一类非自治SIRS传染病模型的持久性与灭绝性研究

本文研究了一类非自治SIRS传染病模型．在比较弱的条件下,我们不仅得到了疾病强一致持续的充分必要条件,还证明了疾病强一致持续与强持续的等价性．本文给出了新的闽值．另外也给出了疾病灭绝的条件．文献[7]中的结论被改进．     

Epidemic cycling in a multi-strain SIRS epidemic network model

Background

One common observation in infectious diseases caused by multi-strain pathogens is that both the incidence of all infections and the relative fraction of infection with each strain oscillate with time (i.e., so-called Epidemic cycling). Many different mechanisms have been proposed for the pervasive nature of epidemic cycling. Nevertheless, the two facts that people contact each other through a network rather than following a simple mass-action law and most infectious diseases involve multiple strains have not been considered together for their influence on the epidemic cycling.

Methods

To demonstrate how the structural contacts among people influences the dynamical patterns of multi-strain pathogens, we investigate a two strain epidemic model in a network where every individual randomly contacts with a fixed number of other individuals. The standard pair approximation is applied to describe the changing numbers of individuals in different infection states and contact pairs.

Results

We show that spatial correlation due to contact network and interactions between strains through both ecological interference and immune response interact to generate epidemic cycling. Compared to one strain epidemic model, the two strain model presented here can generate epidemic cycling within a much wider parameter range that covers many infectious diseases.

Conclusion

Our results suggest that co-circulation of multiple strains within a contact network provides an explanation for epidemic cycling.

     

一类具有暂时免疫传染病模型的Hopf分支

本文应用Hassard的“规范形”方法,讨论了一类具有暂时免疫传染病模型的动力形态．给出了该系统发生Hopf分支的参数曲线,进一步计算出了决定分支方向及稳定性的参数条件,并给出了生态解释．     

The Intensity of a Bean Fusarium Root Rot Epidemic is Dependent on Planting Strategies

Restricting Fusarium root rot (FRR) epidemics and improving productivity using fewer chemicals is a major concern in bean‐growing regions. The main purpose of this research was to identify the planting strategies associated with FRR development and seed production in bean crops. A 2‐year study of 122 commercial bean farms in four major producing regions in Zanjan, Iran, was performed to characterize the associations of farming indicators with FRR and productivity. Linear mixed‐effect models, contingency tables and correspondence analyses were used to describe the variables relationships. Low disease and high productivity were linked to herbicide and manure applications, fungicidal treatment of seeds, manual sowing and sprinkler irrigation. Furrow irrigation, mechanical sowing, planting on raised seedbeds, the lack of fertilizer and herbicide use accounted for high disease and low seed production. The results of the study provide further evidence of factors contributions to the wider FRR spread over bean‐cropping systems. Overall, this suggests that the selected composition of planting strategies can improve bean production and reduce FRR intensity.     

A Discrete Event Simulation Model for Evaluating the Performances of an M/G/C/C State Dependent Queuing System

M/G/C/C state dependent queuing networks consider service rates as a function of the number of residing entities (e.g., pedestrians, vehicles, and products). However, modeling such dynamic rates is not supported in modern Discrete Simulation System (DES) software. We designed an approach to cater this limitation and used it to construct the M/G/C/C state-dependent queuing model in Arena software. Using the model, we have evaluated and analyzed the impacts of various arrival rates to the throughput, the blocking probability, the expected service time and the expected number of entities in a complex network topology. Results indicated that there is a range of arrival rates for each network where the simulation results fluctuate drastically across replications and this causes the simulation results and analytical results exhibit discrepancies. Detail results that show how tally the simulation results and the analytical results in both abstract and graphical forms and some scientific justifications for these have been documented and discussed.     

一类带有脉冲出生和垂直传染的时滞SEIS传染病模型

建立并分析了一个带有脉冲出生、垂直传染和时滞的SEIS传染病模型.利用频闪映射得到了无病周期解的存在性,并得到了两个临界值R~*和R_*,当R~*<1时,无病周期解全局吸引,疾病消失;当R_*>1时,疾病持续.     

