Effect of subtherapeutic administration of antibiotics on the prevalence of antibiotic-resistant Escherichia coli bacteria in feedlot cattle

Antibiotic-resistant Escherichia coli in 300 feedlot steers receiving subtherapeutic levels of antibiotics was investigated through the collection of 3,300 fecal samples over a 314-day period. Antibiotics were selected based on the commonality of use in the industry and included chlortetracycline plus sulfamethazine (TET-SUL), chlortetracycline (TET), virginiamycin, monensin, tylosin, or no antibiotic supplementation (control). Steers were initially fed a barley silage-based diet, followed by transition to a barley grain-based diet. Despite not being administered antibiotics prior to arrival at the feedlot, the prevalences of steers shedding TET- and ampicillin (AMP)-resistant E. coli were >40 and <30%, respectively. Inclusion of TET-SUL in the diet increased the prevalence of steers shedding TET- and AMP-resistant E. coli and the percentage of TET- and AMP-resistant E. coli in the total generic E. coli population. Irrespective of treatment, the prevalence of steers shedding TET-resistant E. coli was higher in animals fed grain-based compared to silage-based diets. All steers shed TET-resistant E. coli at least once during the experiment. A total of 7,184 isolates were analyzed for MIC of antibiotics. Across antibiotic treatments, 1,009 (13.9%), 7 (0.1%), and 3,413 (47.1%) E. coli isolates were resistant to AMP, gentamicin, or TET, respectively. In addition, 131 (1.8%) and 143 (2.0%) isolates exhibited potential resistance to extended-spectrum beta-lactamases, as indicated by either ceftazidime or cefpodoxime resistance. No isolates were resistant to ciprofloxacin. The findings of the present study indicated that subtherapeutic administration of tetracycline in combination with sulfamethazine increased the prevalence of tetracycline- and AMP-resistant E. coli in cattle. However, resistance to antibiotics may be related to additional environmental factors such as diet.     

广义隐Markov模型在基因识别中的应用

广义隐Markov模型(GHMM)是基因识别的一种重要模型,但是其计算量比传统的隐Markov模型大得多,以至于不能直 接在基因识别中使用。根据原核生物基因的结构特点,提出了一种高效的简化算法,其计算量是序列长度的线性函数。在此 基础上,构建了针对原核生物基因的识别程序GeneMiner,对实际数据的测试表明,此算法是有效的。     

Selection and characterization of feather-degrading bacteria from canola meal compost

Canola meal that contains a high level of protein (40% crude protein) was used as compost material for the isolation of feather-degrading bacteria. After 7 and 14 days, bacteria were isolated from compost amended and unamended with soil. Eighty bacterial isolates from canola meal compost were then grown on milk-agar and isolates that produced proteolytic enzymes were identified by the formation of clear haloes around the colonies. A feather medium was chosen for a secondary selection of feather-degrading isolates. Of the eight isolates that hydrolyzed milk protein, five isolates hydrolyzed feathers. Their keratinolytic activities were subsequently confirmed by an assay using azo-keratin as substrate. Seven of the eight bacteria that hydrolyzed milk protein were Bacillus spp, and all five isolates that hydrolyzed feathers were strains of Bacillus licheniformis. Protease inhibition studies indicated that serine proteases are the predominant proteolytic enzymes produced by these feather-degrading isolates. Received 02 April 1999/ Accepted in revised form 17 June 1999     

The evolution of antennal courtship in diplazontine parasitoid wasps (Hymenoptera,Ichneumonidae, Diplazontinae)

Background

As predicted by theory, traits associated with reproduction often evolve at a comparatively high speed. This is especially the case for courtship behaviour which plays a central role in reproductive isolation. On the other hand, courtship behavioural traits often involve morphological and behavioural adaptations in both sexes; this suggests that their evolution might be under severe constraints, for instance irreversibility of character loss. Here, we use a recently proposed method to retrieve data on a peculiar courtship behavioural trait, i.e. antennal coiling, for 56 species of diplazontine parasitoid wasps. On the basis of a well-resolved phylogeny, we reconstruct the evolutionary history of antennal coiling and associated morphological modifications to study the mode of evolution of this complex character system.

Results

Our study reveals a large variation in shape, location and ultra-structure of male-specific modifications on the antennae. As for antennal coiling, we find either single-coiling, double-coiling or the absence of coiling; each state is present in multiple genera. Using a model comparison approach, we show that the possession of antennal modifications is highly correlated with antennal coiling behaviour. Ancestral state reconstruction shows that both antennal modifications and antennal coiling are highly congruent with the molecular phylogeny, implying low levels of homoplasy and a comparatively low speed of evolution. Antennal coiling is lost on two independent occasions, and never reacquired. A zero rate of regaining antennal coiling is supported by maximum parsimony, maximum likelihood and Bayesian approaches.

