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Background Current techniques for measuring absolute lung volumes rely on bulky and expensive equipment and are complicated to use for the operator and the patient. A novel method for measurement of absolute lung volumes, the MiniBox method, is presented. Research Question Across a population of patients and healthy participants, do values for total lung capacity (TLC) determined by the novel compact device (MiniBox, PulmOne Advanced Medical Devices, Ltd.) compare favorably with measurements determined by traditional whole body plethysmography? Study Design and Methods A total of 266 participants (130 men) and respiratory patients were recruited from five global centers (three in Europe and two in the United States). The study population comprised individuals with obstructive (n = 197) and restrictive (n = 33) disorders as well as healthy participants (n = 36). TLC measured by conventional plethysmography (TLCPleth) was compared with TLC measured by the MiniBox (TLCMB). Results TLC values ranged between 2.7 and 10.9 L. The normalized root mean square difference (NSD) between TLCPleth and TLCMB was 7.0% in healthy participants. In obstructed patients, the NSD was 7.9% in mild obstruction and 9.1% in severe obstruction. In restricted patients, the NSD was 7.8% in mild restriction and 13.9% in moderate and severe restriction. No significant differences were found between TLC values obtained by the two measurement techniques. Also no significant differences were found in results obtained among the five centers. Interpretation TLC as measured by the novel MiniBox system is not significantly different from TLC measured by conventional whole body plethysmography, thus validating the MiniBox method as a reliable method to measure absolute lung volumes.

BackgroundAmong the most basic measures of respiratory function is the total lung capacity (TLC). TLC is the pulmonary gas volume at maximal lung inflation, which is the sum of the volume of gas that can be exhaled –the vital capacity (VC)– and the volume of gas that cannot –the residual volume (RV). Determination of VC requires only spirometry whereas determination of RV or TLC requires body plethysmography, gas dilution or washout, or thoracic imaging, each of which is more complex than spirometry, and none of which is suited to routine office practice, population screening, or community medicine. To fill this gap, we describe here a new approach to determine TLC without plethysmography.MethodsIn a heterogeneous population of 434 volunteers (265 male, 169 female; 201 healthy, 170 with airflow obstruction, and 63 with ventilatory restriction), we determined TLC in the standard fashion using conventional body plethysmography (TLCpleth). In the same individuals, we also determined TLC in a novel fashion using the MiniBox ™ (TLCMB). To obtain TLCMB, population-based data from traditional spirometry together with flow-interruption transients were subjected to data mining and machine-learning to create for each individual subject an unbiased statistical determination of TLC.ResultsFor the combined heterogeneous population, we found TLCpleth = 1.02TLCMB −0.091 L, adjusted r2=0.824. For the heterogeneous population as a whole, and for each subpopulation, TLCMB closely tracked TLCpleth. For 26 healthy subjects measured on different days, the coefficient of variation for repeated measurements in was 3.3% for TLCpleth versus 1.6% for TLCMB.ConclusionsThese results establish the validity and potential utility of a new method for rapid, accurate, and repeatable determination of TLC in a heterogeneous patient population, but without the need of a plethysmograph.

Using Triumf's neutral atom trap, Trinat, for nuclear $\ensuremath{\beta}$ decay, we have measured the $\ensuremath{\beta}$ asymmetry with respect to the initial nuclear spin in $^{37}\mathrm{K}$ to be ${A}_{\ensuremath{\beta}}=\ensuremath{-}0.5707{(13)}_{\mathrm{syst}}{(13)}_{\mathrm{stat}}{(5)}_{\mathrm{pol}}$, a 0.3% measurement. This is the best relative accuracy of any $\ensuremath{\beta}$-asymmetry measurement in a nucleus or the neutron, and is in agreement with the standard model prediction $\ensuremath{-}0.5706(7)$. We compare constraints on physics beyond the standard model with other $\ensuremath{\beta}$-decay measurements, and improve the value of ${V}_{\mathrm{ud}}$ measured in this mirror nucleus by a factor of 4.

