Differentiation of crude lipopolysaccharides from Escherichia coli strains using fourier transform infrared spectroscopy and chemometrics

Crude phenol-phase extracts containing bacterial lipopolysaccharides (LPS) from 5 strains of Escherichia coli were investigated to differentiate the strains using Fourier transform infrared (FTIR) spectroscopy and multivariate statistical analysis. The strains used were E. coli K12, E. coli DH5α, E. coli O157:H7, E. coli O157:H12, and E. coli O157:H19. LPS-containing extracts were isolated from each E. coli strain using a hot phenol-water extraction procedure. The extracts were 1st analyzed by deoxycholic acid-polyacrylamide gel electrophoresis and visualized by silver-staining. Analysis of the extracts from E. coli K12 and E. coli DH5α showed rough-type LPS on the lower half of the gel, whereas E. coli O157:H7, E. coli O157:H12, and E. coli O157:H19 yielded abundant smooth LPS (high-molecular-weight LPS that include the O-polysaccharides). Spectra (4000 cm-1 to 700 cm-1) of crude E. coli LPS extracts and intact cells were collected using a FTIR spectrometer. Spectral data were compressed by principle component analysis and analyzed using canonical variate analysis (CVA) of 4000 cm-1 to 700 cm-1 or 1200 cm-1 to 900 cm-1 spectral regions. CVA showed better separation between strains using LPS extracts than intact cells in the 1200 cm-1 to 900 cm-1 spectral region. The same separation trend was found using Mahalanobis distances that quantified spectral differences between the E. coli strains, providing 80% and >95% correct classifications of intact cells and LPS extracts, respectively. This article is the first to report the successful differentiation of E. coli strains at a serotype level using FTIR spectra of bacterial phenol-phase extracts (crude LPS preparations).