Bacteria

About: Bacteria is a research topic. Over the lifetime, 23676 publications have been published within this topic receiving 715990 citations. The topic is also known as: eubacteria.

Survival and implantation of Escherichia coli in the intestinal tract.

Abstract: Preliminary experiments established that a 0.5-ml inoculum that is introduced directly into the stomach of mice was cleared rapidly into the small intestine. Bicarbonate buffer, but not skim milk, protected such an inoculum from stomach acid until at least 90% of it had entered the small intestine. Passage and survival of various Escherichia coli strains through the mouse gut were tested by introducing a buffered bacterial inoculum directly into the stomach, together with the following two intestinal tracers: Cr(51)Cl(3) and spores of a thermophilic Bacillus sp. Quantitative recovery of excreted bacteria was accomplished by collecting the feces overnight in a refrigerated cage pan. The data show that wild-type E. coli strains and E. coli K-12 are excreted rapidly (98 to 100% within 18 h) in the feces without overall multiplication or death. E. coli varkappa1776 and DP50supF, i.e., strains certified for recombinant DNA experiments underwent rapid death in vivo, such that their excretion in the feces was reduced to approximately 1.1 and 4.7% of the inoculum, respectively. The acidity of the stomach had little bactericidal effect on the E. coli K-12 strain tested, but significantly reduced the survival of more acidsensitive bacteria (Vibrio cholerae) under these conditions. Long-term implantation of E. coli strains into continuous-flow cultures of mouse cecal flora or into conventional mice was difficult to accomplish. In contrast, when the E. coli strain was first inoculated into sterile continuous-flow cultures or into germfree mice, which were subsequently associated with conventional mouse cecal flora, the E. coli strains persisted in a large proportion of the animals at levels resembling E. coli populations in conventional mice. Metabolic adaptation contributed only partially to the success of an E. coli inoculum that was introduced first. A mathematical model is described which explains this phenomenon on the basis of competition for adhesion sites in which an advantage accrues to the bacterium which occupies those sites first. The mathematical model predicts that two or more bacterial strains that compete in the gut for the same limiting nutrient can coexist, if the metabolically less efficient strains have specific adhesion sites available. The specific rate constant of E. coli growth in monoassociated gnotobiotic mice was 2.0 h(-1), whereas the excretion rate in conventional animals was -0.23 h(-1). Consequently, limitation of growth must be regarded as the primary mechanism controlling bacterial populations in the large intestine. The beginnings of a general hypothesis of the ecology of the large intestine are proposed, in which the effects of the competitive metabolic interactions described earlier are modified by the effects of bacterial association with the intestinal wall.

CO2 fixation in acetogenic bacteria: Variations on a theme

Abstract: An increasing number of strict anaerobic bacteria are being found which use an alternative pathway to the ubiquitous Calvin cycle for CO2 fixation into cell compounds and the ubiquitous Krebs cycle for acetyl CoA oxidation to CO2. The principles of this non-cyclic pathway, the acetyl CoA pathway, have long been studied in acetogenic bacteria. These bacteria can catalyze the exergonic reduction of 2 CO2 with 8 reducing equivalents to acetate. In this pathway, CO2 reduction is part of a catabolic redox process which functions to accept reducing equivalents from a variety of dehydrogenated substrates. This process yields net ATP generated by electron transport phosphorylation. Acetyl CoA is an intermediate, formed from one CO2 via a tetrahydropteridine-bound 1-carbon unit (methyl group of acetate), and from another CO2 via a bound carbon monoxide (carboxyl group of acetate). The most characteristic and complex enzyme involved in acetyl CoA synthesis is carbon monoxide dehydrogenase (‘acetyl CoA synthase’). The enzymes of this acetyl CoA pathway not only participate in (1) acetate synthesis in energy metabolism of acetogenic bacteria, but also mediate (2) acetyl CoA oxidation in sulfate-reducing bacteria and possibly other anaerobes; (3) acetate disproportionation to CO2 and CH4 in the energy metabolism of many methanogenic bacteria; (4) autotrophic CO2 fixation in autotrophic acetogenic, methanogenic, and most autotrophic sulfate-reducing bacteria; (5) assimilation and/or dissimilation of 1-carbon compounds in many anaerobes; (6) CO oxidation to CO2 in anaerobes. A specialized group of anaerobes performs acetate synthesis from CO2 or from C1 units via a different pathway, the glycine synthase/glycine reductase pathway. Glycine is an intermediate which is formed from 2 C1 compounds, and is then reduced to acetate. The principal features of the two pathways and some open questions are discussed in this review. Emphasis is placed upon the acetyl CoA pathway in acetogenic bacteria, but important advances in the study of other strict anaerobes are also considered.

