How bacteria actively use passive physics to make biofilms.

Modern molecular microbiology elucidates the organizational principles of bacterial biofilms via detailed examination of the interplay between signaling and gene regulation. A complementary biophysical approach studies the mesoscopic dependencies at the cellular and multicellular levels with a distinct focus on intercellular forces and mechanical properties of whole biofilms. Here, motivated by recent advances in biofilm research and in other, seemingly unrelated fields of biology and physics, we propose a perspective that links the biofilm, a dynamic multicellular organism, with the physical processes occurring in the extracellular milieu. Using Bacillus subtilis as an illustrative model organism, we specifically demonstrate how such a rationale explains biofilm architecture, differentiation, communication, and stress responses such as desiccation tolerance, metabolism, and physiology across multiple scales-from matrix proteins and polysaccharides to macroscopic wrinkles and water-filled channels.

Complementary roles of EPS, T3SS and Expansin for virulence of Erwinia tracheiphila, the causative agent of cucurbit wilt.

Erwinia tracheiphila (Smith) is a recently emerged plant pathogen that causes severe economic losses in cucurbit crops in temperate Eastern North America. E. tracheiphila is xylem restricted, and virulence is thought to be related to Exopolysaccharides (EPS) and biofilm formation, which occlude the passage of sap in xylem vessels and causes systemic wilt. However, the role of EPS and biofilm formation, and their contribution to disease in relation to other virulence loci are unknown. Here, we use deletion mutants to explore the roles of EPS, Hrp Type III secretion system (Hrp T3SS) and Expansin in plant colonization and virulence. Then, we quantify the expression of the genes encoding these factors during infection. Our results show that Exopolysaccharides are essential for E. tracheiphila survival in host plants, while Hrp T3SS and Expansin are dispensable for survival but needed for systemic wilt symptom development. EPS and Hrp T3SS display contrasting expression patterns in the plant, reflecting their relevance in different stages of the infection. Finally, we show that expression of the eps and hrpT3SS operons is downregulated in mildly increased temperatures, suggesting a link between expression of these virulence factors and geographic restriction of E. tracheiphila to temperate regions. Our work highlights how E. tracheiphila virulence is a complex trait where several loci are coordinated during infection. These results further shed light into the relationship between virulence factors and the ecology of this pathosystem, which will be essential for developing sustainable management strategies for this emerging pathogen.

Asking a question.

While scientific research should be carried out objectively, the choices of questions asked and approaches taken are deeply personal and subjective. I urge individuals to pursue questions they love and to periodically scrutinize the reasons (the philosophies) that drive that love. As a case study, I scrutinize the "whys" behind some of the scientific questions I pursued during my career.

The New Microbiology: an international lecture course on the island of Spetses.

In September 2022, an international summer course entitled 'The new microbiology' took place in Greece, on the island of Spetses. The organizers aimed to highlight the spectacular advances and the renaissance occurring in Microbiology, driven by developments in genomics, proteomics, imaging techniques, and bioinformatics. Combinations of these advances allow for single cell analyses, rapid and relatively inexpensive metagenomic and transcriptomic data analyses and comparisons, visualization of previously unsuspected mechanisms, and large-scale studies. A 'New Microbiology' is emerging which allows studies that address the critical roles of microbes in health and disease, in humans, animals, and the environment. The concept of one health is now transforming microbiology. The goal of the course was to discuss all these topics with members of the new generation of microbiologists all of whom were highly motivated and fully receptive.

Bacteria grow swiftly and live thriftily.

Bacteria have evolved numerous strategies to use resources efficiently. However, bacterial economies depend on both the physiological context of the organisms as well as their growth state - whether they are growing, non-growing or reinitiating growth. In this essay, we discuss some of the features that make bacteria efficient under these different conditions and during the transitions between them. We also highlight the many outstanding questions regarding the physiology of non-growing bacterial cells. Lastly, we examine how efficiency is apparent in both the mode and tempo of bacterial evolution.

The Spanish Society for Microbiology and Latin American microbiologists: seventy-five years of joint scientific ventures.

As part of this Special Issue of International Microbiology celebrating the 75th anniversary of the founding of the Spanish Society for Microbiology (SEM), Guest Editor Rafael Giraldo invited us to contribute an opinion article on the topic of 75 years of joint scientific ventures between Latin American microbiologists and Spanish microbiologists. Since the creation of SEM in 1945 (Pérez Prieto, NoticiaSEM 98:1, 2016) Latin American microbiologists have been participants, both as individuals and as members of the national associations that are currently integrated into the Latin American Association for Microbiology (ALAM). Thus, the histories of Spanish and Latin American microbiology (Chica and Skinner, Int Microbiol 13:159-164, 2010; Chica, Int Microbiol 11:221-225, 2008) have been closely linked over the last 75 years. In order to provide our perspective on the topic, we decided to provide answers to three questions:What are key aspects of the history of Spanish and Latin American Microbiology interactions? What have been some of our personal experiences in which these interactions shaped our careers? What is our outlook for the future of such interactions?

