Isolation and Identification of Sulphite- and Iron Reducing, Hydrogenase Positive Facultative Anaerobes from Cooling Water Systems

Divergent Nrf Family Proteins and MtrCAB Homologs Facilitate Extracellular Electron Transfer in Aeromonas hydrophila

Extracellular electron transfer (EET) is a strategy for respiration in which electrons generated from metabolism are moved outside the cell to a terminal electron acceptor, such as iron or manganese oxide. EET has primarily been studied in two model systems, Shewanella oneidensis and Geobacter sulfurreducens Metal reduction has also been reported in numerous microorganisms, including Aeromonas spp., which are ubiquitous Gammaproteobacteria found in aquatic ecosystems, with some species capable of pathogenesis in humans and fish. Genomic comparisons of Aeromonas spp. revealed a potential outer membrane conduit homologous to S. oneidensis MtrCAB. While the ability to respire metals and mineral oxides is not widespread in the genus Aeromonas, 90% of the sequenced Aeromonas hydrophila isolates contain MtrCAB homologs. A. hydrophila ATCC 7966 mutants lacking mtrA are unable to reduce metals. Expression of A. hydrophila mtrCAB in an S. oneidensis mutant lacking homologous components restored metal reduction. Although the outer membrane conduits for metal reduction were similar, homologs of the S. oneidensis inner membrane and periplasmic EET components CymA, FccA, and CctA were not found in A. hydrophila We characterized a cluster of genes predicted to encode components related to a formate-dependent nitrite reductase (NrfBCD), here named NetBCD (for Nrf-like electron transfer), and a predicted diheme periplasmic cytochrome, PdsA (periplasmic diheme shuttle). We present genetic evidence that proteins encoded by this cluster facilitate electron transfer from the cytoplasmic membrane across the periplasm to the MtrCAB conduit and function independently from an authentic NrfABCD system. A. hydrophila mutants lacking pdsA and netBCD were unable to reduce metals, while heterologous expression of these genes could restore metal reduction in an S. oneidensis mutant background. EET may therefore allow A. hydrophila and other species of Aeromonas to persist and thrive in iron- or manganese-rich oxygen-limited environments.IMPORTANCE Metal-reducing microorganisms are used for electricity production, bioremediation of toxic compounds, wastewater treatment, and production of valuable compounds. Despite numerous microorganisms being reported to reduce metals, the molecular mechanism has primarily been studied in two model systems, Shewanella oneidensis and Geobacter sulfurreducens We have characterized the mechanism of extracellular electron transfer in Aeromonas hydrophila, which uses the well-studied Shewanella system, MtrCAB, to move electrons across the outer membrane; however, most Aeromonas spp. appear to use a novel mechanism to transfer electrons from the inner membrane through the periplasm and to the outer membrane. The conserved use of MtrCAB in Shewanella spp. and Aeromonas spp. for metal reduction and conserved genomic architecture of metal reduction genes in Aeromonas spp. may serve as genomic markers for identifying metal-reducing microorganisms from genomic or transcriptomic sequencing. Understanding the variety of pathways used to reduce metals can allow for optimization and more efficient design of microorganisms used for practical applications.

A Novel Shewanella Isolate Enhances Corrosion by Using Metallic Iron as the Electron Donor with Fumarate as the Electron Acceptor

The involvement of Shewanella spp. in biocorrosion is often attributed to their Fe(III)-reducing properties, but they could also affect corrosion by using metallic iron as an electron donor. Previously, we isolated Shewanella strain 4t3-1-2LB from an acetogenic community enriched with Fe(0) as the sole electron donor. Here, we investigated its use of Fe(0) as an electron donor with fumarate as an electron acceptor and explored its corrosion-enhancing mechanism. Without Fe(0), strain 4t3-1-2LB fermented fumarate to succinate and CO₂, as was shown by the reaction stoichiometry and pH. With Fe(0), strain 4t3-1-2LB completely reduced fumarate to succinate and increased the Fe(0) corrosion rate (7.0 ± 0.6)-fold in comparison to that of abiotic controls (based on the succinate-versus-abiotic hydrogen formation rate). Fumarate reduction by strain 4t3-1-2LB was, at least in part, supported by chemical hydrogen formation on Fe(0). Filter-sterilized spent medium increased the hydrogen generation rate only 1.5-fold, and thus extracellular hydrogenase enzymes appear to be insufficient to explain the enhanced corrosion rate. Electrochemical measurements suggested that strain 4t3-1-2LB did not excrete dissolved redox mediators. Exchanging the medium and scanning electron microscopy (SEM) imaging indicated that cells were attached to Fe(0). It is possible that strain 4t3-1-2LB used a direct mechanism to withdraw electrons from Fe(0) or favored chemical hydrogen formation on Fe(0) through maintaining low hydrogen concentrations. In coculture with an Acetobacterium strain, strain 4t3-1-2LB did not enhance acetogenesis from Fe(0). This work describes a strong corrosion enhancement by a Shewanella strain through its use of Fe(0) as an electron donor and provides insights into its corrosion-enhancing mechanism.IMPORTANCE Shewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes.

