Anti‐microbial Susceptibility of Streptococcus spp. Isolated from Bovine Mastitis in Argentina

The in vitro susceptibility to penicillin G, erythromycin and clindamycin was determined by the disc diffusion test and by E‐test for a total of 47 streptococcal strains (three Streptococcus uberis, 36 Streptococcus agalactiae, eight Streptococcus dysgalactiae spp. dysgalactiae) isolated from bovine intramammary infections in Argentina. Moreover, resistance phenotypes of erythromycin‐resistant streptococcal isolates was characterized. MIC₉₀ of penicillin G, erythromycin and clindamycin for S. agalactiae were 0.75, 8.0 and 12.0 μg/ml respectively. Resistance to erythromycin and clindamycin was detected in 13 (27.6%) and 12 (25.5%) isolates respectively. No isolate was resistant to penicillin G. Resistance against macrolides, lincosamides and streptogramin B (MLS_B) represented by the constitutive MLS_B phenotype was present in 11 (23.4%) erythromycin‐resistant isolates and two isolates (4.3%) expressed the M phenotype. The inducible MLS_B phenotype was not identified. Results suggest that beta‐lactams are the first‐line antibiotics when treating streptococcal udder infections; however, the continuous monitoring of the antibiotic resistance is essential, as the emergence of resistant strains has become a growing concern on the therapy of bovine mastitis.     

Culture of Differentiated Adult Rabbit Auricular Chondrocytes

Chondrocytes dedifferentiate to a fibroblast‐like phenotype on plastic surfaces. Dedifferentiation is reversible if these cells are then cultured embedded in gels as alginate, agarose or collagen. Chondrocytes cultured in suspension on a non‐adherent surface are also known to form aggregates of differentiated cells. The knowledge of chondrocyte behavior in culture is relevant for tissue engineering purposes. In this report we describe a simple method to culture differentiated or redifferentiated rabbit auricular chondrocytes on plastic surfaces with a stable phenotype. When chondrocyte aggregates formed in suspension are next seeded on plastic surfaces, most of them attach to the plastic as round or polygonal cells, and this morphological differentiation, confirmed by the presence of type II collagen, is stable for long culture periods. We also report that the addition of aggregates to monolayer cultures of dedifferentiated chondrocytes results in their redifferentiation, as is shown by their morphological changes and the synthesis of type II collagen. Therefore, this simple method can be useful for the study of chondrocyte behavior on plastic surfaces and for redifferentiating previously proliferated chondrocytes in tissue engineering techniques. Furthermore, these results demonstrate that, in addition to culture conditions such as cell isolation method or cell‐density, chondrocyte behavior on plastic depends on the presence or absence of aggregates resulting from the dissociation process.     

Occurrence and Distribution of Lactate Dehydrogenase in Knee Joint Carilage in Healthy and Osteoarthrotic Dogs

According to clinical studies, degenerative diseases of canine joints lead to higher lactate dehydrogenase (LDH) levels in synovial fluid. The goal of the present study was to examine the intraarticular distribution of LDH in healthy and osteoarthrotic knee joints in order to identify possible sources of LDH in synovial fluid. As synovial LDH concentrations neither correlate with the number of leukocytes nor with synovitis, our investigation focused on the articular cartilage. Samples from healthy and osteoarthrotic knee joints were fixed and processed for transmission electron microscopy (TEM), immunohistochemistry (IHC), and immunocytochemistry (ICC). In addition, fresh cartilage samples were investigated cytochemically by the tetrazolium‐formazan reaction. Analyses of blood and synovial fluid samples were used to confirm the absence of inflammatory disease. Morphology of articular cartilage was assessed macroscopically and by means of TEM. IHC revealed highest levels of LDH in chondrones and a diffuse labelling of the matrix with a distinctive decrease in signal from superficial to deeper cartilage layers. Ultrastructural localization by ICC showed LDH to be present in the cytoplasm of all chondrocytes and confirmed the density gradient in the matrix. Labelling was absent from nuclei and from pericellular rims. Cytochemistry confirmed the distribution pattern and, thus, expanded our findings beyond immunological evidence by providing proof of enzymatic activity of LDH in articular cartilage. The present results indicate that LDH is transferred from chondrocytes to the cartilaginous matrix. We suggest, therefore, that LDH found in synovial fluid originates from the articular cartilage and that osteoarthrotic processes promote LDH release from the cartilaginous matrix.     

Assessing the potential impacts of alternative landscape designs on amphibian population dynamics

An individual-based, spatially explicit population model was used to predict the consequences of future land-use alternatives for populations of four amphibian species in two central Iowa (midwest USA) agricultural watersheds. The model included both breeding and upland habitat and incorporated effects of climatic variation and demographic stochasticity. Data requirements of the model include life history characteristics, dispersal behavior, habitat affinities, as well as land use and landcover in geographic information systems databases. Future scenarios were ranked according to change in breeder abundance, saturation, and distribution, compared to baseline conditions. Sensitivity of simulation results to changes in model parameters was also examined. Simulated results suggest that while all four species modeled are likely to persist under present and future scenario conditions, two may be more at risk from future landscape change. Although the study species are all widespread generalists regarded as having a low conservation priority, they depend on wetlands and ponds, increasingly endangered habitats in agricultural landscapes. Broader conservation strategies in the region would ensure that these currently common organisms do not become the endangered species of the future.This revised version was published online in May 2005 with corrections to the Cover Date.     

