Plasmid Transfer between Marine Bacteria in the Aqueous Phase and Biofilms in Reactor Microcosms

Plasmid transfer of broad-host-range plasmid RP1 from marine Vibrio sp. strain S14 to marine strain SW5 under optimum conditions on the surface of nutrient plates was improved 2 orders of magnitude by using the plasmid transfer process to select an SW5 recipient more efficient than the wild type in receiving and/or maintaining the plasmid. This recipient strain, SW5H, was used to form biofilms under flow conditions on the surfaces of glass beads in reactors. The S142(RP1) donor strain was introduced to the reactors after either 48 or 170 h of biofilm formation, and production of transconjugants in the aqueous phases and biofilms without selection pressure was assessed. Plasmid transfer to the recipient cells in the biofilm was detected for biofilms formed for 170 h but not in those formed for 48 h. The plasmid transfer frequency was significantly higher (P < 0.05) among cells attached to the bead surfaces in the biofilm than among cells in the aqueous phase.

Determination of nutrients limiting biofilm formation and the subsequent impact on disinfectant decay

Understanding the contribution of both organic and inorganic nutrients to biofilm development and the subsequent impact of developed biofilms on disinfectant decay are important requirements for distribution system management strategies. Nutrient limitation may be one way to control biofilm development without increasing disinfectant dosing. Little is known, however, of the nutrient requirements of biofilms in distribution systems. Indeed, the effects on biofilm development due to the addition of nutrients to distribution systems and what impact biofilm development may have on disinfectant decay is still poorly understood. This study used annular reactors to determine the nutrients limiting for biofilm development in drinking water from two different Sydney sources and the subsequent effects of biofilm development on disinfectant decay. It was found that biofilm development in Sydney water was limited by organic carbon and that biofilm development promoted chloramine decay. Moreover, biofilm development occurred in the presence of chloramine. The ability of biofilms to respond to increases in disinfectant concentrations was dependent on the biomass of the biofilms. In a comparative study using chlorinated drinking water containing very low levels of organic carbon, biofilm development was not detected. Removal of organic carbon resulted in greater persistence of chlorine, which led to greater biofilm control. It was also shown that biofilms could contribute cells to the aqueous phase. The results of the study indicate that treatment and system management strategies should incorporate organic carbon removal to limit biofilm development through a combination of retarding bacterial growth and enhancing disinfectant persistence.

Simultaneous determination of gene expression and bacterial identity in single cells in defined mixtures of pure cultures

A protocol was developed to achieve the simultaneous determination of gene expression and bacterial identity at the level of single cells; a chromogenic beta-galactosidase activity assay was combined with in situ hybridization of fluorescently labelled oligonucleotide probes to rRNA. The method allows monitoring of gene expression and quantification of beta-galactosidase activity in single cells.

Improved methods for in situ enzymatic amplification and detection of low copy number genes in bacteria

We present alternative and improved protocols for in situ analysis of single copy genes in prokaryotes. Primed in situ amplification (PRINS) and cycle PRINS were used to detect, via the incorporation of a fluorescein labelled nucleotide, the presence of specific genes carried on both high and low copy number plasmids in individual cells of Escherichia coli and a marine bacterium, SW5. The optimised protocols described enabled a significant reduction in non-specific signals whilst maintaining high fluorescent activity via labelled nucleotide incorporation. In addition, nucleic acids were amplified linearly and were retained within the permeabilised microbial cells. These methods provide considerable advances in sensitivity, specificity and reliability compared to current protocols for bacterial in situ nucleic acid amplification.

Substratum-induced morphological changes in a marine bacterium and their relevance to biofilm structure

The effects of surfaces on the physiology of bacteria adhering to surfaces or immobilized within biofilms are receiving more interest. A study of the effects of hydrophobic and hydrophilic substrata on the colonization behavior of a marine bacterium, SW5, revealed major differences in the morphology of SW5 on these surfaces. Using epifluorescence, scanning confocal laser, and on-line visualization (time-lapse video) microscopy, the organisms at hydrophobic surfaces were characterized by the formation of tightly packed biofilms, consisting of single and paired cells, whereas those at hydrophilic surfaces exhibited sparse colonization and the formation of chains more than 100 microns long, anchored at the surface by the terminal (colonizing) cell. The results are discussed in terms of the possible factors inducing the observed morphological differences and the significance of these differences in terms of biofilm structure and plasmid transfer when SW5 is the recipient organism.