. Available from: http://www.ijhas.in/text.asp?2016/5/2/111/180428
| Introduction|| |
Staphylococcus aureus is a leading cause of nosocomial and community-acquired infections in every region of the world. The increasing prevalence of methicillin-resistance among staphylococci is a therapeutic threat. This has led to renewed interest in the usage of macrolide–lincosamide–streptogramin B (MLSB) antibiotics to treat S. aureus infections, with clindamycin being the preferable agent due to its excellent pharmacokinetic properties. However, widespread use of MLSB antibiotics has led to an increase in the number of staphylococcal strains acquiring resistance to MLSB antibiotics. In S. aureus, an active efflux mechanism encoded by msrA gene confer resistance to macrolides and streptogramin B antibiotics (so called MS phenotype), and modification of ribosomal target encoded by erm genes cause resistance to MLSB antibiotics; which is called as MLSB resistance. The latter mechanism can be constitutive MLSB (cMLSB), where the rRNA methylase is always produced; or can be inducible MLSB (iMLSB), where methylase is produced only in the presence of an inducing agent. Low levels of erythromycin are the most effective inducer of iMLSB resistance. Previous reports indicated that treatment of patients harboring iMLSB resistant S. aureus with clindamycin might lead to the development of cMLSB resistant strains and subsequently, therapeutic failure. Unfortunately, the iMLSB phenotype cannot be recognized using standard susceptibility test methods, but can be distinguished by erythromycin-clindamycin disk approximation test (D-test). There is no published data on the prevalence of MLSB resistance among clinical isolate of S. aureus in our geographic area. The purpose of this study was to determine the prevalence of resistance to erythromycin and clindamycin in S. aureus isolated from various infections in a tertiary care hospital to assist clinicians in the treatment of these infections by these groups of antibiotics. This study was aimed to find out the percentage of S. aureus having inducible clindamycin resistance (iMLSB) in our geographic area using D-test and to ascertain the relationship between methicillin-resistant S. aureus (MRSA) and inducible clindamycin resistance.
| Materials And Methods|| |
This study was conducted for the period of 1 year and 8 months, from November 2011 to June 2013 at a teaching tertiary care hospital located in the central India. The study was approved by the institutional ethical committee, and the informed consent was obtained from the study subjects. A total of 649 nonduplicate consecutive isolates of S. aureus isolated from clinical specimens such as pus, wound swab, blood, urine, cerebrospinal fluid, sputum and other body fluids were considered for the study. The isolates were identified as S. aureus by standard biochemical techniques  and antibiotic susceptibility tests were performed by the Kirby–Bauer disc diffusion method. Methicillin-resistance was detected using a 30 μg cefoxitin disc and inducible resistance to clindamycin was tested by “D-test” as per Clinical and Laboratory Standards Institute (CLSI) (2011, 2012, and 2013) guidelines. A lawn culture of the isolate which was adjusted to 0.5 McFarland's concentration was made on a Mueller-Hinton agar plate and discs of clindamycin (2 μg) and erythromycin (15 μg) (Hi-Media, Mumbai, India) were placed at a distance of 15 mm (edge to edge) as per the CLSI recommendations, along with routine antibiotic susceptibility testing. Three different phenotypes were appreciated and interpreted. This interpretation was done only for erythromycin-resistant S. aureus strains. MS phenotypes were the staphylococcalisolates exhibiting resistance to erythromycin (zone size ≤13 mm) while sensitive to clindamycin (zone size ≥21 mm) and giving a circular zone of inhibition around clindamycin [Figure 1]a. iMLSB phenotypes were the staphylococcal isolates showing resistance to erythromycin (zone size ≤13 mm) while being sensitive to clindamycin (zone size ≥21 mm) and giving D-shaped zone of inhibition around clindamycin with flattening toward erythromycin disc [Figure 1]b. cMLSB phenotype phenotypes were labeled for erythromycin zone size ≤13 mm and clindamycin zone size ≤14 mm with the circular shape of the zone of inhibition (if any) around clindamycin or with Staphylococcus isolates showing no zones of inhibition around both erythromycin and clindamycin [Figure 1]c. Quality control of the erythromycin and clindamycin discs was performed with S. aureus ATCC 25923, S. aureus ATCC BAA 977: D-test positive and S. aureus ATCC BAA 976-D-test negative.
|Figure 1: Disc diffusion test for inducible clindamycin resistance (a) MS phenotype (b) inducible macrolide–lincosamide–streptogramin B phenotype (c) constitutive macrolide-lincosamide-streptogramin B phenotype|
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| Results|| |
Among the 649 clinical isolates of S. aureus, 245 (37.8%) were MRSA and 404 (62.2%) were methicillin sensitive S. aureus (MSSA) [Table 1].