The History of Microbiology-A Personal Interpretation.

Microbiology began as a unified science using the principles of chemistry to understand living systems. The unified view quickly split into the subdisciplines of medical microbiology, molecular biology, and environmental microbiology. The advent of a universal phylogeny and culture-independent approaches has helped tear down the boundaries separating the subdisciplines. The vision for the future is that the study of the fundamental roles of microbes in ecology and evolution will lead to an integrated biology with no boundary between microbiology and macrobiology.

A horizontally acquired expansin gene increases virulence of the emerging plant pathogen Erwinia tracheiphila.

Erwinia tracheiphila is a bacterial plant pathogen that causes a fatal wilt infection in some cucurbit crop plants. Wilt symptoms are thought to be caused by systemic bacterial colonization through xylem that impedes sap flow. However, the genetic determinants of within-plant movement are unknown for this pathogen species. Here, we find that E. tracheiphila has horizontally acquired an operon with a microbial expansin (exlx) gene adjacent to a glycoside hydrolase family 5 (gh5) gene. Plant inoculation experiments with deletion mutants in the individual genes (Δexlx and Δgh5) and the full operon (Δexlx-gh5) resulted in decreased severity of wilt symptoms, decreased mortality rate, and impaired systemic colonization compared to the Wt strain. Co-inoculation experiments with Wt and Δexlx-gh5 rescued the movement defect of the mutant strain, suggesting that expansin and GH5 function extracellularly. Together, these results show that expansin-GH5 contributes to systemic movement through xylem, leading to rapid wilt symptom development and higher rates of plant death. The presence of expansin genes in diverse species of bacterial and fungal wilt-inducing pathogens suggests that microbial expansin proteins may be an under-appreciated virulence factor for many pathogen species.

The Microbiomes of Seven Lichen Genera Reveal Host Specificity, a Reduced Core Community and Potential as Source of Antimicrobials.

The High Andean Paramo ecosystem is a unique neotropical mountain biome considered a diversity and evolutionary hotspot. Lichens, which are complex symbiotic structures that contain diverse commensal microbial communities, are prevalent in Paramos. There they play vital roles in soil formation and mineral fixation. In this study we analyzed the microbiomes of seven lichen genera in Colombian Paramos using 16S rRNA gene amplicon sequencing and provide the first description of the bacterial communities associated with Cora and Hypotrachyna lichens. Paramo lichen microbiomes varied in diversity indexes and number of OTUs, but were composed predominantly by the phyla Acidobacteria, Actinobacteria, Bacteroidetes, Cyanobacteria, Proteobacteria, and Verrucomicrobia. In the case of Cora and Cladonia, the microbiomes were distinguished based on the identity of the lichen host. While the majority of the lichen-associated microorganisms were not present in all lichens sampled, sixteen taxa shared among this diverse group of lichens suggest a core lichen microbiome that broadens our concept of these symbiotic structures. Additionally, we identified strains producing compounds active against clinically relevant microbial strains. These results indicate that lichen microbiomes from the Paramo ecosystem are diverse and host-specific but share a taxonomic core and can be a source of new bacterial taxa and antimicrobials.

On the possible ecological roles of antimicrobials.

The Introduction of antibiotics into the clinical use in the middle of the 20th century had a profound impact on modern medicine and human wellbeing. The contribution of these wonder molecules to public health and science is hard to overestimate. Much research has informed our understanding of antibiotic mechanisms of action and resistance at inhibitory concentrations in the lab and in the clinic. Antibiotics, however, are not a human invention as most of them are either natural products produced by soil microorganisms or semisynthetic derivatives of natural products. Because we use antibiotics to inhibit the bacterial growth, it is generally assumed that growth inhibition is also their primary ecological function in the environment. Nevertheless, multiple studies point to diverse nonlethal effects that are exhibited at lower levels of antibiotics. Here we review accumulating evidence of antibiosis and of alternative functions of antibiotics exhibited at subinhibitory concentrations. We also speculate on how these effects might alter phenotypes, fitness, and community composition of microbes in the context of the environment and suggest directions for future research.

The environmental occurrence of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is generally described as ubiquitous in natural settings, such as soil and water. However, because anecdotal observations and published reports have questioned whether or not this description is true, we undertook a rigorous study using three methods to investigate the occurrence of P. aeruginosa: We investigated environmental samples, analyzed 16S rRNA data, and undertook a systematic review and meta-analysis of published data. The environmental sample screening identified P. aeruginosa as significantly associated with hydrocarbon and pesticide-contaminated environments and feces, as compared to uncontaminated environments in which its prevalence was relatively low. The 16S rRNA data analysis showed that P. aeruginosa sequences were present in all habitats but were most abundant in samples from human and animals. Similarly, the meta-analysis revealed that samples obtained from environments with intense human contact had a higher prevalence of P. aeruginosa compared to those with less human contact. Thus, we found a clear tendency of P. aeruginosa to be present in places closely linked with human activity. Although P. aeruginosa may be ubiquitous in nature, it is usually scarce in pristine environments. Thus, we suggest that P. aeruginosa should be described as a bacterium largely found in locations associated with human activity.

