Fate of Antibiotic Resistant Bacteria and Genes during Wastewater Chlorination: Implication for Antibiotic Resistance Control

This study investigated fates of nine antibiotic-resistant bacteria as well as two series of antibiotic resistance genes in wastewater treated by various doses of chlorine (0, 15, 30, 60, 150 and 300 mg Cl₂ min/L). The results indicated that chlorination was effective in inactivating antibiotic-resistant bacteria. Most bacteria were inactivated completely at the lowest dose (15 mg Cl₂ min/L). By comparison, sulfadiazine- and erythromycin-resistant bacteria exhibited tolerance to low chlorine dose (up to 60 mg Cl₂ min/L). However, quantitative real-time PCRs revealed that chlorination decreased limited erythromycin or tetracycline resistance genes, with the removal levels of overall erythromycin and tetracycline resistance genes at 0.42 ± 0.12 log and 0.10 ± 0.02 log, respectively. About 40% of erythromycin-resistance genes and 80% of tetracycline resistance genes could not be removed by chlorination. Chlorination was considered not effective in controlling antimicrobial resistance. More concern needs to be paid to the potential risk of antibiotic resistance genes in the wastewater after chlorination.     

Diversity of Culturable Bacteria Associated with Tuber panzhihuanense Pinus armandii Ectomycorrhizosphere Soil

Truffles are edible hypogeous ectomycorrhizal fungi which have great economic importance for their organoleptic properties and have significant ecological interests for forestry. Although some new precious Chinese white truffle have been described constantly, the molecular mechanisms that control truffle body formation are largely unknown. It has been hypothesized that ectomycorrhizosphere soil communities may have influences on truffle production. Thus, isolation and molecular characterisation of culturable bacteria were carried out to investigate the bacteria diversity in mycorrhizosphere soil of Tuber panzhihuanense Pinus armandii in this work. Sequencing results showed a significant presence mostly affiliated with Burkholderia was β Proteobacteria (3098%). The second culturable fraction which dominated by Pseudomonas was γ Proteobacteria (288%) other isolates were mostly Phyllobacterium and Rhizobium, members of α Proteobacteria (1467%), actinobacteria (125%) and Firmicutes (76%) represented by Arthrobacter and Bacillus, respectively. Chryseobacterium ureilyticum was the only bacterial strain belonging to Bacteroidetes. Similarities and differences of culturable bacterial community of ascocarps and ectomycorrhizosphere soil associated with Tuber were discussed.     

Antibiotic Treatment Drives the Diversification of the Human Gut Resistome

Despite the documented antibiotic-induced disruption of the gut microbiota, the impact of antibiotic intake on strain-level dynamics, evolution of resistance genes, and factors influencing resistance dissemination potential remains poorly understood. To address this gap we analyzed public metagenomic datasets from 24 antibiotic treated subjects and controls, combined with an in-depth prospective functional study with two subjects investigating the bacterial community dynamics based on cultivation-dependent and independent methods. We observed that shortterm antibiotic treatment shifted and diversified the resistome composition, increased the average copy number of antibiotic resistance genes, and altered the dominant strain genotypes in an individual-specific manner. More than 30% of the resistance genes underwent strong differentiation at the single nucleotide level during antibiotic treatment. We found that the increased potential for horizontal gene transfer, due to antibiotic administration, was ～3-fold stronger in the differentiated resistance genes than the non-differentiated ones. This study highlights how antibiotic treatment has individualized impacts on the resistome and strain level composition, and drives the adaptive evolution of the gut microbiota.     

