Biology subjects

Kümmerli, R.

Publications and source records attributed to Kümmerli, R..

12 recordsLinked to original sources

bioRxiv2024

Strain identity effects contribute more to Pseudomonas community functioning than strain interactions

Microbial communities can shape key ecological services, but the determinants of their functioning often remain little understood. While traditional research predominantly focuses on effects related to species identity (community composition and species richness), recent work increasingly explores the impact of species interactions on community functioning. Here, we conducted experiments with replicated small communities of fluorescent Pseudomonas bacteria to quantify the relative importance of strain identity versus interaction effects on two important functions, community productivity and siderophore production. By combining supernatant and competition assays with an established linear model method, we show that both factors have significant effects on functioning, but identity effects generally outweigh strain interaction effects. These results hold irrespective of whether strains interactions are inferred statistically or approximated experimentally. Our results have implications for microbiome engineering, as the success of approaches aiming to induce beneficial (probiotic) strain interactions will be sensitive to strain identity effects in many communities.

ecology↗

bioRxiv2024

Interactions between Pseudomonas aeruginosa and six opportunistic pathogens cover a broad spectrum from mutualism to antagonism

Bacterial infections often involve more than one pathogen. While it is known that polymicrobial infections can impact disease outcomes, we have a poor understanding about how pathogens affect each others behaviour and fitness. Here, we used a microscopy approach to explore interactions between Pseudomonas aeruginosa and six opportunistic human pathogens that often co-occur in polymicrobial infections: Acinetobacter baumannii, Burkholderia cenocepacia, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, and Staphylococcus aureus. When following growing micro-colonies on agarose pads over time, we observed a broad spectrum of species-specific ecological interactions, ranging from mutualism to antagonism. For example, P. aeruginosa engaged in a mutually beneficial interaction with E. faecium but suffered from antagonism by E. coli and K. pneumoniae. While we found little evidence for active directional growth towards or away from cohabitants, we observed that certain species increased growth in double layers in co-cultures and that physical forces due to fast colony expansion had a major impact on fitness and interaction patterns. Overall, our work provides an atlas of pathogen interactions, potentially useful to understand species dynamics in polymicrobial infections. We discuss possible mechanisms driving pathogen interactions and offer predictions of how the different ecological interactions could affect virulence.

microbiology↗

bioRxiv2023

A green-fluorescent siderophore protects bacterial communities from UV damage

Sunlight enables virtually all life on earth, but also entails harmful ultraviolet (UV) radiation inducing DNA damage. In response to UV stress, natural selection has favored both curative and preventive measures such as DNA repair mechanisms and UV-absorbing pigments. While UV protection by pigments is well documented in plants, animals and fungi, little is known about their protective role in bacteria. Here, we combine batch-culture and microscopy experiments to show that the siderophore pyoverdine, a fluorescent pigment produced by the opportunistic pathogen Pseudomonas aeruginosa offers high-level protection against UV radiation. Our results reveal that bacteria up-regulate pyoverdine production following UV exposure, seemingly as part of a general stress response. We further found that pyoverdine cannot curatively alleviate UV-damage but protects cells preventively and collectively from oncoming UV exposures through its accumulation in the environment. Altogether, our results reveal a new and non-canonical function of this iron-scavenging molecule, demonstrating that pyoverdine acts as a public sunscreen protecting bacterial populations from UV damage. Given that many bacteria produce pigments, such protection might be widespread in species colonizing habitats exposed to UV radiation.

microbiology↗

bioRxiv2023

RNAseq reveals that Pseudomonas aeruginosa mounts medium-dependent competitive responses when sensing diffusible cues from Burkholderia cenocepacia

Most habitats host diverse bacterial communities, offering opportunities for inter-species interactions. While competition might often dominate such interactions, little is known about whether bacteria can sense competitors and mount adequate responses. The competition-sensing hypothesis proposes that bacteria can use cues such as nutrient stress and cell damage to prepare for battle. Here, we tested this hypothesis by measuring transcriptome changes in Pseudomonas aeruginosa exposed to the supernatant of its competitor Burkholderia cenocepacia. We found that P. aeruginosa exhibited significant and growth-medium-dependent transcriptome changes in response to competition. In iron-rich medium, P. aeruginosa up-regulated genes encoding the type-VI secretion system and the siderophore pyoverdine, whereas genes encoding phenazine toxins and hydrogen cyanide were upregulated under iron-limited conditions. Moreover, general stress response and quorum-sensing regulators were upregulated upon supernatant exposure. Altogether, our results reveal nuanced competitive responses of P. aeruginosa when confronted with B. cenocepacia supernatant, integrating both environmental and social cues.

