Summary
Here, we review the multiple mechanisms that the Gram‐positive bacterium Bacillus subtilis uses to allow it to communicate between cells and establish community structures. The modes of ...action that are used are highly varied and include routes that sense pheromone levels during quorum sensing and control gene regulation, the intimate coupling of cells via nanotubes to share cytoplasmic contents, and long‐range electrical signalling to couple metabolic processes both within and between biofilms. We explore the ability of B. subtilis to detect ‘kin’ (and ‘cheater cells’) by looking at the mechanisms used to potentially ensure beneficial sharing (or limit exploitation) of extracellular ‘public goods’. Finally, reflecting on the array of methods that a single bacterium has at its disposal to ensure maximal benefit for its progeny, we highlight that a large future challenge will be integrating how these systems interact in mixed‐species communities.
‘Bacillus subtilis is a Gram‐positive bacterium that has evolved multiple mechanisms for communication and cooperation. Here we review processes which include division of labour, nanotube formation, metabolic signalling, and quorum sensing’.
Swarming is the collective movement of bacteria across a surface. It requires the production of surfactants (public goods) to overcome surface tension and provides an excellent model to investigate ...bacterial cooperation. Previously, we correlated swarm interaction phenotypes with kin discrimination between B. subtilis soil isolates, by showing that less related strains form boundaries between swarms and highly related strains merge. However, how kin discrimination affects cooperation and territoriality in swarming bacteria remains little explored. Here we show that the pattern of surface colonization by swarming mixtures is influenced by kin types. Closely related strain mixtures colonize the surface in a mixed swarm, while mixtures of less related strains show competitive exclusion as only one strain colonizes the surface. The outcome of nonkin swarm expansion depends on the initial ratio of the competing strains, indicating positive frequency-dependent competition. We find that addition of surfactin (a public good excreted from cells) can complement the swarming defect of nonkin mutants, whereas close encounters in nonkin mixtures lead to territorial exclusion, which limits the exploitation of surfactin by nonkin nonproducers. The work suggests that kin discrimination driven competitive territorial exclusion may be an important determinant for the success of cooperative surface colonization.
We investigated microbial growth in increasing concentrations of hexavalent chromium (Cr(VI)) and its reduction by a microbial community enriched from tannery effluent and by the bacterial strains ...isolated from the enriched community. The bacterial growth was monitored by measuring the optical cell density (OD
), while the Cr(VI) concentration in the samples was determined using spectrophotometry and liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). At a Cr(VI) concentration of 100 mg/L, the isolates affiliated with Pseudomonas aeruginosa (P. aeruginosa) reached higher optical cell densities, but were in general less effective for Cr(VI) reduction than the isolates affiliated with Mammaliicoccus sciuri (M. sciuri). All three M. sciuri isolates and only one of the seven P. aeruginosa isolates were able to reduce 50% of the Cr(VI) with an initial concentration of 100 mg/L within 24 h (pH 7.1), while the six isolates affiliated with P. aeruginosa were less effective. Compared to the isolated, individual bacterial strains, the enriched microbial community was better adapted to the elevated Cr(VI) concentrations, but needed a longer time (48 h) to reduce the Cr(VI) with the same efficacy as the most efficient individual isolates. The ability of the enriched microbial community and the isolated bacterial strains to reduce the Cr(VI) highlights their potential for use in the rapid bioremediation of wastewaters contaminated with Cr(VI).
Abstract
Ammonia oxidation, the first step in nitrification, is performed by autotrophic bacteria and thaumarchaea, whose relative contributions vary in different soils. Distinctive environmental ...niches for the two groups have not been identified, but evidence from previous studies suggests that activity of thaumarchaea, unlike that of bacterial ammonia oxidizers, is unaffected by addition of inorganic N fertilizer and that they preferentially utilize ammonia generated from the mineralization of organic N. This hypothesis was tested by determining the influence of both inorganic and organic N sources on nitrification rate and ammonia oxidizer growth and community structure in microcosms containing acidic, forest soil in which ammonia oxidation was dominated by thaumarchaea. Nitrification rate was unaffected by the incubation of soil with inorganic ammonium but was significantly stimulated by the addition of organic N. Oxidation of ammonia generated from native soil organic matter or added organic N, but not added inorganic N, was accompanied by increases in abundance of the thaumarchaeal amoA gene, a functional gene for ammonia oxidation, but changes in community structure were not observed. Bacterial amoA genes could not be detected. Ammonia oxidation was completely inhibited by 0.01% acetylene in all treatments, indicating ammonia monooxygenase-dependent activity. The findings have implications for current models of soil nitrification and for nitrification control strategies to minimize fertilizer loss and nitrous oxide production.
Abstract
Bacillus subtilis
is a soil bacterium that is competent for natural transformation. Genetically distinct
B. subtilis
swarms form a boundary upon encounter, resulting in killing of one of the ...strains. This process is mediated by a fast-evolving kin discrimination (KD) system consisting of cellular attack and defence mechanisms. Here, we show that these swarm antagonisms promote transformation-mediated horizontal gene transfer between strains of low relatedness. Gene transfer between interacting non-kin strains is largely unidirectional, from killed cells of the donor strain to surviving cells of the recipient strain. It is associated with activation of a stress response mediated by sigma factor SigW in the donor cells, and induction of competence in the recipient strain. More closely related strains, which in theory would experience more efficient recombination due to increased sequence homology, do not upregulate transformation upon encounter. This result indicates that social interactions can override mechanistic barriers to horizontal gene transfer. We hypothesize that KD-mediated competence in response to the encounter of distinct neighbouring strains could maximize the probability of efficient incorporation of novel alleles and genes that have proved to function in a genomically and ecologically similar context.
