Tuning Molecular Motion Enhances Intrinsic Fluorescence in Peptide Amphiphile Nanofibers.

Peptide amphiphiles (PAs) are highly tunable molecules that were recently found to exhibit aggregation-induced emission (AIE) when they self-assemble into nanofibers. Here, we leverage decades of molecular design and self-assembly study of PAs to strategically tune their molecular motion within nanofibers to enhance AIE, making them a highly useful platform for applications such as sensing, bioimaging, or materials property characterization. Since AIE increases when aggregated molecules are rigidly and closely packed, we altered the four most closely packed amino acids nearest to the hydrophobic core by varying the order and composition of glycine, alanine, and valine pairs. Of the six PA designs studied, C16VVAAK2 had the highest quantum yield at 0.17, which is a more than 10-fold increase from other PA designs including the very similar C16AAVVK2, highlighting the importance of precise amino acid placement to anchor rigidity closest to the core. We also altered temperature to increase AIE. C16VVAAK2 exhibited an additional 4-fold increase in maximum fluorescence intensity when the temperature was raised from 5 to 65 °C. As the temperature increased, the secondary structure transitioned from ß-sheet to random coil, indicating that further packing an already aligned molecular system makes it even more readily able to transfer energy between the electron-rich amides. This work both unveils a highly fluorescent AIE PA system design and sheds insights into the molecular orientation and packing design traits that can significantly enhance AIE in self-assembling systems.

CCR5 deficiency normalizes TIMP levels, working memory, and gamma oscillation power in APOE4 targeted replacement mice.

The APOE4 allele increases the risk for Alzheimer's disease (AD) in a dose-dependent manner and is also associated with cognitive decline in non-demented elderly controls. In mice with targeted gene replacement (TR) of murine APOE with human APOE3 or APOE4, the latter show reduced neuronal dendritic complexity and impaired learning. APOE4 TR mice also show reduced gamma oscillation power, a neuronal population activity which is important to learning and memory. Published work has shown that brain extracellular matrix (ECM) can reduce neuroplasticity as well as gamma power, while attenuation of ECM can instead enhance this endpoint. In the present study we examine human cerebrospinal fluid (CSF) samples from APOE3 and APOE4 individuals and brain lysates from APOE3 and APOE4 TR mice for levels of ECM effectors that can increase matrix deposition and restrict neuroplasticity. We find that CCL5, a molecule linked to ECM deposition in liver and kidney, is increased in CSF samples from APOE4 individuals. Levels of tissue inhibitor of metalloproteinases (TIMPs), which inhibit the activity of ECM-degrading enzymes, are also increased in APOE4 CSF as well as astrocyte supernatants brain lysates from APOE4 TR mice. Importantly, as compared to APOE4/wild-type heterozygotes, APOE4/CCR5 knockout heterozygotes show reduced TIMP levels and enhanced EEG gamma power. The latter also show improved learning and memory, suggesting that the CCR5/CCL5 axis could represent a therapeutic target for APOE4 individuals.

Endoprosthetic reconstruction for lower extremity soft tissue sarcomas with bone involvement.

BACKGROUND AND OBJECTIVES: Bone resection and endoprosthetic reconstruction (EPR) in the setting of soft tissue sarcoma (STS) management is rare and incurs unique challenges. We aim to report on the surgical and oncological outcomes of this relatively previously undocumented cohort. METHODS: This is a single-center retrospective review of prospectively collected data for patients who required EPRs following resection of STSs of the lower extremity. Following inclusion criteria, we assessed 29 cases of EPR for primary STS of the lower limb. RESULTS: The mean age was 54 years (range 18-84). Of the 29 patients, there were 6 total femur, 11 proximal femur, 4 intercalary, and 8 distal femur EPRs. Fourteen of 29 patients (48%) underwent re-operations for surgical complications, with 9 relating to infection (31%). When a matched cohort analysis was performed comparing our cohort to STSs that did not necessitate EPR, a reduced rate of overall survival and metastasis-free survival was found in those requiring EPR. CONCLUSION: This series identifies a high rate of complication from EPRs performed for STS. Patients should be cautioned about the high rate of infection, surgical complications, and lower overall survival in this setting.

