Table 4.
Prospective study: resolution of 47 discrepant results comparing conventional phenotypic and MALDI-TOF MS Gram-positive coccus identifications
| No. of isolates | Conventional identification | MALDI-TOF MS identification (Direct transfer formic acid, species cut-off 2.0) | 16S rRNA gene analysis (gold standard) |
|---|---|---|---|
| No reliable identification with MALDI-TOF MS | |||
| 2 | Coagulase-negative staphylococci | No reliable identification | Staphylococcus cohnii |
| 1 | Coagulase-negative staphylococci | No reliable identification | Staphylococcus warneri/pasteurid |
| Identification at the genus level with MALDI-TOF MS | |||
| 18 | Streptococcus dysgalactiae | Streptococcus sp.a | Streptococcus dysgalactiae |
| 1 | Aerococcus viridans | Aerococcus sp.b | Aerococcus viridans |
| 1 | Coagulase-negative staphylococci | Staphylococcus sp.b | Staphylococcus lugdunensis |
| 1 | Coagulase-negative staphylococci | Staphylococcus sp.b | Staphylococcus epidermidis |
| 1 | Staphylococcus aureus | Staphylococcus sp.b | Staphylococcus aureus |
| 1 | Streptococcus mitis/oralis | Streptococcus sp.b | Streptococcus mitis groupc |
| 1 | Streptococcus pyogenes | Streptococcus sp.a | Streptococcus pyogenes |
| Identification at the genus level with conventional methods | |||
| 3 | Enterococcus sp. | Enterococcus faecium | E. faecium/duransd |
| 1 | Enterococcus sp. | Enterococcus casseliflavus | Enterococcus gallinarum/casseliflavusd |
| Other discrepancies | |||
| 9 | Viridans streptococci | Streptococcus pneumoniae | Streptococcus mitis groupc |
| 3 | Streptococcus mitis/oralis | Streptococcus pneumoniae | Streptococcus mitis groupc |
| 1 | Coagulase-negative staphylococci | Staphylococcus aureus | Staphylococcus aureus |
| 1 | Coagulase-negative staphylococci | Staphylococcus lugdunensis | Staphylococcus lugdunensis |
| 1 | Staphylococcus aureus | Staphylococcus warneri | Mixed culturee |
| 1 | Pediococcus pentosaceus | Enterococcus faecium | Enterococcus faecium |
Identification by MALDI-TOF MS on the genus level (consistency category B), as more than one species yielded a score of ≥2.0.
Identification by MALDI-TOF MS on the genus level (consistency category B), as the best matches yielded scores in the range of ≥1.7 to <2.0 only.
The analysis of the 16S rRNA gene is not suited for the differentiation of the S. mitis group (2). Therefore, S. pneumoniae was excluded by biochemical tests.
No differentiation possible with the analysis of the 16S rRNA gene. The E. gallinarum/casseliflavus isolate showed no pigmentation, indicating that it was E. gallinarum (17).
Repetition of conventional and MALDI-TOF MS identification and analysis of the 16S rRNA gene revealed a mixed culture of S. aureus and S. warneri/pasteuri.