microbiology↗

bioRxiv2022

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

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 pathogens 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 target the most virulent species to reduce host morbidity, a prediction we validated experimentally. ImportanceThere is increasing evidence that polymicrobial infections are common and that host morbidity is impacted by interactions between the co-infecting pathogens. In this study, we infected the larvae of the insect host Galleria mellonella with four opportunistic human pathogens as mono or multispecies bacterial infections in all possible combinations and followed host survival and pathogen interactions over time. We discovered that species followed the exact same rank order in terms of their virulence, their growth inside the host, and their competitive ability against co-infecting pathogens. As consequence of this consistent rank order, we found that the most virulent species determines host survival dynamics in mixed infections. Beyond revealing a generalizable predictive pattern for virulence outcomes, our findings also predict that treatments should target the most virulent species in polymicrobial infections, a prediction we validated experimentally.

microbiology↗

bioRxiv2022

Evolution of the quorum sensing systems in Pseudomonas aeruginosa can involve both loss of regulon function and network modulation

Pseudomonas aeruginosa populations evolving in cystic fibrosis (CF) lungs, animal infection models, natural environments or in vitro undergo extensive genetic adaption and diversification. A common mutational target is the quorum sensing (QS) regulon, a three-unit regulatory system that controls the expression of a suite of virulence factors and secreted public goods. Three scenarios have been advocated to explain selection for QS mutants, which include (I) disuse of the regulon, (II) cheating on public goods, or (III) modulation of the network. Here, we test these scenarios by examining a set of 61 QS mutants from an experimental evolution study. We observed non-synonymous mutations in all three QS systems - Las, Rhl and PQS. Most Las mutants carried large deletions, resulting in loss of QS function, and the inability to produce QS-regulated traits (scenario I or II). Conversely, phenotypic and gene expression analyses of Rhl mutants support network modulation (scenario III), as these mutants overexpressed the Las and Rhl regulators and showed an altered QS-regulated trait production portfolio. PQS mutants also showed patterns of network modulation (scenario III), spurring strain diversification and phenotypic trade-offs, where the upregulation of certain QS traits is associated with the downregulation of others. Overall, our results indicate that mutations in different QS systems lead to diverging effects on the social portfolio of bacterial populations. These mutations might not only affect the plasticity and diversity of evolved populations but could also impact bacterial fitness and virulence in infections. ImportancePseudomonas aeruginosa uses quorum sensing (QS), a three-unit multi-layered network, to coordinate expression of traits for growth and virulence in the context of infections. Despite its importance for bacterial fitness, the QS regulon appears to be a common mutational target during long-term adaptation of P. aeruginosa in the host, natural environments and experimental evolutions. This raises the questions why such an important regulatory system is under selection and how mutations change the portfolio of QS-regulated traits. Here, we examine a set of 61 naturally evolved mutants to address these questions. We found that mutations involving the master regulator, LasR, resulted in an almost complete breakdown of QS, while mutations in RhlR and PqsR resulted in modulations of the QS regulon, where both the QS regulon structure and the QS-regulated trait portfolio changed. Our work reveals that natural selection drives diversification in QS activity patterns in evolving populations.

microbiology↗

bioRxiv2021

Single-cell imaging reveals that Staphylococcus aureus is highly competitive against Pseudomonas aeruginosa on surfaces