Antibiotic Resistant Group A Streptococci

A series of Streptococcus pyogenes strains, including strains isolated from patients, mutants which had acquired in vitro resistance to penicillin (Pc), mitomycin C (MC), tetracycline (TC) and chloramphenicol (CM), ultraviolet light induced α hemolytic mutants, as well as β hemolytic mutants (β mutants) derived from α hemolytic mutants (α mutants) were compared as to their antibiotic sensitivity, and physiological, biochemical and serological properties. To obtain β mutants from α mutants the following procedures were employed: (1) serial mouse passage, (2) serial serum-broth transfers, (3) cultivation in heat-killed cultures of parent strains, and (4) cultivation in broth containing bacterial DNA extracted from parent streptococcus cells. From the results obtained these strains could be divided into two major groups, each with two subgroups. Group 1 strains produce soluble hemolysins and are sensitive to Pc. Subgroup 1–1 strains are sensitive to other antibiotics too; subgroup 1–2 are resistant to certain antibiotics other than Pc, bacitracin and MC. Group 2 strains do not produce soluble hemolysins and resistant to Pc. Subgroup 2-1 strains are α hemolytic on horse blood agar and subgroup 2–2 are β hemolytic on the same medium. Pc resistance in group 2 strains was more than 100-fold higher than that of sensitive strains, and was accompanied by MC resistance, but to a lesser degree. Pc resistance in group 2 mutants could be induced by antibiotics other than Pc and also by ultraviolet irradiation. Although group 1 cells retained the characteristics of typical S. pyogenes, group 2 cells, both α and β hemolytic, lost most of the physiological, biochemical and serological properties of this species. The similarity of group 2 strains to group D or group N streptococcal strains in their general properties is discussed.     

The Multi-Drug Resistant Tuberculosis Diagnosis and Treatment Cascade in Bangladesh

Objectives

To determine, in areas supported by BRAC, Bangladesh i) the pre-diagnosis and pre-treatment attrition among presumptive and confirmed Multi-Drug Resistant Tuberculosis (MDR-TB) patients and ii) factors associated with attrition.

Methods

This was a retrospective cohort study involving record review. Presumptive MDR-TB patients from peripheral microscopy centres serving 60% of the total population of Bangladesh were included in the study. Attrition and turnaround time for MDR-TB diagnosis by Xpert MTB/RIF and treatment initiation were calculated between July 2012 and June 2014.

Results

Of 836 presumptive MDR-TB patients referred from 398 peripheral microscopy centres, 161 MDR-TB patients were diagnosed. The number of diagnosed MDR-TB patients was less than country estimates of MDR-TB patients (2000 cases) during the study period. Among those referred, pre-diagnosis and pre-treatment attrition was 17% and 21% respectively. Median turnaround time for MDR-TB testing, result receipt and treatment initiation was four, zero and five days respectively. Farmers (RR=2.3, p=0.01) and daily wage laborers (RR=2.1, p=0.04) had twice the risk of having pre-diagnosis attrition. Poor record-keeping and unreliable upkeep of presumptive MDR-TB patient databases were identified as challenges at the peripheral microscopy centres.

Conclusion

There was a low proportion of pre-diagnosis and pre-treatment attrition in patients with presumptive and confirmed MDR-TB under programmatic conditions. However, the recording and reporting system did not detect all presumptive MDR-TB patients, highlighting the need to improve the system in order to prevent morbidity, mortality and transmission of MDR-TB in the community.     

A Treatment Plant Receiving Waste Water from Multiple Bulk Drug Manufacturers Is a Reservoir for Highly Multi-Drug Resistant Integron-Bearing Bacteria

