Joshua Carter

Background: Legionnaires’ disease (LD) is characterized as a pneumonia secondary to an infection by the bacterium Legionella pneumophila (Lp).¹ Identification of Legionella first occurred after an outbreak of an atypical pneumonia at the 1976 American Legion Convention in Philadelphia, PA – with the offending Gram-negative bacillus, Lp, being named after the disease and the victims.² The most common route of LD infection is through inhalation of aerosolized droplets that contain Lp, typically originating from manmade sources of stagnate water such as plumbing networks and air conditioning systems.³ Although culture of respiratory samples is still considered the gold standard for diagnosis, it presents challenges to providing a timely diagnosis due to the time that it takes and the need for a specific medium containing yeast extract and charcoal.¹ Prior Lp sequencing has yielded certain specific sequences that when present increase the frequency of human infections, with the elevation in pathogenicity being linked to these sequences.⁴

Objective: In this narrative review, we explored the difference in virulence between Lp strains and the epidemiologic implications of expedited genomic analysis.

Search Methods: An outline search in the PubMed database was conducted from 2018 to 2023 using the following keywords: “Legionella”, “Legionnaires’ disease”, “genomic sequencing”, “virulence”.

Results: Studies indicated that virulence is not uniform between Lp strains.^5,6,7 The activation of the NF-κB signaling pathway was investigated and demonstrated that varying strains of Lp1 induced different levels of NF-κB activation in vitro and that the Lp 1 strains that were linked to higher NF-κB activation, were also linked to a higher incidence of severe bodyweight loss and mortality.⁵ The same strains were also linked to an increased level of lung cytokines, with the exception of VEGF, an increase in serum cytokines, a more serious pulmonary effusion, and less alveoli area.⁵ TNF-α secretion from human macrophages in response to infection by 108 different isolates of Lp 1 found that all isolates were able to induce the secretion of TNF-α, but the levels of secretion were highly variable and appear to be linked to the specific genotype of the Lp1 strain.⁶ It was also determined that an increase in TNF-α is linked to an increase in apoptotic cellular death, indicating varying levels of virulence.⁶ Increased LD mortality can also be linked to the induction of a proinflammatory response which is attributed to the presence of a type of IVA secretion system, a secretion system that not all Lp strains possess.⁷

Conclusions: Previous studies have shown that certain strains are linked to increased morbidity and mortality, but the identification of the exact isolate does not typically precede attempts to manage the cluster of infection.⁸ The utilization of Finding low-abundance sequences by hybridization next generation sequencing (FLASH-NGS) of selected Legionella pneumophila genes via a CRISPR/Cas9-based target enrichment system from human samples has emerged as a strategy to identify isolates of concern, decreasing diagnosis time, and potentially improving clinical outcomes.⁸

Works Cited:

1. Viasus D, Gaia V, Manzur-Barbur C, Carratalà J. Legionnaires’ Disease: Update on Diagnosis and Treatment.Infect Dis Ther. 2022;11(3):973-986. doi:10.1007/s40121-022-00635-7
2. Chauhan D, Shames SR. Pathogenicity and Virulence ofLegionella: Intracellular replication and host response. Virulence. 2021;12(1):1122-1144. doi:10.1080/21505594.2021.1903199
3. Mondino S, Schmidt S, Rolando M, Escoll P, Gomez-Valero L, Buchrieser C. Legionnaires’ Disease: State of the Art Knowledge of Pathogenesis Mechanisms of Legionella. Annu Rev Pathol. 2020;15:439-466. doi:10.1146/annurev-pathmechdis-012419-032742
4. Gorzynski J, Wee B, Llano M, et al. Epidemiological analysis of Legionnaires’ disease in Scotland: a genomic study. Lancet Microbe. 2022;3(11):e835-e845. doi:10.1016/S2666-5247(22)00231-2
5. Wang H, Lu J, Li K, et al. The virulence of Legionella pneumophila is positively correlated with its ability to stimulate NF-κB activation.Future Microbiol. 2018;13:1247-1259. doi:10.2217/fmb-2018-0051
6. Guillemot J, Ginevra C, Allam C, et al. TNF-α response in macrophages depends on clinicalLegionella pneumophila isolates genotypes. Virulence. 2022;13(1):160-173. doi:10.1080/21505594.2021.2022861
7. Christensen LM, Sule P, Cirillo SLG, et al. Legionnaires’ Disease Mortality in Guinea Pigs Involves the p45 Mobile Genomic Element.J Infect Dis. 2019;220(10):1700-1710. doi:10.1093/infdis/jiz340
8. Domazetovska A, Jensen SO, Gray M, Radzieta M, Maley M. Culture-Free Phylogenetic Analysis of Legionella pneumophila Using Targeted CRISPR/Cas9 Next-Generation Sequencing. Microbiol Spectr. 2022;10(4):e0035922. doi:10.1128/spectrum.00359-22