|Year : 2017 | Volume
| Issue : 1 | Page : 17-21
Linezolid resistance in Staphylocccus epidermidis: Another armour in the armamentarium of the so called commensal
Simit Kumar, Maitreyi Bandyopadhyay, Abhishek Sengupta, Manas Bandyopadhyay, Mitali Chatterjee
Department of Microbiology, R. G. Kar Medical College and Hospital, Kolkata, West Bengal, India
|Date of Submission||18-Nov-2016|
|Date of Acceptance||09-Dec-2017|
|Date of Web Publication||27-Apr-2018|
Dr. Simit Kumar
Department of Microbiology, R. G. Kar Medical College and Hospital, Kolkata - 700 037, West Bengal
Source of Support: None, Conflict of Interest: None
Whereas previously only regarded as an innocuous commensal microorganism on the human skin, Staphylococcus epidermidis is nowadays seen as an important opportunistic pathogen. In particular, S. epidermidis represents the most common source of infections on indwelling medical devices such as peripheral or central intravenous catheters. Linezolid, the first approved oxazolidinone antibiotic, is a useful therapeutic option in the management of infections caused by multidrug-resistant Gram-positive bacteria. The previous administration of linezolid has been reported to be an independent predictor of linezolid resistance in coagulase-negative staphylococci (CoNS). Cases of patients developing infections with linezolid-resistant CoNS in the absence of prior exposure to linezolid have also been reported. The source of the resistant strain remains undetermined, but the clonal spread of CoNS has been reported to occur within hospitals, and therefore, the possibility of nosocomial transmission from patients colonized with linezolid-resistant CoNS following linezolid exposure needs to be entertained. Besides linezolid resistance, linezolid dependence has also been documented. All harboring linezolid-dependent linezolid-resistant S. epidermidis (LRSE) had prolonged linezolid treatment before yielding LRSE. This exposure also may have fostered the transition from resistance to dependence as suggested previously in vancomycin-dependent enterococci. Therefore, the high intrahospital linezolid consumption may favor not only LRSE selection but also their competitive survival. Should linezolid dependence prove common in highly LRSE isolates, it could explain their increasing clinical occurrence and the emergence of LRSE outbreaks. Our reports describe the first few cases of clinical failure of linezolid treatment due to LRSE in India.
Keywords: CoNS, Linezolid resistance, Staphylococcus epidermidis
|How to cite this article:|
Kumar S, Bandyopadhyay M, Sengupta A, Bandyopadhyay M, Chatterjee M. Linezolid resistance in Staphylocccus epidermidis: Another armour in the armamentarium of the so called commensal. Imam J Appl Sci 2017;2:17-21
|How to cite this URL:|
Kumar S, Bandyopadhyay M, Sengupta A, Bandyopadhyay M, Chatterjee M. Linezolid resistance in Staphylocccus epidermidis: Another armour in the armamentarium of the so called commensal. Imam J Appl Sci [serial online] 2017 [cited 2018 Jul 18];2:17-21. Available from: http://www.e-ijas.org/text.asp?2017/2/1/17/231383
| Introduction|| |
Whereas previously only regarded as an innocuous commensal microorganism on the human skin, Staphylococcus epidermidis is nowadays seen as an important opportunistic pathogen. It is now the most frequent cause of nosocomial infections, at a rate about as high as that due to its more virulent cousin Staphylococcus aureus. In particular, S. epidermidis represents the most common source of infections on indwelling medical devices such as peripheral or central intravenous catheters. Furthermore, S. epidermidis may be involved in prosthetic joint, vascular graft, surgical site, central nervous system shunt, and cardiac device infections. This likely stems from the fact that S. epidermidis is a permanent and ubiquitous colonizer of human skin and the resulting high probability of device contamination during insertion. While S. epidermidis infections only rarely develop into life-threatening diseases, their frequency and the fact that they are extremely difficult to treat represent a serious burden for the public health system. Treatment is complicated by specific antibiotic resistance genes and the formation of biofilms, multicellular agglomerations that have intrinsic resistance to antibiotics, and mechanisms of host defense. Furthermore, recent investigation has identified specific molecular determinants facilitating S. epidermidis immune evasion, ability to cause chronic disease and aiding the noninfectious lifestyle of this microorganism, emphasizing the accidental nature of S. epidermidis infections.
