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Gene flow in environmental Legionella pneumophila leads to genetic and pathogenic heterogeneity within a Legionnaires' disease outbreak.

McAdam PR, Vander Broek CW, Lindsay DS, Ward MJ, Hanson MF, Gillies M, Watson M, Stevens JM, Edwards GF, Fitzgerald JR - Genome Biol. (2014)

Bottom Line: In addition, we discover that some patients were infected with multiple L. pneumophila subtypes, a finding which can affect the certainty of source attribution.Importantly, variation in the complement of type IV secretion systems encoded by different genetic subtypes correlates with virulence in a Galleria mellonella model of infection, revealing variation in pathogenic potential among the outbreak source population of L. pneumophila.Furthermore, our data suggest that in addition to host immune status, pathogen diversity may be an important influence on the clinical outcome of individual outbreak infections.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Legionnaires' disease is a severe form of pneumonia caused by the environmental bacterium Legionella pneumophila. Outbreaks commonly affect people with known risk factors, but the genetic and pathogenic complexity of L. pneumophila within an outbreak is not well understood. Here, we investigate the etiology of the major Legionnaires' disease outbreak that occurred in Edinburgh, UK, in 2012, by examining the evolutionary history, genome content, and virulence of L. pneumophila clinical isolates.

Results: Our high resolution genomic approach reveals that the outbreak was caused by multiple genetic subtypes of L. pneumophila, the majority of which had diversified from a single progenitor through mutation, recombination, and horizontal gene transfer within an environmental reservoir prior to release. In addition, we discover that some patients were infected with multiple L. pneumophila subtypes, a finding which can affect the certainty of source attribution. Importantly, variation in the complement of type IV secretion systems encoded by different genetic subtypes correlates with virulence in a Galleria mellonella model of infection, revealing variation in pathogenic potential among the outbreak source population of L. pneumophila.

Conclusions: Taken together, our study indicates previously cryptic levels of pathogen heterogeneity within a Legionnaires' disease outbreak, a discovery that impacts on source attribution for future outbreak investigations. Furthermore, our data suggest that in addition to host immune status, pathogen diversity may be an important influence on the clinical outcome of individual outbreak infections.

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The virulence ofL. pneumophilaoutbreak isolates correlates with T4SS content.G. mellonella survival curves representing the mean for isolates grouped according to the combination of T4SS, including presence of the Lvh T4ASS Philadelphia (red), Lvh T4ASS novel (blue), and isolates without T4ASS Philadelphia or T4ASS novel (black). Larvae infected with isolates which encoded the Lvh T4ASS novel had a lower survivability compared with those with infected other isolates (P = 0.04).
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Fig3: The virulence ofL. pneumophilaoutbreak isolates correlates with T4SS content.G. mellonella survival curves representing the mean for isolates grouped according to the combination of T4SS, including presence of the Lvh T4ASS Philadelphia (red), Lvh T4ASS novel (blue), and isolates without T4ASS Philadelphia or T4ASS novel (black). Larvae infected with isolates which encoded the Lvh T4ASS novel had a lower survivability compared with those with infected other isolates (P = 0.04).

Mentions: The T4SSs play a central role in the capacity of L. pneumophila to infect free-living amoeba and survive within vacuoles in human alveolar macrophages. Mouse models have traditionally been applied to examine the role of specific Legionella spp. determinants in pathogenesis but an infection model of the G. mellonella (waxmoth) larvae has been developed recently, providing an effective model of L. pneumophila human infection which allows analysis of T4SS-dependent virulence [37]. We used the G. mellonella infection model to examine the virulence of all L. pneumophila clinical isolates obtained in the 2012 Edinburgh outbreak, in addition to a reference strain L. pneumophila Paris of known virulence [37]. Considerable strain-dependent variation in Galleria host survival was observed after infection (Figure 3), and a significant difference in killing capacity was identified between groups of isolates with unique combinations of T4SSs. In particular, strains with the novel putative T4SS resulted in more rapid killing of Galleria larvae than strains without it (P = 0.04; Figure 3). There was no significant difference between isolates containing different variants of the Dot/Icm T4SS (data not shown). Analysis of clinical data for each patient for which there was comparable information (n = 13) was carried out. The small number of patients infected with the more virulent subtype containing the novel Lvh T4SS was not sufficiently powered to facilitate a robust statistical analysis, and there was no statistically significant difference in clinical disease indicators between patients infected with strains containing the novel Lvh T4SS (n = 3), and those infected with strains lacking the Lvh T4SS (n = 10). However, patients with the novel T4SS required more clinical care intervention, including higher intensive care unit (ICU) admission, a higher proportion requiring mechanical ventilation, and fewer ICU-free days. Taken together, we have identified heterogeneity in virulence among closely related L. pneumophila isolates from the same Legionnaires’ disease outbreak that may influence the outcome of infection.Figure 3


Gene flow in environmental Legionella pneumophila leads to genetic and pathogenic heterogeneity within a Legionnaires' disease outbreak.