Various bacterial strains exhibit colonial branching patterns during growth on thin poor substrates. The growth can be either diffusion-limited or kinetic-limited, according to the imposed growth conditions. We present experimental observations of patterns exhibited by the bacterial strains Paenibacillus dendritiformis and Paenibacillus vortex. All manners of branching patterns are observed, the three main being: (I) basic branching; (2) chiral branching; (3) vortex branching. We show that the following biological features can explain the spectrum of observed patterns: (1) Formation of a lubricating fluid. (2) Food chemotactic. (3) Attractive and repulsive chemotactic signaling. (4) Flagella handedness. (5) Transition into pre-spore state. In the theoretical studies we employ knowledge drawn from branching patterning in non-living systems and the mathematical properties of reaction-diffusion models and atomistic models. The above can be used not just to describe existing biological understanding, but also to derive new understanding. For example, reaction-diffusion models that include bacterial density and nutrient concentration, can exhibit branching dynamics if the growth term is a meta-stable fixed point or if the diffusion is state dependent. We show that biologically the growth term has to be an unstable fixed point, but that state-dependent diffusion can represent the lubrication fluid excreted by the bacteria.

We present the morphology diagram (morphologies as function of peptone levels and agar concentrations) observed during colonial development of Paenibacillus alvei bacteria. These bacteria are close to Paenibacillus dendritiformis and have been studied extensively in the past. Like P. dendritiformis, P. alvei produces a layer of lubricating fluid for movement on hard surfaces. Unlike P. dendritiformis which attempts to swim on hard surfaces, P. alvei main movement on hard surfaces is swarming, like the movement of P. vortex. Under some growth conditions P. alvei and P. vortex exhibit collective migration of clusters of bacteria and formation of bacterial vortices. Under different growth conditions P. alvei develop branching patterns. P. alvei also develop new class of patterns which we coin “nebula patterns”. Modeling of the new patterns is still an open challenge.

Bacteria have developed sophisticated modes of cooperative behavior to cope with unfavorable environmental conditions. Here we report the effect of antibiotic stress on the colonial development of Paenibacillus dendritiformis and P. vortex. We focus on the effect of co-trimoxazole on the colonial organization of P. dendritiformis. We find that the exposure to non-lethal concentrations of antibiotic leads to dramatic changes in the colonial growth patterns. Branching, tip-splitting patterns are affected by reduction in the colonial fractal dimension from Df=2.0 to 1.7, appearance of pronounced weak chirality and pronounced radial orientation of the growth. We combine the experimental observations with numerical studies of both discrete and continuous generic models to reveal the causes for the modifications in the patterns. We conclude that the bacteria adjust their chemotactic signaling together with variations in the bacteria length and increase in the metabolic load.

In nature, microorganisms must often cope with hostile environmental conditions. To do so they have developed sophisticated cooperative behaviour and intricate communication capabilities, such as: direct cell-cell physical interactions via extra-membrane polymers, collective production of extracellular 'wetting' fluid for movement on hard surfaces, long range chemical signalling such as quorum sensing and chemotactic (bias of movement according to gradient of chemical agent) signalling, collective activation and deactivation of genes and even exchange of genetic material. Utilizing these capabilities, the colonies develop complex spatio-temporal patterns in response to adverse growth conditions. We present a wealth of beautiful patterns formed during colony development of various microorganisms and for different environmental conditions. Invoking ideas from pattern formation in non-living systems and using 'generic' modelling we are able to reveal novel survival strategies which account for the salient fe...

Segregation of populations is a key question in evolution theory. One important aspect is the relation between spatial organization and the population's composition. Here we study a specific example -- sectors in expanding bacterial colonies. Such sectors are spatially segregated sub-populations of mutants. The sectors can be seen both in disk-shaped colonies and in branching colonies. We study the sectors using two models we have used in the past to study bacterial colonies -- a continuous reaction-diffusion model with non-linear diffusion and a discrete ``Communicating Walkers'' model. We find that in expanding colonies, and especially in branching colonies, segregation processes are more likely than in a spatially static population. One such process is the establishment of stable sub- population having neutral mutation. Another example is the maintenance of wild-type population along side with sub-population of advantageous mutants. Understanding such processes in bacterial colonies is an important subject by itself, as well as a model system for similar processes in other spreading populations.