Linezolid, the first approved oxazolidinone antibiotic, is a useful therapeutic option in the management of infections caused by multidrug-resistant Gram-positive bacteria including methicillin-resistant S. aureus (MRSA), S. epidermidis, and vancomycin-resistant enterococci. Linezolid inhibits bacterial ribosomal protein synthesis by binding to rRNA, specifically to domain V of the 23S rRNA of the 50S ribosomal subunit.
Because linezolid is a purely synthetic antimicrobial, it was thought that preexisting mechanisms of resistance would not be common in nature and hence resistance to the drug would be slow to emerge. However, since its introduction in 2000, resistance to linezolid has been reported in isolates of vancomycin-resistant enterococci, methicillin-resistant staphylococci, and coagulase-negative staphylococci (CoNS). Linezolid resistance has been associated with mutations in the central loop of the domain V region of the 23S rRNA gene,,,, with the G2576T mutation (the substitution of thymine for guanine at position 2576) most frequently reported in Enterococcus faecium,,,,Enterococcus faecalis, and S. aureus isolates.,,, Linezolid resistance has been reported to occur in S. epidermidis isolates, and recently, the specific gene mutation G2576T was also reported to be associated with linezolid resistance in S. epidermidis.
The domain V regions of the 23S rRNA genes of both S. aureus and S. epidermidis are identical. Hence, homologous recombination between the wild-type and mutant genes could be the mechanism by which this mutation is spreading among genetically similar organisms. Alternatively, S. epidermidis may also become resistant to linezolid when it generates its own G2576T mutation in one of the 5–6 alleles of the domain V region of the 23S rRNA gene and then homologous recombination occurs within the same cell. Most organisms carry multiple copies of rRNA genes. S. epidermidis carries 5–6 copies of the 23S rRNA gene. There might be a correlation between the number of genes carrying the G2576T mutation and the minimum inhibitory concentration (MIC) of linezolid for S. epidermidis as has been described for enterococci  and S. aureus.
A new mechanism of linezolid resistance has recently been reported. The mechanism is nonmutational and involves acquisition of a natural resistance gene chloramphenicol-florfenicol resistance (cfr). It appears to be capable of horizontal transfer between staphylococci. The product of the cfr gene is a methyltransferase that catalyzes methylation of A2503 in the 23S rRNA gene of the large ribosomal subunit, conferring resistance to chloramphenicol, florfenicol, and clindamycin.
The previous administration of linezolid has been reported to be an independent predictor of linezolid resistance in CoNS. Cases of patients developing infections with linezolid-resistant CoNS in the absence of prior exposure to linezolid have also been reported. The source of the resistant strain remains undetermined, but the clonal spread of CoNS has been reported to occur within hospitals, and therefore, the possibility of nosocomial transmission from patients colonized with linezolid-resistant CoNS following linezolid exposure needs to be entertained.
Our reports describe the first cases of clinical failure of linezolid treatment due to linezolid-resistant S. epidermidis (LRSE) in India.
| Case Reports|| |
Case number 1
A 33-year-old construction laborer presented to the Orthopaedics Department of our hospital as a case of fracture both bone right leg, without any associated complications. The patient did not have any comorbid systemic diseases or any significant past or family history. His routine preoperative investigations were within normal limits. He was managed surgically by closed reduction and internal fixation by the use of tibial interlocking nail under spinal anesthesia.
On removal of his dressing, a serous discharge was observed from the lower end of the anterior surface of his right leg. Stitches were removed, and dressing was applied over the open wound. He was advised linezolid (600 mg), twice daily orally for 15 days to which he failed to respond and returned with complaints of serous discharge issuing from the same site. There was no associated neurovascular deficit or deformity. The serous discharge was aseptically collected from the open wound and was sent for microbiological analysis.
