Bacteria in nature live in taxonomically complex communities where multitude of species and strains inhabit the same niches and compete for limited resources and space. Surviving in these competitive environments requires mechanisms to recognize and associate with kin and to discriminate against non-kin to increase reproductive success among close relatives. Some of the mechanisms bacteria use to address genetic differences are surface receptors, diffusible signals (e.g. quorum sensing) and toxin-immunity systems (e.g. type VI secretion system (T6SS)). Another way individuals vary within bacterial populations is their physiological states. This means that among clonal cells there is cell-to-cell variability in cells’ proteome, growth rates, age and cell damage loads caused by stochastic differences in gene expression/metabolism and variations in microenvironmental stimuli. While physiological heterogeneity benefits some bacteria by allowing populations to bet-hedge their survival odds in changing environments by expressing different phenotypes, it can also be harmful in cases where fitness depends on coordinated behaviors and synchronized actions by many cells; a function of particular importance to social bacteria. Myxococcus xanthus is a non-pathogenic soil bacterium known for its complex social and coordinated behaviors such as swarming, predation and formation of spore-filled fruiting bodies. These behaviors depend on M. xanthus ability to synchronize the actions of many cells within a population. Considering the collective nature of M. xanthus, we asked how do physiological differences affect cell-cell interactions in this species. To address this question, we investigated the interactions between two genetically related but physiologically distinct populations. We found that M. xanthus uses T6SS to eliminate less fit cells from their population and identified toxic effector and cognate immunity protein (TsxEI) that mediates this sibling antagonism.

Chronologically aging yeast cells are prone to adaptive regrowth, whereby mutants with a survival advantage spontaneously appear and re-enter the cell cycle in stationary phase cultures. Adaptive regrowth is especially noticeable with short-lived strains, including those defective for SNF1, the homolog of mammalian AMP-activated protein kinase (AMPK). SNF1 becomes active in response to multiple environmental stresses that occur in chronologically aging cells, including glucose depletion and oxidative stress. SNF1 is also required for the extension of chronological lifespan (CLS) by caloric restriction (CR) as defined as limiting glucose at the time of culture inoculation. To identify specific downstream SNF1 targets responsible for CLS extension during CR, we screened for adaptive regrowth mutants that restore chronological longevity to a short-lived snf1∆ parental strain. Whole genome sequencing of the adapted mutants revealed missense mutations in TPR motifs 9 and 10 of the transcriptional co-repressor Cyc8 that specifically mediate repression through the transcriptional repressor Mig1. Another mutation occurred in MIG1 itself, thus implicating the activation of Mig1-repressed genes as a key function of SNF1 in maintaining CLS. Consistent with this conclusion, the cyc8 TPR mutations partially restored growth on alternative carbon sources and significantly extended CLS compared to the snf1∆ parent. Furthermore, cyc8 TPR mutations reactivated multiple Mig1-repressed genes, including the transcription factor gene CAT8, which is responsible for activating genes of the glyoxylate and gluconeogenesis pathways. Deleting CAT8 completely blocked CLS extension by the cyc8 TPR mutations on CLS, identifying these pathways as key Snf1-regulated CLS determinants.

Localized and systemic fungal infections caused by Candida albicans can lead to significant mortality and morbidity. However, severe C. albicans infections are relatively rare, occurring mostly in the very young, the very old, and immunocompromised individuals. The fact that these infections are rare is interesting because as much as 80 percent of the population is asymptomatically colonized with C. albicans. It is thought that members of the human microbiota and the immune system work in concert to reduce C. albicans overgrowth through competition and modification of the growth environment. Here, we report that Escherichia coli (strain MG1655) outcompetes and kills C. albicans (strain SC5314) in vitro. We find that E. coli produces a soluble factor that kills C. albicans in a magnesium-dependent fashion such that depletion of available magnesium is essential for toxicity.

New perspectives from South-Y-East, not all about deathA report of the 12th lnternational Meeting on Yeast Apoptosis in Bari, Italy, May 14th-18th, 2017 – Over the last 14 years, the field of yeast regulated cell death (RCD) has been expanding to more and more biomedical research themes, including aging, human diseases, cell stress response, metabolism and systems biology. The 12th International Meeting on Yeast Apoptosis (IMYA12), which was held in Bari, Italy from May 14th to 18th, 2017, nicely (...)