In this paper, we study the effect of discreteness on various models for patterning in bacterial colonies (finite-size effect) and present two types of models to describe the growth of the bacterial colonies. The first model presented is the Communicating Walkers model (CWm), a hybrid model composed of both continuous fields and discrete entities — walkers, which are coarse-graining of the bacteria; coarse-graining may amplify the discreteness inherent to the biological system. Models of the second type are systems of reaction diffusion equations, where the branching of the pattern is due to non-constant diffusion coefficient of the bacterial field. The diffusion coefficient represents the effect of self-generated lubrication fluid on the bacterial movement. The representation of bacteria by a density field neglects their discreteness altogether. We implement the discreteness of the bacteria by introducing a cutoff in the growth term at low bacterial densities. We demonstrate that the cutoff does not improve the models in any way. The cutoff affects the dynamics by decreasing the effective surface tension of the front, making it more sensitive to anisotropy and decreasing the fractal dimension of the evolving patterns. We compare the continuous and semi-discrete models by introducing food chemotaxis and repulsive chemotactic signaling into the models. We find that the growth dynamics of the CWm and the growth dynamics of the Non-Linear Diffusion model (one of the continuous models) are affected in the same manner. From such similarities and from the insensitivity of the CWm to implicit anisotropy, we conclude that even the increased discreteness, introduced by the coarse-graining of the walkers, is small enough to be neglected. There are advantages and disadvantages to the two types of models. Employing both of them in parallel enables us to conclude that the discreteness of the bacteria does not significantly affect the growth dynamics (no finite-size effect).

▪ Abstract In nature, bacteria must often cope with difficult environmental conditions. To do so they have developed sophisticated cooperative behavior and intricate communication pathways. Utilizing these elements, motile microbial colonies frequently develop complex patterns in response to adverse growth conditions on hard surfaces under conditions of energy limitation. We employ the term morphotype to refer to specific properties of colonial development. The morphologies we discuss include a tip-splitting (T) morphotype, chiral (C) morphotype, and vortex (V) morphotype. A generic modeling approach was developed by combining a detailed study of the cellular behavior and dynamics during colonial development and invoking concepts derived from the study of pattern formation in nonliving systems. Analysis of patterning behavior of the models suggests bacterial processes whereby communication leads to self-organization by using cooperative cellular interactions. New features emerging from the model include various modes of cell-cell signaling, such as long-range chemorepulsion, short-range chemoattraction, and, in the case of the V morphotype, rotational chemotaxis. In this regard, pattern formation in microorganisms can be viewed as the result of the exchange of information between the micro-level (the individual cells) and the macro-level (the colony).

Bacterial colonies have developed sophisticated modes of cooperative behavior which enable them to respond to adverse growth conditions. It has been shown that such behavior can be manifested in formation of complex colonial patterns. Certain Bacillus species exhibit collective migration, “turbulent like” flow and emergence of whirlpools during colonial development. Here we present experimental observations of collective behavior and a generic model to explain such behavior. The model incorporates self-propelled and interacting “particles” (swarmers). We show that velocity interaction between the particles can lead to a synchronized movement. To explain vortices formation, we propose a plausible mechanism involving a special chemotactic response (rotational chemotaxis) which is based on speed modulations according to the concentration of a chemoattractant. This mechanism differs from that exhibited by swimming bacteria. We show that the chemomodulation of swarmers' speed together with the velocity interactions impose a torque on the collective motion and can lead to formation of vortices. The inclusion of both attractive and repulsive rotational chemotaxis in the model captures the salient features of the observed growth patterns.

Bacterial colonies have developed sophisticated modes of cooperative behavior which enable them to respond to adverse growth conditions. It has been shown that such behavior can be manifested in the development of complex colonial patterns. Certain bacterial species exhibit formation of branching patterns during colony development. Here we present a generic model to describe such patterning of swimming (tumbling) bacteria on agar surfaces. The model incorporates: (1) food diffusion, (2) reproduction and sporulation of the cells, (3) movement of the bacterial cells within a self-produced wetting fluid and (4) chemotactic signaling. As a plausible explanation for transitions between different branching morphologies, we propose an interplay between chemotaxis towards food, self-produced short range chemoattractant and long range chemorepellent.

Depending on the environmental conditions bacterial colonies growing on agar surfaces can exhibit complex colony formation and various types of collective motion. Experimental results are presented concerning the hydrodynamics (vortices, migration of bacteria in clusters) and colony formation of a morphotype of Bacillus subtilis. Some of these features are not specific to this morphotype but also have been observed in several other bacterial strains, suggesting the presence of universal effects. A simple model of self-propelled particles is proposed, which is capable of describing the hydrodynamics on the intermediate level, including the experimentally observed rotating disks of bacteria. The colony formation is captured by a complex generic model taking into account nutrient diffusion, reproduction, and sporulation of bacteria, extracellular slime deposition, chemoregulation, and inhomogeneous population. Our model also sheds light on some possible biological benefits of this ``multicellular behavior.'' \textcopyright{} 1996 The American Physical Society.