McAdam PR, Vander Broek CW, Lindsay DS, Ward MJ, Hanson MF, Gillies M, Watson M, Stevens JM, Edwards GF, Fitzgerald JR - Genome Biol. (2014)

The virulence ofL. pneumophilaoutbreak isolates correlates with T4SS content.G. mellonella survival curves representing the mean for isolates grouped according to the combination of T4SS, including presence of the Lvh T4ASS Philadelphia (red), Lvh T4ASS novel (blue), and isolates without T4ASS Philadelphia or T4ASS novel (black). Larvae infected with isolates which encoded the Lvh T4ASS novel had a lower survivability compared with those with infected other isolates (P = 0.04).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4256819&req=5

Fig3: The virulence ofL. pneumophilaoutbreak isolates correlates with T4SS content.G. mellonella survival curves representing the mean for isolates grouped according to the combination of T4SS, including presence of the Lvh T4ASS Philadelphia (red), Lvh T4ASS novel (blue), and isolates without T4ASS Philadelphia or T4ASS novel (black). Larvae infected with isolates which encoded the Lvh T4ASS novel had a lower survivability compared with those with infected other isolates (P = 0.04).
Mentions: The T4SSs play a central role in the capacity of L. pneumophila to infect free-living amoeba and survive within vacuoles in human alveolar macrophages. Mouse models have traditionally been applied to examine the role of specific Legionella spp. determinants in pathogenesis but an infection model of the G. mellonella (waxmoth) larvae has been developed recently, providing an effective model of L. pneumophila human infection which allows analysis of T4SS-dependent virulence [37]. We used the G. mellonella infection model to examine the virulence of all L. pneumophila clinical isolates obtained in the 2012 Edinburgh outbreak, in addition to a reference strain L. pneumophila Paris of known virulence [37]. Considerable strain-dependent variation in Galleria host survival was observed after infection (Figure 3), and a significant difference in killing capacity was identified between groups of isolates with unique combinations of T4SSs. In particular, strains with the novel putative T4SS resulted in more rapid killing of Galleria larvae than strains without it (P = 0.04; Figure 3). There was no significant difference between isolates containing different variants of the Dot/Icm T4SS (data not shown). Analysis of clinical data for each patient for which there was comparable information (n = 13) was carried out. The small number of patients infected with the more virulent subtype containing the novel Lvh T4SS was not sufficiently powered to facilitate a robust statistical analysis, and there was no statistically significant difference in clinical disease indicators between patients infected with strains containing the novel Lvh T4SS (n = 3), and those infected with strains lacking the Lvh T4SS (n = 10). However, patients with the novel T4SS required more clinical care intervention, including higher intensive care unit (ICU) admission, a higher proportion requiring mechanical ventilation, and fewer ICU-free days. Taken together, we have identified heterogeneity in virulence among closely related L. pneumophila isolates from the same Legionnaires’ disease outbreak that may influence the outcome of infection.Figure 3

Bottom Line: In addition, we discover that some patients were infected with multiple L. pneumophila subtypes, a finding which can affect the certainty of source attribution.Importantly, variation in the complement of type IV secretion systems encoded by different genetic subtypes correlates with virulence in a Galleria mellonella model of infection, revealing variation in pathogenic potential among the outbreak source population of L. pneumophila.Furthermore, our data suggest that in addition to host immune status, pathogen diversity may be an important influence on the clinical outcome of individual outbreak infections.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Legionnaires' disease is a severe form of pneumonia caused by the environmental bacterium Legionella pneumophila. Outbreaks commonly affect people with known risk factors, but the genetic and pathogenic complexity of L. pneumophila within an outbreak is not well understood. Here, we investigate the etiology of the major Legionnaires' disease outbreak that occurred in Edinburgh, UK, in 2012, by examining the evolutionary history, genome content, and virulence of L. pneumophila clinical isolates.

Results: Our high resolution genomic approach reveals that the outbreak was caused by multiple genetic subtypes of L. pneumophila, the majority of which had diversified from a single progenitor through mutation, recombination, and horizontal gene transfer within an environmental reservoir prior to release. In addition, we discover that some patients were infected with multiple L. pneumophila subtypes, a finding which can affect the certainty of source attribution. Importantly, variation in the complement of type IV secretion systems encoded by different genetic subtypes correlates with virulence in a Galleria mellonella model of infection, revealing variation in pathogenic potential among the outbreak source population of L. pneumophila.

Conclusions: Taken together, our study indicates previously cryptic levels of pathogen heterogeneity within a Legionnaires' disease outbreak, a discovery that impacts on source attribution for future outbreak investigations. Furthermore, our data suggest that in addition to host immune status, pathogen diversity may be an important influence on the clinical outcome of individual outbreak infections.

Show MeSH
Related in: MedlinePlus