A PNPase Dependent CRISPR System in Listeria

The human bacterial pathogen Listeria monocytogenes is emerging as a model organism to study RNA-mediated regulation in pathogenic bacteria. A class of non-coding RNAs called CRISPRs (clustered regularly interspaced short palindromic repeats) has been described to confer bacterial resistance against invading bacteriophages and conjugative plasmids. CRISPR function relies on the activity of CRISPR associated (cas) genes that encode a large family of proteins with nuclease or helicase activities and DNA and RNA binding domains. Here, we characterized a CRISPR element (RliB) that is expressed and processed in the L. monocytogenes strain EGD-e, which is completely devoid of cas genes. Structural probing revealed that RliB has an unexpected secondary structure comprising basepair interactions between the repeats and the adjacent spacers in place of canonical hairpins formed by the palindromic repeats. Moreover, in contrast to other CRISPR-Cas systems identified in Listeria, RliB-CRISPR is ubiquitously present among Listeria genomes at the same genomic locus and is never associated with the cas genes. We showed that RliB-CRISPR is a substrate for the endogenously encoded polynucleotide phosphorylase (PNPase) enzyme. The spacers of the different Listeria RliB-CRISPRs share many sequences with temperate and virulent phages. Furthermore, we show that a cas-less RliB-CRISPR lowers the acquisition frequency of a plasmid carrying the matching protospacer, provided that trans encoded cas genes of a second CRISPR-Cas system are present in the genome. Importantly, we show that PNPase is required for RliB-CRISPR mediated DNA interference. Altogether, our data reveal a yet undescribed CRISPR system whose both processing and activity depend on PNPase, highlighting a new and unexpected function for PNPase in “CRISPRology”.     

一类有投放率系统的控制策略

使用微分方程理论研究了有投放率时一类云杉蚜虫与其天敌模型在相互作用过程中所呈现的动态性质,通过对反馈增益值的选取,得到了系统达到稳定平衡状态的条件,同时保证了实际中蚜虫的密度相对小．     

A Strategy for Eliciting Antibodies against Cryptic,Conserved, Conformationally Dependent Epitopes of HIV Envelope Glycoprotein

Background

Novel strategies are needed for the elicitation of broadly neutralizing antibodies to the HIV envelope glycoprotein, gp120. Experimental evidence suggests that combinations of antibodies that are broadly neutralizing in vitro may protect against challenge with HIV in nonhuman primates, and a small number of these antibodies have been selected by repertoire sampling of B cells and by the fractionation of antiserum from some patients with prolonged disease. Yet no additional strategies for identifying conserved epitopes, eliciting antibodies to these epitopes, and determining whether these epitopes are accessible to antibodies have been successful to date. The defining of additional conserved, accessible epitopes against which one can elicit antibodies will increase the probability that some may be the targets of broadly neutralizing antibodies.

Methodology/Principal Findings

We postulate that additional cryptic epitopes of gp120 are present, against which neutralizing antibodies might be elicited even though these antibodies are not elicited by gp120, and that many of these epitopes may be accessible to antibodies should they be formed. We demonstrate a strategy for eliciting antibodies in mice against selected cryptic, conformationally dependent conserved epitopes of gp120 by immunizing with multiple identical copies of covalently linked peptides (MCPs). This has been achieved with MCPs representing 3 different domains of gp120. We show that some cryptic epitopes on gp120 are accessible to the elicited antibodies, and some epitopes in the CD4 binding region are not accessible. The antibodies bind to gp120 with relatively high affinity, and bind to oligomeric gp120 on the surface of infected cells.

Conclusions/Significance

Immunization with MCPs comprised of selected peptides of HIV gp120 is able to elicit antibodies against conserved, conformationally dependent epitopes of gp120 that are not immunogenic when presented as gp120. Some of these cryptic epitopes are accessible to the elicited antibodies.     

具状态依赖时滞单种群模型周期正解的存在性

用重合度论的连续性定理,本文获得如下具状态依赖时滞的单种群增长模型周期正解的存在性x(t)=x(t)[a(t)+b(t)xp(t-τ(t,x(t)))-c(t)xq(t-τ(t,x(t)))]这里a,b,c∈C((0,∞),R)是周期为ω(ω>0)的连续函数,且a>0,c>0.m,p,q为正整数且q>p.     