Development of Stem-base Pathogens on Different Cultivars of Winter Wheat Determined by Quantitative PCR

The progress of development of stem-base pathogens in crops of second winter wheat was plotted in nine experiments in three years. The amount of each pathogen present was determined by quantitative PCR. Where Tapesia yallundae was present in quantifiable amounts, it usually developed earlier than the other eyespot pathogen, T. acuformis. Both species were usually present in greater amounts on cultivars which are more susceptible to eyespot. The sharp eyespot pathogen, Rhizoctonia cerealis, developed more erratically than either of the Tapesia spp. and there were no consistent effects on different cultivars. Fusarium spp., the cause of brown foot rot, were rarely present in quantifiable amounts, but Microdochium nivale was usually present as one or both of the varieties nivale and majus. Late-season (after anthesis) decreases in M. nivale suggest that any brown foot rot symptoms attributable to this fungus would have fully developed earlier. Cultivar differences in amounts of M. nivale were most clear in stems during internode extension and when relatively large amounts of DNA were present. Such differences approximately reflected eyespot susceptibility, cv. Soissons containing most and cv. Lynx containing least DNA. The results emphasise the difficulty in relating diagnoses, by quantitative PCR or other means, at early growth stages when decisions to apply fungicides against stem-base disease are made, to later disease severity.     

Biological and Physical Constraints on Maize Production in the Humid Forest and Western Highlands of Cameroon

The aim was to identify biological and physical factors responsible for reducing maize yield in Cameroon. Two surveys were conducted in 137 fields in two agroecological zones in 1995–1997. In the Humid Forest (HF), Bipolaris maydis, Stenocarpella macrospora, Puccinia polysora, Rhizoctonia solani and soil fertility were factors that reduced maize production in 1995 and 1996. In the Western Highlands (WHL), Cercospora zeae-maydis, and the interaction between soil fertility and maize variety were the most important constraints to maize production in 1996. In 1997, C. zeae-maydis, S. macrospora, physiological spot and stem borer damage (Busseola fusca) were negatively related to ear weight. The combination of these biological factors (diseases and insects), and the physical parameter of soil fertility were responsible for reducing maize yield in these selected benchmarks of Cameroon. Maximum potential yield reductions were estimated at 68% due to B. maydis and 46% due to S. macrospora, respectively, in the HF in 1995. In 1996, maximum potential yield reductions in the HF were estimated at 34%, 41% and 30% due to S. macrospora, P. polysora and R. solani, respectively. In the WHL, C. zeae-maydis had the potential to cause a yield reduction of 79% in 1996. In the WHL in 1997, the interaction between C. zeae-maydis and B. fusca, stem diseases and the physiological spot caused potential reductions of 52%, 34% and 39%, respectively.     

Identification of Resistance to Common Bacterial Blight Disease on Bean Genotypes Grown in Turkey

Common bacterial blight (CBB) in edible beans (Phaseolus vulgaris), incited Xanthomonas campestris pv. phaseoli, reduces bean yields and seed quality. The main objective of this study was to determine resistance to common bacterial blight in bean genotypes. Twenty-two bean genotypes grown in Turkey including common and snap bean cultivars/lines were collected from different parts of Turkey and tested for resistance against to Xanthomonas campestris pv. phaseoli strain MFD-11. All the common and snap bean lines/cultivars tested were moderately susceptible, susceptible or highly susceptible, except AG-7117 which was found resistant to Xanthomonas campestris pv. phaseoli. This is the first report of a resistance source in a common bean line (AG-7117) against Xanthomonas campestris pv. phaseoli.     

Long-term modelling of weed control strategies: analysis of threshold-based options for weed species with contrasted competitive abilities

A simple life-cycle-based demography model was adapted for two contrasting weed species ( Alopecurus myosuroides and Poa annua ). This model included a seed production function that accounted for population self-regulation through weed:weed interactions. The A. myosuroides version of the model was tested with field data. Long-term simulations of population demography were then performed to investigate the relationship between weed control strategies based on density thresholds and both the frequency of herbicide use and the long-term economic profitability. This study confirms that threshold-based weed management strategies are more cost-effective than spraying every year and may allow important reductions in herbicide use. However, after the first transient years of either systematically spraying or withholding herbicide, the long-term spraying frequency was insensitive to threshold values between 0.01 and 100 plants m⁻². The highest long-term profitability was obtained for the lowest threshold tested, and the profitability decreased rapidly when the threshold was raised above 4–6 and 10–20 plants m⁻² for A. myosuroides and P. annua respectively. The study thus indicates that the exact threshold value is of little importance for the long-term reliance of the system on herbicide, provided that it is reasonably low. For species with low competitive ability, high thresholds may be used in some cropping systems to reduce the spraying frequency for environmental considerations, but those options would also reduce the profitability if no compensatory measures were taken.