Comparison of Xpert MRSA/SA Nasal and MRSA/SA ELITe MGB assays for detection of the mecA gene with susceptibility testing methods for determination of methicillin resistance in Staphylococcus aureus isolates

Abstract

In a series of 82 Staphylococcus strains isolated from culture, 100% were identified as Staphylococcus aureus by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS); 99.9% (77/82) of them were resistant to benzylpenicillin, oxacillin, and cefoxitin, and 6.1% (5/82) were susceptible to methicillin. Xpert MRSA/SA assay results were concordant with the phenotypic results in 76.8% (63/82) of cases and discordant in 23.2% (19/82) of cases. The MRSA/SA ELITe MGB kit results were concordant with phenotypic results in 100% of the cases. When comparing the Xpert MRSA/SA assay results with the MRSA/SA ELITe MGB kit results, 78% (64/82) of the cases were concordant, while 22% (18/82) of the cases were discordant. No statistically significant differences were observed between the two techniques. The PCR protocol that was used to validate the results of these two methods gave the following results: 49 were conventional methicillin-resistant S. aureus (MRSA) isolates (mecA positive and mecALGA251 negative), and 25 were phenotypic MRSA isolates (mecA negative and mecALGA251 positive).

Fig 1

Amplification data obtained with Xpert MRSA/SA assay for 5 of 81 MRSA phenotypes. Shown are a typical MRSA (spa⁺ mecA⁺ SCCmec⁺) (a), false-negative MRSA (spa⁺ mecA⁺ C_T39 SCCmec⁻) (b), spa⁺ mecA⁺ SCCmec⁻ (c), spa⁺ mecA⁻ SCCmec⁺ (d), and spa⁺ mecA⁻ SCCmec⁻ (e) genotypes.

Fig 2

Amplification data obtained with MRSA/SA ELITe MGB kit. Shown are a C_T of −5.37 and a ΔC_T of >2, indicating MRSA (a), a C_T of less than −5.27 and a ΔC_T of >2, indicating MSSA or the presence of two bacteria (the target DNA was incorrectly detected) (b), and a C_T of −0.49 and a ΔC_T value of <2, indicating MSSA (c).

Fig 3

PCR for detection of 16S-23S of the rRNA gene, mecA_LGA251 (primers MFP and MRP), mecA (primers P4 and P7), and mecA_LGA251 (primers FP and RP). Lanes 1 and 6, 100- and 3,000-bp ladder; lane 2, 16S-23S spacer region of the rRNA gene; lane 3, mecA_LGA251 (primers MFP and MRP); lane 4, mecA_LGA251 (primers FP to RP); lane 5, mecA (primers P4 and P7).