Bacteria environmental determinants

Inorganic arsenic oxyanions, frequently present as environmental pollutants, are very toxic for most micro-organisms. Many microbial strains possess genetic determinants that confer resistance. In bacteria, these determinants are often found on plasmids, which has facilitated their smdy to the molecular level. Bacterial plasmids conferring arsenic resistance encode speciflc efflux pumps able to extrude arsenic from the cell cytoplasm, thus lowering the intracellular concentration of the toxic ions. Recently, apparently similar arsenic membrane transport proteins have been found with yeast, plants, and animals (8,9) (see Sec. V, below). 

Another important example of redox titrimetry that finds applications in both public health and environmental analyses is the determination of dissolved oxygen. In natural waters the level of dissolved O2 is important for two reasons it is the most readily available oxidant for the biological oxidation of inorganic and organic pollutants and it is necessary for the support of aquatic life. In wastewater treatment plants, the control of dissolved O2 is essential for the aerobic oxidation of waste materials. If the level of dissolved O2 falls below a critical value, aerobic bacteria are replaced by anaerobic bacteria, and the oxidation of organic waste produces undesirable gases such as CH4 and H2S. 

The dramatic increase of severe or lethal infections caused by antibiotic-resistant bacteria triggered numerous studies on antibiotic resistance, not only from clinical but also from environmental sources. Nowadays it is clear that environment, and water in particular, plays a central role on antibiotic resistance dispersion to and from clinical settings. However, the current state of the art clearly suggests that only a small fraction of the environmental resistome is known. The modes and mechanisms of emergence, evolution and transmission of resistance determinants are still not very well understood. Although environmental pollution is recognized to play an important role on antibiotic resistance evolution and spreading, it is still very difficult to draw cause-effect relationships, which sometimes seems to be strain/species dependent. 

Denitrification occurs only in the presence of oxidized nitrogen and in an environment with limited (whieh prevails in the subsurfaee). Beeause denitrifieation is an enzyme-mediated reaetion, the substrate concentration funetions as a rate-determining factor. The dominant denitrifying bacteria are heterotrophie. The favored environmental conditions for the growth of denitrifying baeteria include a neutral pH (6-8), a favorable water-air (oxygen) ratio, and a subsurface temperature between 20 and 30°C. 

Evidence is accumulating that DOM composition selects specific taxa (see Chapters 9 and 14). Models show that the relative input rates of labile and humic DOM can determine which guilds of bacteria dominate community metabolism (see Chapter 18). Because there may be multiple input sources, varying in magnitude and composition, each community represents a solution to a cacophony of selective pressures. Source heterogeneity in terms of DOM composition or timing probably contributes to greater active diversity, unless response is limited by other environmental factors (e.g., low temperature or low pH). From this perspective, ambient DOM concentration and composition are products of the interaction between input sources and community activity. This conception makes clear the limitations of attempts to model microbial activity in terms of static analyses of DOM abundance or composition. 

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