REVIEW 3 major objections 4 minor 72 references
E. coli bacterium tumbling in bulk and close to surfaces: A simulation study
T0 review · 3 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read Near a bounding surface, E. coli tumble angles shrink and turn forward-biased, explaining reported suppression of tumbling.
desk verdict Near-wall tumbling simulation with a genuinely new in-plane forward-bias result; solid bulk validation, but authors must clarify which flagellum reverses and address the Re=0.4 gap. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is a discrete flagellum built from an extended Kirchhoff-rod elastic energy, in which each segment's rotational strain vector chooses among four polymorphic ground states (normal, coiled, semi-coiled, curly-I) with a smooth transition cost. The flagella are hydrodynamically coupled to the fluid through multiparticle collision dynamics, which supplies the wall-induced flows and drag. The hook is represented by reduced bending rigidity at the motor attachment. This machinery is what lets a single reversal of one motor produce realistic polymorphic transformation, bundle unbundling, and reorientation statistics, both in bulk and at the wall.
What would settle it
Repeat the near-wall tumble simulations with the same bacterium at Reynolds number 0.4 and at about $10^{-5}$ (for example, by raising viscosity or lowering motor torque). If the mean tumble angle near the wall does not stay well below the bulk value of 61 degrees, or if the fraction of in-plane angles below 10 degrees drops from 25%, the central claim fails. On the experimental side, a high-frame-rate (at least 200 Hz) tracking study of individual E. coli near a glass surface that counts every velocity drop would settle whether such small in-plane tumbles actually occur.
Extended reading notes
Core claim
In a simulation of E. coli with four flexible, polymorphic flagella in an MPCD fluid, a tumble event is triggered by reversing one motor. In bulk, the tumble-angle distribution has mean 61 degrees and matches the classic experiment (mean 62 degrees). Near a no-slip surface, the mean drops to 41 degrees, the polar angle peaks at 90 degrees (parallel to the wall), and the in-plane angle distribution is forward-biased: 25% of events have in-plane reorientation below 10 degrees. The authors claim these small forward-biased events are the origin of the experimentally reported 'suppressed' tumbling near surfaces, and that they help bacteria stay at the surface while still exploring it.
Load-bearing premise
The near-wall results assume that the simulated fluid at Reynolds number about 0.4 behaves like real E. coli hydrodynamics at Reynolds number about $10^{-5}$; if inertial effects at 0.4 change the wall-induced drag on flagella, the reduced tumble angles and forward bias could be simulation artifacts.
Editorial extensions
If this is right
- If the wall shifts tumbling to small forward angles, near-wall E. coli effectively reorient in place, which should increase their residence time at surfaces without requiring fewer tumble events.
- Stiffer hooks narrow the tumble-angle distribution and reduce flagellar dispersion; in the model, a 100-fold stiffer hook raises the mean tumble angle and eliminates very large angles.
- Because 53% of near-wall tumbles give a polar angle below 90 degrees, tumbling does provide escape attempts, but escape is not simply correlated with flagellar dispersion.
- The measured bulk agreement pins the model's tumble mechanics to real E. coli, so the surface comparison is the meaningful new quantity.
Reading between the lines
- If the forward-bias is real, then the standard run-and-tumble picture near surfaces should be revised: runs may not be longer, but each tumble changes direction less, so effective exploration is dominated by many small turns.
- A testable extension: track near-wall tumbles with high-speed microscopy and count all velocity drops; the model predicts a population of tumbles with in-plane angle below 10 degrees that previous assays may have discarded.
- Because the simulation runs at Reynolds number about 0.4, roughly 10^4 times the real value, the near-wall forward bias should be re-checked at lower Reynolds numbers before it is used to interpret experiments.
