Identification of Staphylococcus intermedius Group by MALDI-TOF MS

The Staphylococcus intermedius Group includes S. intermedius, S. pseudintermedius and S. delphini, coagulase-positive bacteria commonly isolated from animals. The identification of organisms belonging to this group is presently carried out using molecular methods. This study assessed the suitability of MALDI-TOF MS for their identification. 69 strains of different biological and geographic origins, identified by partial hsp60 gene sequencing as S. intermedius (n = 15), S. pseudintermedius (n = 32) and S. delphini (n = 22), were analyzed by MALDI-TOF MS. The estimated sensitivity, specificity and efficiency were calculated. In addition we computed the agreement between the outcome of MALDI-TOF MS identification and partial hsp60 gene sequencing. The sensitivity of MALDI-TOF MS was higher for S. intermedius [0.95 (95% CI: 0.68–0.99)], than for S. pseudintermedius [0.78 (95% CI: 0.60–0.90)] and S. delphini [0.64 (95% CI: 0.41–0.83)], whereas the specificity was 1 for S. intermedius and S. delphini and 0.97 (95% CI: 0.86–0.99) for S. pseudintermedius. The Cohen's kappa coefficient indicated almost perfect agreement between MALDI-TOF MS and hsp60 gene sequencing for the identification of S. intermedius [0.96 (95% CI: 0.87–1.04)], and substantial agreement for S. delphini and S. pseudintermedius [0.70 (95% CI: 0.52–0.89) and 0.76 (95% CI: 0.616–0.92), respectively]. The overall efficiency of the proteomic identification ranged between 0.88 (95% CI: 0.78–0.95) for S. pseudintermedius and S. delphini and 0.99 (95% CI: 0.92–0.99) for S. intermedius. MALDI-TOF MS is thus a valuable and reliable tool for the rapid and accurate identification of bacteria belonging to the S. intermedius Group.