Novel mechanism for scavenging of hypochlorite involving a periplasmic methionine-rich Peptide and methionine sulfoxide reductase.

UNLABELLED: Reactive chlorine species (RCS) defense mechanisms are important for bacterial fitness in diverse environments. In addition to the anthropogenic use of RCS in the form of bleach, these compounds are also produced naturally through photochemical reactions of natural organic matter and in vivo by the mammalian immune system in response to invading microorganisms. To gain insight into bacterial RCS defense mechanisms, we investigated Azospira suillum strain PS, which produces periplasmic RCS as an intermediate of perchlorate respiration. Our studies identified an RCS response involving an RCS stress-sensing sigma/anti-sigma factor system (SigF/NrsF), a soluble hypochlorite-scavenging methionine-rich periplasmic protein (MrpX), and a putative periplasmic methionine sulfoxide reductase (YedY1). We investigated the underlying mechanism by phenotypic characterization of appropriate gene deletions, chemogenomic profiling of barcoded transposon pools, transcriptome sequencing, and biochemical assessment of methionine oxidation. Our results demonstrated that SigF was specifically activated by RCS and initiated the transcription of a small regulon centering around yedY1 and mrpX. A yedY1 paralog (yedY2) was found to have a similar fitness to yedY1 despite not being regulated by SigF. Markerless deletions of yedY2 confirmed its synergy with the SigF regulon. MrpX was strongly induced and rapidly oxidized by RCS, especially hypochlorite. Our results suggest a mechanism involving hypochlorite scavenging by sacrificial oxidation of the MrpX in the periplasm. Reduced MrpX is regenerated by the YedY methionine sulfoxide reductase activity. The phylogenomic distribution of this system revealed conservation in several Proteobacteria of clinical importance, including uropathogenic Escherichia coli and Brucella spp., implying a putative role in immune response evasion in vivo. IMPORTANCE: Bacteria are often stressed in the environment by reactive chlorine species (RCS) of either anthropogenic or natural origin, but little is known of the defense mechanisms they have evolved. Using a microorganism that generates RCS internally as part of its respiratory process allowed us to uncover a novel defense mechanism based on RCS scavenging by reductive reaction with a sacrificial methionine-rich peptide and redox recycling through a methionine sulfoxide reductase. This system is conserved in a broad diversity of organisms, including some of clinical importance, invoking a possible important role in innate immune system evasion.

Stress-induced changes of phospholipids in betaproteobacterium Aromatoleum aromaticum strain EbN1 due to alkylbenzene growth substrates.

The denitrifying betaproteobacterium strain EbN1 degrades toluene and ethylbenzene under anoxic conditions. Alkylbenzenes are unusual substrates, since their extraordinary chemical stability necessitates complex reactions and their toxic properties as solvents challenge cellular viability. To study the solvent impact on membrane lipid composition, strain EbN1 was grown at low, standard and semi-inhibitory concentrations of toluene (70, 240, and 740 microM) and ethylbenzene (80, 210, and 315 microM). At semi-inhibitory concentrations, phosphatidylglycerol increased at the expense of phosphatidylethanolamine. Moreover, phosphatidylcholine proportions increased and tentatively identified N-hexanoyl-phosphatidylethanolamine was detected. All observed changes in membrane lipid composition are interpreted as the organism's response to prevent the maceration of the cell membrane.

Microbial community associated with glucose-induced enhanced biological phosphorus removal.

The microbial community associated with enhanced biological phosphorus removal with glucose as the main carbon source at 11 degrees C was investigated using microscopy and molecular fingerprinting techniques. The study lasted 77 days and comprised two stages-Stage 1 when the mixture of glucose, yeast and dried milk was the organic carbon source and Stage 2 when glucose was the single carbon source. Rhodocyclus-related polyphosphate accumulating organisms, alpha-Proteobacteria and Bacteroidetes constituted 42% in Stage 1 and 45% in Stage 2, 21% in Stage 1 and 16% in Stage 2, and 10% in Stage 1 and 7% in Stage 2 of the total bacteria, respectively. The Trichococcus genus from the low GC Gram-positive bacteria was possibly responsible for lactic acid production from glucose. The microbial community was gradually changing throughout the experiment and appeared to stabilize towards the end of the experiment. Periods of suboptimal phosphorus removal could have been caused by competition among different microbial communities for carbon substrate.

Perchlorate reduction by autotrophic bacteria in the presence of zero-valent iron.

A series of batch experiments were performed to study the combination of zero-valent iron (ZVI) with perchlorate-reducing microorganisms (PRMs) to remove perchlorate from groundwater. In this method, H2 produced during the process of iron corrosion by water is used by PRMs as an electron donor to reduce perchlorate to chloride. Perchlorate degradation rates followed Monod kinetics, with a normalized maximum utilization rate (rmax) of 9200 microg g(-1) (dry wt) h(-1) and a half-velocity constant (Ks) of 8900 microg L(-1). The overall rate of perchlorate reduction was affected by the biomass density within the system. An increase in the OD600 from 0.025 to 0.08 led to a corresponding 4-fold increase of perchlorate reduction rate. PRM adaptation to the local environment and initiation of perchlorate reduction was rapid under neutral pH conditions. At the initial OD600 of 0.015, perchlorate reduction followed pseudo-first-order reaction rates with constants of 0.059 and 0.033 h(-1) at initial pH 7 and 8, respectively. Once perchlorate reduction was established, the bioreductive process was insensitive to the increases of pH from near neutral to 9.0. In the presence of nitrate, perchlorate reduction rate was reduced, but not inhibited completely.