The arenas and detailed mechanisms for transfer of antibiotic resistance genes between environmental bacteria and pathogens are largely unclear. Selection pressures from antibiotics in situations where environmental bacteria and human pathogens meet are expected to increase the risks for such gene transfer events. We hypothesize that waste-water treatment plants (WWTPs) serving antibiotic manufacturing industries may provide such spawning grounds, given the high bacterial densities present there together with exceptionally strong and persistent selection pressures from the antibiotic-contaminated waste. Previous analyses of effluent from an Indian industrial WWTP that processes waste from bulk drug production revealed the presence of a range of drugs, including broad spectrum antibiotics at extremely high concentrations (mg/L range). In this study, we have characterized the antibiotic resistance profiles of 93 bacterial strains sampled at different stages of the treatment process from the WWTP against 39 antibiotics belonging to 12 different classes. A large majority (86%) of the strains were resistant to 20 or more antibiotics. Although there were no classically-recognized human pathogens among the 93 isolated strains, opportunistic pathogens such as Ochrobactrum intermedium, Providencia rettgeri, vancomycin resistant Enterococci (VRE), Aerococcus sp. and Citrobacter freundii were found to be highly resistant. One of the O. intermedium strains (ER1) was resistant to 36 antibiotics, while P. rettgeri (OSR3) was resistant to 35 antibiotics. Class 1 and 2 integrons were detected in 74/93 (80%) strains each, and 88/93 (95%) strains harbored at least one type of integron. The qPCR analysis of community DNA also showed an unprecedented high prevalence of integrons, suggesting that the bacteria living under such high selective pressure have an appreciable potential for genetic exchange of resistance genes via mobile gene cassettes. The present study provides insight into the mechanisms behind and the extent of multi-drug resistance among bacteria living under an extreme antibiotic selection pressure.     

The Serum Resistome of a Globally Disseminated Multidrug Resistant Uropathogenic Escherichia coli Clone

Escherichia coli ST131 is a globally disseminated, multidrug resistant clone responsible for a high proportion of urinary tract and bloodstream infections. The rapid emergence and successful spread of E. coli ST131 is strongly associated with antibiotic resistance; however, this phenotype alone is unlikely to explain its dominance amongst multidrug resistant uropathogens circulating worldwide in hospitals and the community. Thus, a greater understanding of the molecular mechanisms that underpin the fitness of E. coli ST131 is required. In this study, we employed hyper-saturated transposon mutagenesis in combination with multiplexed transposon directed insertion-site sequencing to define the essential genes required for in vitro growth and the serum resistome (i.e. genes required for resistance to human serum) of E. coli EC958, a representative of the predominant E. coli ST131 clonal lineage. We identified 315 essential genes in E. coli EC958, 231 (73%) of which were also essential in E. coli K-12. The serum resistome comprised 56 genes, the majority of which encode membrane proteins or factors involved in lipopolysaccharide (LPS) biosynthesis. Targeted mutagenesis confirmed a role in serum resistance for 46 (82%) of these genes. The murein lipoprotein Lpp, along with two lipid A-core biosynthesis enzymes WaaP and WaaG, were most strongly associated with serum resistance. While LPS was the main resistance mechanism defined for E. coli EC958 in serum, the enterobacterial common antigen and colanic acid also impacted on this phenotype. Our analysis also identified a novel function for two genes, hyxA and hyxR, as minor regulators of O-antigen chain length. This study offers novel insight into the genetic make-up of E. coli ST131, and provides a framework for future research on E. coli and other Gram-negative pathogens to define their essential gene repertoire and to dissect the molecular mechanisms that enable them to survive in the bloodstream and cause disease.     

