Accuracy of the MicroScan WalkAway system to identify coagulase-negative staphylococci

Abstract number: p1734

Sáez  A., Ruiz  B., Martínez-Martínez  L.

Objective: 

To determine the reliability of the identification of coagulase-negative staphylococci (CoNS) with the MicroScan WalkAway 96 (WA, Dade Behring) system at species level when a > = 90% probability is obtained, considering as a reference the results of molecular identification.

Methods: 

One hundred and sixty-eight isolates of CoNS from clinical samples (October 2003-May 2005) for which the identification with the WA system was >= 90%, and 9 ATCC type strains were evaluated. Bacteria were identified with the WA system using Pos Combo 1S panels. Absence of coagulase was determined with a latex assay (Pastorex^® staph-plus, Bio-Rad). Reference identification was established by sequencing of the 16S rRNA; when identification with WA and 16S rRNA disagree, definitive identification was defined after sequencing of the sodA and tuf genes, as previously described (Drancourt et al. JCM 2000; 38: 3623–30 and Heikens el al. JCM 2005; 43: 2286–90). For identification, the sequences of 16S rRNA, sodA and tuf were compared with those in GeneBank. Homologies values above 97% were considered reliable.

Results: 

All 9 type strains were correctly identified by 16S rRNA sequencing as named by the ATCC. Among the 168 clinical isolates, the molecular method identified the following species (number): S. hominis (39), S. haemolyticus (35), S. saprophyticus (30), S.epidermidis (27), S. lugdunensis (12), S. schleiferi (7), S. capitis (7), S. simulans (4), S. pasteuri (2), S. warneri (2), S. intermedius (2) and S. equorum (1). The WA system correctly identified 8 out of the 9 ATCC strains. S. pasteuri is not included in the WA database, and the corresponding ATCC strain was misidentified as S. warneri. One hundred and fifty-seven out of the 168 (93.4%) clinical isolates were correctly identified by the WA. Five S. haemolyticus were identified by WA as S. auricularis (2), S.simulans (2) and S. warneri (1). Other errors corresponded to: two S. pasteuri misidentified as S. warneri, one S. epidermidis as S. hominis, one S. lugdunensis as S.schleiferi, one S. hominis as S. haemolyticus and one S. equorum as S. cohnii. All isolates of S. saprophyticus, S. schleiferi, S. capitis, S. simulans, S. warneri and S. intermedius were correctly identified by the WA system.

Conclusions: 

The MicroScan WalkAway 96 is reliable to identify CoNS at species level when a probability of > = 90% is obtained. S. pasteuri should be incorporated to the WA database in order to improve its performance.