Abstract
Bacillus are aerobic spore-forming bacteria that are known to lead to specific diseases, such as anthrax and food poisoning. This study focuses on the characterization of these bacteria by the detection of lipids extracted from 33 well-characterized strains from the Bacillus and Brevibacillus genera, with the aim to discriminate between the different species. For the purpose of analysing the lipids extracted from these bacterial samples, two rapid physicochemical techniques were used: matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) and liquid chromatography in conjunction with mass spectrometry (LC-MS). The findings of this investigation confirmed that MALDI-TOF-MS could be used to identify different bacterial lipids and, in combination with appropriate chemometrics, allowed for the discrimination between these different bacterial species, which was supported by LC-MS. The average correct classification rates for the seven species of bacteria were 62.23 and 77.03 % based on MALDI-TOF-MS and LC-MS data, respectively. The Procrustes distance for the two datasets was 0.0699, indicating that the results from the two techniques were very similar. In addition, we also compared these bacterial lipid MALDI-TOF-MS profiles to protein profiles also collected by MALDI-TOF-MS on the same bacteria (Procrustes distance, 0.1006). The level of discrimination between lipids and proteins was equivalent, and this further indicated the potential of MALDI-TOF-MS analysis as a rapid, robust and reliable method for the classification of bacteria based on different bacterial chemical components. Graphical abstractMALDI-MS has been successfully developed for the characterization of bacteria at the subspecies level using lipids and benchmarked against HPLC Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-016-9890-4) contains supplementary material, which is available to authorized users.
Highlights
Classification of bacteria has recently received increasing attention, most likely arising from public health concerns, environmental monitoring, food safety monitoring, taxonomic identification and differentiation of pathogenic species from non-pathogenic species, as well as for the identification of biological threat agents [1,2,3]
Our observations show that samples collected after 10 h of cell culture (ESM Fig. S2A, B) generated better separation for the three species due to there being a sufficient amount of biomass that is needed for lipid extraction, which was evident from the Optical density (OD)
MALDI-TOF-MS is an important technique in analysing biomolecular compounds and has been proven to be useful for discriminating between different microorganisms, and its use in bacterial profiling is common in clinical microbiology testing laboratories [59,60,61]
Summary
Classification of bacteria has recently received increasing attention, most likely arising from public health concerns, environmental monitoring, food safety monitoring, taxonomic identification and differentiation of pathogenic species from non-pathogenic species, as well as for the identification of biological threat agents [1,2,3]. Lipids are important components in bacterial cell membranes as they form lipid bilayers responsible for cell integrity [14, 15]. These cell components have various structures, and several factors can affect lipid synthesis such as culture media, temperature and physical dynamics during cell growth [16]. Interest in the analysis of lipid profiles from bacterial cells for taxonomic identification is increasing [11]. Do lipids play a structural role in the integrity of cell membranes, but they contribute to other cellular processes such as metabolic and signalling pathways [19, 20]
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