Conclusions

Our study provides the first comparative evidence for a tight correlation between male-specific antennal modifications and the use of the antennae during courtship. Antennal coiling in Diplazontinae evolved at a comparatively low rate, and was never reacquired in any of the studied taxa. This suggests that the loss of antennal coiling is irreversible on the timescale examined here, and therefore that evolutionary constraints have greatly influenced the evolution of antennal courtship in this group of parasitoid wasps. Further studies are needed to ascertain whether the loss of antennal coiling is irreversible on larger timescales, and whether evolutionary constraints have influenced courtship behavioural traits in a similar way in other groups.

     

Phytase activity of Selenomonas ruminantium: a preliminary characterization

Obligately anaerobic ruminal bacteria have been found to possess phytase activity, in particular, Selenomonas ruminantium . The phytase activity of S. ruminantium JY35 was produced late in growth and required neither phytate for induction nor phosphate limitation for derepression. The activity was completely cell-associated with a significant fraction extractable by a magnesium chloride solution. Zymogram analysis suggested that the activity was the result of a single gene product of a monomeric nature and approximately 46 kDa in size. The phytase had a temperature optimum of 50–55 °C, but activity dropped off sharply at 60 °C. Phytase activity was optimal over the pH range of 4·0–5·5, and dependent on the nature of the buffer used. Activity was inhibited by citric acid buffer and by the addition of 5 mmol l⁻¹ Fe²⁺, Fe³⁺, Cu²⁺, Zn²⁺ and Hg²⁺. The addition of 5 mmol l^–1 Pb²⁺ to the enzyme assay appeared to enhance activity of the enzyme.     

A method for the selective enumeration and isolation of ruminal Lactobacillus and Streptococcus

Ruminal lactic acid-producing bacteria were selectively isolated and enumerated using a one hour aerobic exposure prior to incubation on a semi-selective Lactobacillus medium, MRS, under anaerobic conditions. The technique allowed growth of pure cultures of ruminal Lactobacillus spp. and Streptococcus bovis without supporting the growth of pure cultures of any of the prominent ruminal bacterial species. In mixed cultures, the one hour aerobic pre-incubation inhibited the growth of the obligate anaerobic ruminal bacteria which can otherwise grow on the MRS medium, and the subsequent anaerobic incubation permitted maximal recovery of the weakly aerotolerant ruminal lactic acid-producing Lactobacillus spp. and Streptococcus spp. The efficacy of this technique in selecting exclusively for the lactic acid-producing bacteria was also demonstrated from populations of rumen bacteria from mixed culture end-point in vitro fermentation, continuous in vitro culture and isolations from fresh ruminal samples.     

On the empirical choice of the time window for restricted mean survival time

The t-year mean survival or restricted mean survival time (RMST) has been used as an appealing summary of the survival distribution within a time window [0, t]. RMST is the patient's life expectancy until time t and can be estimated nonparametrically by the area under the Kaplan-Meier curve up to t. In a comparative study, the difference or ratio of two RMSTs has been utilized to quantify the between-group-difference as a clinically interpretable alternative summary to the hazard ratio. The choice of the time window [0, t] may be prespecified at the design stage of the study based on clinical considerations. On the other hand, after the survival data have been collected, the choice of time point t could be data-dependent. The standard inferential procedures for the corresponding RMST, which is also data-dependent, ignore this subtle yet important issue. In this paper, we clarify how to make inference about a random “parameter.” Moreover, we demonstrate that under a rather mild condition on the censoring distribution, one can make inference about the RMST up to t, where t is less than or even equal to the largest follow-up time (either observed or censored) in the study. This finding reduces the subjectivity of the choice of t empirically. The proposal is illustrated with the survival data from a primary biliary cirrhosis study, and its finite sample properties are investigated via an extensive simulation study.     

钙离子对PSII颗粒放氧活性的影响

外加5 mmol/L Ca~(2 )可以使菠菜PSⅡ颗粒的放氧活性增高。PSⅡ颗粒经EGTA透析、低pH值、光照、2 mol/L NaCl等处理后,放氧活性下降,同时,这些颗粒的钙含量也相应降低。但当外加 5 mmol/L Ca~(2 )时,可使这些颗粒全部或部分地恢复放氧活性。PSⅡ颗粒中存在的钙对放氧起着重要作用;钙在PSⅡ颗粒中的结合位点不止一个,其结合状态有紧密和松散之别。