Complex flows of an Escherichia coli living culture

Transcription

1 Complex flows of an Escherichia coli living culture Catarina Leal Raquel Portela, Rita Sobral, Pedro Almeida, Pedro Patrício José Maria Franco Rotational tumbling of Escherichia coli aggregates under shear R. Portela, P. Patrício, P. L. Almeida, R. G. Sobral, J. M. Franco, and C. R. Leal PHYSICAL REVIEW E 94, (2016)

2 Main goals Rheological characterization of the growth process of an Escherichia coli culture Focus on the aggregation patterns and related motion developed during time, under shear Microscopic model attempt to describe the observed behaviour

3 Escherichia coli Figure 1 Fluorescent optical microscopy of Escherichia coli DHSα (Invitrogen, USA).

4 OD 620nm 3,5 3 E. Coli / LB 2,5 OD 620 nm 2 1,5 lag phase late phase 1 0,5 exponential phase time (min) Figure 2 - E.coli culture in LB medium characterized by optical densities (OD 620nm ) during growth; dashed lines separate distinct growth phases: lag, exponential and late phases. All measurements were performed at 37 C.

5 Viscosity growth curve 0,015 Culture medium LB E.coli/LB iscosity (Pa.s) 0,01 0,005 lag phase exponential phase late phase time (min) Figure 3 E. coli culture steady-state shear growth curve, η(t) measured at a constant shear rate of 10s 1 in LB culture medium (dark grey) and culture medium LB (light gray) (representative curves), at 37 C.

6 Viscosity growth curve 0,02 Culture medium LB E.coli/LB 0,015 viscosity (Pa.s) 0,01 0, time (min) Figure 3 E. coli culture steady-state shear growth curve, η(t) measured at a constant shear rate of 10s 1 in LB culture medium (dark grey) and culture medium LB (light gray) (representative curves), at 37 C. In-set: schematic set-up for video acquisition during growth at some time interval during the transition from the exponential phase to the late phase, performed with a RheoScope Haake.

7 Viscosity growth curve Rheo-imaging E.coli video 1: obtained during the steady-state shear viscosity measurement, started in the late phase of growth, of a E. coli culture, at a constant shear rate of 10 s 1. Measurements were performed at 37 C.

8 Viscosity growth curve Rheo-imaging Figure 4: Image sequences extracted from E.coli video 1, at specific time intervals to illustrate the aggregates rotation motion (full rotation), during the steady-state shear viscosity measurement of a E. coli culture, at a constant shear rate of 10 s 1. Measurements were performed at 37 C.

9 Microscopic Model Consider the cells as small individual particles (not perturbing the flow and inertia effect neglected), connected by rigid links or filaments, thus forming a global rigid body. In particular, if the vorticity direction coincides with one of the principal directions of the aggregate s moment of inertia, the aggregate may rotate only in the plane of the flow, according to the classic equation: dθ dt = γ! (1 d cos2θ) 2 θ! m =! γ 2 1 d 2 (Jeffery s equation) d = (I 2 I 1 ) / (I 1 + I 2 ) d 1 is related to the principal second order moments in the rotational plane, I 1 and I 2

10 Microscopic Model The average value of the angular velocity of the aggregates was estimated to be: 2, 2 ± 0, 7 rad/s (the error associated to this estimation is mainly due to the acquisition frame rate, which is 1 frame/s), and is roughly 1/4 of the shear rate (10 ± /s). This would correspond to a fairly asymmetric aggregate (in the rotational plane, which is perpendicular to the plane of the images, and leading to a large value of d), not suggested by the pictures.

11 Main Ideas In this study, real-time and in situ rheo-imaging rheology was applied to the animal commensal bacteria E. coli during cell growth. As the density of bacteria in the medium increases, cells may rearrange themselves in different aggregates, capable of strongly influencing their environment, and leading to three different physical rheological responses, corresponding to the three distinct phases of growth, the lag, exponential and late phases.

12 Main Ideas In particular, in the late phase of growth, the viscosity increase slows down, reaching an intermittent plateau of maximum viscosity, with several drops and recoveries. In this phase, the highest bacteria density is attained bacteria still grow and divide, but at a lower rate. Big and irregular bacteria aggregates are observed, which keep moving in suspension. No significant sedimentation is observed. The aggregates present translational motion in the shear flow direction, and rotational motion in the vorticity direction. The aggregates become larger in time, due to the incorporation of smaller aggregates and due to the rotational motion, the aggregates become elongated along the rotational axis. Apparently, the size of the aggregates does not influence the rotational motion, since almost all aggregates rotate with the same angular velocity, which is related with the applied shear rate.

13 Main Ideas In spite of the well-known E. coli intrinsic motility, we do not observe an explicit individual motion of the cells within each aggregate, suggesting they are connected by adhesive factors we interpret this behaviour in the light of a simple rigid-body motion. From the theoretical point of view, we could think of more developed models, using cell s intrinsic motility and chemioattractive interactions (a la Keller-Segel type), or using active Brownian forces, expressing the cell s collective interaction, added to the equation of motion of each individual cell. Nevertheless, from our images, we have the impression that the motion of the aggregates resembles the motion of a rigid body.

14 Thank you!