阿尔山不同时期落叶松根际土壤固氮菌的多样性研究

利用传统的细菌分离培养,结合16S rDNA序列分析等方法,对阿尔山地区不同时期落叶松根际可培养固氮菌群落的多样性进行分析,以揭示落叶松根际固氮菌的多样性及群落结构的季节变化规律,为森林生态系统的可持续发展提供理论依据。结果显示:(1)从阿尔山落叶松根际土壤中共计分离纯化细菌菌株112株,分属于14属41种,包括假单胞菌属、伯克氏菌属、根瘤菌属、叶杆菌属、节杆菌属、类芽孢杆菌属、沙雷菌属、欧文菌属、短小杆菌属、芽孢杆菌属、肠杆菌属、不动杆菌属、柄杆菌属、红球菌属;其中优势菌群为假单胞菌属,次优势菌群为叶杆菌属、伯克氏菌属和节杆菌属。(2)季节变化对落叶松固氮菌群的变化有显著影响,表现为4月份和10月份最优势类群为γ-变形菌纲(γ-proteobacteria)中的假单胞菌属,6月份和8月份的最优势类群相同,但组成有所差别,其中6月份优势菌群包括假单胞菌属、短小杆菌属、红球菌属、节杆菌属,8月份的优势菌群为假单胞菌属、不动杆菌属、肠杆菌属、短小杆菌属、红球菌属和节杆菌属。(3)不同时期的物种均匀度指数(McIntosh index)差异显著,8月份最大,4月份最小,变化范围在0.83～1.164之间;物种丰富度指数(Shannon-Wiener index) 6月份和8月份显著高于4月份和10月份;优势度指数(Simpson index) 4月份和10月份显著高于6月份和8月份。研究表明,阿尔山地区落叶松根际微生物的多样性较高,群落相对复杂,分离的14个菌属多为根际促生菌,且不同时期固氮菌的群落组成受季节的影响明显。     

Linkage Between Culturable Mineral-Weathering Bacteria and their Weathering Effectiveness Along a Soil Profile

In this study, we used culture-dependent methodology to characterize the weathering effectiveness and community of culturable mineral-weathering bacteria in an ultisol profile. A total of 261 isolates were obtained and found to have the ability to weather biotite. The proportions of the highly effective Si and Al solubilizers were significantly higher in the D and E horizons than in the A, B, C, and F horizons, while the A, B, C, and F horizons had the similar proportion of the highly effective Si solubilizers. The B and F horizons had the lowest proportion of the highly effective Al solubilizers. The D horizon had the maximum proportion of the highly effective Fe solubilizers. Lowest proportion of the highly effective Fe solubilizers was observed in the A and F horizons. The 261 mineral-weathering isolates were affiliated with 39 bacterial species within 19 genera. Burkholderia anthina from the A and B horizons, Burkholderia stabilis from the C, D, and E horizons, and Curtobacterium citreum from the F horizon had the significantly higher ability to release Si, Al, and Fe from biotite. The results showed the diverse mineral-weathering bacteria and the linkage between the weathering species and their weathering effectiveness along a soil profile.     

Depth-Related Changes in Community Structure of Culturable Mineral Weathering Bacteria and in Weathering Patterns Caused by Them along Two Contrasting Soil Profiles

Bacteria play important roles in mineral weathering and soil formation. However, few reports of mineral weathering bacteria inhabiting subsurfaces of soil profiles have been published, raising the question of whether the subsurface weathering bacteria are fundamentally distinct from those in surface communities. To address this question, we isolated and characterized mineral weathering bacteria from two contrasting soil profiles with respect to their role in the weathering pattern evolution, their place in the community structure, and their depth-related changes in these two soil profiles. The effectiveness and pattern of bacterial mineral weathering were different in the two profiles and among the horizons within the respective profiles. The abundance of highly effective mineral weathering bacteria in the Changshu profile was significantly greater in the deepest horizon than in the upper horizons, whereas in the Yanting profile it was significantly greater in the upper horizons than in the deeper horizons. Most of the mineral weathering bacteria from the upper horizons of the Changshu profile and from the deeper horizons of the Yanting profile significantly acidified the culture media in the mineral weathering process. The proportion of siderophore-producing bacteria in the Changshu profile was similar in all horizons except in the Bg2 horizon, whereas the proportion of siderophore-producing bacteria in the Yanting profile was higher in the upper horizons than in the deeper horizons. Both profiles existed in different highly depth-specific culturable mineral weathering community structures. The depth-related changes in culturable weathering communities were primarily attributable to minor bacterial groups rather than to a change in the major population structure.     

