Staphylococcus aureus is one of the most common skin-colonizing organisms and leading source of nosocomial and community-acquired infections throughout the world. The emergence of methicillin resistance among staphylococcal infections has become a serious issue among hospitalized patients and especially concerning is the ever-expanding pool of community-associated methicillin-resistant S. aureus (CA-MRSA) among ambulatory patients. This has led to an upsurge in the use of safe and effective agents to treat both systemic and localized staphylococcal infections. In contrast to health-care-associated MRSA, CA-MRSA is often susceptible to trimethoprim–sulfamethoxazole, clindamycin, doxycycline or minocycline, and fluoroquinolones with variable susceptibility.[2,3] Therefore, there is increasing interest in the use of macrolide–lincosamide–streptogramin B (MLSB) antibiotics to treat S. aureus infections with clindamycin being an excellent option because of its pharmacokinetic properties which include the availability of both oral and parenteral formulations with swank bioavailability (90% oral bioavailability) and wonderful penetration of the drug into the skin and soft tissue which is not hampered by high bacterial burden at the infection site (unlike β-lactams). Clindamycin has also been reported to decrease or inhibit the production of staphylococcal toxins such as PV toxin and toxic shock staphylococcal toxin. All these attributes make clindamycin an effective therapeutic agent to manage toxigenic staphylococcal infections and also provide sufficient captivation to clinicians for its empiric use. Clindamycin is also an alternate antibiotic for patients with Type-I hypersensitivity to beta-lactam antibiotics.
However, the widespread use of MLSB antibiotics has led to clindamycin resistance in the form of inducible (iMLSB) and constitutive (cMLSB) resistance which is a major concern.[6,7]
The mechanism of resistance is due to the presence of erm genes that encodes methylation of 23s rRNA-binding sites which is shared by three classes of drugs – MLSB. Staphylococcal strains with iMLSB are difficult to detect in the routine laboratory as they appear erythromycin resistant and clindamycin sensitive in vitro when not placed adjacent to each other. In such cases, if clindamycin therapy is given in vivo may select constitutive erm mutants leading to therapeutic failure. A simple method to detect inducible clindamycin resistance is by a D test as per CLSI guidelines 2021. This study aimed to find the prevalence of inducible clindamycin resistance in S. aureus isolates from various clinical samples in a tertiary care hospital.
Materials and Methods
This is a cross-sectional study conducted for a period of 1 year, from January 2020 to December 2020 at a tertiary care hospital in Delhi. A total of 1000 isolates of S. aureus isolated from various clinical specimens such as pus, wound swab, aspirates, blood, and sterile fluids were tested in the department of microbiology.
The isolates were identified as S. aureus by standard microbiological techniques and antibiotic susceptibility tests were performed by Kirby–Bauer disk diffusion method (as per CLSI 2021 guidelines).
Methicillin resistance was detected using a 30-μg cefoxitin disk and cefoxitin E strip.
Inducible resistance to clindamycin was tested by the “D test” (as per CLSI 2021 guidelines).
For the D test, an erythromycin disk (15 μg) was placed at a distance of 15–26 mm from the clindamycin (2 μg) disk on Mueller–Hinton agar plate and incubated overnight at 35°C ± 2°C. After overnight incubation, results are interpreted as inducible clindamycin resistance if there is flattening of the zone of inhibition adjacent to the erythromycin disk (referred to as D-zone) [Figure 1].
Results were calculated and presented in percentages.
Quality control (QC) of the erythromycin and clindamycin disks was performed with S. aureus ATCC25923 according to the standard disk diffusion QC procedure.
Out of the 1000 isolates of S. aureus, 556 (55.6%) isolates were methicillin-sensitive S. aureus (MSSA) and 444 (44.4%) isolates were MRSA [Table 1].
The total percentage of inducible clindamycin resistance was found to be 21.9%.
The prevalence of inducible clindamycin resistance was higher among MRSA 144 (32.4%) isolates as compared to MSSA 75 (13.5%) leaving us with fewer therapeutic options [Table 1].