Bioinformatics has revealed the presence of putative laccase genes in diverse bacteria, including extremophiles, autotrophs, and, interestingly, anaerobes. Integrity of laccase genes in anaerobes has ...been questioned, since laccases oxidize a variety of compounds using molecular oxygen as the electron acceptor. The genome of the anaerobe
Geobacter metallireducens
GS-15 contains five genes for laccase-like multicopper oxidases. In order to show whether one of the predicted genes encodes a functional laccase, the protein encoded by GMET_RS10855 was heterologously expressed in
Escherichia coli
cells. The His
6
-tagged enzyme (named GeoLacc) was purified to a large extent in the apoprotein, inactive form: incubation with CuSO
4
allowed a 43-fold increase of the specific activity yielding a metallo-enzyme. The purified enzyme oxidized some of the typical laccase substrates, including 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), syringaldazine, and 2,6-dimethoxyphenol (2,6-DMP), along with pyrogallol and K
4
Fe(CN)
6
. Temperature optimum was 75 °C and pH optimum for ABTS and 2,6-DMP oxidation was ~ 6.0. As observed for other laccases, the enzyme was inhibited by halide anions and was sensitive to increasing concentrations of dimethyl sulfoxide and Tween-80. Notably, GeoLacc possesses a very high affinity for dioxygen: a similar activity was measured performing the reaction at air-saturated or microaerophilic conditions.
Kin discrimination, broadly defined as differential treatment of conspecifics according to their relatedness, could help biological systems direct cooperative behavior toward their relatives. Here we ...investigated the ability of the soil bacterium Bacillus subtilis to discriminate kin from nonkin in the context of swarming, a cooperative multicellular behavior. We tested a collection of sympatric conspecifics from soil in pairwise combinations and found that despite their history of coexistence, the vast majority formed distinct boundaries when the swarms met. Some swarms did merge, and most interestingly, this behavior was only seen in the most highly related strain pairs. Overall the swarm interaction phenotype strongly correlated with phylogenetic relatedness, indicative of kin discrimination. Using a subset of strains, we examined cocolonization patterns on plant roots. Pairs of kin strains were able to cocolonize roots and formed a mixed-strain biofilm. In contrast, inoculating roots with pairs of nonkin strains resulted in biofilms consisting primarily of one strain, suggestive of an antagonistic interaction among nonkin strains. This study firmly establishes kin discrimination in a bacterial multicellular setting and suggests its potential effect on ecological interactions.
In this study, microcosms were used to investigate the influence of temperature (4 and 28 °C) and water content (45% and 90% WHC) on microbial communities and activities in carbon-rich fen soil. ...Bacterial, archaeal and denitrifier community composition was assessed during incubation of microcosms for 12 weeks using terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA and nitrous oxide reductase (nosZ) genes. In addition, microbial and denitrifier abundance, potential denitrification activity and production of greenhouse gases were measured. No detectable changes were observed in prokaryote or denitrifier abundance. In general, cumulatively after 12 weeks more carbon was respired at the higher temperature (3.7 mg CO₂ g⁻¹ soil), irrespective of the water content, whereas nitrous oxide production was greater under wet conditions (98-336 μg N₂O g⁻¹ soil). After an initial lag phase, methane emissions (963 μg CH₄ g⁻¹ soil) were observed only under warm and wet conditions. T-RFLP analyses of bacterial 16S rRNA and nosZ genes revealed small or undetectable community changes in response to temperature and water content, suggesting that bacterial and denitrifying microbial communities are stable and do not respond significantly to seasonal changes in soil conditions. In contrast, archaeal microbial community structure was more dynamic and was strongly influenced by temperature.
The occurrence and environmental toxicity of pharmaceuticals have recently attracted increasing attention. Diclofenac is a highly consumed non-steroidal anti-inflammatory drug, which is often ...detected in wastewaters, but investigations of its influence on bacteria are scarce.
We investigated the influence of this pharmaceutical on bacterial community in activated sludge exposed to increasing concentrations of diclofenac in fed-batch reactors over 41 days. Nitrification activity of the activated sludge was measured and changes in bacterial community structure were followed using culture-independent molecular method (terminal restriction fragment length polymorphism, T-RFLP) and by the cultivation approach.
Nitrification activity was not detectably influenced by the addition of diclofenac, while the main change of the bacterial community structure was detected only at the end of incubation (after 41 days) when diclofenac was added to artificial wastewater as the only carbon source. Changes in community composition due to enrichment were observed using cultivation approach. However, taxonomic affiliation of isolates did not match taxons identified by T-RFLP community profiling. Isolates obtained from activated sludge used as inoculum belonged to five genera:
,
,
r,
and
, known for their potential to degrade aromatic compounds. However, only
species were isolated after the last enrichment step on minimal agar plates with diclofenac added as the sole carbon source.
Our results suggest that the selected recalcitrant and commonly detected pharmaceutical does not strongly influence the sensitive and important nitrification process of wastewater treatment. Moreover, the isolated strains obtained after enrichment procedure that were able to grow on minimal agar plates with diclofenac added as the only carbon source could serve as potential model bacteria to study bacterial diclofenac degradation.
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