Rapid identification of coagulase-negative staphylococci by Fourier transform infrared spectroscopy Nassim M. Amiali a,b, ⁎ , Michael R. Mulvey c , Jacqueline Sedman b , Marie Louie d , Andrew E. Simor d , Ashraf A. Ismail b a Quelab Laboratories Inc., Montreal, QC, Canada b McGill IR Group, McGill University, Montreal, QC, Canada c Nosocomial Infections, National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, MB, Canada d Sunnybrook and Women's College, Health Science Center, Toronto, ON, Canada Received 16 May 2006; received in revised form 21 July 2006; accepted 11 August 2006 Available online 16 October 2006 Abstract 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. © 2006 Elsevier B.V. All rights reserved. Keywords: FTIR spectroscopy; Identification; Staphylococci; CNS 1. Introduction Coagulase-negative staphylococci (CNS) have been known for decades as commensals on human skin. Formerly regarded as clinically insignificant contaminants transferred during specimen collection and sometimes interfering with blood culture tests for more virulent staphylococci, CNS are now recognized as significant contributors to the increasing incidence of both community-acquired and nosocomial blood- stream infections throughout the world (Sheagren, 1984a,b; Pfaller and Herwadt, 1988; Brumfitt and Hamilton-Miller, 1989; Grosserode and Wenzel, 1991). Given the differences in virulence between Staphylococcus aureus and CNS (Sheagren, 1984a,b; Pfaller and Herwadt, 1988; Brumfitt and Hamilton- Miller, 1989), it has become essential to distinguish between them in clinical samples. Misidentification of S. aureus as CNS can lead to costly efforts to identify the cause of serious infections or to unwarranted broad-spectrum empiric antibiotic treatments (Papasian and Garrison, 1999). It has therefore become increasingly important to identify CNS isolates accu- rately, both to determine their clinical significance and to manage relapses of CNS infections. Phenotypic methods based on bacterial culturing and bio- chemical tests have provided the standard means of differentiating S. aureus from CNS. These methods are time-consuming and occasionally produce inaccurate results (Grant et al., 1994; Ieven et al., 1995; NNIS system., 2003). Conventional biochemical tests Journal of Microbiological Methods 68 (2007) 236 – 242 www.elsevier.com/locate/jmicmeth ⁎ Corresponding author. Tel.: +1 514 277 2558x13; fax: +1 514 277 47 14. E-mail address: nassim.amiali@gmail.com (N.M. Amiali). 0167-7012/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mimet.2006.08.010