- The same polymorphic flagellum model could be applied to other peritrichous bacteria to see whether surface-biased tumbling is generic or specific to E. coli.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops a mesoscale MPCD model of E. coli with four flexible flagella modeled by an extended Kirchhoff-rod theory that includes polymorphic transformations, and uses it to study single tumble events in bulk and near a flat no-slip wall. In bulk, for a fixed tumble time of 30 bundle periods the simulated tumble-angle distribution (mean 61°, standard deviation 34°) is compared with Berg and Brown's experimental values (mean 62°, standard deviation 26°); increasing the hook bending rigidity narrows the distribution and reduces flagellar dispersion. Near a wall, independent tumble simulations yield a mean tumble angle of 41° (versus 61° in bulk), an in-plane reorientation distribution in which 25% of events have an in-plane angle below 10°, and a polar-angle distribution peaking near 90°. The authors propose that these small in-plane reorientations explain the experimentally reported suppression of tumbling and prolonged run times near surfaces.
Significance. If the near-wall statistics are robust, this is a useful mechanistic hypothesis for why tumbling near surfaces appears suppressed: tumbles still occur but reorient the cell mostly within the surface plane and often with very small heading change. The model's strengths are its explicit treatment of flagellar polymorphism, flexible hooks, and hydrodynamic coupling, and the bulk tumble-angle statistics are based on 932–1032 independent events and agree well with a classic experiment. The near-wall predictions, however, rest on two load-bearing assumptions that are not yet verified: the fluid dynamics is computed at Reynolds number about 0.4 rather than the Stokes regime of real E. coli, and the near-wall tumble statistics may be conditional on a single flagellar motor position because of the assumed fourfold axisymmetry. These issues, rather than the bulk phenomenology, determine whether the quantitative claims (41° mean angle, 25% forward bias) can be transferred to real bacteria.
major comments (3)
- [Section 2.4 and Section 4.2] Section 2.4 reports Re≈0.4 for the MPCD fluid, roughly four orders of magnitude above the value for E. coli. The near-surface effects in Fig. 8 — reduced tumble angle, forward-biased in-plane reorientation, and reduced flagellar dispersion — are attributed to wall-induced hydrodynamic interactions and enhanced drag. At Re=0.4 inertial contributions are not negligible, so the quantitative distributions may not represent the Stokes regime that governs real E. coli. Please either perform a convergence study at lower Re (for example by increasing the viscosity or reducing the lattice constant) and show that P(γ), P(φ), and ⟨I1⟩/I_bundle^1 are unchanged, or provide a quantitative argument that the wall-induced flows relevant to tumbling are Stokes-like despite the nominal Reynolds number.
- [Section 2.2 and Section 4.2] Section 2.2 states that the bacterium is 'fully axisymmetric' so that the authors can avoid 'randomly choosing the specific position of the reversely rotating flagellum.' This is a deliberate modeling choice, but it makes the bulk and near-wall cases different: in bulk the four flagella are equivalent, while near a wall one flagellum is closest to the surface and the others are not. Section 4.2 says only that 'tumbling is initiated by reversing the rotation of one flagellum' and does not report whether all four motor positions were sampled, whether the initial roll angle about the body axis was randomized, or whether the same flagellum was used throughout. If a single body-fixed flagellum was used, the 41° mean and the 25% below-10° fraction in Fig. 8 are conditional distributions, not the marginal distributions that experiments observe where motor reversals are stochastic across flagella. The eight near-wall events in Fig. 7 are too few to average over motor identity. Please report the sampling protocol and, if only one motor was used, provide the marginal P(γ), P(φ), and P(θ) over the four motors or justify that the conditional statistics are representative.
- [Section 4.2] Unlike the bulk case, where the text reports N=932 and N=1032 for the two tumble-time protocols, Section 4.2 does not state the number of independent near-wall tumble events used for Fig. 8, and no error bars are shown. The headline numbers — mean γ=41°, 25% of events with φ<10°, and 53% escape-oriented events — need statistical support. Please report N and uncertainties (for example, bootstrap confidence intervals) for all three distributions.
minor comments (4)
- [Section 2.1] Equation (3) includes a term A/2 ξ²(∂sΩ)² that is said to allow smooth transitions between polymorphic regions of size ξ, but the numerical value of ξ is never given; please state it in Section 2.4.
