Capillary bacterial migration on non-nutritive solid surfaces

Abstract

Here we describe an additional type of bacterial migration in which bacterial cells migrate vertically across a non-nutritive solid surface carried by capillary forces. Unlike standard motility experiments, these were run on a glass slide inserted into a Falcon tube, partly immersed in a nutrient medium and partly exposed to air. Observations revealed that capillary forces initiated upward cell migration when biofilm was formed at the border between liquid and air. The movement was facilitated by the production of extracellular polymeric substances (EPS). This motility differs from earlier described swarming, twitching, gliding, sliding, or surfing, although these types of movements are not excluded. We therefore propose to call it "capillary movement of biofilm". This phenomenon may be an ecologically important mode of bacterial motility on solid surfaces.

Keywords: Acinetobacter; Bacillus; Pseudomonas; Staphylococcus; air/liquid interface; biofilm; microscopy; surface motility.

Figure 1

Experimental setup for growing bacterial biofilms on an air/liquid interface

Figure 2

Biofilms grown on glass microscopy slides at the air/liquid interface after 7 days of incubation. Far right: macroscopic view of biofilm formed at air/liquid interface. NS – experiments without shaking; S – experiments with shaking. Scale bar=50 μm

Figure 3

Biofilm of P. aeruginosa at the air/liquid interface after 7 days of incubation under confocal microscopy. Left: autoflorescence; Right: bright field. Scale bar=50 μm

Figure 4

Migration of B. cereus (A), A. junii (B), and S. aureus cells (C) across the air-exposed section of the glass slide imaged after 7 days of incubation. Measures in mm designate the distance crossed up the slide from the air/liquid interface. The “topmost visible microcolony” shows a microcolony or aggregate of cells reaching the farthest away from the interface

Figure 5

Migration of A. baumannii cells across the air zone of glass microscopy slide imaged after 7 days of incubation. Measures in mm designate the distance crossed up the slide from the air/liquid interface. Upper row shows the “topmost visible microcolony”, a microcolony or aggregate of cells reaching the farthest away from the interface. This figure shows several snapshots from the same area (height)

Figure 6

Migration of P. aeruginosa cells across the air-exposed section of the glass slide at 20 mm above the air/liquid interface. Image on the right shows superimposed autofluorescence. Scale bar=50 μm

Figure 7

Microcolonies of B. cereus (A and B) and B. thuringiensis (C) on the air-exposed section of the glass slide after 7 days of incubation. Images on the right show magnified cut-outs. EPS – extracellular polymeric substances. Scale bar=25 μm

Figure 8

Typical microcolony of B. cereus spotted high in the air-exposed section of the glass slide. Scale bar=25 μm

Figure 9

Migration of A. junii (A) and A. baumannii (B) from the air/liquid interface to the air-exposed section of the glass slide. EPS is dyed in blue/purple to better show spreading up the slide [A. junii (C); A. baumannii (D)]

Figure 10

Migration of carbol-fuchsin (left) and A. baumannii cells from nutrient media or saline (right) up the glass slide, recreated from experiments listed in Table 1. Starting bacterial concentrations were either 10³ or 10⁷ CFU/mL. The dashed line marks the farthest point reached on the given days of incubation

Figure 11

Migration of carbol-fuchsin (left) and B. cereus cells from nutrient media or saline (right) up the glass slide, recreated from experiments listed in Table 1. Starting bacterial concentrations were either 10³ or 10⁷ CFU/mL. The dashed line marks the farthest point reached on the given days of incubation