Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll‐a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.     

Division of Labor during Biofilm Matrix Production

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Probiotic Lactobacillus sp. inhibit growth,biofilm formation and gene expression of caries‐inducing Streptococcus mutans

Streptococcus mutans contributes significantly to dental caries, which arises from homoeostasic imbalance between host and microbiota. We hypothesized that Lactobacillus sp. inhibits growth, biofilm formation and gene expression of Streptococcus mutans. Antibacterial (agar diffusion method) and antibiofilm (crystal violet assay) characteristics of probiotic Lactobacillus sp. against Streptococcus mutans (ATCC 25175) were evaluated. We investigated whether Lactobacillus casei (ATCC 393), Lactobacillus reuteri (ATCC 23272), Lactobacillus plantarum (ATCC 14917) or Lactobacillus salivarius (ATCC 11741) inhibit expression of Streptococcus mutans genes involved in biofilm formation, quorum sensing or stress survival using quantitative real‐time polymerase chain reaction (qPCR). Growth changes (OD600) in the presence of pH‐neutralized, catalase‐treated or trypsin‐treated Lactobacillus sp. supernatants were assessed to identify roles of organic acids, peroxides and bacteriocin. Susceptibility testing indicated antibacterial (pH‐dependent) and antibiofilm activities of Lactobacillus sp. against Streptococcus mutans. Scanning electron microscopy revealed reduction in microcolony formation and exopolysaccharide structural changes. Of the oral normal flora, L. salivarius exhibited the highest antibiofilm and peroxide‐dependent antimicrobial activities. All biofilm‐forming cells treated with Lactobacillus sp. supernatants showed reduced expression of genes involved in exopolysaccharide production, acid tolerance and quorum sensing. Thus, Lactobacillus sp. can inhibit tooth decay by limiting growth and virulence properties of Streptococcus mutans.     

第三臼齿退化及其在人类演化上的意义

对第三臼齿退化在世界范围内各人类群体分类数据的分析表明：人类第三臼齿先天缺失是伴随整个人类进化而发生的牙齿退化现象的一部分,这一特片在居住于不同的地理区域的人类始发的时间及增加的速率不尽一致。     

儿童下牙弓发育的测量

本研究测量了１９３６名６－１２岁男女儿童的下颌牙弓的宽度和弦长。儿童前牙弓在７－９岁时增长最快,为此时恒尖牙萌出所致,１０岁以后各年龄组均数差异检验没有显著性意义。在整个替牙期,儿童前牙弓宽度约增加２ｍｍ,弦长增加１．８ｍｍ,儿童后牙弓的宽度和弦长约增加１ｍｍ左右。男孩前牙弓宽度比女孩大１ｍｍ左右。后牙弓的宽度和弦箕各年龄组均为男大于女,约１ｍｍ在右。可以把小学年龄阶段儿童的牙列分为乳牙弓、第１恒     

Membrane-attached biofilms for VOC wastewater treatment. II: Effect of biofilm thickness on performance

This article reports a study of the performance of membrane-attached biofilms grown in a single tube extractive membrane bioreactor (STEMS) used for the treatment of a synthetic wastewater containing a toxic VOC (1,2-dichloroethane [DCE]). Mass balances show that complete mineralization of DCE was achieved, and that the biofilms were effective in reducing air stripping to negligible levels. Experimental results are presented showing the evolution over time of biofilm thickness and its influence on the flux of DCE across the membrane. It has been found that a trade-off exists between the positive influence of biofilms in reducing air-stripping of DCE, and the negative influence of biofilms in reducing DCE flux across the membrane. These considerations lead to an optimal biofilm thickness in the region of 200 to 400 mum being recommended for this system. (c) 1995 John Wiley & Sons, Inc.     

Biofilm parameters influencing biocide efficacy

The influence of biofilm areal cell density, species composition, and the presence of abiotic particles on the disinfection and removal of bacterial biofilms by monochloramine was investigated. Mono- and binary population biofilms of Pseudomonas aeruginosa and Klebsiella pneumoniae were grown on stainless-steel slides in a continuous flow annular reactor. Biofilms were treated in the reactor with a pulse/step dose of 4 mg/L monochloramine for 2 h. Biofilm samples were disaggregated and assayed for colony formation on R2A agar and for total cell numbers by acridine orange direct counts. These data were used to determine apparent first order rate coefficients for the processes of disinfection and detachment. Disinfection rate coefficients exceeded detachment rate coefficients by as much as an order of magnitude and the two coefficients were poorly correlated (r = 0.272). The overall decay rate coefficient (disinfection plus detachment) depended strongly on the initial biofilm areal cell density. It displayed a parabolic dependence on cell density with a maximum near 10(8) cfu/cm(2). This result points to multiple factors influencing biofilm susceptibility to antimicrobial challenge. Decay rates of K. pneumoniae measured in binary population biofilms were comparable with those measured in monopopulation biofilms (p = 0.61). P. aeruginosa decayed more slowly in biofilsm dominated by K. pneumoniae (p = 0.028), indicating some interaction between species. The presence of kaolin and calcium carbonate particles in the biofilm reduced disinfection efficacy. (c) 1995 John Wiley & Sons, Inc.     

A modeling study of anaerobic biofilm systems: I. Detailed biofilm modeling

A detailed model acetate-utilizing methanogenic biofilms accounting for the diffusion of neutral and ionic species, chemical equilibrium, electroneutrality, gas production within the biofilm, pH-dependent Monod kinetics, and the presence of a concentration boundary layer is presented. The model qualitatively fits the pH profiles that are reported for acetate-utilizing methanogenic aggregates. A sensitivity analysis on the biological parameters showed that the flux of acetate is sensitive to the maximum utilization rate, half-saturation constant, and biofilm density for the bulk conditions investigated. Criteria when traditional biofilm models can be used to predict the flux of acetate into the biofilm are established. If the maximum pH change predicted using a hypothetical system is within +/-0.05, the traditional model predicts the flux to within +/-5% of the value calculated with the model developed in this study. (c) 1995 John Wiley & Sons, Inc.