Thomas G. Platt

Microbial Population and Community Dynamics on Plant Roots and Their Feedbacks on Plant Communities

The composition of the soil microbial community can be altered dramatically due to association with individual plant species, and these effects on the microbial community can have important feedbacks on plant ecology. Negative plant-soil feedback plays primary roles in maintaining plant community diversity, whereas positive plant-soil feedback may cause community conversion. Host-specific differentiation of the microbial community results from the trade-offs associated with overcoming plant defense and the specific benefits associated with plant rewards. Accumulation of host-specific pathogens likely generates negative feedback on the plant, while changes in the density of microbial mutualists likely generate positive feedback. However, the competitive dynamics among microbes depends on the multidimensional costs of virulence and mutualism, the fine-scale spatial structure within plant roots, and active plant allocation and localized defense. Because of this, incorporating a full view of microbial dynamics is essential to explaining the dynamics of plant-soil feedbacks and therefore plant community ecology.

Social evolution: Kin preference in a social microbe

Kin recognition helps cooperation to evolve in many animals, but it is uncertain whether microorganisms can also use it to focus altruistic behaviour on relatives. Here we show that the social amoeba Dictyostelium purpureum prefers to form groups with its own kin in situations where some individuals die to assist others. By directing altruism towards kin, D. purpureum should generally avoid the costs of chimaerism experienced by the related D. discoideum.

What's in a name? The semantics of quorum sensing

The expression of many bacterial phenotypes is regulated according to the concentration of chemical cues that they or other bacteria produce, a process often termed quorum sensing (QS). Many aspects of the environment can affect cue concentration. Thus these molecules might be indirect proxies for any one or combination of environmental factors. Recent research suggests that the adaptive significance of QS varies depending on its evolutionary and ecological context. Consequently, some researchers have proposed new terms, each emphasizing different adaptive functions, for the QS process. However, these new terms generate potential for a semantic quagmire and perpetuate the questionable notion that we can identify a single, dominant environmental feature to which the microbes respond. In fact, the ecological context of QS regulation, like the process itself, is complex and impacted by multiple aspects of natural environments.

Kin competition and the evolution of cooperation.

Kin and multilevel selection theories predict that genetic structure is required for the evolution of cooperation. However, local competition among relatives can limit cooperative benefits, antagonizing the evolution of cooperation. We show that several ecological factors determine the extent to which kin competition constrains cooperative benefits. In addition, we argue that cooperative acts that expand local carrying capacity are less constrained by kin competition than other cooperative traits, and are therefore more likely to evolve. These arguments are particularly relevant to microbial cooperation, which often involves the production of public goods that promote population expansion. The challenge now is to understand how an organisms ecology influences how much cooperative groups contribute to future generations and thereby the evolution of cooperation.

FEASIBILITY OF STERNAL INTRAOSSEOUS ACCESS BY EMERGENCY MEDICAL TECHNICIAN STUDENTS

Objective. Emergency medical technician–basic (EMT-B) providers are not trained to establish vascular or intraosseous (IO) access on critically ill patients. This study was conducted to examine the feasibility of training EMT-B students to correctly place a commercial sternal IO infusion device (FAST-1). Methods. Twenty-nine EMT-B students attended a two-hour training session. Subjects were subsequently tested in FAST-1 application using a modified resuscitation mannequin permitting IO needle deployment. Two observers assessed correct IO application andtechnique. Results were analyzed using descriptive statistics (binomial proportions andmedians with 95% confidence intervals). Inter-rater agreement of observations was evaluated using kappa statistics andintraclass correlation coefficients (ICCs). Results. Inter-rater agreement ranged from fair to excellent (kappa = 0.37–1.00) for all parameters except sternal notch identification (kappa = −0.03). Reliabilities of elapsed times were good (ICC = 0.83, 0.31). Correct identification of the sternal notch was accomplished by 28 of 29 students (96.6%; 95% CI: 82.2–99.9%). Correct application of the IO target patch was achieved by 29 of 29 (100.0%; 88.1–100.0%). First-attempt successful IO needle deployment was achieved by 16 of 29 (55.2%; 35.7–73.6%). Overall successful IO needle deployment within four attempts was achieved by 27 of 29 (93.1%; 77.2–99.2%). The protective dome was correctly applied by 27 of 29 (93.1%; 77.2–99.2%). The median time to needle deployment was 27.5 seconds (95% CI: 24–31). The median time to dome placement was 50 seconds (95% CI: 42–55). Conclusions. EMT-B students with minimal training demonstrated limited success with applying a commercial sternal IO device. Clinical application by EMT-Bs on critically ill patients may be possible with more intensive training.

A cooperative virulence plasmid imposes a high fitness cost under conditions that induce pathogenesis

Harbouring a plasmid often imposes a fitness cost on the bacterial host. Motivated by implications for public health, the majority of studies on plasmid cost are focused on elements that impart antibiotic resistance. Plasmids, however, can provide a wide range of ecologically important phenotypes to their bacterial hosts—such as virulence, specialized catabolism and metal resistance. The Agrobacterium tumefaciens tumour-inducing (Ti) plasmid confers both the ability to infect dicotyledonous plants and to catabolize the metabolites that plants produce as a result of being infected. We demonstrate that this virulence and catabolic plasmid imposes a measurable fitness cost on host cells under resource-limiting, but not resource replete, environmental conditions. Additionally, we show that the expression of Ti-plasmid-borne pathogenesis genes necessary to initiate cooperative pathogenesis is extremely costly to the host cell. The benefits of agrobacterial pathogenesis stem from the catabolism of public goods produced by infected host plants. Thus, the virulence-plasmid-dependent costs we demonstrate constitute costs of cooperation typically associated with the ability to garner the benefits of cooperation. Interestingly, genotypes that harbour derived opine catabolic plasmids minimize this trade-off, and are thus able to freeload upon the pathogenesis initiated by other individuals.