Phylogenetic Diversity and Population Densities of Culturable Cellulolytic Soil Bacteria across an Agricultural Encatchment

Abstract A typical, small encatchment (catena B?lkendorf) in the moraine, northeast German agricultural landscape Schorfheide-Chorin was studied with respect to summit, midslope, and foot-slope positions at northern and southern slope exposure, respectively, including a central noncultivated kettle hole position (pot hole). Across the sequence of seven distinct sampling positions, soil organic carbon and total nitrogen contents, soil gravimetric water content, and soil microbial biomass displayed maxima at the kettle hole position. Soil pH revealed a decreasing trend at the northern exposed slope and a minimum at the kettle hole position. Against this background, the population density of total culturable bacteria clearly displayed a minimum at the kettle hole position, whereas the population density of carboxymethylcellulose decomposing bacteria was not clearly differentiated in relation to sampling positions. To study the phylogenetic diversity of culturable cellulolytic bacteria, 311 isolates were obtained from the sampling positions across the entire encatchment and examined by restriction analysis of PCR-amplified 16S rDNA. Using the restriction enzyme ScrFI, isolates were classified into 31 pattern groups. Although the ratio of actinomycetes within total isolates ranged from 0.73 to 0.94, only 16 pattern groups originated from actinomycetes, but 15 from other bacteria. At all sampling positions, a dominant pattern group was identified, containing 38 to 65% of total isolates. Two site-specific pattern groups could be identified, representing significant parts of the total population, which were highly specific for the kettle hole (19% of total isolates) and for foot- and midslope positions (15–18% of total isolates), respectively. In general, the composition of cellulolytic isolates across the encatchment displayed differences with respect to slope positions, but was not significantly affected by soil properties. Based on 16S rDNA sequence analysis, isolates of the dominant as well as the specific pattern groups could be assigned to the genus Streptomyces. Furthermore, sequencing of 16S rDNA of isolates of another three pattern groups revealed a high phylogenetic diversity among these isolates, including cellulomonads and bacilli. Received: 12 August 1998; Accepted: 1 February 1999     

Distribution and Quantification of Antibiotic Resistant Genes and Bacteria across Agricultural and Non-Agricultural Metagenomes

There is concern that antibiotic resistance can potentially be transferred from animals to humans through the food chain. The relationship between specific antibiotic resistant bacteria and the genes they carry remains to be described. Few details are known about the ecology of antibiotic resistant genes and bacteria in food production systems, or how antibiotic resistance genes in food animals compare to antibiotic resistance genes in other ecosystems. Here we report the distribution of antibiotic resistant genes in publicly available agricultural and non-agricultural metagenomic samples and identify which bacteria are likely to be carrying those genes. Antibiotic resistance, as coded for in the genes used in this study, is a process that was associated with all natural, agricultural, and human-impacted ecosystems examined, with between 0.7 to 4.4% of all classified genes in each habitat coding for resistance to antibiotic and toxic compounds (RATC). Agricultural, human, and coastal-marine metagenomes have characteristic distributions of antibiotic resistance genes, and different bacteria that carry the genes. There is a larger percentage of the total genome associated with antibiotic resistance in gastrointestinal-associated and agricultural metagenomes compared to marine and Antarctic samples. Since antibiotic resistance genes are a natural part of both human-impacted and pristine habitats, presence of these resistance genes in any specific habitat is therefore not sufficient to indicate or determine impact of anthropogenic antibiotic use. We recommend that baseline studies and control samples be taken in order to determine natural background levels of antibiotic resistant bacteria and/or antibiotic resistance genes when investigating the impacts of veterinary use of antibiotics on human health. We raise questions regarding whether the underlying biology of each type of bacteria contributes to the likelihood of transfer via the food chain.     

