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1. LINEZOLID SUSCEPTIBILITY TESTING IN DRUG- RESISTANT GRAM POSITIVE COCCI ARYA J ROLL NUMBER : 2008P2MLT01 MSc MLT MICROBIOLOGY FINAL YEAR UNDER GUIDENCE OF : Dr. NANDITA SHASHINDRAN ( ASSI .PROF) AMRITA INSTITUTE OF MEDICAL SCIENCES KOCHI

2. INTRODUCTION Linezolid, the first oxazolidinone to be used clinically, is effective in the treatment of infections caused by various Gram-positive pathogens, including multidrug resistant enterococci and methicillin-resistant Staphylococus aureus . Linezolid resistance in Gram-positive cocci has been encountered clinically as well as in vitro, but it is still a rare phenomenon. Antimicrobial susceptibility testing of resistant Gram positive cocci for linezolid is of importance because it is one of the therapeutic options for MRSA and VRE infections. Linezolid susceptibility can be tested by Disc diffusion ,MIC by broth micro dilution , E test and using the Automated system VITEK 2 COMPACT system

3. AIMS AND OBJECTIVES To determine the susceptibility of linezolid in drug-resistant Gram positive cocci like MRSA, VRE and methicillin-resistant CONS. To correlate the MIC value by broth micro dilution and E test with VITEK 2 COMPACT SYSTEM MATERIALS AND METHODS SAMPLE SIZE :25 METHOD : Minimum inhibitory concentration was detected using either Etest or broth microdilution for all the strains.

4. RESULTS ORGANISM DISTRIBUTION Out of 25 Gram positive cocci , 14 are VRE, 6 are MRSA and 5 are CONS

5. GPC STRAINS SUSCEPTIBLE BY DISC DIFFUSION AND RESISTANT BY VITEK

6. MIC by E test in strain Resistant by Vitek and Susceptible by disc diffusion Here out of 25 total strains ,14 strains are sensitive by disc diffusion and resistant by Vitek . Etest is done for 10 strains and MIC sensitive shown by 3 strains which include 2 MRSA and 1 CONS

7. Linezolid susceptibility by E-Test Ten out of 14 strains which showed discordance between Disc diffusion (S) and Vitek-2 (R) underwent MIC by E Test . Of these 10 strains , 7 strains are Resistant by Etest whereas 3 of the strains are Sensitive by E test. E-test showed more concordance with Vitek-2 than with disc diffusion . Strains Linezolid-susceptible by E-test Linezolid-resistant by E-test MRSA 2 2 (MICs 8 and 12 microgram/ ml) VRE 4 (MICs 12 microgram/ ml and above) Methicillin-resistant CONS 1 1 (MIC 8 microgram/ ml ) Total (10) 3 7

8. LINEZOLID MIC BY BROTH MICRODILUTION Strains Linezolid – Susceptible by BMD Linezolid – Resistant by BMD MRSA 2 VRE 6 4 Methicillin-resistant CONS 1 2 Total (15) 9 6 - Broth microdilution was done for 15 strains. - Nine out of 15 strains ( 60%) tested susceptible . - These strains included 3 which showed discordance between disc diffusion (S) and Vitek-2 (R). Two of these 3 strains tested susceptible by broth microdilution .

9. DISCUSSION Linezolid is an oxazolidinone agent that inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, blocking the formation of the initiation complex. During this study period 25 resistant Gram positive cocci were collected from various samples . Our study includes 14 VRE ,6 MRSA , 5 CONS. Of the 15 strains whose linezolid MICs were tested by broth microdilution , 9 (60%) were susceptible. Out of 10 discordant strains whose MICs were detected by Etest , 7 (70%) were resistant with MICs greater than 8 μg /ml . Thus on MIC detection by Etest , it appears that Vitek-2 is more reliable than disc diffusion for detection of linezolid resistance The 6 strains that were resistant to linezolid by both disc diffusion and Vitek were also resistant by broth microdilution (100% concordance) .

10. LIMITATIONS OF THE STUDY PCR for detection of resistance genes could not be done Either E-test or Broth microdilution for MIC determination was done for MIC determination. Both methods could not be done for all strains .

11. CONCLUSIONS Linezolid is central to the treatment of infections caused by resistant Gram-positive bacteria, including VRE and Methicillin resistant Staphylococci , emphasizing the importance of surveillance of linezolid-resistant strains . MICs of linezolid in 12 of the 25 strains (48%) were susceptible as determined by either E test or broth microdilution . In 70% of the strains that test susceptible by disc diffusion and resistant by Vitek-2, Etest shows resistant MICs . Hence Vitek-2 is more reliable than disc diffusion for the routine detection of linezolid resistance.