A Mechanism for Protein Monoubiquitination Dependent on a trans-Acting Ubiquitin-binding Domain

The length of the ubiquitin chain on a substrate dictates various functional outcomes, yet little is known about its regulation in vivo. The yeast arrestin-related protein Rim8/Art9 is monoubiquitinated in vivo by the Rsp5 ubiquitin ligase. This also requires Vps23, a protein that displays an ubiquitin-E2 variant (UEV) domain. Here, we report that binding of the UEV domain to Rim8 interferes with ubiquitin chain elongation and directs Rim8 monoubiquitination. We propose that Vps23 UEV competes with Rsp5 HECT N-lobe for binding to the first conjugated ubiquitin, thereby preventing polyubiquitination. These findings reveal a novel mechanism to control ubiquitin chain length on substrates in vivo.     

Evaluating Paratransgenesis as a Potential Control Strategy for African Trypanosomiasis

Genetic-modification strategies are currently being developed to reduce the transmission of vector-borne diseases, including African trypanosomiasis. For tsetse, the vector of African trypanosomiasis, a paratransgenic strategy is being considered: this approach involves modification of the commensal symbiotic bacteria Sodalis to express trypanosome-resistance-conferring products. Modified Sodalis can then be driven into the tsetse population by cytoplasmic incompatibility (CI) from Wolbachia bacteria. To evaluate the effectiveness of this paratransgenic strategy in controlling African trypanosomiasis, we developed a three-species mathematical model of trypanosomiasis transmission among tsetse, humans, and animal reservoir hosts. Using empirical estimates of CI parameters, we found that paratransgenic tsetse have the potential to eliminate trypanosomiasis, provided that any extra mortality caused by Wolbachia colonization is low, that the paratransgene is effective at protecting against trypanosome transmission, and that the target tsetse species comprises a large majority of the tsetse population in the release location.     

Threshold Conditions for a Non-Autonomous Epidemic System Describing the Population Dynamics of Dengue

A non-autonomous dynamical system, in which the seasonal variation of a mosquito vector population is modeled, is proposed to investigate dengue overwintering. A time-dependent threshold, R(t), is deduced such that when its yearly average, denoted by , is less than 1, the disease does not invade the populations and when is greater than 1 it does. By not invading the population we mean that the number of infected individuals always decrease in subsequent seasons of transmission. Using the same threshold, all the qualitative features of the resulting epidemic can be understood. Our model suggests that trans-ovarial infection in the mosquitoes facilitates dengue overwintering. We also explain the delay between the peak in the mosquitoes population and the peak in dengue cases. An erratum to this article can be found at     

A Strategy for Rotation of Different Bacteriophage Defenses in a Lactococcal Single-Strain Starter Culture System

A new strategy for starter culture rotations was developed for a series of phage-resistant clones genetically derived from a single strain of Lactococcus lactis subsp. lactis. Phage-resistant derivatives carrying different defense systems were constructed via conjugation with various plasmids encoding abortive infection (Abi/Hsp) and/or restriction and modification (R/M) systems of different specificity. The plasmids included pTR2030 (Hsp⁺ R⁺/M⁺), pTN20 (Abi⁺ R⁺/M⁺), pTRK11 (R⁺/M⁺), and pTRK68 (R⁺/M⁺). Selected phage-resistant transconjugants or transformants were evaluated in different rotation sequences through cycles of the Heap-Lawrence starter culture activity test in milk contaminated with phage and whey from the previous cycle. When used in consecutive sequence, derivative strains carrying the R/M systems encoded by pTN20, pTRK11, and pTRK68 retarded phage development when the initial levels of phage contamination were below 10² PFU/ml but not when levels were increased to 10³ PFU/ml. Use of a derivative bearing pTR2030 (Hsp⁺ R⁺/M⁺) at the beginning of the rotation prevented phage development, even when the initial levels of phage contamination were high (10⁶ PFU/ml). Alternating the type and specificity of R/M and Abi defenses through the rotation prevented phage proliferation and in some cases eliminated contaminating phages. A model rotation sequence for the phage defense rotation strategy was developed and performed successfully over nine cycles of the Heap-Lawrence starter culture activity test in the presence of high-titer commercial phage composites. This phage defense rotation strategy is designed to protect a highly specialized Lactococcus strain from phage attack during continuous and extended use in the dairy industry.