Detection of inducible clindamycin resistance in Staphylococcus aureus and coagulase-negative staphylococci - a study from South India
S Kumar, S Umadevi, N Joseph, A Kali, J Easow, S Srirangaraj, G Kandhakumari, R Singh, P Charles, S Stephen
clindamycin, constitutive resistance, d-test, inducible resistance, staphylococcus aureus
S Kumar, S Umadevi, N Joseph, A Kali, J Easow, S Srirangaraj, G Kandhakumari, R Singh, P Charles, S Stephen. Detection of inducible clindamycin resistance in Staphylococcus aureus and coagulase-negative staphylococci - a study from South India. The Internet Journal of Microbiology. 2010 Volume 9 Number 2.
Although erythromycin and clindamycin are in separate antimicrobial agent classes, macrolides and lincosamides, respectively, their mechanisms of action (inhibition of protein synthesis) and mechanisms of resistance are similar . The cross-resistance for 3 antibiotic families (macrolides e.g., erythromycin, clarithromycin, azithromcyin; lincosamides e.g., clindamycin; and group B streptogrammins e.g., quinupristin) that share a common binding site is called as the MLSB phenotype . The two main mechanisms of resistance are production of methylase enzyme encoded by a multiallele plasmid-borne gene
It is important to determine if resistance (whether inducible or constitutive) to clindamycin exists when it is being considered for therapy. Antimicrobial susceptibility data are important for the management of infections, but false susceptibility results may be obtained if staphylococci are not tested for inducible CL resistance by the disk diffusion induction test (D-test). We performed this study to determine the prevalence of inducible clindamycin resistance in clinical isolates of
Material and Methods
Study design and setting
This cross-sectional study was conducted in the Department of Microbiology of Mahatma Gandhi Medical College and Research Institute (MGMC & RI), a 700-bedded tertiary care super-specialty hospital with teaching facility, located in Pondicherry, India. This study was approved by the Research and Ethical committees of our institute and informed consent was obtained from each patient.
Clinical samples and bacterial isolates
Three hundred isolates of
Identification of staphylococcal isolates was done based on colony morphology on 5% sheep blood agar, Gram stain and catalase test. Coagulase test by the plasma tube method and sugar fermentation tests were done to distinguish between
Those isolates which were erythromycin resistant were subjected to 'D test' as per CLSI guidelines . A 0.5 McFarland suspension of staphylococci was inoculated on Mueller Hinton agar plate. The test was performed with erythromycin (15 µg) disc placed at a distance of 15mm (edge to edge) from clindamycin (2 µg) disc, followed by overnight incubation at 37oC. Three different phenotypes were interpreted as follows :
1. cMLS B phenotype – isolates showing resistance to both erythromycin (zone size ≤13mm) and clindamycin (zone size ≤14mm) with circular shape of zone of inhibition if any around clindamycin.
2. iMLS B phenotype – isolates showing resistance to erythromycin (zone size ≤13mm), while being sensitive to clindamycin (zone size ≥21mm) with a D shaped zone of inhibition around clindamycin with flattening towards erythromycin disc.
3. MS phenotype – isolates showing resistance to erythromycin (zone size ≤13mm) while being sensitive to clindamycin (zone size ≥21mm) with a circular zone of inhibition around clindamycin.
The demographic details of the patients included in the study are summarized in Table 1. A total of 300 staphylococci were isolated from various types of clinical samples obtained from these patients. Of these 300 isolates, 176 were identified as
Of the 300 staphylococcal isolates, 121 (40.33%) were erythromycin resistant and clindamycin sensitive (Table 2). These were subjected to D- test for detecting inducible clindamycin resistance. The rates of inducible clindamycin resistance of the different staphylococcal isolates are shown in Table 3. The inducible clindamycin resistance was significantly more among MRSA compared to methicillin sensitive
The antibiotic susceptibility patterns of the different staphylococcal isolates are summarized in Table 4. Majority of the MRSA isolates were susceptible to clindamycin, vancomycin and linezolid, while most of them were resistant to erythromycin, gentamicin, ciprofloxacin, tetracycline and sulfamethoxazole-trimethoprim.
The performance of antimicrobial susceptibility testing remains a crucial component of the microbiology laboratory. Due to the emergence of methicillin resistance in
Among the 176
In the present study the constitutive clindamycin resistance was present in 2.9% of MRSA and 4.7% of MSSA isolates. This trend is in contrast with other studies from Korea where the majority of MRSA had constitutive resistance (cMLSB) . This indicates that the incidence of constitutive and inducible resistance in staphylococcal isolates varies widely by hospital and geographic region. The low constitutive clindamycin resistance in our study may also be attributed to the fact that drug is not commonly used and hence there is less selection of resistant strains.
In our study, majority of the MRSA isolates were susceptible to clindamycin, vancomycin and linezolid, while most of them were resistant to erythromycin, gentamicin, ciprofloxacin, tetracycline and sulfamethoxazole-trimethoprim, similar to the study by Mallick et al . In our study,
In conclusion, resistance to antimicrobials such as macrolides might not be readily apparent by routine testing. The D-test is easy to perform and inexpensive for practical work. We feel that this test should be made mandatory as a routine work in clinical microbiology laboratories. Therapeutic failures can be prevented if clindamycin is not used for treatment of patients with infections caused by staphylococci with inducible clindamycin resistance.