A national qualitative investigation of the impact of service change on doctors' training during Covid-19.

BACKGROUND: The Covid-19 crisis sparked service reconfigurations in healthcare systems worldwide. With postgraduate medical education sitting within these systems, service reconfigurations substantially impact trainees and their training environment. This study aims to provide an in-depth qualitative understanding of the impact of service reconfiguration on doctors' training during the pandemic, identifying opportunities for the future as well as factors that pose risks to education and training and how these might be mitigated. METHODS: Qualitative parallel multi-centre case studies examined three Trusts/Health Boards in two countries in the United Kingdom. Data were collected from online focus groups and interviews with trainees and supervisors using semi-structured interview guides (September to December 2020). A socio-cultural model of workplace learning, the expansive-restrictive continuum, informed data gathering, analysis of focus groups and coding. RESULTS: Sixty-six doctors participated, representing 25 specialties/subspecialties. Thirty-four participants were male, 26 were supervisors, 17 were specialty trainees and 23 were foundation doctors. Four themes described the impact of pandemic-related service reconfigurations on training: (1) Development of skills and job design, (2) Supervision and assessments, (3) Teamwork and communication, and (4) Workload and wellbeing. Service changes were found to both facilitate and hinder education and training, varying across sites, specialties, and trainees' grades. Trainees' jobs were redesigned extensively, and many trainees were redeployed to specialties requiring extra workforce during the pandemic. CONCLUSIONS: The rapid and unplanned service reconfigurations during the pandemic caused unique challenges and opportunities to doctors' training. This impaired trainees' development in their specialty of interest, but also presented new opportunities such as cross-boundary working and networking.

Management of meningeal solitary fibrous tumors/hemangiopericytoma; surgery alone or surgery plus postoperative radiotherapy?

INTRODUCTION: A meningeal solitary fibrous tumor (SFT), also called hemangiopericytoma, is a rare mesenchymal malignancy. Due to anatomic constrains, even after macroscopic complete surgery with curative intent, the local relapse risk is still relatively high, thus increasing the risk of dedifferentiation and metastatic spread. This study aims to better define the role of postoperative radiotherapy (RT) in meningeal SFTs. PATIENTS AND METHODS: A retrospective study was performed across seven sarcoma centers. Clinical information was retrieved from all adult patients with meningeal primary localized SFT treated between 1990 and 2018 with surgery alone (S) compared to those that also received postoperative RT (S + RT). Differences in treatment characteristics between subgroups were tested using independent samples t-test for continuous variables and chi-square tests for proportions. Local control (LC) and overall survival (OS) rates were calculated as time from start of treatment until progression or death from any cause. LC and OS in groups receiving S or S + RT were compared using Kaplan-Meier survival curves. RESULTS: Among a total of 48 patients, 7 (15%) underwent S and 41 (85%) underwent S + RT. Median FU was 65 months. LC was significantly associated with treatment. LC after S at 60 months was 60% versus 90% after S + RT (p = 0.052). Furthermore, R1 resection status was significantly associated with worse LC (HR 4.08, p = 0.038). OS was predominantly associated with the mitotic count (HR 3.10, p = 0.011). CONCLUSION: This retrospective study, investigating postoperative RT in primary localized meningeal SFT patients, suggests that combining RT to surgery in the management of this patient population may reduce the risk for local failures.

Combining Lower Extremity Radiographic Markers Begets More Accurate Predictions of Remaining Skeletal Growth.