细菌抗生素和重金属协同选择抗性机制研究进展

随着各类抗生素和新型复合金属材料的不断开发和使用,环境中抗生素和重金属离子协同污染的机率不断提高,对环境选择最为敏感的细菌通过自身的进化和发展形成了二者协同选择的抗性机制,如协同抗性、交叉抗性、协同调控和生物膜诱导机制。     

成都某三甲医院多重耐药菌的临床分布特点及耐药性分析

摘要 目的：探讨我院分离多重耐药菌（MDRO）的临床分布及耐药变化,为临床抗菌药物的合理使用提供依据。方法：回顾性分析我院2019年1月～2019年12月住院患者送检标本分离的病原菌,对MDRO分布及其耐药性进行统计分析。结果：我院住院患者共分离出病原菌4399株,MDRO 459株,MDRO检出率为10.43%。MDRO前5位分别是碳青霉烯类耐药铜绿假单胞菌（CR-PAE,125株,占27.23%）、碳青霉烯类耐药鲍曼不动杆菌（CR-ABA,123株,占26.80%）、碳青霉烯类耐药肺炎克雷伯菌（CR-KPN,118株,占25.71%）、耐甲氧西林金黄色葡萄球菌（MRSA,74株,占16.12%）和碳青霉烯类耐药肠杆科细菌（CRE,13株,占2.83%）。在标本类型上,痰液中MDRO占所有MDRO分离株的60.57%,其次为尿液、肺泡灌洗液、分泌物和血液标本。MDRO分离株占前4位的科室分别为重症医学科、神经外科、康复医学科和干部病房。在耐药性方面,CR-ABA和CR-KPN是我院耐药最严重的菌株。CR-ABA对头孢他啶、头孢曲松和头孢吡肟、氟喹诺酮类药物（环丙沙星）、氨基糖苷类（庆大霉素和妥布霉素）均显示出较高的耐药性,为85.00%以上;对碳青霉烯类药物（亚胺培南）的耐药率在99.00%以上,替加环素对该耐药菌的敏感性较高。CR-KPN对复方新诺明、氨曲南、庆大霉素、喹诺酮类（环丙沙星和左氧氟沙星）、头孢烯类（头孢唑啉、头孢曲松、头孢西丁、头孢吡肟、头孢他啶）、β-内酰胺类与酶抑制剂复合物（氨苄西林/舒巴坦、阿莫西林/克拉维酸和哌拉西林/他唑巴坦）及碳青霉烯类抗菌药物（厄他培南和亚胺培南）的耐药率均在90.00%以上。结论：我院MDRO检出率较高,应加强MDRO耐药性监测,及时向临床医师反馈,建立沟通协作机制,促进合理选用抗菌药物,防止MDRO的感染和蔓延。     

大莲湖池杉林湿地土壤可培养细菌的分离与鉴定

采用牛肉膏蛋白胨培养基培养,从大莲湖池杉林土壤中共分离得到20个菌落形态不同的菌株。通过对这些菌株的形态、培养特征、生理生化特征的研究以及16S rDNA序列分析,初步确定这些菌株分别属于假单胞菌属(Pseudomonas)、芽胞杆菌属(Bacillus)、红球菌属(Rhodococcus)、北里孢菌属(Kitasatosporia)、金黄杆菌属(Chryseobacterium)、不动杆菌属(Acinetobacter)、黄杆菌属(Flavobacterium)、鞘氨醇杆菌属(Sphingobacte-rium)和丛毛单胞菌属(Comamonas)等9个属细菌。其中芽胞杆菌属和不动杆菌属细菌是优势菌,分离到的红球菌属、北里孢菌属、鞘氨醇杆菌属和丛毛单胞菌属细菌在国内湿地土壤中报道较少。     

淋病流行株多重耐药性与mtrRCDE外排系统的关系

为了探讨mtrRCDE外排系统与淋病流行株多重耐药性的关系 ,应用K B法和琼脂稀释法从湛江地区分离出 6 2株淋球菌多重耐药株。利用煮沸法提取细菌DNA ,PCR扩增mtrR基因 ,并对扩增产物测序 ,比较敏感株与多重耐药株的差异。结果显示 :5株敏感株和 2株耐青霉素淋球菌无mtrR的突变 ,10株多重耐药株均有mtrR基因的突变 ,表现为第 12 4位和第 139位G→A。从而提示 ,mtrRCDE外排系统与淋病流行株的多重耐药性密切相关。     

