Broadcasting of amplitude- and frequency-modulated c-di-GMP signals facilitates cooperative surface commitment in bacterial lineages.

Work on surface sensing in bacterial biofilms has focused on how cells transduce sensory input into cyclic diguanylate (c-di-GMP) signaling, low and high levels of which generally correlate with high-motility planktonic cells and low-motility biofilm cells, respectively. Using Granger causal inference methods, however, we find that single-cell c-di-GMP increases are not sufficient to imply surface commitment. Tracking entire lineages of cells from the progenitor cell onward reveals that c-di-GMP levels can exhibit increases but also undergo oscillations that can propagate across 10 to 20 generations, thereby encoding more complex instructions for community behavior. Principal component and factor analysis of lineage c-di-GMP data shows that surface commitment behavior correlates with three statistically independent composite features, which roughly correspond to mean c-di-GMP levels, c-di-GMP oscillation period, and surface motility. Surface commitment in young biofilms does not correlate to c-di-GMP increases alone but also to the emergence of high-frequency and small-amplitude modulation of elevated c-di-GMP signal along a lineage of cells. Using this framework, we dissect how increasing or decreasing signal transduction from wild-type levels, by varying the interaction strength between PilO, a component of a principal surface sensing appendage system, and SadC, a key hub diguanylate cyclase that synthesizes c-di-GMP, impacts frequency and amplitude modulation of c-di-GMP signals and cooperative surface commitment.

Interaction between the type 4 pili machinery and a diguanylate cyclase fine-tune c-di-GMP levels during early biofilm formation.

To initiate biofilm formation, it is critical for bacteria to sense a surface and respond precisely to activate downstream components of the biofilm program. Type 4 pili (T4P) and increasing levels of c-di-GMP have been shown to be important for surface sensing and biofilm formation, respectively; however, mechanisms important in modulating the levels of this dinucleotide molecule to define a precise output response are unknown. Here, using macroscopic bulk assays and single-cell tracking analyses of Pseudomonas aeruginosa, we uncover a role of the T4P alignment complex protein, PilO, in modulating the activity of the diguanylate cyclase (DGC) SadC. Two-hybrid and bimolecular fluorescence complementation assays, combined with genetic studies, are consistent with a model whereby PilO interacts with SadC and that the PilO-SadC interaction inhibits SadC's activity, resulting in decreased biofilm formation and increased motility. Using single-cell tracking, we monitor both the mean c-di-GMP and the variance of this dinucleotide in individual cells. Mutations that increase PilO-SadC interaction modestly, but significantly, decrease both the average and variance in c-di-GMP levels on a cell-by-cell basis, while mutants that disrupt PilO-SadC interaction increase the mean and variance of c-di-GMP levels. This work is consistent with a model wherein P. aeruginosa uses a component of the T4P scaffold to fine-tune the levels of this dinucleotide signal during surface commitment. Finally, given our previous findings linking SadC to the flagellar machinery, we propose that this DGC acts as a bridge to integrate T4P and flagellar-derived input signals during initial surface engagement.

Roadmap on emerging concepts in the physical biology of bacterial biofilms: from surface sensing to community formation.

Bacterial biofilms are communities of bacteria that exist as aggregates that can adhere to surfaces or be free-standing. This complex, social mode of cellular organization is fundamental to the physiology of microbes and often exhibits surprising behavior. Bacterial biofilms are more than the sum of their parts: single-cell behavior has a complex relation to collective community behavior, in a manner perhaps cognate to the complex relation between atomic physics and condensed matter physics. Biofilm microbiology is a relatively young field by biology standards, but it has already attracted intense attention from physicists. Sometimes, this attention takes the form of seeing biofilms as inspiration for new physics. In this roadmap, we highlight the work of those who have taken the opposite strategy: we highlight the work of physicists and physical scientists who use physics to engage fundamental concepts in bacterial biofilm microbiology, including adhesion, sensing, motility, signaling, memory, energy flow, community formation and cooperativity. These contributions are juxtaposed with microbiologists who have made recent important discoveries on bacterial biofilms using state-of-the-art physical methods. The contributions to this roadmap exemplify how well physics and biology can be combined to achieve a new synthesis, rather than just a division of labor.

Mood moderates the effect of aesthetic appeal on performance.

Aesthetically appealing stimuli can improve performance in demanding target localisation tasks compared to unappealing stimuli. Two search-and-localisation experiments were carried out to examine the possible underlying mechanism mediating the effects of appeal on performance. Participants (N = 95) were put in a positive or negative mood prior to carrying out a visual target localisation task with appealing and unappealing targets. In both experiments, positive mood initially led to faster localisation of appealing compared to unappealing stimuli, while an advantage for appealing over unappealing stimuli emerged over time in negative mood participants. The findings are compatible with the idea that appealing stimuli may be inherently rewarding, with aesthetic appeal overcoming the detrimental effects of negative mood on performance.