Pseudomonas aeruginosa and Staphylococcus aureus frequently occur together in polymicrobial infections, and their interactions can complicate disease progression as well as treatment options. While interactions between P. aeruginosa and S. aureus have been extensively described using planktonic batch cultures, little is known about whether and how individual cells interact with each other on solid substrates. This is important, because in infections, both species frequently colonize surfaces to form microcolony aggregates and biofilms. Here, we performed single-cell time-lapse fluorescence microscopy combined with automated image analysis to describe interactions between P. aeruginosa PAO1 with three different S. aureus strains (Cowan I, 6850, JE2) during microcolony growth on agarose surfaces. While P. aeruginosa is usually considered the dominant species, we found that the competitive balance tips in favor of S. aureus on surfaces. We observed that all S. aureus strains accelerated the onset of microcolony growth in competition with P. aeruginosa and significantly compromised P. aeruginosa growth prior to physical contact. These results suggest that S. aureus deploys mechanisms of both resource competition and interference competition via diffusible compounds. JE2 was the most competitive S. aureus strain, simply usurping P. aeruginosa microcolonies when coming into direct contact, while 6850 was the weakest competitor itself suppressed by P. aeruginosa. Moreover, P. aeruginosa reacted to the assault of S. aureus by showing increased directional growth and expedited expression of quorum sensing regulators controlling the synthesis of competitive traits. Altogether, our results reveal that quantitative single-cell live imaging has the potential to uncover microbial behaviors that cannot be predicted from batch culture studies, and thereby contribute to our understanding of interactions between pathogens that co-colonize host-associated surfaces during polymicrobial infections.

microbiology↗

bioRxiv2020

Impact of biotic and abiotic factors on competitive interactions between Pseudomonas aeruginosa and Staphylococcus aureus

Bacterial communities in the environment and in infections are typically diverse, yet we know little about the factors that determine interspecies interactions. Here, we apply concepts from ecological theory to understand how biotic and abiotic factors affect interaction patterns between the two opportunistic human pathogens Pseudomonas aeruginosa and Staphyloccocus aureus, which often co-occur in polymicrobial infections. Specifically, we conducted a series of short- and long-term competition experiments between P. aeruginosa PAO1 (as our reference strain) and three different S. aureus strains (Cowan I, 6850, JE2) at three starting frequencies and under three environmental (culturing) conditions. We found that the competitive ability of P. aeruginosa strongly depended on the strain background of S. aureus, whereby P. aeruginosa dominated against Cowan I and 6850, but not against JE2. In the latter case, both species could end up as winners depending on conditions. Specifically, we observed strong frequency-dependent fitness patterns, including positive frequency dependence, where P. aeruginosa could dominate JE2 only when common, but not when rare. Finally, changes in environmental (culturing) conditions fundamentally altered the competitive balance between the two species, in a way that P. aeruginosa dominance increased when moving from shaken to static environments. Altogether, our results highlight that ecological details can have profound effects on the competitive dynamics between co-infecting pathogens, and determine whether two species can co-exist or invade each others populations from rare. Moreover, our findings might parallel certain dynamics observed in chronic polymicrobial infections. ImportanceBacterial infections are frequently caused by more than one species and such polymicrobial infections are often considered more virulent and more difficult to treat than the respective monospecies infections. Pseudomonas aeruginosa and Staphyloccocus aureus are among the most important pathogens in polymicrobial infections and their co-occurrence is linked to worse disease outcome. There is great interest in understanding how these two species interact with each other and what the consequences for the host are. While previous studies have mainly looked at molecular mechanisms implicated in interactions between P. aeruginosa and S. aureus, here we show that ecological factors such as strain background, species frequency and environmental conditions are important elements determining population dynamics and species co-existence patterns. We propose that the uncovered principles may also play major roles in infections, and therefore proclaim that an integrative approach combining molecular and ecological aspects is required to fully understand polymicrobial infections.

microbiology↗

bioRxiv2018

Pervasive positive linkage between bacterial social traits suggests the predominance of competitive interactions in natural Pseudomonas communities

Bacteria frequently cooperate by sharing secreted metabolites such as enzymes and siderophores. The expression of different public good traits can be interdependent, and studies on laboratory systems have shown that such trait linkage affects eco-evolutionary dynamics within bacterial communities. Here, we examine whether linkage among social traits occurs in natural Pseudomonas communities by examining investment levels and correlations between five public goods: biosurfactants, biofilm components, proteases, pyoverdines, and toxic compounds. Our phenotypic assays involving 315 isolates from soil and freshwater communities revealed that their social trait expression profiles varied dramatically, and that correlations between traits were frequent, exclusively positive, and sometimes habitat-specific. Our results indicate that Pseudomonas communities are dominated by isolates lying on a continuum between a social type producing multiple public goods, and an asocial type showing low investment into social traits. This segregation into different social types could reflect local adaptation to different microhabitats, or emerge from competition between different (social) strategies. Moreover, our results show that isolates with specialized trait repertoires are rare, suggesting limited scope for the mutual exchange of different public goods between isolates. Overall, our work indicates that complex interdependencies among social traits influence the evolution of microbial lifestyles in nature.

