pSM19035 of the pathogenic bacterium Streptococcus pyogenes is a low-copy-number plasmid carrying erythromycin resistance, stably maintained in a broad range of gram-positive bacteria. We show here that the --operon of this plasmid constitutes a novel proteic plasmid addiction system in which the and genes encode an antitoxin and toxin, respectively, while plays an autoregulatory function. Expression of toxin Zeta is bactericidal for the gram-positive Bacillus subtilis and bacteriostatic for the gram-negative Escherichia coli. The toxic effects of gene expression in both bacterial species are counteracted by proper expression of . The -toxin-antitoxin cassette stabilizes plasmids in E. coli less efficiently than in B. subtilis.Bacterial plasmids are generally inherited in a very stable manner and independently of the cell chromosome. The specific mechanisms of stable plasmid maintenance have been studied mainly for plasmids replicating in gram-negative bacteria (23,44). In the majority of high-copy-number plasmids, the copy number and cell division control in combination with the multimer resolution system ensure a very low frequency of plasmid loss. For low-copy-number plasmids, mechanisms exist which enable their maintenance during cell growth in nonselective conditions. While the active partitioning process precisely distributes plasmid copies to each daughter cell at division (33, 64), plasmid addiction systems (also called toxinantitoxin TA cassettes) kill or reduce the growth of plasmidfree descendant cells (1, 35). The molecular basis of the postsegregational killing (PSK) requires the existence of at least two plasmid genes: one specifying a stable toxic agent and another coding for an unstable factor which prevents the lethal action of the toxin. While the toxin is always a protein, the antidote is either antisense RNA (which inhibits the translation of toxin mRNA) or a protein (35). Many such systems and their chromosomal analogues have been described for different gram-negative bacteria (6,24,28,45). Antisense RNA-regulated stabilization systems constitute a well-conserved group called the hok-sok family (the name reflecting the functions of the host killing and suppression of killing genes from plasmid R1) (22).Some common features can be indicated for proteic plasmid addiction systems (PPAS): organization in operons, autoregulation, formation of the antidote-toxin complex, and different decay rates of the two proteins involved (23,29,67). In contrast to the hok-sok family, there is no significant sequence similarity among PPAS genes. The specific mechanisms leading to the noxious effects of the toxin are known for few systems only. The first identified and best understood is ccd of Escherichia coli F factor (34). The CcdB toxin is a gyrase poison able to bind the free GyrA subunit and to trap cleaved DNA-gyrase complex, leading to the induction of SOS response and subsequent cell death (3). Gyrase is also the target for the ParE toxin of the broad-host-range RK2 plasmid parDE system (37), although t...

Relatively stable physical conditions in caves allow for the examination of the relationship between geochemical processes and the activity of microorganisms, reflected in substantial rock alterations, formation of new structures, surface deterioration and cave expansion. Although caves are considered as extreme environments, they are inhabited by microbial communities with unexpected diversity. While Proteobacteria and Actinobacteria are the most ubiquitous groups, also the presence of Archaea has been frequently noted recently. Here, we present a summary of results on diversity of cave microorganisms in the context of taxon distribution as well as the contribution and role of individual taxa in cave ecosystems.

No abstract

A system for biohydrogen production was developed based on long-term continuous cultures grown on sugar beet molasses in packed bed reactors. In two separate cultures, consortia of fermentative bacteria developed as biofilms on granitic stones. In one of the cultures, a granular sludge was also formed. Metagenomic analysis of the microbial communities by 454-pyrosequencing of amplified 16S rDNA fragments revealed that the overall biodiversity of the hydrogen-producing cultures was quite small. The stone biofilm from the culture without granular sludge was dominated by Clostridiaceae and heterolactic fermentation bacteria, mainly Leuconostocaeae. Representatives of the Leuconostocaeae and Enterobacteriaceae were dominant in both the granules and the stone biofilm formed in the granular sludge culture. The culture containing granular sludge produced hydrogen significantly more effectively than that containing only the stone biofilm: 5.43 vs. 2.8 mol H(2)/mol sucrose from molasses, respectively. The speculations that lactic acid bacteria may favor hydrogen production are discussed.

The stable inheritance of bacterial plasmids is achieved by a number of different mechanisms. Among them are resolution of plasmid oligomers into monomers, active plasmid partitioning into dividing cells and selective killing of plasmid-free segregants. A special focus is given to the last mechanism. It involves a stable toxin and an unstable antidote. The antidotes neutralize their cognate toxins or prevent their synthesis. The different decay rates of the toxins and the antidotes underlie molecular mechanisms of toxin activation in plasmid-free cells. By eliminating of plasmid-free cells from the population of plasmid-bearing ones the toxin-antidote couples therefore act as plasmid addiction systems.

No abstract

Toxin-antitoxin systems are widely distributed among many bacterial species, including human pathogens. Typically, these systems consist of two genes in an operon which encodes a stable toxin disrupting essential cellular processes and a labile antitoxin preventing toxicity. Regulation of type II TA system in which both components are proteins, relies on proteolysis. In this paper, we outline the significant features of antitoxin proteins important for proteolysis. We present examples of best known processes of antitoxin degradation by specific proteases mainly in Escherichia coli, but are also included intensively studied systems from other bacteria. The effect of environmental conditions on regulation and activity of TA systems and on consequences of proteolytic activity are discussed.

Anaerobic digestion is a complex process involving hydrolysis, acidogenesis, acetogenesis and methanogenesis. The separation of the hydrogen-yielding (dark fermentation) and methane-yielding steps under controlled conditions permits the production of hydrogen and methane from biomass. The characterization of microbial communities developed in bioreactors is crucial for the understanding and optimization of fermentation processes. Previously we developed an effective system for hydrogen production based on long-term continuous microbial cultures grown on sugar beet molasses. Here, the acidic effluent from molasses fermentation was used as the substrate for methanogenesis in an upflow anaerobic sludge blanket bioreactor. This study focused on the molecular analysis of the methane-yielding community processing the non-gaseous products of molasses fermentation. The substrate for methanogenesis produces conditions that favor the hydrogenotrophic pathway of methane synthesis. Methane production results from syntrophic metabolism whose key process is hydrogen transfer between bacteria and methanogenic Archaea. High-throughput 454 pyrosequencing of total DNA isolated from the methanogenic microbial community and bioinformatic sequence analysis revealed that the domain Bacteria was dominated by Firmicutes (mainly Clostridia), Bacteroidetes, δ- and γ-Proteobacteria, Cloacimonetes and Spirochaetes. In the domain Archaea, the order Methanomicrobiales was predominant, with Methanoculleus as the most abundant genus. The second and third most abundant members of the Archaeal community were representatives of the Methanomassiliicoccales and the Methanosarcinales. Analysis of the methanogenic sludge by scanning electron microscopy with Energy Dispersive X-ray Spectroscopy and X-ray diffraction showed that it was composed of small highly heterogeneous mineral-rich granules. Mineral components of methanogenic granules probably modulate syntrophic metabolism and methanogenic pathways. A rough functional analysis from shotgun data of the metagenome demonstrated that our knowledge of methanogenesis is poor and/or the enzymes responsible for methane production are highly effective, since despite reasonably good sequencing coverage, the details of the functional potential of the microbial community appeared to be incomplete.