Microbial community modulates growth of symbiotic fungus required for stingless bee metamorphosis.

The Brazilian stingless bee Scaptotrigona depilis requires the brood cells-associated fungus Zygosaccharomyces sp. as steroid source for metamorphosis. Besides the presence of Zygosaccharomyces sp., other fungi inhabit S. depilis brood cells, but their biological functions are unknown. Here we show that Candida sp. and Monascus ruber, isolated from cerumen of S. depilis brood provisions, interact with Zygosaccharomyces sp. and modulate its growth. Candida sp. produces volatile organic compounds (VOCs) that stimulate Zygosacchromyces sp. development. Monascus ruber inhibits Zygosacchromyces sp. growth by producing lovastatin, which blocks steroid biosynthesis. We also observed that in co-cultures M. ruber inhibits Candida sp. through the production of monascin. The modulation of Zygosaccharomyces sp. growth by brood cell-associated fungi suggests their involvement in S. depilis larval development. This tripartite fungal community opens new perspectives in the research of microbial interactions with bees.

When We Stop Thinking about Microbes as Cells.

Microbes are often thought of as individual cells. However, in their natural habitats, they typically exist in the context of other cells, be they of the same or different species. How these cells interact in space and time is key to their ecology and evolution. In this perspective, we consider the implications of this in terms of the future of microbiological research. This article is part of the special issue "Microbiology: how to bridge mechanisms and phenomenology" edited by Suckjoon Jun.

PolyGlcNAc-containing exopolymers enable surface penetration by non-motile Enterococcus faecalis.

Bacterial pathogens have evolved strategies that enable them to invade tissues and spread within the host. Enterococcus faecalis is a leading cause of local and disseminated multidrug-resistant hospital infections, but the molecular mechanisms used by this non-motile bacterium to penetrate surfaces and translocate through tissues remain largely unexplored. Here we present experimental evidence indicating that E. faecalis generates exopolysaccharides containing ß-1,6-linked poly-N-acetylglucosamine (polyGlcNAc) as a mechanism to successfully penetrate semisolid surfaces and translocate through human epithelial cell monolayers. Genetic screening and molecular analyses of mutant strains identified glnA, rpiA and epaX as genes critically required for optimal E. faecalis penetration and translocation. Mechanistically, GlnA and RpiA cooperated to generate uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that was utilized by EpaX to synthesize polyGlcNAc-containing polymers. Notably, exogenous supplementation with polymeric N-acetylglucosamine (PNAG) restored surface penetration by E. faecalis mutants devoid of EpaX. Our study uncovers an unexpected mechanism whereby the RpiA-GlnA-EpaX metabolic axis enables production of polyGlcNAc-containing polysaccharides that endow E. faecalis with the ability to penetrate surfaces. Hence, targeting carbohydrate metabolism or inhibiting biosynthesis of polyGlcNAc-containing exopolymers may represent a new strategy to more effectively confront enterococcal infections in the clinic.

Draft Genome Sequence of an Erwinia tracheiphila Isolate from an Infected Muskmelon (Cucumis melo).

Erwinia tracheiphila is a bacterial plant pathogen emerging in eastern North America. To aid in understanding genetic variation within E. tracheiphila, here we sequence the first reference genome of an infected muskmelon (Cucumis melo). The genome assembles into a single chromosomal contig, three plasmid contigs, and one bacteriophage contig.

A single mutation in rapP induces cheating to prevent cheating in Bacillus subtilis by minimizing public good production.

Cooperation is beneficial to group behaviors like multicellularity, but is vulnerable to exploitation by cheaters. Here we analyze mechanisms that protect against exploitation of extracellular surfactin in swarms of Bacillus subtilis. Unexpectedly, the reference strain NCIB 3610 displays inherent resistance to surfactin-non-producing cheaters, while a different wild isolate is susceptible. We trace this interstrain difference down to a single amino acid change in the plasmid-borne regulator RapP, which is necessary and sufficient for cheater mitigation. This allele, prevalent in many Bacillus species, optimizes transcription of the surfactin operon to the minimum needed for full cooperation. When combined with a strain lacking rapP, NCIB 3610 acts as a cheater itself-except it does not harm the population at high proportions since it still produces enough surfactin. This strategy of minimal production is thus a doubly advantageous mechanism to limit exploitation of public goods, and is readily evolved from existing regulatory networks.

