Sam Dukan

Differential oxidative damage and expression of stress defence regulons in culturable and non-culturable Escherichia coli cells

Potentially pathogenic bacteria, such as Escherichia coli and Vibrio cholerae, become non‐culturable during stasis. The analysis of such cells has been hampered by difficulties in studying bacterial population heterogeneity. Using in situ detection of protein oxidation in single E. coli cells, and using a density‐gradient centrifugation technique to separate culturable and non‐culturable cells, we show that the proteins in non‐culturable cells show increased and irreversible oxidative damage, which affects various bacterial compartments and proteins. The levels of expression of specific stress regulons are higher in non‐culturable cells, confirming increased defects relating to oxidative damage and the occurrence of aberrant, such as by amino‐acid misincorporation, proteins. Our data suggest that non‐culturable cells are produced due to stochastic deterioration, rather than an adaptive programme, and pinpoint oxidation management as the ‘Achilles heel’ of these cells.

Dynamic modelling of bacterial growth in drinking water networks

Abstract Numerous biological and physicochemical reactions take place in drinking water distribution systems, and give rise to phenomena whereby the organoleptic or bacteriological characteristics of the distributed water are modified. Drinking water may contain residual biodegradable dissolved organic compounds which provide a primary source for the formation of a trophic chain inside the pipes. Bacterial biomasses develop mainly on the internal surface of the pipes, where they are relatively well protected from the action of chlorination agents. The detachment of these biomasses is responsible for most of the bacterial proliferation observed in water samples taken in distribution systems, and also contributes to the installation of undesirable metazoea such as Asellus aquaticus . Combatting these biological developments calls for the application of preventive and remedial treatments, and these can be studied more closely by the use of modelling. This article proposes a model for the study of the behaviour of bacterial biomasses in distribution networks, taking into account the various major parameters which govern their structure, such as the ratio of biodegradable dissolved organic carbon (BDOC), temperature, residual chlorine, pH and the hydraulic conditions of each pipe. The model makes use of the data supplied by the Piccolo hydraulic modelling software, which can provide predictive mapping of the situation of each section of the network. What is more, by taking into account the physicochemical and biological variations in the water at the intake to the network, this dynamic model forecasts the evolution of the variables depending on residence time but also on time, thus enabling better visualisation of a disruption in the system in real time. We discuss the influence of the expression of the detachment of fixed bacteria on solutions of the system of differential equations. Use of the model reveals threshold values of temperature and BDOC which can enable a natural limitation of bacterial biomasses in the network without the use of chlorine.

Simultaneous effects of environmental factors on motile Aeromonas dynamics in an urban effluent and in the natural seawater

Seasonal dynamics of motile Aeromonas in a treated urban effluent and in natural seawater along the Sfax coast (Mediterranean sea, Tunisia) were measured over a year concurrently with seven environmental factors, and compared with those of faecal coliforms. Counts for Aeromonas from a standard plate count method, ranged from 1.48 x 10(5)CFU.100 ml(-1) to 2.2 x 10(8)CFU.100 ml(-1) in the effluent and from 7.9 x 10(3)CFU.100 ml(-1) to undetectable level in the surface marine waters. Contrary to faecal coliforms, the Aeromonas dynamics exhibited a seasonal distribution in seawater which was inverse of the seasonal distribution in the sewage: From the end of November 1998 to April 1999 (cold period), Aeromonas counts increased in the treated effluent, while it decreased very rapidly in seawater. From May to October (warm period), Aeromonas abundance decreased in the effluent but showed an increasing fluctuating trend in the marine waters with a maximum in late summer/early autumn when the temperatures were around 22-23 degrees C. Multiple correlation and regression analyses suggest, by the coefficient of determination (R(2)), that 42% of variance in Aeromonas number changes in the treated effluent, may be explained by only turbidity, radiation and Aeromonas density in the previous sample, while 37% of variance in marine ecosystem were explained by radiance and conductivity. Furthermore, the t statistics and their p values and the coefficient of partial determination (r(2)) indicated that radiance contributed the most (r(2)=0.3184, t=-3.2, p=0.0041) to the dynamics of motile Aeromonas in seawater, when combined with conductivity. The models relevant for changes in faecal coliforms abundance incorporated turbidity, radiance in the effluent and conductivity, pH, radiance, turbidity in coastal marine environment. These models explain 66% and 73% of the observed cell number fluctuation, with turbidity (r(2)=0.529, t=5.08, p=0.0001) and conductivity (r(2)=0.5407, t=4.97, p=0.0001) as dominant factors in the multivariate model proposed, respectively, for the two sampling sites. The results presented here suggest that the combination of negative effects of sunlight and conductivity in natural seawater mainly affects the colony-forming capacity and make the motile Aeromonas nonrecoverable during cold months.