BACKGROUND: Radiographic markers of skeletal maturity are vital to the prediction and interpretation of skeletal growth patterns. Accurate predictions of skeletal maturity factor into the management of common musculoskeletal disorders. Bone age is conventionally measured using hand and wrist radiographs. The primary study objective was to optimize skeletal maturity estimates based on the morphology of markers at the hip, knee, and foot rather than conventional upper extremity radiographs. METHODS: This was a retrospective analysis of children from the Bolton-Brush collection with anteroposterior radiographs of the hip and anteroposterior and lateral radiographs of the knee and foot, and heights recorded at the time of each radiograph. The percent growth achieved (%GA) was calculated as a function of final patient height. Poor quality radiographs were excluded, leaving 50 patients-32 females and 18 males-and 1068 radiographs for analysis. Skeletal maturity was evaluated using the Oxford bone, O'Connor knee, and calcaneal apophyseal scores. Interrater and intrarater reliability analyses were performed for hip and knee scores. Multiple linear regressions were conducted on these scores and chronologic age as predictors of %GA. Mean differences were calculated between actual and estimated %GAs. All analyses were performed in Prism 8.0. RESULTS: Each lower extremity skeletal maturity score served as statistically significant, independent predictors of %GA, the accuracy and strength of which increased with the addition of chronologic age. The integration of all 3 systems and chronologic age yielded the most predictive, accurate model predictive of %GA, which can be used to determine percent growth remaining. However, this fully integrated system was not statistically superior to the combination of knee and foot scores and knee score and chronologic age, which yield similarly accurate %GA predictions. The hip and knee systems demonstrated good to excellent interrater and intrarater reliability. CONCLUSIONS: Integration of bone age scores across different regions of the lower extremity has the potential to facilitate orthopaedic decision-making using radiographs already obtained in the treatment of pediatric musculoskeletal conditions. LEVEL OF EVIDENCE: Level IV.

Developing a neurally informed ontology of creativity measurement.

A central challenge for creativity research-as for all areas of experimental psychology and cognitive neuroscience-is to establish a mapping between constructs and measures (i.e., identifying a set of tasks that best captures a set of creative abilities). A related challenge is to achieve greater consistency in the measures used by different researchers; inconsistent measurement hinders progress toward shared understanding of cognitive and neural components of creativity. New resources for aggregating neuroimaging data, and the emergence of methods for identifying structure in multivariate data, present the potential for new approaches to address these challenges. Identifying meta-analytic structure (i.e., similarity) in neural activity associated with creativity tasks might help identify subsets of these tasks that best reflect the similarity structure of creativity-relevant constructs. Here, we demonstrated initial proof-of-concept for such an approach. To build a model of similarity between creativity-relevant constructs, we first surveyed creativity researchers. Next, we used NeuroSynth meta-analytic software to generate maps of neural activity robustly associated with tasks intended to measure the same set of creativity-relevant constructs. A representational similarity analysis-based approach identified particular constructs-and particular tasks intended to measure those constructs-that positively or negatively impacted the model fit. This approach points the way to identifying optimal sets of tasks to capture elements of creativity (i.e., dimensions of similarity space among creativity constructs), and has long-term potential to meaningfully advance the ontological development of creativity research with the rapid growth of creativity neuroscience. Because it relies on neuroimaging meta-analysis, this approach has more immediate potential to inform longer-established fields for which more extensive sets of neuroimaging data are already available.

Localization of keyhole limpet hemocyanin-like immunoreactivity in the nervous system of Biomphalaria alexandrina.

