Browsing by Author "Chen, Xiao"

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Biofilm removal caused by chemical treatments
(2000-12) Chen, Xiao; Stewart, Philip S.
Biofilm protein removal by a variety of chemical treatments was investigated. Binary population biofilms of P. aeruginosa and K. pneumoniae were grown in continuous flow annular reactors for 7–9 days prior to a 1-h treatment period. Treatments that caused removal of more than 25% of the biomass (as total protein) included NaCl and CaCl2, two chelating agents (EDTA and Dequest 2006), surfactants (SDS, Tween 20, and Triton X-100), a pH increase, lysozyme, hypochlorite, monochloramine, and concentrated urea. Treatments that caused little removal (less than 25%) included a control, MgCl2, sucrose, nutrient upshifts and downshifts, and a pH decrease. The amount of biofilm protein removal and the reduction in viable cell numbers in the biofilm were not correlated. Some treatments caused significant killing but not much removal while other treatments caused removal with little killing. These results underscore the fact that biofilm removal and killing are distinct processes. The chemical diversity of agents that bring about biofilm removal suggests that multiple interactive forces contribute to biofilm cohesion. No pattern of differential removal of the two microbial species could be discerned.
Chemically induced biofilm detachment
(Montana State University - Bozeman, College of Engineering, 1998) Chen, Xiao
Chlorine penetration into artificial biofilm is limited by a reaction-diffusion interaction
(1996-01) Chen, Xiao; Stewart, Philip S.
The retarded penetration of chlorine into artificial biofilms of Pseudomonas aeruginosa entrapped in agarose gel slabs was investigated experimentally and shown to be consistent with an unsteady reaction−diffusion model. A chlorine microelectrode was used to measure transient chlorine concentration profiles in artificial biofilms in a flow cell. While chlorine penetrated relatively quickly into pure agarose films (∼15 min), its penetration into biofilms was greatly retarded when cells were present. The degree of retardation was proportional to the initial cell density in the biofilm. After 3 h of treatment with a flowing chlorine solution, the chlorine concentration at the substratum under a 526 μm thick biofilm containing 14 400 mg L-1 cell mass had only risen to 10% of the bulk solution value. A mathematical model of the transient reaction−diffusion interaction correctly captured the qualitative behavior of experimentally measured chlorine concentration profiles. Parameter values for the simulations were obtained from the literature and from independent investigations of biomass−chlorine reactions using well-mixed suspensions. Kinetic and stoichiometric coefficients for the reactions of agarose and cell mass with chlorine were obtained by fitting a simple first-order (in both reactants) kinetic model to chlorine versus time data. The reaction rate coefficient for chlorine−cell reaction (1.1 × 10-3 L mg-1 s-1) exceeded that of chlorine−agarose reaction (3.7 × 10-6 L mg-1 s-1) by 2 orders of magnitude. The yield coefficient relating the amount of cell mass consumed to the amount of chlorine consumed ranged from 0.6 to 4.3 mg mg-1, depending on the duration of the experiment. This study shows that the reaction rate of chlorine with cellular biomass is fast enough that diffusion of this disinfectant into the biofilm readily becomes rate limiting. This reaction−diffusion interaction affords an excellent explanation for the poor efficacy of chlorine when used against biofilm microorganisms.
Role of electrostatic interactions in cohesion of bacterial biofilms
(2002-07) Chen, Xiao; Stewart, Philip S.
Significant decreases in the apparent viscosity of a bacterial biofilm suspension were measured following addition of sodium, potassium, magnesium, or calcium salts, whereas iron salts increased the viscosity. Electrostatic interactions contribute to biofilm cohesion and iron cations are potent crosslinkers of the biofilm matrix.
Transport limitation of chlorine disinfection of pseudomonas aeruginosa entrapped in alginate beads
(1996-01) Xu, Xiaoming; Stewart, Philip S.; Chen, Xiao
An artificial biofilm system consisting of Pseudomonas aeruginosa entrapped in alginate and agarose beads was used to demonstrate transport limitation of the rate of disinfection of entrapped bacteria by chlorine. Alginate gel beads with or without entrapped bacteria consumed chlorine. The specific rate of chlorine consumption increased with increasing cell loading in the gel beads and decreased with increasing bead radius. The value of an observable modulus comparing the rates of reaction and diffusion ranged from less than 0.1 to 8 depending on the bead radius and cell density. The observable modulus was largest for large (3-mm-diameter) beads with high cell loading (1.8 × 109 cfu/cm3) and smallest for small beads (0.5 mm diameter) with no cells added. A chlorine microelectrode was used to measure chlorine concentration profiles in agarose beads (3.0 mm diameter). Chlorine fully penetrated cell-free agarose beads rapidly; the concentration of chlorine at the bead center reached 50% of the bulk concentration within approximately 10 min after immersion in chlorine solution. When alginate and bacteria were incorporated into an agarose bead, pronounced chlorine concentration gradients persisted within the gel bead. Chlorine did gradually penetrate the bead, but at a greatly retarded rate; the time to reach 50% of the bulk concentration at the bead center was approximately 46 h. The overall rate of disinfection of entrapped bacteria was strongly dependent on cell density and bead radius. Small beads with low initial cell loading (0.5 mm diameter, 1.1 × 107 cfu/cm3) experienced rapid killing; viable cells could not be detected (<1.6 × 105 cfu/cm3) after 15 min of treatment in 2.5 mg/L chlorine. In contrast, the number of viable cells in larger beads with a higher initial cell density (3.0 mm diameter, 2.2 × 109 cfu/cm3) decreased only about 20% after 6 h of treatment in the same solution. Spatially nonuniform killing of bacteria within the beads was demonstrated by measuring the transient release of viable cells during dissolution of the beads. Bacteria were killed preferentially near the bead surface. Experimental results were consistent with transport limitation of the penetration of chlorine into the artificial biofilm arising from a reaction–diffusion interaction. The methods reported here provide tools for diagnosing the mechanism of biofilm resistance to reactive antimicrobial agents in such applications as the treatment of drinking and cooling waters.