In this study the importance of Providencia species as a cause of travellers' diarrhoea was examined using a selective medium developed by the authors. Providencia species could easily be distinguished from other enteric pathogens by the colour of the colonies obtained. Nine strains of Providencia alcalifaciens, nine of Providencia rettgeri and five of Providencia stuartii were isolated from 130 specimens, representing a surprisingly high incidence of infection compared with other pathogens isolated on SS agar and TCBS agar. Patients infected with P. rettgeri complained of abdominal pain, as for other Providencia species, but also of vomiting, which is rather characteristic of P. rettgeri infection. To analyse the pathogenicity of these isolates, their invasiveness was examined using Caco-2 cells. Most of the P. rettgeri strains invaded Caco-2 cells. Random amplified polymorphic DNA (RAPD) fingerprinting showed the same profile for two P. rettgeri isolates from individuals travelling in the same tour group. The results show that Providencia species, especially P. rettgeri, might cause diarrhoea, and that these species are important pathogens.

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An improved simulation model (PCPF-1) has been evaluated for the prediction of the fate of mefenacet in an experimental paddy field. This model simulates the fate and transport of pesticide in paddy water and the top 1 cm of paddy soil. Observed concentrations of mefenacet in the paddy water and the surface soil exponentially decreased from their maximum concentrations of 0.70 mg litre(-1) and 11.3 mg kg(-1), respectively. Predicted mefenacet concentrations both in the water and surface soil were in excellent agreement with those measured during the first 2 weeks after herbicide application, but concentrations in paddy water were appreciably overestimated thereafter. The model simulated mefenacet losses through runoff, percolation and degradation to be respectively 41.9%, 6.4% and 57.3% of applied, and the mass balance error was about -6%. The model simulation implied that drainage and seepage control, especially shortly after application when herbicide concentrations are high, is essential for preventing pesticide losses from paddy fields. In focusing on pesticide concentrations in this early period the PCPF-1 model can be a beneficial tool for risk assessment of pesticide losses and in the evaluation of agricultural management for reducing pesticide pollution associated with paddy rice production.

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A novel aerobic pentachloronitrobenzene-degrading bacterium, Nocardioides sp. strain PD653, was isolated from an enrichment culture in a soil-charcoal perfusion system. The bacterium also degraded hexachlorobenzene, a highly recalcitrant environmental pollutant, accompanying the generation of chloride ions. Liberation of 14 CO 2 from [U-ring-14 C]hexachlorobenzene was detected in a culture of the bacterium and indicates that strain PD653 is able to mineralize hexachlorobenzene under aerobic conditions. The metabolic pathway of hexachlorobenzene is initiated by oxidative dechlorination to produce pentachlorophenol. As further intermediate metabolites, tetrachlorohydroquinone and 2,6-dichlorohydroquinone have been detected. Strain PD653 is the first naturally occurring aerobic bacteria capable of mineralizing hexachlorobenzene.

Isolation and functional analysis of microbes mediating the methylation of arsenic (As) in paddy soils is important for understanding the origin of dimethylarsinic acid (DMA) in rice grains. Here, we isolated from the rice rhizosphere a unique bacterium responsible for As methylation. Strain GSRB54, which was isolated from the roots of rice plants grown in As-contaminated paddy soil under anaerobic conditions, was classified into the genus Streptomyces by 16S ribosomal RNA sequencing. Sequence analysis of the arsenite S-adenosylmethionine methyltransferase (arsM) gene revealed that GSRB54 arsM was phylogenetically different from known arsM genes in other bacteria. This strain produced DMA and monomethylarsonic acid when cultured in liquid medium containing arsenite [As(III)]. Heterologous expression of GSRB54 arsM in Escherichia coli promoted methylation of As(III) by converting it into DMA and trimethylarsine oxide. These results demonstrate that strain GSRB54 has a strong ability to methylate As. In addition, DMA was detected in the shoots of rice grown in liquid medium inoculated with GSRB54 and containing As(III). Since Streptomyces are generally aerobic bacteria, we speculate that strain GSRB54 inhabits the oxidative zone around roots of paddy rice and is associated with DMA accumulation in rice grains through As methylation in the rice rhizosphere.

A new coupled model (PCPF-SWMS) was developed for simulating fate and behavior of pollutant in paddy water and paddy soil. The model coupled the PCPF-1, a lumped model simulating pesticide concentrations in paddy water and 1 cm-surface sediment compartment, and the SWMS-2D, a finite element numerical model solving Richard's and advection-dispersion equations for solute transport in soil compartment. The coupling involved improvements on interactions of the water flow and the concentration the pollutant of at the soil interface between both compartments. The monitoring data collected from experimental plots in Tsukuba, Japan in 1998 and 1999 were used to parameterise and calibrate hydraulic functioning, hydrodynamic and hydrodispersive parameters of the paddy soil. The analysis on the hydraulic functioning of paddy soil revealed that the hard pan layer was the key factor controlling percolation rate and tracer transport. Matric potential and tracer monitoring highlighted the evolution of saturated hydraulic conductivity (K S ) of hard pan layer during the crop season. K S slightly decreased after puddling by clay clogging and strongly increased after mid term drainage by drying cracks. The model was able to calculate residential time in every soil layers. Residential time of tracer in top saturated layers was evaluated to be less than 40 days. It took 60 days to reach the unsaturated layers below hardpan layer.