Serotype discrimination of encapsulated Streptococcus pneumoniae strains by Fourier-transform infrared spectroscopy and chemometrics

•FTIR for serotyping encapsulated Streptococcus pneumonia strains•FTIR method correctly predicts 100% of the tested serogroups.•Closely related serotypes were accurately discriminated.•FTIR proved to be a promising alternative for serotyping S. pneumonia.

Fourier-transform infrared (FTIR) spectroscopy was evaluated as an alternative method for serotyping encapsulated Streptococcus pneumonia strains. Sixty-nine invasive strains from the Microbiology Laboratory of the Medicine Faculty of University of Lisbon were selected for the analysis. The encapsulated strains used in this work (9N, 9V, 14, 19A, 19F, 23A, 23B and 23F) were serotyped by the standard capsular reaction test using the chessboard method and specific sera. FTIR spectra were analysed with chemometric methods. Results showed that the best spectral region unveiling serotype difference is located at 1185–900 cm− 1. A partial least squares discriminant analysis model was calibrated using this spectral region against the chessboard method serotyping. FTIR spectroscopy proved to be able to correctly predict 100% of strains according to the serogroups (9, 14, 19, 23). A segregated analysis showed that it was also possible to differentiate serotypes belonging to the same serogroup although spectral differences are less pronounced as was initially expected.