Microbial activity in drinking water-associated biofilms

Microbial biofilms from surfaces in contact with water may play a beneficial role in drinking water treatment as biological filters. However, detrimental effects such as biofouling (i.e., biocorrosion and water quality deterioration) may also occur. In this study microbiological processes and factors influencing the activity of bacteria in biofilms were investigated by conventional cultivation methods. The presence of bacteria belonging to different ecophysiological groups was assessed during drinking water treatment, in biofilms developed on concrete, steel and sand surfaces. Influences of the treatment process, type of immersed material and physico-chemical characteristics of raw/bulk water and biofilms upon the dynamics of bacterial communities were evaluated. Results revealed intense microbial activity in biofilms occurring in the drinking water treatment plant of Cluj. Ammonification, iron reduction and manganese oxidation were found to be the predominant processes. Multiple significant correlations were established between the evolution of biofilm bacteria and the physico-chemical parameters of raw/ bulk water. The type of immersed material proved to have no significant influence upon the evolution of microbial communities, but the treatment stage, suggesting that the processes applied restrict microbial growth not only in bulk fluid but in biofilms, too.

Genome sequence of Aeromonas hydrophila ATCC 7966T: jack of all trades

The complete genome of Aeromonas hydrophila ATCC 7966(T) was sequenced. Aeromonas, a ubiquitous waterborne bacterium, has been placed by the Environmental Protection Agency on the Contaminant Candidate List because of its potential to cause human disease. The 4.7-Mb genome of this emerging pathogen shows a physiologically adroit organism with broad metabolic capabilities and considerable virulence potential. A large array of virulence genes, including some identified in clinical isolates of Aeromonas spp. or Vibrio spp., may confer upon this organism the ability to infect a wide range of hosts. However, two recognized virulence markers, a type III secretion system and a lateral flagellum, that are reported in other A. hydrophila strains are not identified in the sequenced isolate, ATCC 7966(T). Given the ubiquity and free-living lifestyle of this organism, there is relatively little evidence of fluidity in terms of mobile elements in the genome of this particular strain. Notable aspects of the metabolic repertoire of A. hydrophila include dissimilatory sulfate reduction and resistance mechanisms (such as thiopurine reductase, arsenate reductase, and phosphonate degradation enzymes) against toxic compounds encountered in polluted waters. These enzymes may have bioremediative as well as industrial potential. Thus, the A. hydrophila genome sequence provides valuable insights into its ability to flourish in both aquatic and host environments.

Evolutionary and diagnostic implications of intragenomic heterogeneity in the 16S rRNA gene in Aeromonas strains

Sequencing 16S rRNA genes (SSU) cloned from Aeromonas strains revealed that strains contained up to six copies differing by < or = 1.5%. The SSU copies from Aeromonas veronii LMG13695 clustered with sequences from four Aeromonas species. These results demonstrate intragenomic heterogeneity of SSU and suggest caution when using SSU to identify aeromonads.

Tracking the concentration of heterotrophic plate count bacteria from the source to the consumer's tap

The goal of this project was to quantify the concentration of heterotrophic plate count (HPC) bacteria within water reaching consumer's taps, and from the sources used by a major utility serving the City of Tucson, AZ. With this information, the amounts and sources of HPC bacteria consumed at the tap could be determined. Samples of water were collected on a monthly basis from two well fields, the CAVSARP recovery well field and Southern Avra Valley well field which serves as one of the groundwater sources for Tucson, AZ, and the distribution system which serves the same homes from which tap water was also tested. The average concentration of HPC in source waters within Southern Avra Valley Wells was 56 CFU/ml (range 1-1995/ml). From the CAVSARP recovery well field, corresponding values were 38 CFU/ml (1 to 502 CFU/ml). Unblended groundwater in the chlorinated distribution system averaged 22 CFU/ml (range 1-794). Blended water at the chlorinated distribution site averaged 47 CFU/ml (range 10-158). There was a major shift in the percentage of gram negative to gram-positive bacteria from the wells to the distribution system, to the tap. In the surface CAP source water, 76% of the bacteria were gram-negative compared to 27% gram-negative in the CAVSARP recovery wells. In contrast, Avra Valley wells contained 17% gram-negative bacteria. In both the Tucson groundwater distribution sites and blended distribution sites, the corresponding number of gram negative bacteria was 12%. Finally at the tap, only 0.2% of the bacteria were gram-negative. The average number of bacteria in household taps averaged 3072 HPC/ml and was equal or greater than 500 ml 68% of the time. This study shows that the number of HPC bacteria increases dramatically from the distribution system to the consumers tap. Thus, the major source of bacteria ingested by the average consumer in Tucson originates from bacteria within the household distribution system or the household tap, rather than from source waters or the distribution system. It is also clear that consumers' regularly consume more than 500 HPC/ml from drinking water taken from the household tap.