Surveillance of bacterial susceptibility to five antimicrobial agents was performed during a 1-year period in and around four freshwater fish farms situated along a stream in western Denmark. Besides assessing the levels of antibiotic resistance among the culturable fraction of microorganisms in fish, water, and sediment samples, two major fish pathogens (88 Flavobacterium psychrophilum isolates and 134 Yersinia ruckeri isolates) and 313 motile Aeromonas isolates, representing a group of ubiquitous aquatic bacteria, were isolated from the same samples. MICs were obtained applying a standardized agar dilution method. A markedly decreased susceptibility of F. psychrophilum isolates to most antimicrobial agents presently available for use in Danish aquaculture was detected, while the collected Y. ruckeri isolates remained largely sensitive to all therapeutic substances. Comparing the inlet and outlet samples, the increase of the antibiotic-resistant proportions observed among the culturable microflora was more pronounced and statistically significant among the motile aeromonads. High levels of individual and multiple antimicrobial resistances were demonstrated within the collected flavobacteria and aeromonads, thus indicating a substantial impact of fish farming on several groups of bacteria associated with aquacultural environments.

A collection of 313 motile aeromonads isolated at Danish rainbow trout farms was analyzed to identify some of the genes involved in high levels of antimicrobial resistance found in a previous field trial (A. S. Schmidt, M. S. Bruun, I. Dalsgaard, K. Pedersen, and J. L. Larsen, Appl. Environ. Microbiol. 66:4908-4915, 2000), the predominant resistance phenotype (37%) being a combined oxytetracycline (OTC) and sulphadiazine/trimethoprim resistance. Combined sulphonamide/trimethoprim resistance (135 isolates) appeared closely related to the presence of a class 1 integron (141 strains). Among the isolates containing integrons, four different combinations of integrated resistance gene cassettes occurred, in all cases including a dihydrofolate reductase gene and a downstream aminoglycoside resistance insert (87 isolates) and occasionally an additional chloramphenicol resistance gene cassette (31 isolates). In addition, 23 isolates had "empty" integrons without inserted gene cassettes. As far as OTC resistance was concerned, only 66 (30%) out of 216 resistant aeromonads could be assigned to resistance determinant class A (19 isolates), D (n ‫؍‬ 6), or E (n ‫؍‬ 39); three isolates contained two tetracycline resistance determinants (AD, AE, and DE). Forty OTC-resistant isolates containing large plasmids were selected as donors in a conjugation assay, 27 of which also contained a class 1 integron. Out of 17 successful R-plasmid transfers to Escherichia coli recipients, the respective integrons were cotransferred along with the tetracycline resistance determinants in 15 matings. Transconjugants were predominantly tetA positive (10 of 17) and contained class 1 integrons with two or more inserted antibiotic resistance genes. While there appeared to be a positive correlation between conjugative R-plasmids and tetA among the OTCresistant aeromonads, tetE and the unclassified OTC resistance genes as well as class 1 integrons were equally distributed among isolates with and without plasmids. These findings indicate the implication of other mechanisms of gene transfer besides plasmid transfer in the dissemination of antibiotic resistance among environmental motile aeromonads.

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

The significance of Aeromonas hydrophila in association with disease outbreaks in aquaculture production in the Zhejiang province of China was investigated. Bacteriological examination of moribund fish and crabs resulted in 95 bacterial isolates: 88 bacterial isolates from fish and 7 isolates from crabs. PCR and traditional biochemical methods were used for identification of A. hydrophila. Out of 69 motile aeromonads, 35 isolates were identified as A. hydrophila by biochemical tests. However, 6 of those were not identified as A. hydrophila by a species specific PCR method. Serotyping revealed 2 dominant serotypes (O9 and O97) among A. hydrophila isolates. The data presented show that approximately 42% of the motile aeromonads isolated from disease outbreaks among various fish species were A. hydrophila. It is noteworthy that A. hydrophila accounted for more than 50% of the isolated aeromonands isolated from crucian carp Carassius carassius and Wuchang bream Megalobrama amblycephala with haemorrhagic septicaemia. Although this species was the most frequently isolated organism from internal organs of diseased fish and crabs in the present study, other motile Aeromonas spp. were also found. The PCR assay was useful in preventing misidentification of A. hydrophila, which may occur when only phenotypic tests are employed. KEY WORDS: Aeromonas hydrophila · PCR · China · Haemorrhagic septicaemia · Fish diseases Resale or republication not permitted without written consent of the publisherDis Aquat Org 46: [23][24][25][26][27][28][29] 2001 primary pathogen of freshwater fish or a secondary opportunistic pathogen of compromised or stressed hosts (Jeney & Jeney 1995). A. hydrophila has been associated with tail and fin rot, haemorrhagic septicaemia and epizootic ulcerative syndrome (EUS) (Austin & Adams 1996, Roberts 1997. A. hydrophila has also been described as the dominant infectious agent of 'fish-bacterial-septicaemia' in freshwater cultured cyprinid fishes, mainly crucian carp Carassius carassius, Wuchang bream Megalobrama amblycephala and silver carp Hypophthalmichthys molitrix in the Zhejiang province and other provinces in the Southeast of China between 1989to 1993(Qian et al. 1997. 'Fish-bacterial-septicaemia' occurs each summer in the Zhejiang province and results in significant losses for the fish farmers of the region; from 1989 to 1991 the losses were estimated to have been approximately 2200 tons of fish per year (D. Qian pers. comm.).The taxonomy of the genus Aeromonas has been revised, and new motile, mesophilic species have been identified: A. allosaccharophila, A. veronii biogroups sobria and veronii and A. encheleia have been reported as fish pathogens (Toranzo et al. 1989, Paniagua et al. 1990, Joseph & Carnahan 1994, Esteve et al. 1995. Still, A. hydrophila is regarded as the predominant fish pathogen within the mesophilic aeromonads, although its importance may have been overestimated in the past.Thus, the purpose of the present study was to estimate the actual significance of Aeromonas...

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Class 1 integrons were found in 26 of 40 antibiotic-resistant isolates of the fish pathogen Aeromonas salmonicida from Northern Europe and North America. Three different dhfr genes, conferring trimethoprim resistance, and one ant(3")1a aminoglycoside resistance gene were identified as gene inserts. The gene cassettes tended to be conserved among isolates from a particular geographical area. Nineteen isolates transferred R-plasmids carrying different tet determinants to Escherichia coli in filter mating assays, and in 15 cases, the class 1 integrons were co-transferred. Transferable sulphadiazine, trimethoprim and streptomycin resistances were invariably encoded by integrons. It thus appears that integron-encoded antibiotic resistance genes contribute substantially to the horizontal spread of antimicrobial resistance within this species, being associated with conjugative plasmids.

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The oxygen isotope composition of cellulose (δ O ) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ O of CO (δ O ), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ O , O-enrichment of leaf water (Δ O ) and cellulose (Δ O ) relative to source water, and p p , the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C grass (Cleistogenes squarrosa). δ O and N supply, and their interactions with VPD, had no effect on δ O , Δ O , Δ O and p p . Δ O and Δ O increased with VPD, while p p decreased. That VPD-effect on p p was supported by sensitivity tests to variation of Δ O and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not O relations; conversely, VPD had no effect on growth or morphology, but controlled O relations. The work implies that reconstructions of VPD from Δ O would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p p is ignored. Progress in understanding the relationship between Δ O and Δ O will require separate investigations of p and p and of their responses to environmental conditions.