Evaluation of colorimetric nitrate reductase assay for rapid detection of methicillin resistance in clinical isolates of Staphylococcus aureus

Background & objectives:

Methicillin resistant Staphylococcus aureus (MRSA) remains a major cause of health care-associated infections. Rapid detection of MRSA facilitates the early initiation of appropriate treatment and infection control. Hence, the present study was undertaken to standardize and evaluate the performance of rapid colorimetric nitrate reductase assay (NRA) for determining methicillin resistance in S.aureus.

Methods:

A total of 160 clinical isolates of S. aureus, (80 each of methicillin susceptible and methicillin resistant) were included in the study. Minimum inhibitory concentration (MIC) was determined by NRA and reference broth micro dilution (BMD) methods. Results of NRA were compared with BMD and analyzed.

Results:

For MRSA, the MIC values ranged from 4 to ≥ 16 μg/ml and for MSSA, ≤ 0.5 to 2 μg/ml. Category and essential agreement for NRA as compared with BMD were found to be 99.4 and 89.7 per cent, respectively. No minor or major discrepancy was observed. A single resistant isolate showed very major discrepancy.

Interpretation & conclusions:

Colorimetric NRA being an inexpensive test requiring no special equipment can be employed as an alternative method for rapid detection of MRSA in resource limited settings.

Worrisome trends in rising minimum inhibitory concentration values of antibiotics against methicillin resistant Staphylococcus aureus – Insights from a tertiary care center, South India

Introduction

Appearance of isolated reports of resistance to anti-methicillin-resistant Staphylococcus aureus (MRSA) drugs is worrisome underscoring the need to continuously monitor the susceptibility of clinical MRSA isolates to these drugs. Hence, the present study is conducted to determine the susceptibility of MRSA isolates to various classes of anti-MRSA drugs such as vancomycin (glycopeptide), daptomycin (lipopeptide), tigecycline (glycylcycline), and linezolid (oxazolidinone) to determine the MIC₅₀ and MIC₉₀ values, and to observe MIC creep over a three year period, if any, with respect to these drugs.

Methods

A total of 200 isolates of MRSA obtained from clinical specimens were included. MIC was determined by E-test for anti-MRSA antibiotics vancomycin, linezolid, daptomycin, and tigecycline. Non-parametric methods (Kruskal–Wallis and Chi-square test) were used to assess MIC trends over time. In addition, MIC₅₀ and MIC₉₀ values were also calculated.

Results

No isolate was found resistant to vancomycin, daptomycin, or linezolid; five isolates were resistant to tigecycline. Seven VISA isolates were encountered with the MIC value for vancomycin of 4 μg/mL. MIC values for vancomycin, tigecycline, linezolid showed a definite increase over a 3-year period which was statistically significant with p-values <0.0001, 0.0032, 0.0242, respectively. When the percentage of isolates with a median MIC value less than or equal to that of the index year was calculated, the change was most striking with vancomycin. The proportion of isolates with higher MIC values was greater in 2014 than 2012 and 2013.

Conclusion

MIC creep was notably observed with vancomycin, and to some extent with tigecycline and linezolid. Selection pressure may result in creeping MICs, which may herald the emergence of resistant organisms.