In nature, bacterial colonies often must cope with hostile environmental conditions. To do so they have developed sophisticated cooperative behavior and intricate communication channels on all levels. The result is that a profusion of complex patterns are formed during growth of various bacterial strains and for different environmental conditions. Some qualitative features of the complex morphologies may be accounted for by invoking ideas from pattern formation in non-living systems together with a simplified model of chemotactic “feedback”. We present a non-local communicating walkers model to study the effect of local bacterium-bacterium interaction and communication via chemotaxis signaling. The model is an hybridization of the continuous approach (to handle chemicals’ diffusion) and the atomistic approach (each “atom” or “walker” represents 104–105 bacteria). Using the model we demonstrate how communication enables the colony to develop complex patterns in response to adverse growth conditions. Efficient response of the colony requires self-organization on all levels, which can be achieved only via cooperative behavior of the bacteria. It can be viewed as the action of an interplay between the micro-level (the individual bacterium) and the macro-level (the colony) in the determination of the emerging pattern. We show that seemingly unrelated patterns can result from the employment of the same generic strategies.

A simple model with a novel type of dynamics is introduced in order to investigate the emergence of self-ordered motion in systems of particles with biologically motivated interaction. In our model particles are driven with a constant absolute velocity and at each time step assume the average direction of motion of the particles in their neighborhood with some random perturbation $(\ensuremath{\eta})$ added. We present numerical evidence that this model results in a kinetic phase transition from no transport (zero average velocity, $|{\mathbf{v}}_{a}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$) to finite net transport through spontaneous symmetry breaking of the rotational symmetry. The transition is continuous, since $|{\mathbf{v}}_{a}|$ is found to scale as $({\ensuremath{\eta}}_{c}\ensuremath{-}\ensuremath{\eta}{)}^{\ensuremath{\beta}}$ with $\ensuremath{\beta}\ensuremath{\simeq}0.45$.

Bacterial colonies must often cope with unfavourable environmental conditions. To do so, they have developed sophisticated modes of cooperative behaviour. It has been found that such behaviour can cause bacterial colonies to exhibit complex growth patterns similar to those observed during non-equilibrium growth processes in non-living systems; some of the qualitative features of the latter may be invoked to account for the complex patterns of bacterial growth. Here we show that a simple model of bacterial growth can reproduce the salient features of the observed growth patterns. The model incorporates random walkers, representing aggregates of bacteria, which move in response to gradients in nutrient concentration and communicate with each other by means of chemotactic 'feedback'. These simple features allow the colony to respond efficiently to adverse growth conditions, and generate self-organization over a wide range of length scales.

We present a study of interfacial pattern formation during growth of bacterial colonies. Growth of bacterial colony bears similarities to but presents an inherent additional level of complexity compared to non-living systems. In the former case, the building blocks themselves are living systems each with its own autonomous self-interest and internal degrees of freedom. At the same time, efficient adaptation of the colony to adverse growth conditions requires self-organization on all levels — which can be achieved only via cooperative behavior of the bacteria. To do so, the bacteria have developed sophisticated communication channels on all levels. Here we present a non-local communicating walkers model to study the effect of local bacterium-bacterium interaction and communication via chemotaxis signaling. We demonstrate how communication enables the colony to develop complex patterns in response to adverse growth conditions. Efficient response of the colony requires self-organization on all levels, which can be achieved only via cooperative behavior of the bacteria. It can be viewed as the action of an interplay between the micro-level (the individual bacterium) and the macro-level (the colony) in the determination of the emerging pattern. Some qualitative features of the complex morphologies can be accounted for by invoking ideas from pattern formation in non-living systems together with a simplified model of chemotactic "feedback."

