Predation by Myxococcus xanthus Induces Bacillus subtilis To Form Spore-Filled Megastructures

Abstract

bBiofilm formation is a common mechanism for surviving environmental stress and can be triggered by both intraspecies and interspecies interactions. Prolonged predator-prey interactions between the soil bacterium Myxococcus xanthus and Bacillus subtilis were found to induce the formation of a new type of B. subtilis biofilm, termed megastructures. Megastructures are treelike brachiations that are as large as 500 m in diameter, are raised above the surface between 150 and 200 m, and are filled with viable endospores embedded within a dense matrix. Megastructure formation did not depend on TasA, EpsE, SinI, RemA, or surfactin production and thus is genetically distinguishable from colony biofilm formation on MSgg medium. As B. subtilis endospores are not susceptible to predation by M. xanthus, megastructures appear to provide an alternative mechanism for survival. In addition, M. xanthus fruiting bodies were found immediately adjacent to the megastructures in nearly all instances, suggesting that M. xanthus is unable to acquire sufficient nutrients from cells housed within the megastructures. Lastly, a B. subtilis mutant lacking the ability to defend itself via bacillaene production formed megastructures more rapidly than the parent. Together, the results indicate that production of the megastructure facilitates B. subtilis escape into dormancy via sporulation. Biofilm formation is part of the life cycle for many bacterial species, leading to the accumulation of cells or spores embedded within a matrix (1, 2). Biofilms can be multispecies in composition (3) or comprised of a single species, as observed for Bacillus subtilis and Myxococcus xanthus. For both B. subtilis and M. xanthus, biofilms arise in response to environmental challenges, such as interspecies interactions or competition for nutrients, and culminate in sporulation, an escape into dormancy as a long-term survival mechanism. Entry into the dormant state is an energyconsuming process, and thus, the decision whether to enter the dormant state is critical for cells and is controlled by complex regulatory networks (4). An advantage gained by B. subtilis is that spores are widely resistant to predation by the protozoan Tetrahymena thermophile, the bacterivorous nematode Caenorhabditis elegans (5, 6), as well as the predatory bacterium M. xanthus (7). An advantage gained by M. xanthus spore production within fruiting bodies is that subsequent germinating populations are at critical numbers for group behavior, including predation.For both B. subtilis and M. xanthus, the biofilms that house dormant spores display varied structural complexity. The various biofilms produced by myxobacteria range from tree-like structures in Stigmatella aurantiaca (8) to domes for M. xanthus (9). These structures are called fruiting bodies, as they contain quiescent spores capable of germinating after extended periods of dormancy. For B. subtilis, cells produce wrinkled biofilms containing spores on solid surfaces (colony biofilms or fruiting bodies) or at an air-liquid interfa...

Identification of Functions Affecting Predator-Prey Interactions between Myxococcus xanthus and Bacillus subtilis

“…Each plate contained both a negative (loss-of-function [LOF]) control, as well as the M. xanthus parent for comparison. The LOF control used here is the mglAB mutant that was previously characterized for its loss of motility and inability to carry out predation (7). The predation plates were incubated at 32°C, and each individual prey spot was screened by microscopy each day for 5 days.…”

“…During prolonged exposure to predation, B. subtilis produces megastructures composed of viable spores embedded within what appears to be a dense matrix of unknown composition. Our model suggests that megastructure formation provides an opportunity for B. subtilis NCIB3610 to produce spores, allowing for long-term survival of cells (7).…”

“…M. xanthus motility requires type IV pilus (T4P) motility and another machine involved in gliding (e.g., focal adhesion complexes and exopolysaccharide [EPS]). It has been shown that efficient predation requires the regulation of both motility systems, which are controlled, in part, by the Frz chemosensory system (7,11,12). Upon encountering prey, M. xanthus cells display coordinated rippling in which groups of cells move back and forth trapping prey cells to enhance predation, a phenomenon called predataxis (12).…”

“…Another ubiquitous microbial predator is Myxococcus xanthus, which secrets lytic enzymes and specialized metabolites and hunts in groups and consumes prey (4)(5)(6). M. xanthus is able to consume a diverse repertoire of microbes ranging from phage to bacterial plant pathogens and clinical isolates, and it uses the resulting nutrients to sustain growth (7)(8)(9).…”

Biofilms 2015: Multidisciplinary Approaches Shed Light into Microbial Life on Surfaces

“…Bacillus subtilis also produces the metabolite bacillaene to fend off predatory M. xanthus. Antibiotic production at the interface between layers of M. xanthus and B. subtilis spores may result in a "standoff" between the two organisms (52).…”