Evolution of the Insertion-Deletion Mutation Rate Across the Tree of Life

Mutations are the ultimate source of variation used for evolutionary adaptation, while also being predominantly deleterious and a source of genetic disorders. Understanding the rate of insertion-deletion mutations (indels) is essential to understanding evolutionary processes, especially in coding regions, where such mutations can disrupt production of essential proteins. Using direct estimates of indel rates from 14 phylogenetically diverse eukaryotic and bacterial species, along with measures of standing variation in such species, we obtain results that imply an inverse relationship of mutation rate and effective population size. These results, which corroborate earlier observations on the base-substitution mutation rate, appear most compatible with the hypothesis that natural selection reduces mutation rates per effective genome to the point at which the power of random genetic drift (approximated by the inverse of effective population size) becomes overwhelming. Given the substantial differences in DNA metabolism pathways that give rise to these two types of mutations, this consistency of results raises the possibility that refinement of other molecular and cellular traits may be inversely related to species-specific levels of random genetic drift.

Feasibility of Basic Emergency Medical Technicians to Perform Selected Advanced Life Support Interventions

Objective. Emergency medical technician-basic (EMT-B) providers often provide the initial care to victims of out-of-hospital cardiac arrest. While automated external defibrillators enable EMT-B providers to deliver rescue shocks, patients in cardiac arrest may require additional interventions that EMT-B providers may not presently deliver. We sought to evaluate the feasibility of training EMT-B providers to provide additional cardiac resuscitation procedures using the laryngeal mask airway (LMA) andintraosseous (IO) access. Methods. In this prospective observational study, we trained 18 EMT-B providers to use the LMA andIO drill (EZ-IO) in a three-hour educational session. Working in two-person teams, the rescuers performed a simulated ventricular fibrillation resuscitation. We evaluated placement success as well as elapsed time to placement of the LMA andEZ-IO. Results. EMT-B providers successfully placed the LMA in 14 of 18 scenarios (78%; 95% confidence interval, 52% to 94%), with a mean of two attempts for placement. Subjects successfully placed the EZ-IO in 17 of 18 scenarios (94%; 95% confidence interval, 73% to 100%), all on the first attempt. The median time to LMA placement following the third shock was 109 seconds (interquartile range, 58–158) andthe median time to EZ-IO placement was 72 seconds (interquartile range, 50–93) after LMA placement. Conclusions. EMT-B providers demonstrated moderate success in performing advanced-level cardiac resuscitation interventions. These observations suggest potential for expanding the role of basic-level rescuers in cardiopulmonary resuscitation. Key words: cardiopulmonary resuscitation; emergency medical services; simulation; out-of-hospital cardiac arrest; intraosseous; laryngeal mask airway.

RESOURCE AND COMPETITIVE DYNAMICS SHAPE THE BENEFITS OF PUBLIC GOODS COOPERATION IN A PLANT PATHOGEN

Cooperative benefits depend on a variety of ecological factors. Many cooperative bacteria increase the population size of their groups by making a public good available. Increased local population size can alleviate the constraints of kin competition on the evolution of cooperation by enhancing the between‐group fitness of cooperators. The cooperative pathogenesis of Agrobacterium tumefaciens causes infected plants to exude opines—resources that provide a nearly exclusive source of nutrient for the pathogen. We experimentally demonstrate that opines provide cooperative A. tumefaciens cells a within‐group fitness advantage over saprophytic agrobacteria. Our results are congruent with a resource–consumer competition model, which predicts that cooperative, virulent agrobacteria are at a competitive disadvantage when opines are unavailable, but have an advantage when opines are available at sufficient levels. This model also predicts that freeloading agrobacteria that catabolize opines but cannot infect plants competitively displace the cooperative pathogen from all environments. However, we show that these cooperative public goods also promote increased local population size. A model built from the Price Equation shows that this effect on group size can contribute to the persistence of cooperative pathogenesis despite inherent kin competition for the benefits of pathogenesis.

Non-additive costs and interactions alter the competitive dynamics of co-occurring ecologically distinct plasmids.

Plasmids play an important role in shaping bacterial evolution and adaptation to heterogeneous environments. As modular genetic elements that are often conjugative, the selective pressures that act on plasmid-borne genes are distinct from those that act on the chromosome. Many bacteria are co-infected by multiple plasmids that impart niche-specific phenotypes. Thus, in addition to host–plasmid dynamics, interactions between co-infecting plasmids are likely to be important drivers of plasmid population dynamics, evolution and ecology. Agrobacterium tumefaciens is a facultative plant pathogen that commonly harbours two distinct megaplasmids. Virulence depends on the presence of the tumour-inducing (Ti) plasmid, with benefits that are primarily restricted to the disease environment. Here, we demonstrate that a second megaplasmid, the At plasmid, confers a competitive advantage in the rhizosphere. To assess the individual and interactive costs of these plasmids, we generated four isogenic derivatives: plasmidless, pAt only, pTi only and pAtpTi, and performed pairwise competitions under carbon-limiting conditions. These studies reveal a low cost to the virulence plasmid when outside of the disease environment, and a strikingly high cost to the At plasmid. In addition, the costs of pAt and pTi in the same host were significantly lower than predicted based on single plasmid costs, signifying the first demonstration of non-additivity between naturally occurring co-resident plasmids. Based on these empirically demonstrated costs and benefits, we developed a resource–consumer model to generate predictions about the frequencies of these genotypes in relevant environments, showing that non-additivity between co-residing plasmids allows for their stable coexistence across environments.