Coordinated surface activities in Variovorax paradoxus EPS

Abstract

Background: Variovorax paradoxus is an aerobic soil bacterium frequently associated with important biodegradative processes in nature. Our group has cultivated a mucoid strain of Variovorax paradoxus for study as a model of bacterial development and response to environmental conditions. Colonies of this organism vary widely in appearance depending on agar plate type.

Results: Surface motility was observed on minimal defined agar plates with 0.5% agarose, similar in nature to swarming motility identified in Pseudomonas aeruginosa PAO1. We examined this motility under several culture conditions, including inhibition of flagellar motility using Congo Red. We demonstrated that the presence of a wetting agent, mineral, and nutrient content of the media altered the swarming phenotype. We also demonstrated that the wetting agent reduces the surface tension of the agar. We were able to directly observe the presence of the wetting agent in the presence and absence of Congo Red, and found that incubation in a humidified chamber inhibited the production of wetting agent, and also slowed the progression of the swarming colony. We observed that swarming was related to both carbon and nitrogen sources, as well as mineral salts base. The phosphate concentration of the mineral base was critical for growth and swarming on glucose, but not succinate. Swarming on other carbon sources was generally only observed using M9 salts mineral base. Rapid swarming was observed on malic acid, d-sorbitol, casamino acids, and succinate. Swarming at a lower but still detectable rate was observed on glucose and sucrose, with weak swarming on maltose. Nitrogen source tests using succinate as carbon source demonstrated two distinct forms of swarming, with very different macroscopic swarm characteristics. Rapid swarming was observed when ammonium ion was provided as nitrogen source, as well as when histidine, tryptophan, or glycine was provided. Slower swarming was observed with methionine, arginine, or tyrosine. Large effects of mineral content on swarming were seen with tyrosine and methionine as nitrogen sources. Biofilms form readily under various culture circumstances, and show wide variance in structure under different conditions. The amount of biofilm as measured by crystal violet retention was dependent on carbon source, but not nitrogen source. Filamentous growth in the biofilm depends on shear stress, and is enhanced by continuous input of nutrients in chemostat culture.

Conclusion: Our studies have established that the beta-proteobacterium Variovorax paradoxus displays a number of distinct physiologies when grown on surfaces, indicative of a complex response to several growth parameters. We have identified a number of factors that drive sessile and motile surface phenotypes. This work forms a basis for future studies using this genetically tractable soil bacterium to study the regulation of microbial development on surfaces.

Figure 1

Variovorax paradoxus displays swarming motility. Swarming plates with glucose and casamino acids inoculated with drops of P. aeruginosa PAO-1 (A), V. paradoxus EPS (B), or E. coli S17-1 (C).

Figure 2

Swarming of V. paradoxus EPS is inhibited in a dose dependent manner by the presence of Congo Red in the agar. Plates containing doses of Congo Red ranging from 1–1000 μg/L were incubated at 30°C either A) under ambient atmospheric humidity or B) in a humidified glass dish. Symbols in both panels: No CR (black diamond), 1 μg/L CR (open square), 10 μg/L CR (filled triangle), 50 μg/L CR (×), 100 μg/L(*), 500 μg/L CR (open circle), 1000 μg/L (+). Swarm diameter measured in triplicate, reported as mean ± SEM.

Figure 3

Humidity affects response to Congo Red swarming inhibition. A-D) gross morphology of V. paradoxus EPS on plates incubated at 30°C on media containing 0, 10,100, and 500 μg/L CR after 48 h. E-H) Edge images from the same culture conditions at 24 h. I-L) gross morphology of 48 h cultures on identical media incubated at 30°C in a humidified chamber. M-P) edge images from the humidified chamber incubated cultures at 24 h. Scale bar = 25 microns.

Figure 4

A wetting agent is present beyond the edge of the swarm. Colony photography using reflected light (A, B) illustrating the presence of a wetting agent (arrows) preceding the spreading colony on (A) FW medium with succinate and NH₄Cl as C, N source. B) Colony spread is limited by 500 μg/L CR, but wetting agent spreads as above. C) Drop collapse assay using dilute methylene blue solution showing the reduced surface tension in the wetting agent zone (left of the black line).

Figure 5

Nutrient dependence of swarming motility. A) Swarm diameter at 24 h (blue bars) or 48 h (red bars) using several carbon sources on FW (F) or M9 (M) base. F/M-S = succinate, F/M-G = glucose, F-G-P = glucose + 2 mM phosphate buffer (pH7), M-M = maltose, F/M-CAA = casamino acids (C+N), M-Ma = malic acid, M-So = sorbitol, M-Su = sucrose. * indicates that swarms merged by 48 h. B) Swarm diameter at 24 h (blue bars) or 48 h (red bars) using several nitrogen sources on FW (F) or M9 (M) base. All swarms measured in triplicate, with error in all cases ± SEM.

Figure 6

Edges of swarms are affected by nutrients, basal medium. Swarming edge images after 24 h on a variety of media. FW base medium was used for (A, B, D, J, K, L) with M8/M9 base medium used for the other panels. Succinate is the C source in all panels except B (glucose) and C (maltose). For growth on FW-glucose, 2 mM sodium phosphate buffer (pH 7) was added. NH₄Cl was the N source in (A-C), with alternative N sources methionine (D, E), arginine (F), tyrosine (G, J), tryptophan (H, K), and histidine (I, L). Arrows point to extruded material from swarm edges under certain conditions. Scale bar = 25 microns.

Figure 7

Gross swarm morphology is affected by nutrients, basal medium. Colony morphologies after 1d on A) FW-succinate-NH₄Cl and B) FW-casamino acids. C) After 3d on FW-succinate-methionine, a "rare branch" phenotype was observed. D) Slower swarming on M9-succinate-tyrosine was characterized by a less well defined swarm with altered structure. Stark differences in extent and form of swarming were observed on E) FW-succinate-tryptophan and F) M9-succinate-tryptophan. G) After an extended incubation, swarms on FW-succinate-NH₄Cl display a mutually repellent morphology with distinct internal and external edges.

Figure 8

Nutrient dependence of batch biofilm formation. A) Biofilm formation with succinate as carbon source is not dependent on nitrogen source. N1 = methionine, N2 = tyrosine, N3 = tryptophan, N4 = NH₄SO₄, N5 = glycine, N6 = arginine, N7 = histidine, N8 = NH₄Cl. B) Biofilm formation on variable carbon sources with NH₄Cl as nitrogen source. C1 = glucose, C2 = casamino acids, C3 = succinate, C4 = maleic acid, C5 = d-sorbitol, C6 = maltose, C7 = benzoate, C8 = mannitol, C9 = malic acid, C10 = sucrose. In both instances measurements were taken after 24 h (blue bars) and 48 h (red bars). Error is computed as ± SEM.

Figure 9

Static and Stirred batch biofilms. A) A static biofilm grown for 48 h in a Nunc one-well plate shows characteristic biofilm forms near the air-broth interface when stained with 1% crystal violet. B) V. paradoxus EPS from a stirred batch bioreactor on a glass slide show a strong propensity toward filamentous morphology. Both images at 1000× magnification. Scale bar = 10 microns.

Figure 10

Biofilms cultivated in a CDC stirred biofilm reactor. V. paradoxus EPS was cultured from a broth inoculum for 18 h under stirred batch conditions (A, B), followed by 24 h (C, D) or 48 h (E, F) under continuous flow conditions (2 ml/min). BacLight staining with PI (red, dead cells) and Syto9 (green, live cells). 100×, scale bar = 100 microns (A, C, E). 400×, scale bar = 25 microns (B, D, F).