Browsing by Author "Srinivasan, Rohini"

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Action of monochloramine on biofilms of Pseudomonas aeruginosa and Klebsiella pneumoniae
(Montana State University - Bozeman, College of Engineering, 1994) Srinivasan, Rohini
Analysis of biofilm disinfection by monochloramine and free chlorine
(1993) Griebe, Thomas; Chen, Ching-I; Srinivasan, Rohini; Stewart, Philip S.
Biofilm parameters influencing biocide efficacy
(1995-06) Srinivasan, Rohini; Stewart, Philip S.; Griebe, Thomas; Chen, Ching-I; Xu, Xiaoming
The influence of biofilm areal cell density, species composition, and the presence of abiotic particles on the disinfection and removal of bacterial biofilms by monochloramine was investigated. Mono- and binary population biofilms of Pseudomonas aeruginosa and Klebsiella pneumoniae were grown on stainless-steel slides in a continuous flow annular reactor. Biofilms were treated in the reactor with a pulse/step dose of 4 mg/L monochloramine for 2 h. Biofilm samples were disaggregated and assayed for colony formation on R2A agar and for total cell numbers by acridine orange direct counts. These data were used to determine apparent first order rate coefficients for the processes of disinfection and detachment. Disinfection rate coefficients exceeded detachment rate coefficients by as much as an order of magnitude and the two coefficients were poorly correlated (r = 0.272). The overall decay rate coefficient (disinfection plus detachment) depended strongly on the initial biofilm areal cell density. It displayed a parabolic dependence on cell density with a maximum near 108 cfu/cm2. This result points to multiple factors influencing biofilm susceptibility to antimicrobial challenge. Decay rates of K. pneumoniae measured in binary population biofilms were comparable with those measured in monopopulation biofilms (p = 0.61). P. aeruginosa decayed more slowly in biofilsm dominated by K. pneumoniae (p = 0.028), indicating some interaction between species. The presence of kaolin and calcium carbonate particles in the biofilm reduced disinfection efficacy. © 1995 John Wiley & Sons, Inc.
Biofilm structural heterogeneity visualized by three microscopic methods
(1995-08) Stewart, Philip S.; Murga, Ricardo; Srinivasan, Rohini; de Beer, Dirk
The structural heterogeneity of a microbial biofilm was demonstrated by scanning electron microscopy, confocal scanning laser microscopy (CSLM), and cryoembedding followed by sectioning and microscopic examination. Biofilm was composed of a binary population of Pseudomonas aeruginosa and Klebsiella pneumoniae grown in a continuous flow annular reactor. The three microscopic methods provided a consistent picture of the biofilm as a non-uniform structure characterized by variable thickness and variable local cell and polymer densities. Significant changes in these parameters occurred in the biofilm over distances of 10 μm or less. Though the biofilm was several hundred microns thick in places, areas of bare substratum were also observed on the same sample coupon. Cell-free pores and channels in the biofilm interior were evident. Specific staining of cellular nucleic acids with ethidium bromide and extracellular polymeric substances (EPS) with calcofluor showed that cell and EPS distributions did not always overlap. The ethidium bromide-stained region was contained within the larger region of calcofluor staining; thus, some cell-free areas actually were filled with EPS. CSLM and cryoembedding approaches are superior to SEM in their ability to image the biofilm interior and in their potential to provide quantitative information.
Comparison of respiratory activity and culturability during monochloramine disinfection of binary population biofilms
(1994-05) Stewart, Philip S.; Griebe, Thomas; Srinivasan, Rohini; Chen, Cheng-Sao; Yu, Feipeng Philip; de Beer, Dirk; McFeters, Gordon A.
Biofilm bacteria challenged with monochloramine retained significant respiratory activity, even though they could not be cultured on agar plates. Microbial colony counts on agar media declined by approximately 99.9% after 1 h of disinfection, whereas the number of bacteria stained by a fluorescent redox dye experienced a 93% reduction. Integrated measures of biofilm respiratory activity, including net oxygen and glucose utilization rates, showed only a 10 to 15% reduction. In this biofilm system, measures of microbial respiratory activity and culturability yielded widely differing estimates of biocide efficacy.
Direct measurement of chlorine penetration into biofilms during disinfection
(1994-12) de Beer, Dirk; Srinivasan, Rohini; Stewart, Philip S.
Effects of various metal substrata on accumulation of pseudomonas aeruginosa biofilms and the efficacy of monochloramine as a biocide
(1993-11) Chen, Ching-I; Griebe, Thomas; Srinivasan, Rohini; Stewart, Philip S.