|Table 1: Occurrence of methicillin-resistant Staphylococcus aureus and methicillin-sensitive Staphylococcus aureus isolates in Staphylococcus aureus|
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Of the 649 S. aureus isolates, 323 (49.8%) had the erythromycin-sensitive and clindamycin-sensitive phenotype, 175 (27%) had a constitutive resistance phenotype (cMLSB), 96 (14.8%) had the inducible resistance (iMLSB) phenotype, and 55 (8.4%) had an MSB phenotype [Figure 2].
|Figure 2: Pi-graph showing phenotypic pattern of inducible clindamycin resistance|
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Among the 245 MRSA isolates, 156 (64.8%) had the constitutive, 61 (25%) had the iMLSB resistance and 20 (8%) had the MS phenotype. Among the 404 MSSA isolates, 19 (4.7%) and 35 (8.7%) isolates were found to have the cMLSB and iMLSB resistance phenotypes respectively whereas 35 (8.7%) exhibited the MS phenotype [Table 2].
|Table 2: Susceptibility to erythromycin and clindamycin among all Staphylococcus aureus isolates|
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| Discussion|| |
Clindamycin is a drug which is useful for treating both methicillin-susceptible and resistant staphylococcal infections. Good oral absorption makes it an important option in outpatient therapy as a follow-up after intravenous therapy. Clindamycin is also a good alternative antibiotic for the penicillin-allergic patients. Since the iMLSB resistance mechanism is not recognized using standard susceptibility test methods and its prevalence varies according to geographic location and even from hospital to hospital, D-test becomes an imperative part of routine antimicrobial susceptibility test for all clinical isolates of S. aureus.
In this study, of 649 S. aureus studied over a period of 1 year and 8 months, Erythromycin resistance was seen in 326 (50.2%) isolates. Among the erythromycin-resistant S. aureus, iMLSB resistance was observed in 96 (29.4%) isolates. Investigators such as Prabhu et al., (10%), Ciraj et al. (13.1%), Deotale et al., (14.5%) have reported a lower incidence of inducible clindamycin resistance while Ajantha et al., showed 63% of inducible resistance. Our findings are consistent with the results of Pal et al., (24.63%), Gupta et al., and Kaur and Khare (21.1%) These observations suggest that if the D-test would not have been performed, one-third of the erythromycin resistant isolates would have been misidentified as clindamycin sensitive resulting in the therapeutic failure.
The incidence of cMLSB is very high (53.8%) in our hospital setting, similar to those observed by Pal et al., (46.97%). Studies made by Ciraj et al., found iMLSB and cMLSB phenotypes equal to 5.4% and 43.7% and Kaur and Khare  found iMLSB and cMLSB phenotypes equal to 21.1% and 53.8%, respectively. Interestingly, a study by Angel et al., detected no cMLSB resistance. Deotale et al., found constitutive resistance in 7.3% of MRSA isolates. Ciraj et al., found 15.3% of cMLSB phenotype among the MRSA strains. In this study, constitutive phenotype predominated over the inducible phenotype among MRSA (64.8% vs. 2.5%) and in MSSA isolates inducible phenotype predominated over the constitutive phenotype (14.08% vs. 7.7%) contrary to Angel et al., who did not find constitutive phenotypes in any of their isolates.
The incidence of iMLSB and cMLSB is higher among MRSA (25% and 64.8%) as compared to MSSA (8.7% and 4.6%), respectively, in our setting. However, Kaur and Khare  reported (39.43% and 13.38%) in MRSA and (14.08% and 7.7%) in MSSA, respectively. Ajantha et al., also showed 74% iMLSB in MRSA and 45% in MSSA. On the contrary, Levin et al., showed a higher percentage of inducible resistance in MSSA (68%) as compared to MRSA (12.5%). Uzunović et al., also showed higher inducible resistances in MSSA as compared to MRSA.
In this study, 16.9% of erythromycin-resistant staphylococcal isolates showed true clindamycin susceptibility (MS phenotype). Patients with infections caused by such isolates can be treated with clindamycin without the emergence of resistance during therapy. The high frequency of methicillin-resistant isolates (37.8%) with in vitro inducible clindamycin resistance at our institute raises the concern of clindamycin treatment failures with methicillin-resistant infections.
| Conclusion|| |
High prevalence of clindamycin resistance, especially cMLSB resistance, in our community shows that antimicrobial susceptibility test is essential when clindamycin is an option for therapy of S. aureus infection. So, clinical microbiology laboratories should report inducible clindamycin resistance in S. aureus, and D-test can be used as a simple, auxiliary, and reliable method to delineate inducible and constitutive clindamycin resistance in routine testing. Therefore, inducible clindamycin detection should be the part of S. aureus sensitivity testing in all the microbiology laboratories.
The authors would like to thank the Chairperson and Dean of the institute for providing laboratory facilities and healthy working atmosphere during the study period. The authors are also thankful to the technical staff of the institute for providing necessary helping hand during the endeavor.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]