microbiology↗

bioRxiv2018

Virulence evolution in the opportunistic bacterial pathogen Pseudomonas aeruginosa

Bacterial opportunistic pathogens are feared for their difficult-to-treat nosocomial infections and for causing morbidity in immunocompromised patients. Here, we study how such a versatile opportunist, Pseudomonas aeruginosa, adapts to conditions inside and outside its model host Caenorhabditis elegans, and use phenotypic and genotypic screens to identify the mechanistic basis of virulence evolution. We found that virulence significantly dropped in unstructured environments both in the presence and absence of the host, but remained unchanged in spatially structured environments. Reduction of virulence was either driven by a substantial decline in the production of siderophores (in treatments without hosts) or toxins and proteases (in treatments with hosts). Whole-genome sequencing of evolved clones revealed positive selection and parallel evolution across replicates, and showed an accumulation of mutations in regulator genes controlling virulence factor expression. Our study identifies the spatial structure of the non-host environment as a key driver of virulence evolution in an opportunistic pathogen.

microbiology↗

bioRxiv2017

The path to re-evolve cooperation is constrained in Pseudomonas aeruginosa

BackgroundA common form of cooperation in bacteria is based on the secretion of beneficial metabolites, shareable as public good among cells at the group level. Because cooperation can be exploited by \"cheat\" mutants, which contribute less or nothing to the public good, there has been great interest in understanding the conditions required for cooperation to remain evolutionarily stable. In contrast, much less is known about whether cheats, once fixed in the population, are able to revert back to cooperation when conditions change. Here, we tackle this question by subjecting experimentally evolved cheats of Pseudomonas aeruginosa, partly deficient for the production of the iron-scavenging public good pyoverdine, to conditions previously shown to favor cooperation.\n\nResultsFollowing approximately 200 generations of experimental evolution, we screened 720 evolved clones for changes in their pyoverdine production levels. We found no evidence for the re-evolution of full cooperation, even in environments with increased spatial structure, and reduced costs of cooperation - two conditions that have previously been shown to maintain cooperation. In contrast, we observed selection for complete abolishment of pyoverdine production. The patterns of complete trait degradation were likely driven by \"cheating on cheats\" in unstructured, iron-limited environments where pyoverdine is important for growth, and selection against a maladaptive trait in iron-rich environments where pyoverdine is superfluous.\n\nConclusionsOur study shows that the path to re-evolve cooperation seems constrained. One reason might be that the number of mutational targets potentially leading to reversion is limited. Alternatively, it could be that the selective conditions required for revertants to spread from rare are much more stringent than those needed to maintain cooperation.

evolutionary biology↗

bioRxiv2016

Individual- versus group-optimality in the production of secreted bacterial compounds

How unicellular organisms optimize the production of compounds is a fundamental biological question. While it is typically thought that production is optimized at the individual-cell level, secreted compounds could also allow for optimization at the group level, leading to a division of labor where a subset of cells produces and shares the compound with everyone. Using mathematical modelling, we show that the evolution of such division of labor depends on the cost function of compound production. Specifically, for any trait with saturating benefits, linear costs promote the evolution of uniform production levels across cells. Conversely, production costs that diminish with higher output levels favor the evolution of specialization - especially when compound shareability is high. When experimentally testing these predictions with pyoverdine, a secreted iron-scavenging compound produced by Pseudomonas aeruginosa, we found linear costs and, consistent with our model, detected uniform pyoverdine production levels across cells. We conclude that for shared compounds with saturating benefits, the evolution of division of labor is facilitated by a diminishing cost function. More generally, we note that shifts in the level of selection from individuals to groups do not solely require cooperation, but critically depend on mechanistic factors, including the distribution of compound synthesis costs.

evolutionary biology↗