Recent years have led to increased effort to describe and understand the peripheral nervous system and its influence on central mechanisms and behavior in gastropod molluscs. This study revealed that an antibody raised against keyhole limpet hemocyanin (KLH) cross-reacts with an antigen(s) found extensively in both the central and the peripheral nervous systems of Biomphalaria alexandrina. The results revealed KLH-like immunoreactive (LIR) neurons in the cerebral, pedal, buccal, left pleural, right parietal, and visceral ganglion within the CNS with fibers projecting throughout all the peripheral nerves. Numerous KLH-LIR peripheral sensory neurons located in the foot, lips, tentacles, mantle, esophagus, and penis exhibited a bipolar morphology with long tortuous dendrites. KLH-LIR cells were also present in the eye and statocyst, thus suggesting the labeling of multiple sensory modalities/cell types. KLH-LIR cells did not co-localize with tyrosine hydroxylase (TH)-LIR cells, which have previously been described in this and other gastropods. The results thus provide descriptions of thousands of peripheral sensory neurons, not previously described in detail. Future research should seek to pair sensory modalities with peripheral cell type and attempt to further elucidate the nature of KLH-like reactivity. These findings also emphasize the need for caution when analyzing results obtained through use of antibodies raised against haptens conjugated to carrier proteins, suggesting the need for stringent controls to help limit potential confounds caused by cross-reactivity. In addition, this study is the first to describe neuronal cross-reactivity with KLH in Biomphalaria, which could provide a substrate for host-parasite interactions with a parasitic trematode, Schistosoma.

Management of organ transplant recipients attending a high-throughput skin cancer surgery and surveillance clinic in Queensland.

BACKGROUND: The incidence of skin cancer in organ transplant recipients (OTRs) is very high, due mainly to long-term immunosuppressive therapy. The problem is particularly severe for OTRs living in Queensland, Australia, and results in significant mortality. OBJECTIVES: To describe the experience of the first dedicated outpatient high-throughput transplant skin clinic in Queensland. METHODS: This prospective evaluation study was conducted at a newly established, outpatient transplant skin cancer surgery and surveillance clinic. Participants (89 OTRs and 12 non-OTRs) were referred to the Princess Alexandra Hospital Transplant Skin Clinic during December 2016 to May 2017, and were each followed for 3 months. Self-completed questionnaires were administered at baseline and the end of follow-up (n = 94), and details of any skin cancers occurring in that period were extracted from hospital records. RESULTS: In the 3-month follow-up of 101 participants, a total of 615 skin lesions were detected in the 3-month follow-up of 101 participants, of which 478 (78%) were treated in the clinic and 55 (9%) were referred to another specialist. Of the 478 treated lesions, 268 were histopathologically confirmed skin cancers, equivalent to 2·7 (95% confidence interval 2·5-2·8) skin cancers per participant per 3 months. The overall number needed to treat for any skin cancer was 1·4 (95% confidence interval 1·3-1·5). Three-quarters (374) of in-clinic treatments were surgical, and most (90%) were complete excisions. The median time from detection of skin cancer to excision was 7 days. CONCLUSIONS: This high-volume surgical outpatient transplant skin clinic enables efficient treatment of skin cancers in very-high-risk OTRs.

Life cycle expression of inbreeding depression in Eucalyptus regnans and inter-generational stability of its mixed mating system.

BACKGROUND AND AIMS: Many plants exhibit a mixed mating system. Published models suggest that this might be an evolutionarily stable rather than a transitional state despite the presence of inbreeding depression, but there is little empirical evidence. Through field experimentation, we studied the role of inbreeding depression in eliminating inbred progeny from the reproductive cohort of the forest tree Eucalyptus regnans, and demonstrate a stable mixed primary mating system over two successive generations. METHODS: Two field experiments were conducted using seed from natural populations. We sowed open-pollinated seeds to simulate a natural regeneration event and determined isozyme genotypes of dominant and suppressed individuals over 10 years. We also planted a mixture of open-pollinated, outcross and selfed families with common maternal parentage; monitored survival of cross types over 29 years; and determined the percentage of outcrosses in open-pollinated seed from a sample of reproductively mature trees using microsatellite analysis. KEY RESULTS: Both experiments demonstrated progressive competitive elimination of inbred plants. By 29 years, the reproductive cohort in the planted experiment consisted only of outcrosses which produced seed which averaged 66 % outcrosses, similar to the estimate for the parental natural population (74 %). CONCLUSIONS: Selective elimination of inbred genotypes during the intense intra-specific competition characteristic of the pre-reproductive phase of the life cycle of E. regnans results in a fully outcrossed reproductive population, in which self-fertility is comparable with that of its parental generation. The mixed mating system may be viewed as an unavoidable consequence of the species' reproductive ecology, which includes the demonstrated effects of inbreeding depression, rather than a strategy which is actively favoured by natural selection.