The glycerophospholipid cardiolipin is a unique constituent of bacterial and mitochondrial membranes. It is involved in forming and stabilizing high molecular mass membrane protein complexes and in maintaining membrane architecture. Absence of cardiolipin leads to reduced efficiency of the electron transport chain, decreased membrane potential, and, ultimately, impaired respiratory metabolism. For the protozoan parasite Trypanosoma brucei cardiolipin synthesis is essential for survival, indicating that the enzymes involved in cardiolipin production represent potential drug targets. T. brucei cardiolipin synthase (TbCLS) is unique as it belongs to the family of phospholipases D (PLD), harboring a prokaryotic-type cardiolipin synthase (CLS) active site domain. In contrast, most other eukaryotic CLS, including the yeast ortholog ScCrd1, are members of the CDP-alcohol phosphatidyltransferase family. To study if these mechanistically distinct CLS enzymes are able to catalyze cardiolipin production in a cell that normally expresses a different type of CLS, we expressed TbCLS and ScCrd1 in CLS-deficient yeast and trypanosome strains, respectively. Our results show that TbCLS complemented cardiolipin production in CRD1 knockout yeast and partly restored wild-type colony forming capability under stress conditions. Remarkably, CL remodeling appeared to be impaired in the transgenic construct, suggesting that CL production and remodeling are tightly coupled processes that may require a clustering of the involved proteins into specific CL-synthesizing domains. In contrast, no complementation was observed by heterologous expression of ScCrd1 in conditional TbCLS knockout trypanosomes, despite proper mitochondrial targeting of the protein.

Viruses manipulate cellular processes to create an environment favorable to replication. For most viruses, this includes subverting the expression of interferon (IFN), a signaling molecule that can stimulate production of a vast array of antiviral gene products. Rotavirus, a segmented double-stranded RNA virus that causes acute gastroenteritis in infants and young children, inhibits IFN expression through its nonstructural protein NSP1. This viral protein stifles IFN expression by inducing the degradation of host factors that are necessary for upregulating the activity of IFN genes. In the case of nearly all human and porcine rotavirus strains, NSP1 induces the ubiquitination-dependent proteasomal degradation of β-transducin repeat containing protein (β-TrCP), a host factor that plays an essential role in activating the IFN-transcription factor, NF-κB. Key to the process is the presence of a decoy sequence (degron) at the C-terminus of NSP1 that causes β-TrCP to mistakenly bind NSP1 instead of its natural target, inhibitor-of-κB (IκB). In a recent report published by Davis et al [2017; mBio 8(4): e01213-17], we describe molecular requirements that govern NSP1 recognition of β-TrCP, including an essential degron phosphorylation event, and the step-wise incorporation of NSP1 into hijacked cullin-RING E3 ligases (CRLs) that ubiquitinate and tag β-TrCP for degradation. Notably, although β-TrCP is chiefly recognized for its role as a master regulator of NF-κB signaling and IFN expression, β-TrCP also controls the stability of checkpoint proteins implicated in numerous other cellular pathways with antiviral activities, including autophagy and apoptosis. Thus, the impact of NSP1 on creating an intracellular environment favorable to virus replication may extend well beyond the IFN signaling pathway.

Fungal pathogens rely on the production of specific virulence factors during infection. Inhibiting such factors generally results in reduced fungal pathogenicity. Most studies in the past have focused on understanding the molecular mechanisms of fungal virulence factor expression during mono-culture, or during interaction with the host. However, a potentially important, second type of interaction has been less well studied thus far - the interplay of fungal pathogens of humans with other microbes found in their natural habitat. Specifically, whether environmental bacteria may impact fungal virulence factor production is largely unknown. In our recent work, we have identified the soil bacterium, Bacillus safensis, as a potent inhibitor of virulence factor production by two major fungal pathogens of humans, Cryptococcus neoformans, and Candida albicans. We determined that the anti-virulence factor mechanism is, at least in part, based on production of bacterial chitinases that target and destabilize the fungal cell surface. These findings describe a cross-kingdom interaction between an environmental bacterium and pathogenic fungi, and highlight the fungal cell wall as an attractive antifungal drug target.

Growth and death are both fundamental macroscopic properties for all living matters, and thus cell division and mortality rates are good parameters for characterizing cellular physiology in a given environment. While population growth rates in various conditions have been reported in literature, death rate is rarely measured, especially in favorable culture conditions where cells grow exponentially. In our recent study (Nakaoka and Wakamoto, 2017), we developed a microfluidics-based platform to track multiple single cell lineages until death. The system enabled us to monitor both cell growth and death in controlled steady environments, and we confirmed the absence of replicative aging in fission yeast old-pole cell lineages by showing remarkable constancy both in cell division and mortality rates. Furthermore, we revealed a growth-death trade-off relation in non-stressed conditions. The phenomenological law that constrains macroscopic physiological parameters could provide a new quantitative insight into possible balanced-growth states in various environments.