Force-Induced Changes of PilY1 Drive Surface Sensing by Pseudomonas aeruginosa.

During biofilm formation, the opportunistic pathogen Pseudomonas aeruginosa uses its type IV pili (TFP) to sense a surface, eliciting increased second-messenger production and regulating target pathways required to adapt to a surface lifestyle. The mechanisms whereby TFP detect surface contact are still poorly understood, although mechanosensing is often invoked, with few data supporting this claim. Using a combination of molecular genetics and single-cell analysis, with biophysical, biochemical, and genomics techniques, we show that force-induced changes mediated by the von Willebrand A (vWA) domain-containing, TFP tip-associated protein PilY1 are required for surface sensing. Atomic force microscopy shows that TFP/PilY1 can undergo force-induced, sustained conformational changes akin to those observed for mechanosensitive proteins like titin. We show that mutation of a single cysteine residue in the vWA domain of PilY1 results in modestly lower surface adhesion forces, reduced sustained conformational changes, and increased nanospring-like properties, as well as reduced c-di-GMP signaling and biofilm formation. Mutating this cysteine has allowed us to genetically separate a role for TFP in twitching motility from surface-sensing signaling. The conservation of this Cys residue in all P. aeruginosa PA14 strains and its absence in the ∼720 sequenced strains of P. aeruginosa PAO1 may contribute to explaining the observed differences in surface colonization strategies observed for PA14 versus PAO1. IMPORTANCE Most bacteria live on abiotic and biotic surfaces in surface-attached communities known as biofilms. Surface sensing and increased levels of the second-messenger molecule c-di-GMP are crucial to the transition from planktonic to biofilm growth. The mechanism(s) underlying TFP-mediated surface detection that triggers this c-di-GMP signaling cascade is unclear. Here, we provide key insight into this question; we show that the eukaryote-like vWA domain of the TFP tip-associated protein PilY1 responds to mechanical force, which in turn drives the production of a key second messenger needed to regulate surface behaviors. Our studies highlight a potential mechanism that may account for differing surface colonization strategies.

Smoking expectancies for flavored and non-flavored cigarettes among college students.

Several tobacco companies have introduced specially flavored cigarettes, yet little is known about their appeal among college student nonsmokers, regular smokers, and those susceptible to smoking. Undergraduates (N=424) rated 12 brands of cigarettes on multiple attributes based on manufacturer advertisements. This paper focused on two brands with flavored and non-flavored versions (Camel and Salem). Despite brand, regular smokers and those susceptible to smoking initiation had higher positive expectancies and lower negative expectancies about smoking than nonsmokers. Flavored cigarettes elicited higher positive expectancies than non-flavored counterparts across all groups, including nonsmokers. Indeed, the degree to which flavored Camels had higher positive expectancies than Camel Lights was at least as large in a group of susceptible nonsmokers and experimenters (susceptible/experimenters). Despite being present in nonsmokers and susceptible/experimenters, negative expectancies were significantly lower for flavored versus non-flavored brands. Logistic regressions revealed that positive expectancies predicted "intention to try" each brand for regular smokers and susceptible/experimenters. These findings suggest that targeting the marketing of positive attributes may be useful in preventing smoking behavior.

College students' expectancies for light cigarettes and potential reduced exposure products.

OBJECTIVES: To determine positive and negative beliefs about light cigarettes and potential reduced exposure products (PREPs) among college student smokers and non-smokers. METHODS: A web-based survey conducted in October-November 2004 among 424 students rating 5 advertisements for cigarette brands (Marlboro Red, Light, and Ultralight; Quest; Eclipse) on 28 items tapping positive and negative product expectancies. RESULTS: Marlboro Light and Ultralight were rated more positively and less negatively than their Red counterpart. PREPs showed low positive and negative ratings relative to Marlboro Light. Positive expectancies were significantly related to willingness to try each brand. CONCLUSIONS: Advertising plays a role in influencing how college students view light and PREP cigarette brands.

Perception of initial moving target signals: support for a cumulative lateral inhibition theory.

The cumulative lateral inhibition (CLI) theory of the Fröhlich effect (perceptual mislocalization of the starting position of a moving target in the direction of movement) proposes that the target is difficult to see early in its trajectory because inhibitory feedback from later target views weakens initial target representations. In contrast, attention shift explanations contend that a phenomenal representation of the target is unavailable until attention has shifted to the vicinity of the target. Experiments 1-3 demonstrated that instructing naive undergraduate participants to accept different degrees of target clarity before making their response can alter the magnitude (and presence) of the Fröhlich effect. Experiments 2-4 showed that increasing movement distance by adding target presentations reduced the visibility of the target at early positions. These results are difficult to reconcile with an attention shift account.

