Phenotypic Diversity as a Mechanism to Exit Cellular Dormancy

SummaryMicroorganisms can facilitate their survival in stressful environments by entering a state of metabolic inactivity or dormancy [1]. However, this state impairs the function of the very sensory systems necessary to detect favorable growth conditions. Thus, how can a metabolically quiescent cell accurately monitor environmental conditions in order to best decide when to exit dormancy? One strategy employed by microbes to deal with changing environments is the generation of phenotypes that may be less well adapted to a current condition but might confer an advantage in the future [2, 3]. This bet-hedging depends on phenotypic diversity in the population [4], which itself can derive from naturally occurring stochastic differences in gene expression [5, 6]. In the case of metabolic dormancy, a bet-hedging strategy that has been proposed is the “scout model” where cells comprising a fraction of the dormant population reinitiate growth stochastically, independent of environmental cues [7, 8]. Here, we provide experimental evidence that such a mechanism exists in dormant spores produced by the ubiquitous soil bacterium Bacillus subtilis. We observe that these spores reinitiate growth at a low but measureable frequency even in the absence of an inducing signal. This phenomenon is the result of phenotypic variation in the propensity of individual spores to reinitiate growth spontaneously. Since this bet-hedging mechanism produces individuals that will either grow under favorable conditions or die under unfavorable conditions, a population can properly respond to environmental changes despite the impaired sensory ability of individual cells

Detection of fungal and bacterial carbohydrates: Do the similar structures of chitin and peptidoglycan play a role in immune dysfunction?

Carbohydrate recognition is fundamental to a wide variety of interkingdom interactions. For example, bacterial peptidoglycan, an N-acetyl-D-glucosamine (GlcNAc)–N-acetylmuramic acid (MurNAc) polymer, and fungal chitin, a GlcNAc polymer, are both immunostimulatory to vertebrates. In addition, bacterially produced Nodulation (Nod) factors, which consist of a modified GlcNAc, play a key role in symbiotic interactions with plants. The structural similarity of these GlcNAc-containing molecules suggests possible overlap in their physiological action, although many of the mechanisms underlying the detection of these molecules in different organisms appear unrelated. However, a single phylogenetically conserved domain, called Lysin (LysM), is found in specific receptors in signaling pathways responsive to one or more of these three related carbohydrates in bacteria, plants, and fungi and possibly mammalian systems. Thus, promiscuous activation could occur when a structurally similar but physiologically inappropriate ligand binds and thereby aberrantly activates an incorrect LysM domain-containing receptor. Here, I will discuss this possibility and its implications for immune pathologies such as asthma in which chitin is relevant

PSICIC: Noise and Asymmetry in Bacterial Division Revealed by Computational Image Analysis at Sub-Pixel Resolution

Live-cell imaging by light microscopy has demonstrated that all cells are spatially and temporally organized. Quantitative, computational image analysis is an important part of cellular imaging, providing both enriched information about individual cell properties and the ability to analyze large datasets. However, such studies are often limited by the small size and variable shape of objects of interest. Here, we address two outstanding problems in bacterial cell division by developing a generally applicable, standardized, and modular software suite termed Projected System of Internal Coordinates from Interpolated Contours (PSICIC) that solves common problems in image quantitation. PSICIC implements interpolated-contour analysis for accurate and precise determination of cell borders and automatically generates internal coordinate systems that are superimposable regardless of cell geometry. We have used PSICIC to establish that the cell-fate determinant, SpoIIE, is asymmetrically localized during Bacillus subtilis sporulation, thereby demonstrating the ability of PSICIC to discern protein localization features at sub-pixel scales. We also used PSICIC to examine the accuracy of cell division in Esherichia coli and found a new role for the Min system in regulating division-site placement throughout the cell length, but only prior to the initiation of cell constriction. These results extend our understanding of the regulation of both asymmetry and accuracy in bacterial division while demonstrating the general applicability of PSICIC as a computational approach for quantitative, high-throughput analysis of cellular images

Developmental Commitment in a Bacterium

SummaryWe investigated developmental commitment during sporulation in Bacillus subtilis. Sporulation is initiated by nutrient limitation and involves division of the developing cell into two progeny, the forespore and the mother cell, with different fates. Differentiation becomes irreversible following division when neither the forespore nor the mother cell can resume growth when provided with nutrients. We show that commitment is governed by the transcription factors σF and σE, which are activated in the forespore and the mother cell, respectively. We further show that commitment involves spoIIQ, which is under the control of σF, and spoIIP, which is under the control of both σF and σE. In the presence of nutrients, the forespore can exhibit rodlike, longitudinal growth when SpoIIQ and SpoIIP are absent, whereas the mother cell can do so when SpoIIP alone is absent. Thus, developmental commitment of this single-celled organism, like that of the cells of complex, multicellular organisms, ensures that differentiation is maintained despite changes in the extracellular milieu

Tunability and Noise Dependence in Differentiation Dynamics

The dynamic process of differentiation depends on the architecture, quantitative parameters, and noise of underlying genetic circuits. However, it remains unclear how these elements combine to control cellular behavior. We analyzed the probabilistic and transient differentiation of Bacillus subtilis cells into the state of competence. A few key parameters independently tuned the frequency of initiation and the duration of competence episodes and allowed the circuit to access different dynamic regimes, including oscillation. Altering circuit architecture showed that the duration of competence events can be made more precise. We used an experimental method to reduce global cellular noise and showed that noise levels are correlated with frequency of differentiation events. Together, the data reveal a noise-dependent circuit that is remarkably resilient and tunable in terms of its dynamic behavior

Partial penetrance facilitates developmental evolution in bacteria

Development normally occurs similarly in all individuals within an isogenic population, but mutations often affect the fates of individual organisms differently. This phenomenon, known as partial penetrance, has been observed in diverse developmental systems. However, it remains unclear how the underlying genetic network specifies the set of possible alternative fates and how the relative frequencies of these fates evolve. Here we identify a stochastic cell fate determination process that operates in Bacillus subtilis sporulation mutants and show how it allows genetic control of the penetrance of multiple fates. Mutations in an intercompartmental signalling process generate a set of discrete alternative fates not observed in wild-type cells, including rare formation of two viable 'twin' spores, rather than one within a single cell. By genetically modulating chromosome replication and septation, we can systematically tune the penetrance of each mutant fate. Furthermore, signalling and replication perturbations synergize to significantly increase the penetrance of twin sporulation. These results suggest a potential pathway for developmental evolution between monosporulation and twin sporulation through states of intermediate twin penetrance. Furthermore, time-lapse microscopy of twin sporulation in wild-type Clostridium oceanicum shows a strong resemblance to twin sporulation in these B. subtilis mutants. Together the results suggest that noise can facilitate developmental evolution by enabling the initial expression of discrete morphological traits at low penetrance, and allowing their stabilization by gradual adjustment of genetic parameters