American Society for Microbiology

Polyhydroxyalkanoates (PHAs), of which polyhydroxybutyrate (PHB) is the most abundant, are bacterial carbon and energy reserve materials of widespread occurrence. They are composed of 3-hydroxyacid monomer units and exist as a small number of cytoplasmic granules per cell. The properties of the C4 homopolymer PHB as a biodegradable thermoplastic first attracted industrial attention more than 20 years ago. Copolymers of C4 (3-hydroxybutyrate [3HB]) and C5 (3-hydroxyvalerate [3HV]) monomer units have modified physical properties; e.g., the plastic is less brittle than PHB, whereas PHAs containing C8 to C12 monomers behave as elastomers. This family of materials is the centre of considerable commercial interest, and 3HB-co-3HV copolymers have been marketed by ICI plc as Biopol. The known polymers exist as 2(1) helices with the fiber repeat decreasing from 0.596 nm for PHB to about 0.45 nm for C8 to C10 polymers. Novel copolymers with a backbone of 3HB and 4HB have been obtained. The native granules contain noncrystalline polymer, and water may possibly act as a plasticizer. Although the biosynthesis and regulation of PHB are generally well understood, the corresponding information for the synthesis of long-side-chain PHAs from alkanes, alcohols, and organic acids is still incomplete. The precise mechanisms of action of the polymerizing and depolymerizing enzymes also remain to be established. The structural genes for the three key enzymes of PHB synthesis from acetyl coenzyme A in Alcaligenes eutrophus have been cloned, sequenced, and expressed in Escherichia coli. Polymer molecular weights appear to be species specific. The factors influencing the commercial choice of organism, substrate, and isolation process are discussed. The physiological functions of PHB as a reserve material and in symbiotic nitrogen fixation and its presence in bacterial plasma membranes and putative role in transformability and calcium signaling are also considered.

The ecology of hydrocarbon degradation by microbial populations in the natural environment is reviewed, emphasizing the physical, chemical, and biological factors that contribute to the biodegradation of petroleum and individual hydrocarbons. Rates of biodegradation depend greatly on the composition, state, and concentration of the oil or hydrocarbons, with dispersion and emulsification enhancing rates in aquatic systems and absorption by soil particulates being the key feature of terrestrial ecosystems. Temperature and oxygen and nutrient concentrations are important variables in both types of environments. Salinity and pressure may also affect biodegradation rates in some aquatic environments, and moisture and pH may limit biodegradation in soils. Hydrocarbons are degraded primarily by bacteria and fungi. Adaptation by prior exposure of microbial communities to hydrocarbons increases hydrocarbon degradation rates. Adaptation is brought about by selective enrichment of hydrocarbon-utilizing microorganisms and amplification of the pool of hydrocarbon-catabolizing genes. The latter phenomenon can now be monitored through the use of DNA probes. Increases in plasmid frequency may also be associated with genetic adaptation. Seeding to accelerate rates of biodegradation has been shown to be effective in some cases, particularly when used under controlled conditions, such as in fermentors or chemostats.

Nhiễm trùng đường hô hấp do Pseudomonas aeruginosa và Burkholderia cepacia đóng vai trò chính trong sinh bệnh học của xơ nang (CF). Bài tổng quan này tóm tắt những tiến bộ mới nhất trong việc hiểu mối tương tác giữa vật chủ và mầm bệnh trong CF với sự nhấn mạnh vào vai trò và kiểm soát của sự chuyển đổi thành dạng nhầy trong P. aeruginosa, hiện tượng này biểu hiện sự thích ứng của loại mầm bệnh cơ hội này với quá trình nhiễm trùng mãn tính trong CF, và sức đề kháng tự nhiên với kháng sinh của B. cepacia, sự lây lan giữa người với người, và đôi khi gây tử vong nhanh chóng do loại vi khuẩn này gây ra. Mặc dù việc hiểu cơ chế chuyển đổi thành dạng nhầy trong P. aeruginosa đã tiến đến mức mà hiện tượng này đã trở thành một hệ thống mô hình để nghiên cứu phản ứng căng thẳng của vi khuẩn trong sinh bệnh học vi sinh, thách thức gần đây hơn với B. cepacia, vốn nổi lên như một mầm bệnh thực sự mạnh mẽ của CF, được thảo luận trong bối cảnh các vấn đề lâm sàng, phân loại, truyền nhiễm và các phương thức tiềm năng của kiểu bệnh lý.

Bacteria continuously adapt to changes in their environment. Responses are largely controlled by signal transduction systems that contain two central enzymatic components, a protein kinase that uses adenosine triphosphate to phosphorylate itself at a histidine residue and a response regulator that accepts phosphoryl groups from the kinase. This conserved phosphotransfer chemistry is found in a wide range of bacterial species and operates in diverse systems to provide different regulatory outputs. The histidine kinases are frequently membrane receptor proteins that respond to environmental signals and phosphorylate response regulators that control transcription. Four specific regulatory systems are discussed in detail: chemotaxis in response to attractant and repellent stimuli (Che), regulation of gene expression in response to nitrogen deprivation (Ntr), control of the expression of enzymes and transport systems that assimilate phosphorus (Pho), and regulation of outer membrane porin expression in response to osmolarity and other culture conditions (Omp). Several additional systems are also examined, including systems that control complex developmental processes such as sporulation and fruiting-body formation, systems required for virulent infections of plant or animal host tissues, and systems that regulate transport and metabolism. Finally, an attempt is made to understand how cross-talk between parallel phosphotransfer pathways can provide a global regulatory curcuitry.