Effects of a Surgical Receipt Program on the Supply Costs of Five General Surgery Procedures.

BACKGROUND: Surgical supplies occupy a large portion of health care expenditures but is often under the surgeon's control. We sought to assess whether an automated, surgeon-directed, cost feedback system can decrease supply expenditures for five common general surgery procedures. MATERIALS AND METHODS: An automated "surgical receipt" detailing intraoperative supply costs was generated and emailed to surgeons after each case. We compared the median cost per case for 18 mo before and after implementation of the surgical receipt. We controlled for price fluctuations by applying common per-unit prices in both periods. We also compared the incision time, case length booking accuracy, length of stay, and postoperative occurrences. RESULTS: Median costs decreased significantly for open inguinal hernia ($433.45 to $385.49, P < 0.001), laparoscopic cholecystectomy ($886.77 to $816.13, P = 0.002), and thyroidectomy ($861.21 to $825.90, P = 0.034). Median costs were unchanged for laparoscopic appendectomy and increased significantly for lumpectomy ($325.67 to $420.53, P < 0.001). There was an increase in incision-to-closure minutes for open inguinal hernia (71 to 75 min, P < 0.001) and laparoscopic cholecystectomy (75 to 96 min, P < 0.001), but a decrease in thyroidectomy (79 to 73 min, P < 0.001). There was an increase in booking accuracy for laparoscopic appendectomy (38.6% to 55.0%, P = 0.001) and thyroidectomy (32.5% to 48.1%, P = 0.001). There were no differences in postoperative occurrence rates and length of stay duration. CONCLUSIONS: An automated surgeon-directed surgical receipt may be a useful tool to decrease supply costs for certain procedures. However, curtailing surgical supply costs with surgeon-directed cost feedback alone is challenging and a multimodal approach may be necessary.

Effect of Biochar on Microbial Growth: A Metabolomics and Bacteriological Investigation in E. coli.

Biochar has been proposed as a soil amendment in agricultural applications due to its advantageous adsorptive properties, high porosity, and low cost. These properties allow biochar to retain soil nutrients, yet the effects of biochar on bacterial growth remain poorly understood. To examine how biochar influences microbial metabolism, Escherichia coli was grown in a complex, well-defined media and treated with either biochar or activated carbon. The concentration of metabolites in the media were then quantified at several time points using NMR spectroscopy. Several metabolites were immediately adsorbed by the char, including l-asparagine, l-glutamine, and l-arginine. However, we find that biochar quantitatively adsorbs less of these metabolic precursors when compared to activated carbon. Electron microscopy reveals differences in surface morphology after cell culture, suggesting that Escherichia coli can form biofilms on the surfaces of the biochar. An examination of significant compounds in the tricarboxylic acid cycle and glycolysis reveals that treatment with biochar is less disruptive than activated carbon throughout metabolism. While both biochar and activated carbon slowed growth compared to untreated media, Escherichia coli in biochar-treated media grew more efficiently, as indicated by a longer logarithmic growth phase and a higher final cell density. This work suggests that biochar can serve as a beneficial soil amendment while minimizing the impact on bacterial viability. In addition, the experiments identify a mechanism for biochar's effectiveness in soil conditioning and reveal how biochar can alter specific bacterial metabolic pathways.

An immunohistochemical analysis of peptidergic neurons apparently associated with reproduction and growth in Biomphalaria alexandrina.

