Rapid identification of coagulase-negative staphylococci by Fourier transform infrared spectroscopy

Coagulase-negative staphylococci (CNS), frequently associated with both community-acquired and nosocomial bloodstream infections, must be distinguished from Staphylococcus aureus for clinical purposes. Conventional methods are too laborious and time-consuming and often lack sensitivity to CNS. Fourier transform infrared (FTIR) spectroscopy combined with the use of a universal growth medium (Que-Bact Universal Medium No. 2) and chemometrics was evaluated for its potential as a rapid and simple clinical tool for making this distinction. FTIR spectra of 11 methicillin-sensitive and 11 methicillin-resistant CNS isolates as well as 25 methicillin-sensitive, 47 methicillin-resistant, 34 borderline oxacillin-resistant and 35 glycopeptide intermediate S. aureus isolates were obtained from dried films of stationary-phase cells grown on the universal medium. Principal component analysis (PCA), self-organizing maps, and the K-nearest neighbor algorithm were employed to cluster the different phenotypes based on similarity of their FTIR spectra. PCA of the first-derivative normalized spectral data from a single narrow region (2888-2868 cm(-1)) yielded complete differentiation of CNS from both methicillin-sensitive and methicillin-resistant S. aureus. The rate of correct classification was somewhat reduced, from 100% to 90%, after inclusion of borderline oxacillin-resistant and glycopeptide intermediate S. aureus strains in the data set. Differentiation based on the data in broader spectral regions was much less reliable. The results of this study indicate that with proper spectral region selection, FTIR spectroscopy and cluster analysis may provide a simple and accurate means of CNS species identification.