Comparison of S-layer secretion genes in freshwater caulobacters

Polysaccharides and proteins added to flowing drinking water at microgram-per-liter levels promote the formation of biofilms predominated by bacteroidetes and proteobacteria

Biopolymers are important substrates for heterotrophic bacteria in (ultra)oligotrophic freshwater environments, but information about their utilization at microgram-per-liter levels by attached freshwater bacteria is lacking. This study aimed at characterizing biopolymer utilization in drinking-water-related biofilms by exposing such biofilms to added carbohydrates or proteins at 10 μg C liter(-1) in flowing tap water for up to 3 months. Individually added amylopectin was not utilized by the biofilms, whereas laminarin, gelatin, and caseinate were. Amylopectin was utilized during steady-state biofilm growth with simultaneously added maltose but not with simultaneously added acetate. Biofilm formation rates (BFR) at 10 μg C liter(-1) per substrate were ranked as follows, from lowest to highest: blank or amylopectin (≤6 pg ATP cm(-2) day(-1)), gelatin or caseinate, laminarin, maltose, acetate alone or acetate plus amylopectin, and maltose plus amylopectin (980 pg ATP cm(-2) day(-1)). Terminal restriction fragment length polymorphism (T-RFLP) and 16S rRNA gene sequence analyses revealed that the predominant maltose-utilizing bacteria also dominated subsequent amylopectin utilization, indicating catabolic repression and (extracellular) enzyme induction. The accelerated BFR with amylopectin in the presence of maltose probably resulted from efficient amylopectin binding to and hydrolysis by inductive enzymes attached to the bacterial cells. Cytophagia, Flavobacteriia, Gammaproteobacteria, and Sphingobacteriia grew during polysaccharide addition, and Alpha-, Beta-, and Gammaproteobacteria, Cytophagia, Flavobacteriia, and Sphingobacteriia grew during protein addition. The succession of bacterial populations in the biofilms coincided with the decrease in the specific growth rate during biofilm formation. Biopolymers can clearly promote biofilm formation at microgram-per-liter levels in drinking water distribution systems and, depending on their concentrations, might impair the biological stability of distributed drinking water.

Colonization of overlaying water by bacteria from dry river sediments

We studied the diversity, community composition and activity of the primary microbial colonizers of the water above freshly re-wetted sediments from a temporary river. Dried sediments, collected from Mulargia River (Sardinia, Italy), were covered with sterile freshwater in triplicate microcosms, and changes of the planktonic microbial assemblage were monitored over a 48 h period. During the first 9 h bacterial abundance was low (1.5 x 10(4) cells ml(-1)); it increased to 3.4 x 10(6) cells ml(-1) after 28 h and did not change thereafter. Approximately 20% of bacteria exhibited DNA de novo synthesis already after 9 h of incubation. Changes of the ratios of (3)H-leucine to (3)H-thymidine incorporation rates indicated a shift of growth patterns during the experiment. Extracellular enzyme activity showed a maximum at 48 h with aminopeptidase activity (430.8 +/- 22.6 nmol MCA l(-1) h(-1)) significantly higher than alkaline phosphatase (98.6 +/- 4.3 nmol MUF l(-1) h(-1)). The primary microbial colonizers of the overlaying water - as determined by 16S rRNA gene sequence analysis - were related to at least six different phylogenetic lineages of Bacilli and to Alphaproteobacteria (Brevundimonas spp. and Caulobacter spp.). Large bacterial cells affiliated to one clade of Bacillus sp. were rare in the dried sediments, but constituted the majority of the planktonic microbial assemblage and of cells with detectable DNA-synthesis until 28 h after re-wetting. Their community contribution decreased in parallel with a rise of flagellated and ciliated protists. Estimates based on cell production rates suggested that the rapidly enriched Bacillus sp. suffered disproportionally high loss rates from selective predation, thus favouring the establishment of a more heterogenic assemblage of microbes (consisting of Proteobacteria, Actinobacteria and Cytophaga-Flavobacteria). Our results suggest that the primary microbial colonizers of the water above dried sediments are passively released into the plankton and that their high growth potential is counteracted by the activity of bacterivorous protists.

S-layer anchoring and localization of an S-layer-associated protease in Caulobacter crescentus

The S-layer of the gram-negative bacterium Caulobacter crescentus is composed of a single protein, RsaA, that is secreted and assembled into a hexagonal crystalline array that covers the organism. Despite the widespread occurrence of comparable bacterial S-layers, little is known about S-layer attachment to cell surfaces, especially for gram-negative organisms. Having preliminary indications that the N terminus of RsaA anchors the monomer to the cell surface, we developed an assay to distinguish direct surface attachment from subunit-subunit interactions where small RsaA fragments are incubated with S-layer-negative cells to assess the ability of the fragments to reattach. In doing so, we found that the RsaA anchoring region lies in the first approximately 225 amino acids and that this RsaA anchoring region requires a smooth lipopolysaccharide species found in the outer membrane. By making mutations at six semirandom sites, we learned that relatively minor perturbations within the first approximately 225 amino acids of RsaA caused loss of anchoring. In other studies, we confirmed that only this N-terminal region has a direct role in S-layer anchoring. As a by-product of the anchoring studies, we discovered that Sap, the C. crescentus S-layer-associated protease, recognized a cleavage site in the truncated RsaA fragments that is not detected by Sap in full-length RsaA. This, in turn, led to the discovery that Sap was an extracellular membrane-bound protease, rather than intracellular, as previously proposed. Moreover, Sap was secreted to the cell surface primarily by the S-layer type I secretion apparatus.