- [Section 3.3, Fig. 5] The text says that for hook rigidities of 100Ah and 400Ah the tumble-angle distributions 'become more narrow,' but the reported mean angles of 87° and 69° are above the reference mean of 61°; please clarify whether 'narrow' refers to the variance and report the standard deviations for these distributions.
- [Section 5 and Data availability] The model relies on several phenomenological parameters (ground-state energies δ(n), hook rigidity Ah, motor reversal protocol) and the paper does not assess sensitivity of the near-wall results to these choices; a brief sensitivity discussion would strengthen the conclusions.
- [Data availability] The data availability statement says data are available upon request from one of the authors; for reproducibility, please consider depositing the simulation code and parameter files in a public repository.
Circularity Check
No significant circularity: the near-wall tumble statistics are emergent simulation outputs benchmarked against independent experiments.
full rationale
The paper's central claims—the bulk tumble-angle distribution, the hook-stiffness dependence, and the near-surface shift to smaller tumble angles with in-plane forward bias—are outputs of an MPCD + Kirchhoff-rod simulation, not fits to the target statistics. The only explicitly calibrated quantity, the ground-state energy offsets δ(n), is chosen to reproduce the curly-I polymorphic state during reverse motor rotation, a qualitative input about flagellar conformation; it does not by construction determine the reported mean tumble angle (61° bulk, 41° near wall) or the 25% in-plane forward-bias fraction. The bulk P(γ) is benchmarked against the independent Berg & Brown measurements, and the near-surface trends are compared with separate experimental studies (Molaei et al.; Junot et al.; Lemelle et al.). The self-citations (Vogel & Stark, Adhyapak & Stark, Zantop & Stark) are methodological references for the extended Kirchhoff-rod model, ground-state vectors, and MPCD implementation; they are not invoked as uniqueness theorems or as substitutes for the new simulation evidence. The in-plane angle φ is defined from the simulated final orientation and is a derived statistic, not an input parameter. No equation is visibly identical to an input, and no fitted parameter is renamed as a prediction. Any concerns about Reynolds-number mismatch or conditional motor choice near the wall are correctness/robustness issues, not circularity.
Assumptions & free parameters
free parameters (4)
- Polymorphic ground state energies delta(n) =
{0.0, 25, 10, 0.0} kBT
- Hook bending rigidity Ah =
2.5 kBT a0
- Motor torque reversal time =
3.5 tau_b
- Gamma distribution shape parameter k =
2
assumptions (4)
- domain assumption The extended Kirchhoff rod free energy with a min over polymorphic ground states (eqn. 3) quantitatively captures polymorphic transformations during tumbling.
- domain assumption The flagellar hook can be represented by a short Kirchhoff rod segment with reduced bending and twisting rigidity, rotating about a fixed motor axis tilted at 55 degrees.
- domain assumption The symmetrically attached four-flagella geometry is representative of E. coli tumbling behavior.
- domain assumption MPCD at the chosen collision rule and density reproduces the relevant low-Reynolds-number hydrodynamics of swimming E. coli.