Influence of environment on the content and composition of microbial free amino acid pools.

Abstract: SUMMARY: The free amino acid pool contents of Gram-negative bacteria (Aerobacter aerogenes, Erwinia carotovora, Pseudomonas fluorescens) were studied as functions of the growth environment and were compared with those from correspondingly grown cultures of Gram-positive bacteria (Bacillus subtilis var. niger, B. megaterium, B. polymyxa) and the yeast Saccharomyces cerevisiae. Although the pools of the Gram-positive bacteria and the yeast contained five to 20 times the concentration of free amino acids present in the pools of Gram-negative bacteria, all pools were similar in containing only a limited range of detectable amino acids. Glutamate invariably predominated and generally accounted for over 50% of the total amino acid content of the pool. The contents and composition of pools from micro-organisms maintained in steady states in chemostat cultures did not vary with time, but changed significantly with changes in either growth rate or the nature of the growth limitation. However, these pool variations were small compared with those resulting from addition of 2% (w/v) NaCl to a culture of growing bacteria. With cultures of Gram-negative bacteria, sudden changes in medium salinity effected marked and rapid changes in free glutamate content; with Gram-positive bacteria, similar changes occurred, but extremely slowly. Addition of 4% (w/v) NaCl to growing yeast cultures brought about no observed changes in pool size or composition. These results are discussed with reference to the involvement of free amino acids in synthesis and functioning of microorganisms.

Genes involved in arsenic transformation and resistance associated with different levels of arsenic-contaminated soils.

Abstract: Arsenic is known as a toxic metalloid, which primarily exists in inorganic form [As(III) and As(V)] and can be transformed by microbial redox processes in the natural environment. As(III) is much more toxic and mobile than As(V), hence microbial arsenic redox transformation has a major impact on arsenic toxicity and mobility which can greatly influence the human health. Our main purpose was to investigate the distribution and diversity of microbial arsenite-resistant species in three different arsenic-contaminated soils, and further study the As(III) resistance levels and related functional genes of these species. A total of 58 arsenite-resistant bacteria were identified from soils with three different arsenic-contaminated levels. Highly arsenite-resistant bacteria (MIC > 20 mM) were only isolated from the highly arsenic-contaminated site and belonged to Acinetobacter, Agrobacterium, Arthrobacter, Comamonas, Rhodococcus, Stenotrophomonas and Pseudomonas. Five arsenite-oxidizing bacteria that belonged to Achromobacter, Agrobacterium and Pseudomonas were identified and displayed a higher average arsenite resistance level than the non-arsenite oxidizers. 5 aoxB genes encoding arsenite oxidase and 51 arsenite transporter genes [18 arsB, 12 ACR3(1) and 21 ACR3(2)] were successfully amplified from these strains using PCR with degenerate primers. The aoxB genes were specific for the arsenite-oxidizing bacteria. Strains containing both an arsenite oxidase gene (aoxB) and an arsenite transporter gene (ACR3 or arsB) displayed a higher average arsenite resistance level than those possessing an arsenite transporter gene only. Horizontal transfer of ACR3(2) and arsB appeared to have occurred in strains that were primarily isolated from the highly arsenic-contaminated soil. Soils with long-term arsenic contamination may result in the evolution of highly diverse arsenite-resistant bacteria and such diversity was probably caused in part by horizontal gene transfer events. Bacteria capable of both arsenite oxidation and arsenite efflux mechanisms had an elevated arsenite resistance level.