Chemical Changes On, and Through, The Bacterial Envelope in <i>Escherichia coli</i> Mutants Exhibiting Impaired Plasmid Transfer Identified Using Time-of-Flight Secondary Ion Mass Spectrometry

Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) using a (CO₂)_6k⁺ gas cluster ion beam (GCIB) was used to analyze <i>Escherichia coli</i> mutants previously identified as having impaired plasmid transfer capability related to the spread of antibiotic resistance. The subset of mutants selected were expected to result in changes in the bacterial envelope composition through the deletion of genes encoding for FabF, DapF, and Lpp, where the surface sensitivity of ToF-SIMS can be most useful. Analysis of arrays of spotted bacteria allowed changes in the lipid composition of the bacteria to be elucidated using multivariate analysis and confirmed through imaging of individual ion signals. Significant changes in chemical composition were observed, including a surprising loss of cyclopropanated fatty acids in the <i>fabF</i> mutant where FabF is associated with the elongation of FA(16:1) to FA(18:1) and not cyclopropane formation. The ability of the GCIB to generate increased higher mass signals from biological samples allowed intact lipid A (<i>m</i>/<i>z</i> 1796) to be detected on the bacteria and, despite a 40 keV impact energy, depth profiled through the bacterial envelope along with other high mass ions including species at <i>m</i>/<i>z</i> 1820 and 2428, attributed to ECA_CYC, that were only observed below the surface of the bacteria and were notably absent in the depth profile of the <i>lpp</i> mutant. The analysis provides new insights into the action of the specific pathways targeted in this study and paves the way for whole new avenues for the characterization of intact molecules within the bacterial envelope.