Bactericidal Efficacy of Oxidized Silver against Biofilms Formed by Curtobacterium flaccumfaciens pv. flaccumfaciens.

Bacterial wilt is a re-emerging disease on dry bean and can affect many other crop species within the Fabaceae. The causal agent, Curtobacterium flaccumfaciens pv. flaccumfaciens (CFF), is a small, Gram-positive, rodshaped bacterium that is seed-transmitted. Infections in the host become systemic, leading to wilting and economic loss. Clean seed programs and bactericidal seed treatments are two critical management tools. This study characterizes the efficacies of five bactericidal chemicals against CFF. It was hypothesized that this bacterium was capable of forming biofilms, and that the cells within biofilms would be more tolerant to bactericidal treatments. The minimum biocide eradication concentration assay protocol was used to grow CFF biofilms, expose the biofilms to bactericides, and enumerate survivors compared to a non-treated control (water). Streptomycin and oxysilver bisulfate had EC95 values at the lowest concentrations and are likely the best candidates for seed treatment products for controlling seed-borne bacterial wilt of bean. The results showed that CFF formed biofilms during at least two phases of the bacterial wilt disease cycle, and the biofilms were much more difficult to eradicate than their planktonic counterparts. Overall, biofilm formation by CFF is an important part of the bacterial wilt disease cycle in dry edible bean and antibiofilm bactericides such as streptomycin and oxysilver bisulfate may be best suited for use in disease management.

Mixed-species biofilms cultured from an oil sand tailings pond can biomineralize metals.

Here, we used an in vitro biofilm approach to study metal resistance and/or tolerance of mixed-species biofilms grown from an oil sand tailings pond in northern Alberta, Canada. Metals can be inhibitory to microbial hydrocarbon degradation. If microorganisms are exposed to metal concentrations above their resistance levels, metabolic activities and hydrocarbon degradation can be slowed significantly, if not inhibited completely. For this reason, bioremediation strategies may be most effective if metal-resistant microorganisms are used. Viability was measured after exposure to a range of concentrations of ions of Cu, Ag, Pb, Ni, Zn, V, Cr, and Sr. Mixed-species biofilms were found to be extremely metal resistant; up to 20 mg/L of Pb, 16 mg/L of Zn, 1,000 mg/L of Sr, and 3.2 mg/L of Ni. Metal mineralization was observed by visualization with scanning electron microscopy with metal crystals of Cu, Ag, Pb, and Sr exuding from the biofilms. Following metal exposure, the mixed-species biofilms were analyzed by molecular methods and were found to maintain high levels of species complexity. A single species isolated from the community (Rhodococcus erythropolis) was used as a comparison against the mixed-community biofilm and was seen to be much less tolerant to metal stress than the community and did not biomineralize the metals.

Evaluation of microbial biofilm communities from an Alberta oil sands tailings pond.

Bitumen extraction from the oil sands of Alberta has resulted in millions of cubic meters of waste stored on-site in tailings ponds. Unique microbial ecology is expected in these ponds, which may be key to their bioremediation potential. We considered that direct culturing of microbes from a tailings sample as biofilms could lead to the recovery of microbial communities that provide good representation of the ecology of the tailings. Culturing of mixed species biofilms in vitro using the Calgary Biofilm Device (CBD) under aerobic, microaerobic, and anaerobic growth conditions was successful both with and without the addition of various growth nutrients. Denaturant gradient gel electrophoresis and 16S rRNA gene pyrotag sequencing revealed that unique mixed biofilm communities were recovered under each incubation condition, with the dominant species belonging to Pseudomonas, Thauera, Hydrogenophaga, Rhodoferax, and Acidovorax. This work used an approach that allowed organisms to grow as a biofilm directly from a sample collected of their environment, and the biofilms cultivated in vitro were representative of the endogenous environmental community. For the first time, representative environmental mixed species biofilms have been isolated and grown under laboratory conditions from an oil sands tailings pond environment and a description of their composition is provided.

Can filamentous fungi form biofilms?

The discovery of biofilm formation in bacteria and yeasts has led to a better understanding of microbial ecology and to new insights into the mechanisms of virulence and persistence of pathogenic microorganisms. However, it is generally assumed that filamentous fungi, some of which have a significant impact on our health or our economy, do not form biofilms. In contrast to this assumption, here we discuss recent findings supporting the hypothesis that surface-associated filamentous fungi can form biofilms. Based on these findings and on previous models for bacterial and yeast systems, we propose preliminary criteria and a model for biofilm formation by filamentous fungi.

The GacS sensor kinase controls phenotypic reversion of small colony variants isolated from biofilms of Pseudomonas aeruginosa PA14.

The GacS/GacA two-component regulatory system in pseudomonads regulates genes involved in virulence, secondary metabolism and biofilm formation. Despite these regulatory functions, some Pseudomonas species are prone to spontaneous inactivating mutations in gacA and gacS. A gacS(-) strain of Pseudomonas aeruginosa PA14 was constructed to study the physiological role of this sensor histidine kinase. This loss-of-function mutation was associated with hypermotility, reduced production of acylhomoserine lactones, impaired biofilm maturation, and decreased antimicrobial resistance. Biofilms of the gacS(-) mutant gave rise to phenotypically stable small colony variants (SCVs) with increasing frequency when exposed to silver cations, hydrogen peroxide, human serum, or certain antibiotics (tobramicin, amikacin, azetronam, ceftrioxone, oxacilin, piperacillin or rifampicin). When cultured, the SCV produced thicker biofilms with greater cell density and greater antimicrobial resistance than did the wild-type or parental gacS(-) strains. Similar to other colony morphology variants described in the literature, this SCV was less motile than the wild-type strain and autoaggregated in broth culture. Complementation with gacS in trans restored the ability of the SCV to revert to a normal colony morphotype. These findings indicate that mutation of gacS is associated with the occurrence of stress-resistant SCV cells in P. aeruginosa biofilms and suggests that in some instances GacS may be necessary for reversion of these variants to a wild-type state.