Using WGS as surveillance strategy for Legionella pneumophila.

Introduction: Legionella pneumophila is an intracellular pathogen, best known for causing a Pneumonia – Legionnaires´ disease. As it is ubiquitously found in the environment and genetically very divers, progress in typing and sequencing technologies contributes increasingly to understand its general distribution and specifically outbreaks. The seven gene sequence-based typing scheme (SBT), as the currently most widely used method for genotyping, bears limitations regarding the discrimination of several sequence types (STs). Therefore, sequence comparisons from modern DNA sequencing methods and genomic sequence analysis correlated with epidemiological information could indicate connections, such as common sources or unnoticed contacts.
Goals: Our goal was to generate a survey of all patient isolates from 2023, to depict the current surveillance status of legionellae infections. We aimed to extract the STs and mAb-types from the generated WGS data. Using WGS, possible matches of patients and water isolates (possible infection sources) are more definite confirmable or deniable than by only using mAb-typeing and SBT.
Methods: The mAb-type of L. pneumophila colonies (152 samples) was validated via ELISA (Dresden-Panel) and the ST was determined via sanger sequencing. All confirmed colonies from 2023 were sequenced by WGS (Illumina, coverage >30). Generated sequences were analyzed in the Ridom SeqSphere+ software (version 4.0). WGS data were correlated to L. pneumophila mAb-types and STs, generating a clonal map of isolates from 2023.
Results: In 2023 the majority of confirmed L. pneumophila belongs to SG1 (88%, diverse mAb-types). The most abundant mAb-type (32%) was Knoxville, belonging to the highly virulent Pontiac-Group. Among the 137 determined STs (at least 33 different), ST9 was most abundant. The samples of ST9 cluster in diverse clouds, having up to 200 different alleles revealing thereby limitations of the SBT with regard to a clear identification infection sources. We obtained water isolates corresponding to 38 patients to look for possible matches. Only for 14 of the isolates, a match in mAb-type and ST with the corresponding water sample was confirmed.