The pus sample showed growth of S. epidermidis, which was identified by standard laboratory procedures. The antibiotic susceptibility of the isolate was performed by Kirby–Bauer disc diffusion technique as per Clinical and Laboratory Standards Institute (CLSI guidelines), which showed the organism to be resistant to linezolid [Figure 1]. Disc diffusion testing was performed with 30 μg linezolid discs (BBL, Becton Dickinson). The MIC of the isolate was further determined by agar dilution method (using 2, 4, 8, 16, 32, 64, 128, and 256 μg/mL) in accordance with CLSI standards  using a linezolid preparation obtained from the manufacturer (Pfizer, India) which showed the MIC of the isolate to be 256 μg/mL. Repeat sample of pus collected from the discharge showed similar findings.
|Figure 1: Linezolid resistance in Staphylococcus epidermidis isolate by disc diffusion technique by Stoke's Method|
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The patient was advised dressing of the wound, active knee bending, and static quadriceps exercises. Linezolid was stopped, and the patient was started on intravenous vancomycin, to which the patient responded well, with the resolution of the wound.
Case number 2
A 40-year-old female patient presented to the Emergency Department of our hospital with acute onset fever and weakness in all four limbs. On examination, there was ptosis with reduced vision and jerky movements in her left eye, which however was reacting normally to light. The upper limbs had Grade 2 power while the tone was reduced in both the lower limbs. She had upper motor neuron type of seventh nerve palsy. The patient had no history of associated comorbid conditions. The patient was admitted to the Intensive Care Unit (ICU) of our hospital.
The blood investigations showed that the patient had slightly elevated levels of liver enzymes with the alanine aminotransferase levels being 50 U/L and the aspartate aminotransferase levels being 55 U/L. The serum lipase levels were 55 U/L and amylase levels were 112U/L. The patient had hyponatremia with serum sodium levels of 126 meq/L while potassium levels were elevated at 5.5 meq/L. All the other routine blood tests were within normal limits. Cerebrospinal fluid analysis revealed 4 cells/mm3, predominantly lymphocytes with elevated protein levels of 127 mg/dl and glucose levels of 112 mg/dl. Computed tomography scan of the brain revealed infarction in the left posterior parietal lobe, while magnetic resonance imaging brain revealed T2 flare with hyperintense foci of white matter with bilateral lesion in cerebellum and occipital cortex. Nerve conduction velocity tests were within normal limits.
The patient was diagnosed as a case of Guillain–Barre syndrome and was started on intravenous immunoglobulin, methylcobalamin, piperacillin and tazobactam, and linezolid. Urine culture sent on the day of admission for microbiological analysis revealed the growth of Enterococcus spp. and identified by standard laboratory procedures. The antibiotic susceptibility of the isolate was performed by Kirby–Bauer disc diffusion technique as per CLSI guidelines, which showed the organism to be sensitive to routinely administered antibiotics.
However, the set of three blood cultures sent on the day of admission, revealed the growth of S. epidermidis, and identified by standard laboratory procedures. The antibiotic susceptibility of the isolate was performed by Kirby–Bauer disc diffusion technique as per CLSI guidelines, which showed the organism to be resistant to linezolid. Disc diffusion testing was performed with 30 μg linezolid discs (BBL, Becton Dickinson). The MIC of the isolate was further determined by agar dilution method (using 2, 4, 8, 16, 32, 64, 128, and 256 μg/mL) in accordance with CLSI standards  using a linezolid preparation obtained from the manufacturer (Pfizer, India) which showed the MIC of the isolate to be 256 μg/mL.
After the microbiological report was available, linezolid was stopped, and the patient was started on intravenous vancomycin to which the patient responded well, with subsidence of fever and improvement in consciousness following which the patient was later shifted to the ward, after 5 days of stay in the ICU. The patient later had an uneventful recovery.
| Discussion|| |
We report the first few identified cases of linezolid-resistant S. epidermidis in India in two distinctly different cases admitted in different wards of our hospital, one with a prior exposure to the drug and the other without it.
