Identification and Subtyping of Salmonella Isolates Using Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI–TOF) Mass Spectrometry

Introduction: Subtyping of isolates of the same genus and species is an important tool in epidemiological investigations. A number of phenotypic and genotypic subtyping methods are available; for example, serotyping is the most widely used initial characterization performed on foodborne pathogens, such as Salmonella enterica subsp enterica. Unfortunately, serotyping is labor-intensive, costly, time-consuming, requires considerable operator skill and a wealth of reagents. A potential alternative to bacterial serotyping is subtyping using Matrix Assisted Laser Desorption/Ionization Time of Flight (MALDI–TOF) Mass Spectrometry. This is a library-based approach, offering a rapid, reproducible method for bacterial identification with a high sensitivity and specificity and at minimal cost.

Purpose: The purpose of this study was to determine the feasibility of using MALDI-TOF to differentiate five known Salmonella serovars recovered from experimental microcosms inoculated with known strains of Salmonella.

Methods: A reference library for each of the five known Salmonella isolates was created using a minimum of 30 spectral readings. A total of 965 unknown Salmonella isolates were then identified using MALDI-ToF.  The identities of the isolates were confirmed using conventional methods (serotyping/PCR). Sensitivity and specificity data were calculated and McNemar’s tests used to determine any statistical difference between identification using MALDI-TOF and conventional methods (P < 0.05).

Results: MALDI TOF identified all 965 isolates as being Salmonella species. Among them, 938 of these isolates were correctly subtyped using MALDI-ToF (Biotyping score ≥ 2.5).  Sensitivity ranged from 82 -100% and specificity for all serotypes was approximately 99%. There was no statistical difference between results obtained using MALDI-TOF and conventional methods.  

Significance: This study demonstrates that MALDI-TOF is a viable alternative for the rapid identification and differentiation of Salmonella subtypes.