An Introduced Crop Plant Is Driving Diversification of the Virulent Bacterial Pathogen Erwinia tracheiphila.

Erwinia tracheiphila is the causal agent of bacterial wilt of cucurbits, an economically important phytopathogen affecting an economically important phytopathogen affecting few cultivated Cucurbitaceae few cultivated Cucurbitaceae host plant species in temperate eastern North America. However, essentially nothing is known about E. tracheiphila population structure or genetic diversity. To address this shortcoming, a representative collection of 88 E. tracheiphila isolates was gathered from throughout its geographic range, and their genomes were sequenced. Phylogenomic analysis revealed three genetic clusters with distinct hrpT3SS virulence gene repertoires, host plant association patterns, and geographic distributions. Low genetic heterogeneity within each cluster suggests a recent population bottleneck followed by population expansion. We showed that in the field and greenhouse, cucumber (Cucumis sativus), which was introduced to North America by early Spanish conquistadors, is the most susceptible host plant species and the only species susceptible to isolates from all three lineages. The establishment of large agricultural populations of highly susceptible C. sativus in temperate eastern North America may have facilitated the original emergence of E. tracheiphila into cucurbit agroecosystems, and this introduced plant species may now be acting as a highly susceptible reservoir host. Our findings have broad implications for agricultural sustainability by drawing attention to how worldwide crop plant movement, agricultural intensification, and locally unique environments may affect the emergence, evolution, and epidemic persistence of virulent microbial pathogens.IMPORTANCEErwinia tracheiphila is a virulent phytopathogen that infects two genera of cucurbit crop plants, Cucurbita spp. (pumpkin and squash) and Cucumis spp. (muskmelon and cucumber). One of the unusual ecological traits of this pathogen is that it is limited to temperate eastern North America. Here, we complete the first large-scale sequencing of an E. tracheiphila isolate collection. From phylogenomic, comparative genomic, and empirical analyses, we find that introduced Cucumis spp. crop plants are driving the diversification of E. tracheiphila into multiple lineages. Together, the results from this study show that locally unique biotic (plant population) and abiotic (climate) conditions can drive the evolutionary trajectories of locally endemic pathogens in unexpected ways.

Amycomicin is a potent and specific antibiotic discovered with a targeted interaction screen.

The rapid emergence of antibiotic-resistant pathogenic bacteria has accelerated the search for new antibiotics. Many clinically used antibacterials were discovered through culturing a single microbial species under nutrient-rich conditions, but in the environment, bacteria constantly encounter poor nutrient conditions and interact with neighboring microbial species. In an effort to recapitulate this environment, we generated a nine-strain actinomycete community and used 16S rDNA sequencing to deconvolute the stochastic production of antimicrobial activity that was not observed from any of the axenic cultures. We subsequently simplified the community to just two strains and identified Amycolatopsis sp. AA4 as the producing strain and Streptomyces coelicolor M145 as an inducing strain. Bioassay-guided isolation identified amycomicin (AMY), a highly modified fatty acid containing an epoxide isonitrile warhead as a potent and specific inhibitor of Staphylococcus aureus Amycomicin targets an essential enzyme (FabH) in fatty acid biosynthesis and reduces S. aureus infection in a mouse skin-infection model. The discovery of AMY demonstrates the utility of screening complex communities against specific targets to discover small-molecule antibiotics.

Quantification of the Composition Dynamics of a Maize Root-associated Simplified Bacterial Community and Evaluation of Its Biological Control Effect.

Besides analyzing the composition and dynamics of microbial communities, plant microbiome research aims to understanding the mechanism of plant microbiota assembly and their biological functions. Here, we describe procedures to investigate the role of bacterial interspecies interactions in root microbiome assembly and the beneficial effects of the root microbiota on hosts by using a maize root-associated simplified seven-species (Stenotrophomonas maltophilia, Ochrobactrum pituitosum, Curtobacterium pusillum, Enterobacter cloacae, Chryseobacterium indologenes, Herbaspirillum frisingense and Pseudomonas putida) synthetic bacterial community described in our previous work. Surface-sterilized maize seeds were grown in a gnotobiotic system based on double-tube growth chambers after being soaked in suspensions containing multiple species of bacteria. The dynamics of the composition of the bacterial communities colonized on maize roots were tracked by a culture-dependent method with a selective medium for each of the seven strains. The impact of bacterial interactions on the community assembly was evaluated by monitoring the changes of community structure. The plant-protection effects of the simplified seven-species community were assessed by quantifying (1) the growth of a fungal phytopathogen, Fusarium verticillioides on the surfaces of the seeds and (2) the severity of seedling blight disease the fungus causes, in the presence and absence of the bacterial community. Our protocol will serve as useful guidance for studying plant-microbial community interactions under the laboratory conditions.