Cooperative secretions facilitate host range expansion in bacteria

The majority of emergent human pathogens are zoonotic in origin, that is, they can transmit to humans from other animals. Understanding the factors underlying the evolution of pathogen host range is therefore of critical importance in protecting human health. There are two main evolutionary routes to generalism: organisms can tolerate multiple environments or they can modify their environments to forms to which they are adapted. Here we use a combination of theory and a phylogenetic comparative analysis of 191 pathogenic bacterial species to show that bacteria use cooperative secretions that modify their environment to extend their host range and infect multiple host species. Our results suggest that cooperative secretions are key determinants of host range in bacteria, and that monitoring for the acquisition of secreted proteins by horizontal gene transfer can help predict emerging zoonoses

Trans-mitochondrial coordination of cristae at regulated membrane junctions

Reminiscent of bacterial quorum sensing, mammalian mitochondria participate in inter-organelle communication. However, physical structures that enhance or enable interactions between mitochondria have not been defined. Here we report that adjacent mitochondria exhibit coordination of inner mitochondrial membrane cristae at inter-mitochondrial junctions (IMJs). These electron-dense structures are conserved across species, resistant to genetic disruption of cristae organization, dynamically modulated by mitochondrial bioenergetics, independent of known inter-mitochondrial tethering proteins mitofusins and rapidly induced by the stable rapprochement of organelles via inducible synthetic linker technology. At the associated junctions, the cristae of adjacent mitochondria form parallel arrays perpendicular to the IMJ, consistent with a role in electrochemical coupling. These IMJs and associated cristae arrays may provide the structural basis to enhance the propagation of intracellular bioenergetic and apoptotic waves through mitochondrial networks within cells

Optimised chronic infection models demonstrate that siderophore ‘cheating’ in Pseudomonas aeruginosa is context specific

The potential for siderophore mutants of Pseudomonas aeruginosa to attenuate virulence during infection, and the possibility of exploiting this for clinical ends, have attracted much discussion. This has largely been based on the results of in vitro experiments conducted in iron-limited growth medium, in which siderophore mutants act as social ‘cheats:’ increasing in frequency at the expense of the wild type to result in low-productivity, low-virulence populations dominated by mutants. We show that insights from in vitro experiments cannot necessarily be transferred to infection contexts. First, most published experiments use an undefined siderophore mutant. Whole-genome sequencing of this strain revealed a range of mutations affecting phenotypes other than siderophore production. Second, iron-limited medium provides a very different environment from that encountered in chronic infections. We conducted cheating assays using defined siderophore deletion mutants, in conditions designed to model infected fluids and tissue in cystic fibrosis lung infection and non-healing wounds. Depending on the environment, siderophore loss led to cheating, simple fitness defects, or no fitness effect at all. Our results show that it is crucial to develop defined in vitro models in order to predict whether siderophores are social, cheatable and suitable for clinical exploitation in specific infection contexts

Defining motility in the Staphylococci

The ability of bacteria to move is critical for their survival in diverse environments and multiple ways have evolved to achieve this. Two forms of motility have recently been described for Staphylococcus aureus, an organism previously considered to be non-motile. One form is called spreading, which is a type of sliding motility and the second form involves comet formation, which has many observable characteristics associated with gliding motility. Darting motility has also been observed in Staphylococcus epidermidis. This review describes how motility is defined and how we distinguish between passive and active motility. We discuss the characteristics of the various forms of Staphylococci motility, the molecular mechanisms involved and the potential future research directions

Pseudomonas aeruginosa aggregate formation in an alginate bead model system exhibits in vivo-like characteristics

© 2017 American Society for Microbiology. All Rights Reserved. Alginate beads represent a simple and highly reproducible in vitro model system for diffusion-limited bacterial growth. In this study, alginate beads were inoculated with Pseudomonas aeruginosa and followed for up to 72 h. Confocal microscopy revealed that P. aeruginosa formed dense clusters similar in size to in vivo aggregates observed ex vivo in cystic fibrosis lungs and chronic wounds. Bacterial aggregates primarily grew in the bead periphery and decreased in size and abundance toward the center of the bead. Microsensor measurements showed that the O2 concentration decreased rapidly and reached anoxia ~100 μm below the alginate bead surface. This gradient was relieved in beads supplemented with NO3- as an alternative electron acceptor allowing for deeper growth into the beads. A comparison of gene expression profiles between planktonic and alginate-encapsulated P. aeruginosa confirmed that the bacteria experienced hypoxic and anoxic growth conditions. Furthermore, alginate-encapsulated P. aeruginosa exhibited a lower respiration rate than the planktonic counterpart and showed a high tolerance toward antibiotics. The inoculation and growth of P. aeruginosa in alginate beads represent a simple and flexible in vivo-like biofilm model system, wherein bacterial growth exhibits central features of in vivo biofilms. This was observed by the formation of small cell aggregates in a secondary matrix with O2-limited growth, which was alleviated by the addition of NO3 - as an alternative electron acceptor, and by reduced respiration rates, as well as an enhanced tolerance to antibiotic treatment