Clindamycin is a lincosamide antibiotic that has been approved for the treatment of anaerobic and staphylococcal infections. At present, clindamycin has become an excellent drug for methicillin-sensitive and resistant staphylococcal infections. However, clindamycin resistance can develop in staphylococcal infections with inducible phenotypes or spontaneous constitutive resistant mutants that may arise during clindamycin therapy. Reporting susceptibility to clindamycin without checking for inducible clindamycin resistance may result in inappropriate use of clindamycin. Routine antibiotic susceptibility test methods are unable to detect iMLSB, so erythromycin–clindamycin disk approximation test or D-test becomes an important and reliable part of antimicrobial susceptibility testing of all clinical isolates of S. aureus.
In our study, 1000 staphylococcal isolates were obtained from various clinical samples over a period of 1 year, in which 556 (55.6%) isolates were methicillin-sensitive whereas 444 (44.4%) isolates were methicillin-resistant. Out of these isolates, a total of 846 isolates were erythromycin resistant. Out of which 150 isolates showed cMLSB (erythromycin-R and clindamycin-R). When the D test was performed on the remaining 696 isolates, inducible clindamycin resistance was detected in 219 (21.9%) isolates. Various studies by Prabhu et al. (10%), Ciraj et al. (13.1%), and Deotale et al. (14.5%) have reported a lower incidence of inducible clindamycin resistance, whereas Ajantha et al. showed a high incidence of 63% in their study. Our findings are consistent with those of Pal et al. (24.63%), Gupta et al. (18%), Singh et al. (29.4%), and Kaur and Khare (21.11%). This difference in susceptibility might be due to geographical variation and lack of uniformity in testing protocols for the detection of inducible clindamycin resistance. These inducible clindamycin-resistant isolates may have been missed if the D-test would not have been performed.
MRSA isolates showed a higher incidence of iMLSB (32.4%) as compared to methicillin-sensitive isolates (13.5%) leaving with fewer therapeutic options for MRSA infections. Similarly, few studies have reported a higher incidence of iMLSB among MRSA as compared to MSSA.[7,9,15]
Therapeutic failure of clindamycin in iMLSB has also been documented by various researchers.
McGehee et al. reported two cases of erythromycin-resistant staphylococcal infections that were treated with lincomycin (predecessor of clindamycin) which were susceptible to lincomycin before starting therapy but rapidly became resistant to lincomycin.
Another study by Rao reported three cases of S. aureus infection with iMLSB when treated with clindamycin therapy, one patient showed complete success, another showed clinical success but with bacterial persistence, and the third patient showed relapse with cMLSB. This calls into question the efficacy of clindamycin in iMLSB strains. Many other studies were conducted over a period of time where patients with iMLSB infections when managed with clindamycin showed clinical failure or relapse with cMLSB.[18,19,20]
This is due to the presence of inducers which lead to the rearrangement of mRNA, allowing the ribosome to translate methylase coding sequence, in turn leading to clindamycin resistance.
Clindamycin has been an attractive option in the treatment of skin and soft-tissue infections, mixed infections, and serious infections because of its efficacy against MRSA and MSSA, as well as anaerobes. However, this emerging inducible clindamycin resistance has led to questioning the efficacy of clindamycin use against any erythromycin-resistant Staphylococci spp. However, if inducible resistance can be reliably detected by a simple test on a routine basis in clinically significant isolates, clindamycin can be safely and effectively used in patients with true clindamycin-susceptible strains without the emergence of resistance during therapy. By using clindamycin, the use of vancomycin can be avoided. The high frequency of methicillin-resistant isolates with in vitro clindamycin resistance at our institute raises the concern of failure of clindamycin therapy in methicillin-resistant isolates.
MRSA is a formidable, multifaceted, and unpredictable pathogen which has a wonderful capacity to upgrade its pathogenic potential and remains a major threat to human health. One of the major concerning issues is the pathogenicity attributable to the production of toxins by this organism. Emergence of multidrug resistance among S. aureus is another impediment in the management of infections caused by them. Pharmacokinetics and pharmacodynamics of this drug make this a wonderful candidate for empiric use but contravenes the provisions of rational use of antimicrobials. Furthermore, the emergence of inducible clindamycin resistance would require factoring in while considering the option of using this drug.
D test for detection of inducible clindamycin resistance is a simple test which can provide necessary real-time inputs to the treating clinicians regarding the judicious use of this drug. Therefore, it would be scientifically highly prudent if the detection of inducible clindamycin resistance is included in the drug sensitivity testing protocols of S. aureus in the interest of patient management and to avoid empiric use of this valuable therapeutic intervention in this era of the ever-expanding problem of drug resistance.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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