Diversity of Culturable Bacteria Isolated from Highland Barley Cultivation Soil in Qamdo,Tibet Autonomous Region

The soil bacterial communities have been widely investigated. However, there has been little study of the bacteria in Qinghai-Tibet Plateau, especially about the culturable bacteria in highland barley cultivation soil. Here, a total of 830 individual strains were obtained at 4°C and 25°C from a highland barley cultivation soil in Qamdo, Tibet Autonomous Region, using fifteen kinds of media. Seventy-seven species were obtained, which belonged to 42 genera and four phyla; the predominant phylum was Actinobacteria (68.82%), followed by Proteobacteria (15.59%), Firmicutes (14.29%), and Bacteroidetes (1.30%). The predominant genus was Streptomyces (22.08%, 17 species), followed by Bacillus (6.49%, five species), Micromonospora (5.19%, four species), Microbacterium (5.19%, four species), and Kribbella (3.90%, three species). The most diverse isolates belonged to a high G+C Gram-positive group; in particular, the Streptomyces genus is a dominant genus in the high G+C Gram-positive group. There were 62 species and 33 genera bacteria isolated at 25°C (80.52%), 23 species, and 18 genera bacteria isolated at 4°C (29.87%). Meanwhile, only eight species and six genera bacteria could be isolated at 25°C and 4°C. Of the 77 species, six isolates related to six genera might be novel taxa. The results showed abundant bacterial species diversity in the soil sample from the Qamdo, Tibet Autonomous Region.     

福州森林红壤细菌的菌群结构及其功能分析

福建地区是我国的第二大林区,土壤类型主要表现为红壤.对福州地区森林中因木材腐朽而形成的树洞中的土壤样品进行筛选,构建了一份具有强木质纤维素降解能力的酸性红壤的细菌16S rKNA基因文库,利用限制性片段长度多态性(RFLP)对随机克隆进行筛选;对该生态环境下的细菌菌群进行了系统发育分析.结果表明土壤pH偏酸性严重影响了细菌类群的多样性,酸杆菌门Acidobacteria(71.5%)和变形菌门Proteobacteria(24.1%)是该酸性土样中的优势菌群,其中酸杆菌门在该生态系统中占有绝对优势.变形菌菌群中的主要类群为光合细菌,固氮细菌和溶菌细菌.研究表明,在木质纤维素自然降解的过程中,存在于酸性树洞土壤中的细菌参与了碳素和氮素的固定与循环,并对其微生态的稳定起到重要作用.     

The Genetic Diversity of Culturable Nitrogen-Fixing Bacteria in the Rhizosphere of Wheat

A total of 17 culturable nitrogen-fixing bacterial strains associated with the roots of wheat growing in different regions of Greece were isolated and characterized for plant-growth-promoting traits such as auxin production and phosphate solubilization. The phylogenetic position of the isolates was first assessed by the analysis of the PCR-amplified 16S rRNA gene. The comparative sequence analysis and phylogenetic analysis based on 16S rRNA gene sequences show that the isolates recovered in this study are grouped with Azospirillum brasilense, Azospirillum zeae, and Pseudomonas stutzeri. The diazotrophic nature of all isolates was confirmed by amplification of partial nifH gene sequences. The phylogenetic tree based on nifH gene sequences is consistent with 16S rRNA gene phylogeny. The isolates belonging to Azospirillum species were further characterized by examining the partial dnaK gene phylogenetic tree. Furthermore, it was demonstrated that the ipdC gene was present in all Azospirillum isolates, suggesting that auxin is mainly synthesized via the indole-3-pyruvate pathway. Although members of P. stutzeri and A. zeae are known diazotrophic bacteria, to the best of our knowledge, this is the first report of isolation and characterization of strains belonging to these bacterial genera associated with wheat.