Diagnostic MALDI-TOF MS can differentiate between high and low toxic Staphylococcus aureus bacteraemia isolates as a predictor of patient outcome

Abstract

Staphylococcus aureus bacteraemia (SAB) is a major cause of blood-stream infection (BSI) in both healthcare and community settings. While the underlying comorbidities of a patient significantly contributes to their susceptibility to and outcome following SAB, recent studies show the importance of the level of cytolytic toxin production by the infecting bacterium. In this study we demonstrate that this cytotoxicity can be determined directly from the diagnostic MALDI-TOF mass spectrum generated in a routine diagnostic laboratory. With further development this information could be used to guide the management and improve the outcomes for SAB patients.

Keywords: bacteraemia; Agr; MALDI-TOF MS diagnosis; Staphylococcus aureus; toxicity.

Fig. 1.

Diagnostic MALDI-TOF can differentiate between S. aureus isolates with and without a functional Agr system. (a) Comparison of relative spectra produced for an isogenic pair of a wild-type (blue) and an agr mutant strain (red), based on eight repeated runs, with the peaks at 3000 Da highlighted. (b) Average peak differences between the relative MALDI-TOF spectra of the wild-type and agr mutant, colour-coded by statistical significance of the difference in mean height (Welch two sample t-test, corrected for multiple testing using Benjamini-Hochberg with N(peaks)=53). A number of significant differences were detected with the biggest difference identified as the peak at 3000 Da. (c) Comparison of relative MALDI-TOF spectra of an isogenic pair of a wild-type and a Hld mutant strain identifying the 3000 Da peak (highlighted in white) as the Hld toxin.

Fig. 2.

The 3000 Da peak is strongly associated with cytotoxicity and is predictive of 30 day mortality following SAB. (a) Relative peak heights at position 3 000 Da shows a bimodal distribution with some strains lacking this peak altogether (blue circles). (b) There is a significant association of the 3 000 Da peak and the cytotoxicity of 136 SAB isolates, as demonstrated by means of percentage of cell lysis (toxicity) stratified by presence or absence of the 3 000 Da peak. (c) Receiver operating characteristic (ROC) curves of fitted logistic regression models of patient age, patient comorbidity and the presence/absence of the 3 000 Da peak (black lines) compared with a model considering patient age and comorbidity alone (red lines), demonstrating improved discrimination of patient outcome (30 day mortality) when considering the toxicity-associated MALDI-TOF peak. Thick lines are based on the respective models fitted to all data; thin lines correspond to ROC curves based on a single run of a three-fold cross-validation, with reported AUCs based on 100 repeat runs.