Cheaters, diffusion and nutrients constrain decomposition by microbial enzymes in spatially structured environments

Extracellular enzymes allow microbes to acquire carbon and nutrients from complex molecules and catalyse the rate-limiting step in nutrient mineralization. Because the factors regulating enzyme production are poorly understood, I used a simulation model to examine how competition, nutrient availability and spatial structure affect microbial growth and enzyme synthesis. In simulations where enzyme-producing microbes competed with cheaters (who do not synthesize enzymes but take-up product), higher enzyme costs favoured cheaters, while lower rates of enzyme diffusion favoured producers. Cheaters and producers coexisted in highly organized spatial patterns at intermediate enzyme costs and diffusion rates. Simulations with varying nutrient inputs showed that nitrogen supply can limit carbon mineralization, microbial growth and enzyme production because of the nitrogen-demanding stoichiometry of enzymes (C : N =  c. 3.5 : 1). These results suggest that competition from cheaters, slow diffusion and nitrogen limitation may constrain microbial foraging and the enzymatic decomposition of complex compounds in natural environments.     

The Actinorhizal Symbiosis

Abstract The term ``actinorhiza' refers both to the filamentous bacteria Frankia, an actinomycete, and to the root location of nitrogen-fixing nodules. Actinorhizal plants are classified into four subclasses, eight families, and 25 genera comprising more than 220 species. Although ontogenically related to lateral roots, actinorhizal nodules are characterized by differentially expressed genes, supporting the idea of the uniqueness of this new organ. Two pathways for root infection have been described for compatible Frankia interactions: root hair infection or intercellular penetration. Molecular phylogeny groupings of host plants correlate with morphologic and anatomic features of actinorhizal nodules. Four clades of actinorhizal plants have been defined, whereas Frankia bacteria are classified into three major phylogenetic groups. Although the phylogenies of the symbionts are not fully congruent, a close relationship exists between plant and bacterial groups. A model for actinorhizal specificity is proposed that includes different levels or degrees of specificity of host-symbiont interactions, from fully compatible to incompatible. Intermediate, compatible, but delayed or limited interactions are also discussed. Actinorhizal plants undergo feedback regulation of symbiosis involving at least two different and consecutive signals that lead to a mechanism controlling root nodulation. These signals mediate the opening or closing of the window of susceptibility for infection and inhibit infection and nodule development in the growing root, independently of infection mechanism. The requirement for at least two molecular recognition steps in the development of actinorhizal symbioses is discussed.     

Mutation in IRO1 tightly linked with URA3 gene reduces virulence of Candida albicans

Gene deletion in the pathogenic fungus Candida albicans has relied heavily on a URA3 cassette and a recipient delta ura3 strain CAI4. The IRO1 gene adjacent to URA3 was inadvertently deleted during construction of CAI4. We report here that a mutation in IRO1 reduces virulence of C. albicans.     

Isolation of a unique membrane protein from Naegleria fowleri

Naegleria fowleri, an amoeboflagellate, is the causative agent of Primary Amoebic Meningoencephalitis, a fulminating disease of the central nervous system. In order to elucidate the mechanisms of pathogenicity of this amoeba, a cDNA expression library was prepared from N. fowleri RNA. A specific protein was found to be expressed from a cDNA clone designated Mp2CL5. Northern blot analysis showed that the Mp2CL5 mRNA was expressed in pathogenic N. fowleri but was not expressed in non-pathogenic Naegleria species nor in Acanthamoeba. Western blot analysis using anti-N. fowleri antiserum demonstrated that IPTG-induced Escherichia coli Mp2CL5 expressed a 23-kDa recombinant protein. The Mp2CL5 recombinant protein was histidine-tagged and purified to homogeneity from E. coli. A polyclonal rabbit antiserum was prepared against the purified Mp2CL5 recombinant protein. This antibody was used to further characterize the Mp2CL5 native protein expressed by N. fowleri. Western blot analysis in conjunction with immunofluorescence microscopy demonstrated the presence of a native protein of 17 kDa on the plasma membrane of N. fowleri trophozoites. The native N. fowleri protein was expressed in the logarithmic phase of trophozoite growth and the production of this protein increased through the stationary phase of growth. Studies are in progress to examine further its role as a virulence factor.     

细菌基因工程杀虫剂研究进展

本文综述了在细菌基因工程杀虫菌构建过程中涉及到或可能涉及到的杀虫基因或因素的种类和特性 ,描述了工程菌构建的方法及策略 ,以及目前所获得的各类工程杀虫菌 ;讨论了杀虫工程菌的应用潜力和发展方向。     

13年败血症的致病菌变迁和耐药谱研究

目的 阐明败血症多得耐药菌分布。方法 对我院１９８６年１月至１９９８年１２月１３年住院的１７０５例经血培养阳性证实的败血症进行致病菌分布与年代比较、耐药性分析。结果 发现以金葡萄为主的Ｇ＾＋菌占４９．５５％、大肠杆菌为主的Ｇ菌占４９．０６％、Ｌ型菌０．９３％、真菌０．４４％,复数菌４．５１％。葡萄球菌７２５（４０．００％）占首位,有上升趋势;不动杆菌、克雷伯菌、嗜麦芽黄单胞菌等崛起并增多;大肠杆菌     

Genetic and phenotypic characterization of Xanthomonas axonopodis pv. maculifoliigardeniae causing bacterial leaf spot of Ixora in Taiwan

Ixora spp. are Rubiaceae plants commonly planted as hedges or potted flower. Recently, incidents of bacterial leaf spot of Ixora were observed in central parts of Taiwan. Previous research on the disease has been scarce and focused mainly on its diagnosis. Therefore, many characteristics of the causal agent remain unclear. The present study aims to improve our understanding of this lesser-characterized pathogen and provide information useful for its identification and management. Bacterial strains Ixo1, Ixo2 and Ixo3 were isolated from infected Ixora x westii. All three isolates were able to grow and induce leaf spot symptoms on Ixora. They also exhibited morphological and physiological characteristics typical of Xanthomonads. Biolog analysis indicated that Ixo1 to Ixo3 have metabolic fingerprints similar to X. axonopodis pv. poinsettiicola. Multilocus sequence analysis and inoculation assays identified Ixo1 to Ixo3 as X. axonopodis pv. maculifoliigardeniae, albeit their gene sequences were very similar to other species/pathovars belonging to the X. euvesicatoria species complex; members of this species complex have different plant hosts, yet share similar housekeeping gene sequences. A semi-specific PCR assay evaluated in this work was able to differentiate Ixo1 to Ixo3 from bacteria not belonging to the X. euvesicatoria species complex, suggesting that the assay may be used in diagnosing bacterial leaf spot of ixoras. Finally, the sensitivity of the isolated pathogen to multiple commercial pesticides was tested, and the results showed that the bacterium is sensitive to streptomycin + tetracycline (10％ SP), thiophanate methyl + streptomycin (68.8％ WP) and oxolinic acid (20% WP), but more tolerant against copper-based chemicals. Overall, the findings from this work may facilitate the identification and management of X. axonopodis pv. maculifoliigardeniae.