Cite this review
Pith. "Pith review of E. coli bacterium tumbling in bulk and close to surfaces: A simulation study." pith.science (2026). https://pith.science/paper/WLS66WFN
@misc{pith2026250420893,
author = {Pith},
title = {Pith review of: E. coli bacterium tumbling in bulk and close to surfaces: A simulation study},
year = {2026},
howpublished = {\url{https://pith.science/paper/WLS66WFN}},
note = {Machine review of arXiv:2504.20893}
}
read the original abstract
Motility is fundamental to the survival and proliferation of microorganisms. The E. coli bacterium propels itself using a bundle of rotating helical flagella. If one flagellum reverses its rotational direction, it leaves the bundle, performs a polymorphic transformation, and the bacterium tumbles. The E. coli bacterium is hydrodynamically attracted to surfaces. This prolongs its residence time, while tumbling facilitates surface detachment. We develop a model of E. coli that uses an extended Kirchhoff rod theory to implement flagellar flexibility as well as different polymorphic conformations and perform hydrodynamic simulations with the method of multiparticle collision dynamics (MPCD). To establish a reference case, we determine the distribution of tumble angles in the bulk fluid, which shows good agreement with experiments for a fixed tumble time. Increasing the hook stiffness, narrows the tumble angle distribution and reduces the flagellar dispersion during tumbling. Close to a bounding surface, the tumble angle distribution is shifted to smaller angles, while flagellar dispersion is reduced. Reorientation within the plane favors the forward direction, which might be an explanation for prolonged run times observed in experiments
Reference graph
Works this paper leans on
-
[1]
K. M. Ottemann and J. F. Miller, Mol. Microbiol., 1997, 24, 1109–1117
work page 1997
-
[2]
L. A. Pratt and R. Kolter, Mol. Microbiol., 1998, 30, 285–293
work page 1998
- [3]
- [4]
- [5]
- [6]
-
[7]
Bray , Cell Movements: From Molecules to Motility, Garland Science, 2nd edn., 2000
D. Bray , Cell Movements: From Molecules to Motility, Garland Science, 2nd edn., 2000
work page 2000
- [8]
Show all 72 references
-
[9]
H. C. Berg and L. Turner, Biophys. J., 1993, 65, 2201–2216
1993
-
[10]
H. C. Berg, The rotary motor of bacterial flagella, 2003. 10
2003
-
[11]
Turner, W
L. Turner, W. S. Ryu and H. C. Berg, J. Bacteriol., 2000, 182, 2793–2801
2000
-
[12]
H. C. Berg, E. coli in Motion, 1st edn., 2004
2004
-
[13]
N. C. Darnton, L. Turner, S. Rojevsky and H. C. Berg, J. Bac- teriol., 2007, 189, 1756–1764
2007
-
[14]
H. C. Berg and D. A. Brown, Nature, 1972, 239, 500–504
1972
-
[15]
J. W. Costerton, Z. Lewandowski, D. E. Caldwell, D. R. Kor- ber and H. M. Lappin-Scott, Annu. Rev. Microbiol., 1995, 49, 711–745
1995
-
[16]
Van Houdt and C
R. Van Houdt and C. W. Michiels, Res. Microbiol., 2005, 156, 626–633
2005
-
[17]
A. P. Berke, L. Turner, H. C. Berg and E. Lauga, Phys. Rev. Lett., 2008, 101, 038102
2008
-
[18]