Peptide hormones and neurotransmitters involved in reproduction and growth have been studied extensively in certain gastropod molluscs, such as Lymnaea stagnalis and Aplysia californica. The present study employs antisera that have been used to study peptidergic neurons in those species to probe the central nervous system of another gastropod, Biomphalaria alexandrina, an intermediate host of the parasitic trematode that causes schistosomiasis in humans. Whole mount preparations of central ganglia were stained immunohistochemically, and several populations of neurons appeared to be homologous to those forming the neuroendocrine axis that has been previously described in L. stagnalis. These cells include the caudodorsal cells and the light green and canopy cells, which produce hormones that regulate ovulation and growth, respectively. Other populations of cells containing APGWamide, FMRFamide and/or related peptides are consistent with ones that innervate the penis in L. stagnalis and other gastropods. Identification of neurons that might be responsible for the control of reproduction and growth in Biomphalaria provides an important initial step toward the development of novel methods of disease control and pest management directed toward reducing snail populations.

The effects of a multidisciplinary high-throughput skin clinic on healthcare costs of organ transplant recipients.

BACKGROUND: A long-term complication among organ transplant recipients (OTRs) is skin malignancies which are associated with level and duration of immunosuppressive treatment, sun exposure and age. Dermatological surveillance is recommended for OTRs at high risk of skin malignancies, but evidence is lacking on the benefits of such services. OBJECTIVE: To examine the economic impact on patients and on the hospital service of a multidisciplinary high-throughput skin cancer clinic in Brisbane, Australia, dedicated to dermatological and surgical care of high-risk OTRs. METHODS: In a pre/postdesign, hospital admission and cost data were obtained for 101 consecutively enrolled study participants from 12 months prior to the introduction of the clinic (to February 2016), the 3-month 'run-in' period (March to May 2016) and 12 months subsequent (to June 2017). Differences between pre- and post-clinic hospital costs were tested using non-parametric bootstrapping and interrupted time series analysis. A survey of patient out-of-pocket costs and perceived financial burden was also undertaken during the clinic. RESULTS: Overall hospital costs were higher after the clinic but 3-monthly hospital costs for skin procedures trended downwards. Despite 3-monthly mean, hospital visits increasing from 85 to 314, mean 3-monthly costs reduced by AU$1491 (P < 0.001) indicating greater cost efficiency. Total patient out-of-pocket costs were AU$18 377 over 3 months. CONCLUSION: Clinical costing data revealed higher, more rapid throughput and significantly lower per patient costs pre- and postestablishment of a multidisciplinary skin cancer clinic for OTRs.

Bacteriocins and the assembly of natural Pseudomonas fluorescens populations.

When competing for space and resources, bacteria produce toxins known as bacteriocins to gain an advantage over competitors. Recent studies in the laboratory have confirmed theoretical predictions that bacteriocin production can determine coexistence, by eradicating sensitive competitors or driving the emergence of resistant genotypes. However, there is currently limited evidence that bacteriocin-mediated competition influences the coexistence and distribution of genotypes in natural environments, and what factors drive interactions towards inhibition remain unclear. Using natural soil populations of Pseudomonas fluorescens, we assessed the ability of the isolates to inhibit one another with respect to spatial proximity in the field, genetic similarity and niche overlap. The majority of isolates were found to produce bacteriocins; however, widespread resistance between coexisting isolates meant relatively few interactions resulted in inhibition. When inhibition did occur, it occurred more frequently between ecologically similar isolates. However, in contrast with results from other natural populations, we found no relationship between the frequency of inhibition and the genetic similarity of competitors. Our results suggest that bacteriocin production plays an important role in mediating competition over resources in natural settings and, by selecting for isolates resistant to local bacteriocin production, can influence the assembly of natural populations of P. fluorescens.

Light scattering and random lasing in aqueous suspensions of hexagonal boron nitride nanoflakes.