In previous years, many examples of swarming patterns have been found in systems of microorganisms. These examples range from the adaptive branching structures built by surface-resident bacteria to the swirling localized vortices seen during amoeba aggregation in Dictyostelium discoideum colonies. Modeling these systems allows us to understand the connection between microscopic interactions among the cellular constituents and the resultant collective macroscopic patterns. After a very brief review of some of these phenomena, we present new results along two disparate directions: the derivation of a reaction-diffusion equation capable of dealing with chiral branching (i.e. the breaking of left-right symmetry) and the introduction of a new flocking model to study cell-sorting (aka phase separation) in localized vortices

Many bacterial strains exhibit beautiful branching patterns during colonial development. Much effort is devoted to the search for basic principles of self-organization (growth, communication, regulation and control) on the cellular and multi-cellular levels. Our approach is to use the successful conceptual framework for branching growth patterns in non-living systems as a tool to understand their significantly more complex biological counterparts.

In nature, microorganisms must often cope with hostile environmental conditions. To do so they have developed sophisticated cooperative behavior and intricate communication capabilities, such as: direct cell-cell physical interactions via extramembrane polymers, collective production of extracellular “wetting” fluid for movement on hard surfaces, long range chemical signaling such as quorum sensing and chemotactic (bias of movement according to gradient of chemical agent) signaling, collective activation and deactivation of genes and even exchange of genetic material. Utilizing these capabilities, the colonies develop complex spatio-temporal patterns in response to adverse growth conditions. We present a wealth of branching and chiral patterns formed during colonial development of lubricating, swimming bacteria (bacteria that produce a wetting layer of fluid so they can swim in it). Invoking ideas from pattern formation in non-living systems and using “generic” modeling we are able to reveal novel survival strategies which account for the salient features of the evolved patterns. Using the models, we demonstrate how communication leads to self-organization via cooperative behavior of the cells. In this regard, pattern formation in microorganisms can be viewed as the result of the exchange of information between the micro-level (the individual cells) and the macro-level (the colony). We mainly review known results, but include a new model of chiral growth, which enables us to study the effect of chemotactic signaling on the chiral growth. We also introduce a measure for weak chirality and use this measure to compare the results of model simulations with experimental observations.

We investigate a discrete model consisting of self-propelled particles that obey simple interaction rules. We show that this model can self-organize and exhibit coherent localized solutions in one- and in two-dimensions.In one-dimension, the self-organized solution is a localized flock of finite extent in which the density abruptly drops to zero at the edges.In two-dimensions, we focus on the vortex solution in which the particles rotate around a common center and show that this solution can be obtained from random initial conditions, even in the absence of a confining boundary. Furthermore, we develop a continuum version of our discrete model and demonstrate that the agreement between the discrete and the continuum model is excellent., 4 pages, 5 figures

During the course of evolution, bacteria have developed sophisticated cooperative behavior and intricate communication capabilities [1-3]. Utilizing these capabilities, bacterial colonies develop complex spatio-temporal patterns in response to adverse growth conditions. It is now understood that the study of cooperative self-organization of bacterial colonies is an exciting new multidisciplinary field of research, necessitating the merger of biological information with the physics of non-equilibrium processes and the mathematics of non-linear dynamics. At this stage, several experimental systems have been identified, and preliminary modeling efforts are making significant progress in providing a framework for the understanding of experimental observations [4-12]. This endeavour is not limited to bacteria alone. Studies have been performed of other types of microorganisms as well, such as amoeba [13] and yeast [14]

Various bacterial strains (e.g. strains belonging to the genera Bacillus, Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns during growth on poor semi-solid substrates. These patterns reflect the bacterial cooperative self-organization. Central part of the cooperation is the collective formation of lubricant on top of the agar which enables the bacteria to swim. Hence it provides the colony means to advance towards the food. One method of modeling the colonial development is via coupled reaction-diffusion equations which describe the time evolution of the bacterial density and the concentrations of the relevant chemical fields. This idea has been pursued by a number of groups. Here we present an additional model which specifically includes an evolution equation for the lubricant excreted by the bacteria. We show that when the diffusion of the fluid is governed by nonlinear diffusion coefficient branching patterns evolves. We study the effect of the rates of emission and decomposition of the lubricant fluid on the observed patterns. The results are compared with experimental observations. We also include fields of chemotactic agents and food chemotaxis and conclude that these features are needed in order to explain the observations., Comment: 1 latex file, 16 jpeg files, submitted to Phys. Rev. E