G. Li, J. Bensson, L. Nisimova, D. Munger, P. Mahautmr, J. X. Tang, M. R. Maxey and Y. V. Brun, Phys. Rev. E , 2011, 84, 041932
2011
-
[19]
Drescher, J
K. Drescher, J. Dunkel, L. H. Cisneros, S. Ganguly and R. E. Goldstein, Proc. Natl. Acad. Sci. U. S. A. , 2011, 108, 10940– 10945
2011
-
[20]
Vogel and H
R. Vogel and H. Stark, Phys. Rev. Lett., 2013, 110, 158104
2013
-
[21]
T. C. Adhyapak and H. Stark, Soft Matter , 2016, 12, 5621–5629
2016
-
[22]
C. R. Calladine, Nature, 1975, 255, 121–124
1975
-
[23]
T. C. Adhyapak and H. Stark, Phys. Rev. E, 2015, 92, 052701
2015
-
[24]
W. Lee, Y. Kim, B. E. Griffith and S. Lim, Phys. Rev. E, 2018, 98, 052405
2018
-
[25]
E. E. Riley , D. Das and E. Lauga,Sci. Rep., 2018, 8, 10728
2018
-
[26]
K. Son, J. S. Guasto and R. Stocker, Nat. Phys., 2013, 9, 494– 498
2013
-
[27]
Hirano, S
T. Hirano, S. Yamaguchi, K. Oosawa and S. Aizawa, J. Bacte- riol., 1994, 176, 5439–5449
1994
-
[28]
T. Kato, F. Makino, T. Miyata, P. Horváth and K. Namba,Nat. Commun., 2019, 10, 5295
2019
-
[29]
M. T. Brown, B. C. Steel, C. Silvestrin, D. A. Wilkinson, N. J. Delalez, C. N. Lumb, B. Obara, J. P. Armitage and R. M. Berry , J. Bacteriol., 2012, 194, 3495–3501
2012
-
[30]
Bianchi, F
S. Bianchi, F. Saglimbeni, G. Frangipane, M. C. Cannarsa and R. Di Leonardo, PRX Life, 2023, 1, 013016
2023
-
[31]
Spöring, V
I. Spöring, V. A. Martinez, C. Hotz, J. Schwarz-Linek, K. L. Grady , J. M. Nava-Sedeño, T. Vissers, H. M. Singer, M. Rohde, C. Bourquin, H. Hatzikirou, W. C. K. Poon, Y. S. Dufour and M. Erhardt, PLoS Biol., 2018, 16, 1–19
2018
-
[32]
A. L. Nord, A. Biquet-Bisquert, M. Abkarian, T. Pigaglio, F. Seduk, A. Magalon and F. Pedaci, Nat. Commun., 2022, 13, 2925
2022
-
[33]
Zhang, C
X. Zhang, C. Zhang, R. Zhang and J. Yuan, Phys. Rev. Lett., 2023, 130, 138401
2023
-
[34]
Schaar, A
K. Schaar, A. Zöttl and H. Stark, Phys. Rev. Lett., 2015, 115, 038101
2015
-
[35]
S. M. Mousavi, G. Gompper and R. G. Winkler, Soft Matter, 2020, 16, 4866–4875
2020
-
[36]
Lauga, W
E. Lauga, W. R. DiLuzio, G. M. Whitesides and H. A. Stone, Biophys. J., 2006, 90, 400–412
2006
-
[37]
Perez Ipiña, S
E. Perez Ipiña, S. Otte, R. Pontier-Bres, D. Czerucka and F. Pe- ruani, Nat. Phys., 2019, 15, 610–615
2019
-
[38]
Bianchi, F
S. Bianchi, F. Saglimbeni and R. Di Leonardo, Phys. Rev. X, 2017, 7, 011010
2017
-
[39]
Molaei, M
M. Molaei, M. Barry , R. Stocker and J. Sheng,Phys. Rev. Lett., 2014, 113, 068103
2014
-
[40]
Lemelle, T
L. Lemelle, T. Cajgfinger, C. C. Nguyen, A. Dominjon, C. Place, E. Chatre, R. Barbier, J.-F. Palierne and C. Vaillant, Biophys. J., 2020, 118, 2400–2410
2020
-
[41]
Junot, T
G. Junot, T. Darnige, A. Lindner, V. A. Martinez, J. Arlt, A. Dawson, W. C. K. Poon, H. Auradou and E. Clément,Phys. Rev. Lett., 2022, 128, 248101
2022
-
[42]
Giacché, T
D. Giacché, T. Ishikawa and T. Yamaguchi,Phys. Rev. E, 2010, 82, 056309
2010
-
[43]
J. Hu, A. Wysocki, R. G. Winkler and G. Gompper, Sci. Rep., 2015, 5, 9586
2015
-
[44]
M. Kong, Y. Wu, G. Li and R. G. Larson, Soft Matter, 2015, 11, 1572–1581
