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Exposure to pairs of Aeromonas strains enhances virulence in the Caenorhabditis elegans infection model

View Article: PubMed Central

ABSTRACT

Aeromonad virulence remains poorly understood, and is difficult to predict from strain characteristics. In addition, infections are often polymicrobial (i.e., are mixed infections), and 5–10% of such infections include two distinct aeromonads, which has an unknown impact on virulence. In this work, we studied the virulence of aeromonads recovered from human mixed infections. We tested them individually and in association with other strains with the aim of improving our understanding of aeromonosis. Twelve strains that were recovered in pairs from six mixed infections were tested in a virulence model of the worm Caenorhabditis elegans. Nine isolates were weak worm killers (median time to death, TD50, ≥7 days) when administered alone. Two pairs showed enhanced virulence, as indicated by a significantly shortened TD50 after co-infection vs. infection with a single strain. Enhanced virulence was also observed for five of the 14 additional experimental pairs, and each of these pairs included one strain from a natural synergistic pair. These experiments indicated that synergistic effects were frequent and were limited to pairs that were composed of strains belonging to different species. The genome content of virulence-associated genes failed to explain virulence synergy, although some virulence-associated genes that were present in some strains were absent from their companion strain (e.g., T3SS). The synergy observed in virulence when two Aeromonas isolates were co-infected stresses the idea that consideration should be given to the fact that infection does not depend only on single strain virulence but is instead the result of a more complex interaction between the microbes involved, the host and the environment. These results are of interest for other diseases in which mixed infections are likely and in particular for water-borne diseases (e.g., legionellosis, vibriosis), in which pathogens may display enhanced virulence in the presence of the right partner. This study contributes to the current shift in infectiology paradigms from a premise that assumes a monomicrobial origin for infection to one more in line with the current pathobiome era.

No MeSH data available.


Flowchart showing the sequential nematode killing assays (NKA). The median time to worm death (TD50) determinations were performed for single and paired strains according to the type of pair (naturally recovered or experimentally associated), and, for experimental pairs, according to the type of species pairing (same species or different species).
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Figure 1: Flowchart showing the sequential nematode killing assays (NKA). The median time to worm death (TD50) determinations were performed for single and paired strains according to the type of pair (naturally recovered or experimentally associated), and, for experimental pairs, according to the type of species pairing (same species or different species).

Mentions: In this work, a “natural” pair refers to two distinct Aeromonas strains that were recovered from one single clinical sample (Table 1). An “experimental” pair denotes strains of clinical origin that were not isolated from the same sample and that belonged either to the same or to different species. The design of this study is shown in Figure 1, as follows: (i) the TD50 was determined for each strain individually and for the naturally occurring pairs, as indicated by the clinical sample number (Table 1). (ii) For a natural pair that showed a synergistic effect (i.e., a worm exposure to pairs resulting in time to death and a TD50 that are significantly less than the time to death and TD50 for each individual strain), the additional TD50s were determined after varying the inoculum ratio of the two associated strains from 1:1 to 1:1000. (iii) TD50 values were also determined for experimental pairs. Experimental pairs were designed to establish whether the effect was pair-specific. These included one strain from one synergistic natural pair and either a strain from another synergistic naturally occurring pair or a weak worm killer strain that belonged to a different species or to the same species (Figure 1). All experiments using strain pairs were performed after ensuring that the inoculum ratio was 1:1 for killing assays, except for experiments that studied the effect of the inoculum ratio on synergy.


Exposure to pairs of Aeromonas strains enhances virulence in the Caenorhabditis elegans infection model
Flowchart showing the sequential nematode killing assays (NKA). The median time to worm death (TD50) determinations were performed for single and paired strains according to the type of pair (naturally recovered or experimentally associated), and, for experimental pairs, according to the type of species pairing (same species or different species).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4631986&req=5

Figure 1: Flowchart showing the sequential nematode killing assays (NKA). The median time to worm death (TD50) determinations were performed for single and paired strains according to the type of pair (naturally recovered or experimentally associated), and, for experimental pairs, according to the type of species pairing (same species or different species).
Mentions: In this work, a “natural” pair refers to two distinct Aeromonas strains that were recovered from one single clinical sample (Table 1). An “experimental” pair denotes strains of clinical origin that were not isolated from the same sample and that belonged either to the same or to different species. The design of this study is shown in Figure 1, as follows: (i) the TD50 was determined for each strain individually and for the naturally occurring pairs, as indicated by the clinical sample number (Table 1). (ii) For a natural pair that showed a synergistic effect (i.e., a worm exposure to pairs resulting in time to death and a TD50 that are significantly less than the time to death and TD50 for each individual strain), the additional TD50s were determined after varying the inoculum ratio of the two associated strains from 1:1 to 1:1000. (iii) TD50 values were also determined for experimental pairs. Experimental pairs were designed to establish whether the effect was pair-specific. These included one strain from one synergistic natural pair and either a strain from another synergistic naturally occurring pair or a weak worm killer strain that belonged to a different species or to the same species (Figure 1). All experiments using strain pairs were performed after ensuring that the inoculum ratio was 1:1 for killing assays, except for experiments that studied the effect of the inoculum ratio on synergy.

View Article: PubMed Central

ABSTRACT

Aeromonad virulence remains poorly understood, and is difficult to predict from strain characteristics. In addition, infections are often polymicrobial (i.e., are mixed infections), and 5–10% of such infections include two distinct aeromonads, which has an unknown impact on virulence. In this work, we studied the virulence of aeromonads recovered from human mixed infections. We tested them individually and in association with other strains with the aim of improving our understanding of aeromonosis. Twelve strains that were recovered in pairs from six mixed infections were tested in a virulence model of the worm Caenorhabditis elegans. Nine isolates were weak worm killers (median time to death, TD50, ≥7 days) when administered alone. Two pairs showed enhanced virulence, as indicated by a significantly shortened TD50 after co-infection vs. infection with a single strain. Enhanced virulence was also observed for five of the 14 additional experimental pairs, and each of these pairs included one strain from a natural synergistic pair. These experiments indicated that synergistic effects were frequent and were limited to pairs that were composed of strains belonging to different species. The genome content of virulence-associated genes failed to explain virulence synergy, although some virulence-associated genes that were present in some strains were absent from their companion strain (e.g., T3SS). The synergy observed in virulence when two Aeromonas isolates were co-infected stresses the idea that consideration should be given to the fact that infection does not depend only on single strain virulence but is instead the result of a more complex interaction between the microbes involved, the host and the environment. These results are of interest for other diseases in which mixed infections are likely and in particular for water-borne diseases (e.g., legionellosis, vibriosis), in which pathogens may display enhanced virulence in the presence of the right partner. This study contributes to the current shift in infectiology paradigms from a premise that assumes a monomicrobial origin for infection to one more in line with the current pathobiome era.

No MeSH data available.