Liquid phase exfoliation allows large scale production of 2D materials in solution. The particles are highly anisotropic and strongly scatter light. While spherical particles can be accurately and precisely described by a single parameter-the radius, 2D nanoflakes, however, cannot be so easily described. We investigate light scattering in aqueous solutions of 2D hexagonal boron nitride nanoflakes in the single and multiple scattering regimes. In the single scattering regime, the anisotropic 2D materials show a much stronger depolarization of light when compared to spherical particles of similar size. In the multiple scattering regime, the scattering as a function of optical path for hexagonal boron nitride nanoflakes of a given lateral length was found to be qualitatively equivalent to scattering from spheres with the same diameter. We also report the presence of random lasing in high concentration suspensions of aqueous h-BN mixed with Rhodamine B dye. The h-BN works as a scattering agent and Rhodamine B as a gain medium for the process. We observed random lasing at 587 nm with a threshold energy of 0.8 mJ.

Pyoverdin cheats fail to invade bacterial populations in stationary phase.

Microbes engage in cooperative behaviours by producing and secreting public goods, the benefits of which are shared among cells, and are therefore susceptible to exploitation by nonproducing cheats. In nature, bacteria are not typically colonizing sterile, rich environments in contrast to laboratory experiments, which involve inoculating sterile culture with few bacterial cells that then race to fill the available niche. Here, we study the potential implications of this difference, using the production of pyoverdin, an iron-scavenging siderophore that acts as a public good in the bacteria Pseudomonas aeruginosa. We show that (1) nonproducers are able to invade cultures of producers when added at the start of growth or during early exponential growth phase, but not during late exponential or stationary phase; (2) the producer strain does not produce pyoverdin in the late exponential and stationary phases and so is not paying the cost of cooperating during those phases. These results suggest that whether a nonproducing mutant can invade will depend upon when the mutation arises, as well as the population structure, and raise a potential difficulty with the use of antimicrobial treatment strategies that propose to exploit the invasive abilities of cheats.

Co-evolutionary dynamics between public good producers and cheats in the bacterium Pseudomonas aeruginosa.

The production of beneficial public goods is common in the microbial world, and so is cheating--the exploitation of public goods by nonproducing mutants. Here, we examine co-evolutionary dynamics between cooperators and cheats and ask whether cooperators can evolve strategies to reduce the burden of exploitation, and whether cheats in turn can improve their exploitation abilities. We evolved cooperators of the bacterium Pseudomonas aeruginosa, producing the shareable iron-scavenging siderophore pyoverdine, together with cheats, defective in pyoverdine production but proficient in uptake. We found that cooperators managed to co-exist with cheats in 56% of all replicates over approximately 150 generations of experimental evolution. Growth and competition assays revealed that co-existence was fostered by a combination of general adaptions to the media and specific adaptions to the co-evolving opponent. Phenotypic screening and whole-genome resequencing of evolved clones confirmed this pattern, and suggest that cooperators became less exploitable by cheats because they significantly reduced their pyoverdine investment. Cheats, meanwhile, improved exploitation efficiency through mutations blocking the costly pyoverdine-signalling pathway. Moreover, cooperators and cheats evolved reduced motility, a pattern that likely represents adaptation to laboratory conditions, but at the same time also affects social interactions by reducing strain mixing and pyoverdine sharing. Overall, we observed parallel evolution, where co-existence of cooperators and cheats was enabled by a combination of adaptations to the abiotic and social environment and their interactions.

An experimental test of whether cheating is context dependent.

Microbial cells rely on cooperative behaviours that can breakdown as a result of exploitation by cheats. Recent work on cheating in microbes, however, has produced examples of populations benefiting from the presence of cheats and/or cooperative behaviours being maintained despite the presence of cheats. These observations have been presented as evidence for selection favouring cheating at the population level. This apparent contradiction arises when cheating is defined simply by the reduced expression of a cooperative trait and not in terms of the social costs and benefits of the trait under investigation. Here, we use two social traits, quorum sensing and iron-scavenging siderophore production in Pseudomonas aeruginosa, to illustrate the importance of defining cheating by the social costs and benefits. We show that whether a strain is a cheat depends on the costs and benefits associated with the social and abiotic environment and not the absolute expression of a cooperative trait.