Various bacterial strains exhibit colonial branching patterns during growth on poor substrates. These patterns reflect bacterial cooperative self-organization and cybernetic processes of communication, regulation and control employed during colonial development. One method of modeling is the continuous, or coupled reaction-diffusion approach, in which continuous time evolution equations describe the bacterial density and the concentration of the relevant chemical fields. In the context of branching growth, this idea has been pursued by a number of groups. We present an additional model which includes a lubrication fluid excreted by the bacteria. We also add fields of chemotactic agents to the other models. We then present a critique of this whole enterprise with focus on the models' potential for revealing new biological features., Comment: 1 latex file, 40 gif/jpeg files (compressed into tar-gzip). Physica A, in press

1 Introduction 2 Microevolution in a Petri-dish 2.1 Bursts of Branching Growth 2.2 Cybernators and Cooperative Genetic Changes 2.3 Burst of the Chiral Morphotype 2.4 Bursts of the Vortex Morphotype 3 Observed Complex Patterns 3.1 Patterns of the 7-Morphotype 3.2 Patterns of the C Morphotype 3.3 Closer Look at the V Morphotype 4 Modeling the Growth 4.1 The Generic Modeling Approach 4.2 The Communicating Walkers Model 4.3 From Flagella Handedness to the Macroscopic Chirality 4.4 Cooperative Formation of Vortices 5 Conclusions Acknowledgments References

Bacterial colonies can develop chiral morphology in which the colony consists of twisted branches, all with the same handedness. Microscopic observations of the chiral growth are presented. We propose that the observed (macroscopic) chirality results from the microscopic chirality of the flagella (via handedness in tumbling) together with orientation interaction between the bacteria. The above assumptions are tested using a generalized version of the communicating walkers model.

We study the formation of spot patterns seen in a variety of bacterial species when the bacteria are subjected to oxidative stress due to hazardous byproducts of respiration. Our approach consists of coupling the cell density field to a chemoattractant concentration as well as to nutrient and waste fields. The latter serves as a triggering field for emission of chemoattractant. Important elements in the proposed model include the propagation of a front of motile bacteria radially outward form an initial site, a Turing instability of the uniformly dense state and a reduction of motility for cells sufficiently far behind the front. The wide variety of patterns seen in the experiments is explained as being due the variation of the details of the initiation of the chemoattractant emission as well as the transition to a non-motile phase., Comment: 4 pages, REVTeX with 4 postscript figures (uuencoded) Figures 1a and 1b are available from the authors; paper submitted to PRL.

We present a study of interfacial pattern formation during growth of bacterial colonies. Growth of bacterial colony bears similarities to but presents an inherent additional level of complexity compared to non-living systems. In the former case, the building blocks themselves are living systems each with its own autonomous self-interest and internal degrees of freedom. At the same time, efficient adaptation of the colony to adverse growth conditions requires self-organization on all levels — which can be achieved only via cooperative behavior of the bacteria. To do so, the bacteria have developed sophisticated communication channels on all levels. Here we present a non-local communicating walkers model to study the effect of local bacterium-bacterium interaction and communication via chemotaxis signaling. We demonstrate how communication enables the colony to develop complex patterns in response to adverse growth conditions. Efficient response of the colony requires self-organization on all levels, which can be achieved only via cooperative behavior of the bacteria. It can be viewed as the action of an interplay between the micro-level (the individual bacterium) and the macro-level (the colony) in the determination of the emerging pattern. Some qualitative features of the complex morphologies can be accounted for by invoking ideas from pattern formation in non-living systems together with a simplified model of chemotactic "feedback."

We present a study of interfacial pattern formation during growth of bacterial colonies. Growth of bacterial colonies bears similarities but presents an inherent additional level of complexity in comparison with non-living systems. In the former case, the building blocks themselves are living systems, each with its own autonomous self-interest and internal degrees of freedom. The bacteria have developed sophisticated communication channels, which they utilize when growth conditions are tough. Here we present a non-local communicating walkers model to study the effect of local bacterium-bacterium interaction and communication via chemotaxis signaling. We demonstrate how communication enables the colony to develop complex patterns in response to adverse growth conditions. This self-organization of the colony, which can be achieved only via cooperative behavior of the bacteria, may be viewed as the outcome of an interplay between the micro-level (the individual bacterium) and the macro-level (the colony). Some qualitative features of the complex morphologies can be accounted for by invoking ideas from pattern formation in non-living systems together with a simplified model of chemotactic “feedback”.