No evidence for directional biases in inhibition of return.

The phenomenon of "inhibition of return" (IOR) has been the subject of considerable research interest for nearly 30 years. Two reports claiming directional biases in IOR (Spalek & Hammad, Perception & Psychophysics 66:219-233, 2004, Psychological Science 16:15-18, 2005) were examined more closely, as such findings challenge the theoretical role attributed to IOR and imply that this purported mechanism for the facilitation of visual search would bias search in systematic ways. The data from two new experiments, as well as reanalysis of the original data, showed the reports to result from an unconventional method of calculating IOR that confounded visual field with target location. Although we found significant differences in target detection response times between the visual fields, directional biases were absent from all of the examined data when the conventional method of computing IOR was applied.

Informational affordances: evidence of acquired perception-action sequences for information extraction.

Visual objects can automatically prime actions allowing efficient interaction with them. The present study examined whether object perception can automatically prime actions leading to efficient information extraction. Participants in Experiment 1 learned to rotate a cube in a specific way with the end goal of efficiently revealing object-identifying information. In Experiments 2 and 3, the end goal of obtaining object-identifying information was removed, but the stimulus-response associations were preserved. Only object views associated with actions learned in the context of obtaining identifying information caused response interference and benefits in a subsequent test phase where the object was irrelevant. These results demonstrate the existence of informational affordances: perception-action sequences acquired with the goal of information extraction that are automatically primed during later exposure to the object. Perceptual priming of actions for efficient information extraction is an important component of expert performance and its use of action systems to optimally deal with the world.

Successes and failures in producing attentional object-based cueing effects.

Over 30 years of research using Posner's spatial cueing paradigm has shown that selective attention operates on representations of spatial locations, leading to space-based theories of attention. Manipulations of stimuli and methods have shown this paradigm to be sensitive to several types of object-based representations-providing evidence for theories incorporating object-based attentional selection. This paper critically evaluates the evidence demanding object-based explanations that go beyond positing spatial representations alone, with an emphasis on identifying and interpreting successes and failures in obtaining object-based cueing effects. This overview of current evidence is used to generate hypotheses regarding critical factors in the emergence and influence of object representations-their generation, strength, and maintenance-in the modulation of object-based facilitatory and inhibitory cueing effects.

How does attention spread across objects oriented in depth?

Previous evidence suggests that attention can operate on object-based representations. It is not known whether these representations encode depth information and whether object depth, if encoded, is in viewer- or object-centered coordinates. To examine these questions, we employed a spatial cuing paradigm in which one corner of a 3-D object was exogenously cued with 75% validity. By rotating the object in depth, we can determine whether validity effects are modulated by 2-D or 3-D cue-target distance and whether validity effects depend on the position of the viewer relative to the object. When the image of a 3-D object was present (Experiments 1A and 1B), validity effects were not modulated by changes in 2-D cue-target distance, and shifting attention toward the viewer led to smaller validity effects than did shifting attention away from the viewer. When there was no object in the display (Experiments 2A and 2B), validity effects increased linearly as a function of 2-D cue-target distance. These results demonstrate that attention spreads across representations of perceived objects that encode depth information and that the object's orientation in depth is encoded in viewer-centered coordinates.

There's little return for attentional momentum.

Inhibition of return (IOR) refers to a delay in response time (RT) to targets appearing at a previously cued location. The prevailing view is that IOR reflects visual-motor inhibition. The "attentional momentum" account rejects this idea, and instead proposes that IOR reflects an automatic shift of attention away from the cued location resulting in slower RTs to targets presented there and speeded RTs to targets opposite the cue (an opposite facilitation effect or OFE). The drawback of this account is that J. J. Snyder, W. C. Schmidt, and A. Kingstone (2001) showed that there are few data to support the OFE, and no evidence that the OFE accounts for the IOR effect. Despite this evidence, several recent studies have promoted attentional momentum as a valid explanation for the IOR effect. Reanalysis of these recent studies and new data reveal, again, that IOR routinely occurs in the absence of the OFE, and when the OFE does occur, the IOR effect need not be present. This double dissociation invalidates attentional momentum as an explanation for the IOR effect. Extant data support an inhibitory explanation of the IOR effect.

A server-side program for delivering experiments with animations.

A server-side program for animation experiments is presented. The program is capable of delivering an experiment composed of discrete animation sequences in various file formats, collecting a discrete or continuous response from the observer, evaluating the appropriateness of the response, and ensuring that the user is not proceeding at an unreasonable rate. Most parameters of the program are controllable by experimenter-edited text files or simple switches in the program code, thereby minimizing the need for programming to create new experiments. A simple demonstration experiment is discussed and is freely available.