Shear Rate Moderates Community Diversity in Freshwater Biofilms

Abstract

The development of freshwater multispecies biofilms at solid-liquid interfaces occurs both in quiescent waters and under conditions of high shear rates. However, the influence of hydrodynamic shear rates on bacterial biofilm diversity is poorly understood. We hypothesized that different shear rates would significantly influence biofilm diversity and alter the relative proportions of coaggregating and autoaggregating community isolates. In order to study this hypothesis, freshwater biofilms were developed at five shear rates (<0.1 to 305 S ؊1 ) in a rotating concentric cylinder reactor fed with untreated potable water. Eubacterial diversity was assessed by denaturing gradient gel electrophoresis (DGGE) and culturing on R2A agar. Fifty morphologically distinct biofilm strains and 16 planktonic strains were isolated by culturing and identified by partial 16S rRNA gene sequencing, and their relatedness was determined by the construction of a neighbor-joining phylogenetic tree. Phylogenetic and DGGE analyses showed an inverse relationship between shear rate and bacterial diversity. An in vitro aggregation assay was used to assess the relative proportions of coaggregating and autoaggregating species from each biofilm. The highest proportion of autoaggregating bacteria was present at high shear rates ( Microorganisms colonize a wide range of environments as spatially organized, taxonomically diverse, multispecies biofilm communities (1,20,44). In potable water distribution systems, many bacterial species, including members of the Proteobacteria, Actinobacteria, low-GϩC-content gram-positive bacteria, and Cytophaga-Flavobacterium-Bacterioides group, readily adhere to surfaces to form multispecies biofilms (29,41). The abilities of these taxonomically diverse organisms to attach to surfaces and codevelop within multispecies biofilms are imperative for their survival and persistence within flowing environments (4,33,44). Few published articles have described the effect of shear on the bacterial composition and diversity of freshwater biofilms. Evidence is emerging, however, that at high fluid velocities with associated high shear rates, multispecies communities are less diverse than those developed at lower shear rates (9,23,35,42). In order to adhere to surfaces or surface-attached cells and subsequently to form biofilms, bacteria in high-velocity flowing systems must overcome shear stress at the fluid-surface interface (7, 11). Attachment is facilitated by the expression of cell surface polymers that alter cell surface properties (3, 15). Properties that enhance attachment include increases in whole-cell hydrophobicity (14) and the abilities to coaggregate (25, 36) and autoaggregate (17). Recently, it was shown, using a simple recirculating tank model (35), that freshwater biofilms formed at high shear rates contained a high proportion of bacterial species that were not detectable in the surrounding fluid phase. In addition, a larger proportion of biofilm strains than of their planktonic counterparts were able to coaggr...

Pyrosequencing analysis of eukaryotic and bacterial communities in faucet biofilms

“…Some Acidovorax species isolated from freshwater biofilm communities showed strong autoaggregating abilities, and possessed high whole-cell hydrophobicity which can enhance the autoaggregation interactions. And it is suggested that aggregating abilities and whole-cell hydrophobicity are important for colonization and biofilm development in flowing environments (Rickard et al, 2004). Biofilm formation in oligotrophic drinking water environments also could be viewed as a successional process in structure and species compositions (Martiny et al, 2003).…”

Adhaeribacter aquaticus gen. nov., sp. nov., a Gram-negative isolate from a potable water biofilm

“…Some strains that were isolated on R2A agar (Reasoner & Geldreich, 1985) at the high velocities possessed unusual morphological and biochemical characteristics. In particular, strain MBRG1.5 T possessed a low partial 16S rRNA gene sequence identity (92?7 %) when compared with sequences from species with validly published names in the EMBL database (Rickard et al, 2004). In this study, strain MBRG1.5…”

“…A recent study on the influence of water velocity on the community diversity of potable water biofilms demonstrated that the taxonomic diversity of biofilms developed at high velocities was much lower than that of biofilms developed under conditions of minimal velocity (Rickard et al, 2004). Some strains that were isolated on R2A agar (Reasoner & Geldreich, 1985) at the high velocities possessed unusual morphological and biochemical characteristics.…”

Effect of physiochemical factors on autoaggregation and adhesion of Flavobacterium johnsoniae-like isolates

“…The ability to coaggregate and autoaggregate have been found to enhance bacterial attachment, with a larger proportion of biofilm strains being able to coaggregate and autoaggregate compared to their planktonic counterparts (Rickard et al, 2004). Autoaggregation is a "selfish" mechanism whereby a strain within the biofilm will express polymers to enhance the integration of genetically identical strains (Rickard et al, 2003a) and may thus enhance the development of freshwater biofilms.…”

“…Autoaggregation is a "selfish" mechanism whereby a strain within the biofilm will express polymers to enhance the integration of genetically identical strains (Rickard et al, 2003a) and may thus enhance the development of freshwater biofilms. Autoaggregation interactions are enhanced by increased hydrophobicity and tend to be stronger than coaggregation (Rickard et al, 2004). Autoaggregation can be highly specific and is typically mediated by interactions between "adhesins" or specific carbohydrate-binding proteins (lectins) found on one cell and complementary saccharide "receptors" borne on another (Rickard et al, 2003a).…”