Resistance to linezolid in S. epidermidis has rarely been reported worldwide. However, specific antibiotic resistance genes are widespread in S. epidermidis. High-level resistance to methicillin is encoded on mobile genetic elements (MGEs), namely the staphylococcal cassette chromosome mec (SCCmec), which contains the mecA gene encoding a penicillin-binding protein, PBP2a, with decreased affinity for methicillin compared to other PBPs. In addition to methicillin resistance, S. epidermidis strains have acquired resistance to several other antibiotics including rifamycin, fluoroquinolones, gentamicin, tetracycline, chloramphenicol, erythromycin, clindamycin, and sulphonamides. Very rarely, there is resistance to streptogramins, linezolid, and tigecycline. Most antibiotic resistance genes are plasmid-encoded and more often found in methicillin-resistant than methicillin-susceptible strains. In addition, staphylococcal biofilm formation significantly decreases the activity of vancomycin and other antibiotics.
The frequency of antibiotic resistance in S. epidermidis reflects antibiotic overuse.
Furthermore, the ubiquity of S. epidermidis as a human commensal microorganism renders this bacterium an optimal carrier and reservoir for antibiotic resistance genes, particularly those that do not inflict a major fitness cost to the bacteria, such as SCCmec elements. Accordingly, there is evidence suggesting that methicillin-resistant cassettes are transferred from S. epidermidis to S. aureus. Especially, the acquisition of SCCmec type IV by community-associated MRSA (CA-MRSA) has had an enormous impact on public health. It has enabled the combination of methicillin resistance at no cost for fitness paired with exceptional virulence, which is the main molecular basis of the epidemic caused by CA-MRSA. In addition, there is recent evidence indicating that CA-MRSA acquired other MGEs from S. epidermidis by horizontal gene transfer that may be important for efficient colonization. These findings emphasize an important role of S. epidermidis in human disease by providing a “reservoir” function for the transfer of genetic elements to enhance the pathogenic success of S. aureus. In this regards, the acquisition of linezolid resistance in S. epidermidis is alarming as this could lead to the transfer of resistance among S. aureus, the notorious nosocomial pathogen.
However, not all patients who were colonized with LRSE strain had received linezolid, and it is likely that these patients acquired the resistant S. epidermidis due to cross-infection. S. epidermidis may be shed from the skin into the environment on skin squamae, and it is possible that contaminated environmental surfaces could serve as a potential reservoir for these microorganisms, but the potential role of the inanimate environment as a source of nosocomial CoNS has received little attention. The emergence of linezolid resistance in S. epidermidis is associated with increased usage of linezolid, which may have exerted a selective pressure. Cross-infection is probably related to environmental contamination with the resistant strain and the ease with which S. epidermidis can be transmitted on the hands of health-care workers. One of our patients had an exposure to linezolid, while in the other patient from whom LRSE was isolated, there was no history of linezolid exposure, while the other patient had received linezolid for 15 days.
These considerations highlight the need for prophylactic measures against S. epidermidis infections. Vaccination and decolonization, often discussed for other pathogens including S. aureus, appear not appropriate for S. epidermidis. First, there is no anti-staphylococcal vaccine, and several lines of evidence indicate that it may be very difficult to use traditional active immunization for staphylococci. Second, eradication of S. epidermidis as a common part of the human microflora may not only be difficult to achieve owing to the fact that re-colonization from other individuals will be fast; it may also turn out to be counterproductive as it may allow potentially more harmful microorganisms to take the place of S. epidermidis. Thus, it is commonly agreed upon that the best way to deal with S. epidermidis infections is by prevention, which includes sterilization of medical equipment and body parts of patients and health-care personnel in possible contact with indwelling medical devices during surgery.
Besides linezolid resistance, linezolid dependence has also been documented. All harboring linezolid-dependent LRSE had prolonged linezolid treatment before yielding LRSE. This exposure also may have fostered the transition from resistance to dependence as suggested previously in vancomycin-dependent enterococci. Therefore, the high intrahospital linezolid consumption may favor not only LRSE selection but also their competitive survival. Should linezolid dependence prove common in highly LRSE isolates, it could explain their increasing clinical occurrence and the emergence of LRSE outbreaks.
| Conclusion|| |
To preserve the usefulness of linezolid as a therapeutic agent, judicious use of this antibiotic and careful stewardship of its use within individual institutions and units are important as our experience has demonstrated. Surveillance for the emergence of resistant strains is necessary to identify their emergence at an early stage so that appropriate measures can be taken to prevent their spread.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
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