A new MALDI-TOF approach for the quick sequence type identification of Legionella pneumophila

Recently, MALDI-TOF has emerged as a quick tool for bacterial typing. The aim was to evaluate if MALDI-TOF based typing of Legionella pneumophila can achieve the same discriminatory power as that of the Sequence Based Typing (SBT) method.

The Sequence Type (ST) was obtained from the 90 strains included in the training set and an in-house MALDI-TOF library based on the Main Spectra Profile (MSP) was generated for the identification of such ST. Then, our library was validated by three procedures: a) creating a dendrogram, b) searching for specific peaks present exclusively in each MSP entry, and c) analysing a validation set composed of 14 strains with known ST. Fully characterized L. pneumophila ATCC 33152, which belongs to ST 36, was used as a control strain.

In the training set, 17 strains belonged to ST 1, 1 to ST 20, 63 to ST 22, 1 to ST 146, 6 to ST 578, and 2 to ST 1086. Specific peaks present in each MSPs spectrum, which are considered type-specific biomarkers, ranged from 2 to 11; more concretely, MSP for ST 1 identification shows 2 specific peaks; MSP for ST 20 identification: 9 specific peaks; MSP for ST 22 and ST 36 identification: 11 specific peaks; MSP for ST 146 identification: 5 specific peaks; and MSP for ST 578 and ST 1086 identification: 3 specific peaks. Using the validation set (nine strains belonging to ST 22 and five to ST 1), MALDI-TOF assigned accurately the ST in 30 min per tested strain with a full match.

The ST of L. pneumophila can be identified and reported in few minutes directly from colonies grown on BCYE agar using MALDI-TOF.