The study investigates linezolid susceptibility in drug-resistant gram-positive cocci, specifically MRSA, VRE, and methicillin-resistant CONS. Results show that 60% of strains are susceptible to linezolid using broth microdilution, while the E-test reveals that 70% of discordant strains are resistant, highlighting the reliability of Vitek-2 in detecting linezolid resistance. The findings stress the importance of monitoring linezolid-resistant strains to ensure effective treatment of resistant infections.

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 0 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 0
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.

13. 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.