Negative interactions and virulence differences drive the dynamics in multispecies bacterial infections

Abstract

Bacterial infections are often polymicrobial, leading to intricate pathogen–pathogen and pathogen–host interactions. There is increasing interest in studying the molecular basis of pathogen interactions and how such mechanisms impact host morbidity. However, much less is known about the ecological dynamics between pathogens and how they affect virulence and host survival. Here we address these open issues by co-infecting larvae of the insect model host Galleria mellonella with one, two, three or four bacterial species, all of which are opportunistic human pathogens. We found that host mortality was always determined by the most virulent species regardless of the number of species and pathogen combinations injected. In certain combinations, the more virulent pathogen simply outgrew the less virulent pathogen. In other combinations, we found evidence for negative interactions between pathogens inside the host, whereby the more virulent pathogen typically won a competition. Taken together, our findings reveal positive associations between a pathogen's growth inside the host, its competitiveness towards other pathogens and its virulence. Beyond being generalizable across species combinations, our findings predict that treatments against polymicrobial infections should first target the most virulent species to reduce host morbidity, a prediction we validated experimentally.

Predicting bacterial interaction outcomes from monoculture growth and supernatant assays

“…A line demarks the transition from neutral/positive to negative effects on the other species. (B) Species rank order from in vivo competition experiments in the larvae of G. mellonella 12 hours post-infection from a previous study (22). The latter includes all raw data, while Fig.…”

“…In our study, we combine experiments and computer simulations on a 4-species bacterial consortium to tackle some of these challenges. Our consortium consists of four opportunistic human pathogens ( Burkholderia cenocepacia [B], Cronobacter sakazakii [C], Klebsiella michiganensis [K], Pseudomonas aeruginosa [P]), and has been established as a model community in one of our earlier studies (22). With this model consortium, we pursue four main aims.…”

“…For all experiments, we used the following four bacterial species: Pseudomonas aeruginosa PAO1 [P] (28), Burkholderia cenocepacia K56-2 [B] (29), Klebsiella michiganensis [K], and Cronobacter sakazakii (ATCC29004) [C]. We had established this consortium of four gram-negative bacterial species in an earlier study (22), in which we were interested in interactions between pathogens with different virulence levels in the larvae of the greater wax moth G. mellonella .…”

Interactions betweenPseudomonas aeruginosaand six opportunistic pathogens cover a broad spectrum from mutualism to antagonism

“…aeruginosa also suppressed B. cenocepacia in the host, there was no overall change in virulence in coinfections with the two species (Schmitz et al, 2023). In another study, zebrafish larvae were infected with P. aeruginosa and either A. baumannii or K. pneumoniae (Schmitz et al, 2024).…”

Pathogen Growth and Virulence Dynamics Drive the Host Evolution Against Coinfections

“…For instance, pathogens with divergent antigenic properties, which a single immune strategy cannot control, might lead the host to activate multiple immune components simultaneously during coinfection, increasing the energetic burden ( 13 ) and immunopathological risk ( 14 , 15 ). Moreover, many naturally occurring coinfection can also involve pathogens that vary widely in their growth and virulence dynamics ( 16 ). In such cases, the host might evolve temporally separated immune strategies depending on how and at what rate different pathogens multiply inside the host and manifest their virulence ( 17 ).…”