O-Antigen Typing of Escherichia coli by MALDI-TOF MS Analysis of O-Antigen Polysaccharides

Importance of the Topic

The structural variability of Escherichia coli O-antigens underpins serotyping, an essential activity for outbreak investigation, food safety surveillance, and clinical microbiology. Traditional O‑serotyping relies on large panels of antisera and labor-intensive workflows to discriminate more than 180 O‑serogroups. The use of MALDI‑TOF MS to detect polymeric mass spectral patterns of O‑antigen polysaccharides offers a rapid, reagent‑sparing alternative that can streamline typing and increase laboratory throughput.

Objectives and Study Overview

This application study evaluated whether MALDI‑TOF MS analysis of O‑antigen polysaccharides can discriminate O‑antigen types of E. coli. The authors focused on major enterohemorrhagic E. coli (EHEC) serogroups (O157, O26, O103), comparing type strains and human‑derived isolates. The goal was to establish whether periodic peak intervals in MALDI mass spectra correspond to the calculated masses of repeating monosaccharide units and therefore enable O‑antigen typing with a single reagent and a single assay.

Methodology

Key experimental steps:

• Cultivation: Single colonies grown on LB agar were collected and suspended in purified water.
• Extraction: A two‑step heat treatment at 90 °C (10 min) with intermediate centrifugation. The pellet was treated with HCl (final 100 mM) and NaCl (final 25 mM) followed by a second 90 °C incubation; the supernatant after centrifugation served as the O‑antigen polysaccharide sample.
• Sample preparation for MALDI: 1.0 µL of sample spotted and air‑dried, then 1.0 µL of DHB matrix (2,5‑dihydroxybenzoic acid, 1.25 mg/mL in 50% acetonitrile) was applied and dried.
• Spectral acquisition: Multiple profiles (5 shots × 256 profiles) were recorded and averaged.

Used Instrumentation

The work used a benchtop linear MALDI‑TOF MS system (MALDI‑8020). Typical acquisition parameters reported:

• Ion mode: Linear positive
• m/z range: 500–10,000
• Matrix: DHB
• Pulsed extraction: 2,300 Da
• Laser repetition frequency: 200 Hz; laser intensity: 85
• Rastering: Selected spots with a 1000 µm × 1000 µm raster and 50 µm spacing between points

Main Results and Discussion

Conceptual basis: O‑antigen polysaccharides are linear polymers of repeating oligosaccharide units. In MALDI spectra, polymeric species produce series of peaks separated by the mass of the repeating unit; thus the repeating‑unit mass can be inferred from peak spacing.

Findings by serogroup:

• O157: The calculated repeating‑unit mass for the O157 trisaccharide/oligosaccharide unit (composed of RhaNAc, Fuc, Glc, GalNAc) is ~699 Da (accounting for loss of H2O during glycosidic bond formation). Experimental spectra from type strain and human isolates showed regularly spaced peaks (m/z 1438.9, 2137.5, 2836.0, 3534.6) with intervals ≈699 Da, consistent across isolates.
• O26: Repeating‑unit mass calculated ~536–537 Da. Spectra from the type strain and human isolates displayed periodic peaks spaced by ≈536 Da at multiple charge states and oligomer lengths.
• O103: Repeating‑unit mass calculated ~1003 Da. Spectra exhibited periodic intervals ≈1003 Da in both type and clinical isolates.

Interpretation: For the three serogroups examined, mass spectral periodicity matched the theoretical masses of repeating units, enabling assignment of O‑antigen type by simple inspection of interpeak spacings. The approach was robust for both reference strains and recent human‑derived EHEC isolates, indicating applicability to clinical and surveillance samples.

Key Data Summaries

Prevalence context (Osaka, Japan, 2023): O157 dominated EHEC isolates (≈83.5%), while O26 (≈3.4%) and O103 (≈2.3%) were less common. This underlines the practical value of rapid O157 recognition but also the need to cover diverse serogroups in a typing library.

Spectral characteristics: The MALDI spectra of each serogroup showed multiple oligomer peaks corresponding to different numbers of repeating units; the interpeak intervals provided the diagnostic feature rather than absolute peak positions, which can shift with adducts (e.g., Na+, H+).

Benefits and Practical Applications

The MALDI‑based O‑antigen typing approach offers several operational advantages:

• Single reagent (DHB matrix) and single assay workflow versus large antisera panels required for immunoserotyping.
• Rapid turnaround compatible with routine MALDI workflows already present in many microbiology labs.
• Potential for reduced cost and labor, scalable to surveillance programs and outbreak response.

Practical uses include food‑safety screening, epidemiological surveillance, and preliminary typing in clinical laboratories. However, the authors note that this application note is for research use and not for diagnostic claims.

Limitations and Considerations

Important constraints and caveats:

• Study scope: Demonstrated for three serogroups (O157, O26, O103); broad applicability across >180 E. coli O‑types remains to be established.
• Spectral variability: Ionization efficiency, adduct formation, and sample preparation can affect absolute m/z values; diagnostics should rely on interpeak spacing and include adduct-awareness and calibration.
• Database needs: Robust typing requires a validated spectral library and automated matching algorithms to avoid subjective interpretation.
• Regulatory and clinical validation: Method requires formal validation before use in certified diagnostic workflows.

Future Trends and Opportunities

Paths to development and broader implementation:

• Expansion of a curated spectral database spanning the full diversity of E. coli O‑antigens and related species.
• Integration of automated pattern recognition and machine‑learning classifiers to assign serogroups from peak spacing and spectral fingerprints.
• Application of tandem MS or high‑resolution instruments to dissect atypical or mixed‑serogroup samples and to resolve co‑occurring adducts.
• Miniaturized, high‑throughput sample preparation workflows compatible with colony‑to‑MALDI pipelines for routine surveillance.
• Cross‑platform harmonization and interlaboratory studies to establish reproducibility, limits of detection, and standard operating procedures for regulatory acceptance.

Conclusion

This study demonstrates that MALDI‑TOF MS analysis of extracted O‑antigen polysaccharides can reveal repeating‑unit mass signatures that differentiate E. coli O‑serogroups (demonstrated for O157, O26, O103). The technique can type both reference strains and human‑derived EHEC isolates using a single matrix reagent and a straightforward sample preparation protocol. Further expansion of serogroup coverage, database development, and validation work are necessary to convert this promising approach into a routine, widely adopted typing method.

References

1. Jigyo Nenpo. Osaka Institute of Public Health, FY2023. Annual report (2023).
2. Shogo U., Hiroshi H. MALDI glycotyping of O‑antigens from a single colony of gram‑negative bacteria. Scientific Reports. 2024;14:12719. DOI: 10.1038/s41598-024-62729-1.
3. Liu B., Furevi A., Perepelov A.V., Guo X., Cao H., Wang Q., Reeves P.R., Knirel Y.A., Wang L., Widmalm G. Structure and genetics of Escherichia coli O antigens. FEMS Microbiology Reviews. 2020;44(6):655–683. DOI: 10.1093/femsre/fuz028.