2015
-
[45]
Wu-Zhang, P
B. Wu-Zhang, P. Zhang, R. Baillou, A. Lindner, E. Clément, G. Gompper and D. A. Fedosov,bioRxiv, 2025
2025
-
[46]
Dvoriashyna and E
M. Dvoriashyna and E. Lauga, PLoS ONE, 2021, 16, 1–26
2021
-
[47]
Zöttl and H
A. Zöttl and H. Stark, Eur. Phys. J. E, 2018, 41, 61
2018
-
[48]
J. Hu, M. Yang, G. Gompper and R. G. Winkler, Soft Matter, 2015, 11, 7867–7876
2015
-
[49]
S. B. Babu and H. Stark, New J. Phys., 2012, 14, 085012
2012
-
[50]
A. W. Zantop and H. Stark, Soft Matter , 2020, 16, 6400–6412
2020
-
[51]
Eisenstecken, J
T. Eisenstecken, J. Hu and R. G. Winkler, Soft Matter, 2016, 12, 8316–8326
2016
-
[52]
L. D. Landau and E. M. Lifshitz, Theory of Elasticity, 3rd edn., 1976
1976
-
[53]
A. E. H. Love, A treatise on the mathematical theory of elastic- ity, Cambridge university press, 2013
2013
-
[54]
Vogel and H
R. Vogel and H. Stark, Eur. Phys. J. E: Soft Matter Biol. Phys., 2010, 33, 259–271
2010
-
[55]
Vogel and H
R. Vogel and H. Stark, Eur. Phys. J. E: Soft Matter Biol. Phys., 2012, 35, 15
2012
-
[56]
R. E. Goldstein, A. Goriely , G. Huber and C. W. Wolgemuth, Phys. Rev. Lett., 2000, 84, 1631–1634. 11
2000
-
[57]
P. J. Mears, S. Koirala, C. V. Rao, I. Golding and Y. R. Chemla, eLife, 2014, 3, e01916
2014
-
[58]
Namba and F
K. Namba and F. Vonderviszt, Q. Rev. Biophys. , 1997, 30, 1–65
1997
-
[59]
Malevanets and R
A. Malevanets and R. Kapral, J. Chem. Phys. , 1999, 110, 8605–8613
1999
-
[60]
Malevanets and R
A. Malevanets and R. Kapral, J. Chem. Phys. , 2000, 112, 7260–7269
2000
-
[61]
Gompper, T
G. Gompper, T. Ihle, D. Kroll and R. Winkler, inMulti-Particle Collision Dynamics: A Particle-Based Mesoscale Simulation Ap- proach to the Hydrodynamics of Complex Fluids , ed. C. Holm and K. Kremer, Springer Berlin Heidelberg, Berlin, Heidel- berg, 2009, pp. 1–87
2009
-
[62]
Noguchi, N
H. Noguchi, N. Kikuchi and G. Gompper, Europhys. Lett. , 2007, 78, 10005
2007
-
[63]
I. O. Götze, H. Noguchi and G. Gompper, Phys. Rev. E, 2007, 76, 046705
2007
-
[64]
Ihle and D
T. Ihle and D. M. Kroll, Phys. Rev. E, 2001, 63, 020201
2001
-
[65]
Noguchi and G
H. Noguchi and G. Gompper, Phys. Rev. E, 2008, 78, 016706
2008
-
[66]
Ripoll, K
M. Ripoll, K. Mussawisade, R. G. Winkler and G. Gompper, Phys. Rev. E, 2005, 72, 016701
2005
-
[67]
N. C. Darnton and H. C. Berg, Biophys. J., 2007, 92, 2230– 2236
2007
-
[68]
A. Sen, R. K. Nandy and A. N. Ghosh, J. Electron Microsc. , 2004, 53, 305–309
2004
-
[69]
J. K. Dhont, An Introduction to Dynamics of Colloids, Elsevier, Amsterdam, 1996, vol. 2
1996
-
[70]
Z. Qu, F. Z. Temel, R. Henderikx and K. S. Breuer, Proc. Natl. Acad. Sci. U. S. A., 2018, 115, 1707–1712
2018
-
[71]
Seyrich, Z
M. Seyrich, Z. Alirezaeizanjani, C. Beta and H. Stark, New J. Phys., 2018, 20, 103033
2018
-
[72]
Turner, L
L. Turner, L. Ping, M. Neubauer and H. C. Berg, Biophys. J., 2016, 111, 630–639. 12
2016
Reviewed August 16, 2026 · model on record in the stance chip above.
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