Bacteria lack the cytoskeleton and motors of eukaryotic cells, and their cytoplasm has been considered to be purely fluid. New data show that bacterial cytoplasm can solidify to resemble a soft glass, unless enzymatic activity creates motions to fluidize it. © 2014 Elsevier Ltd

Janmey, P.A.

MacKintosh, F.C.

SummaryBacteria lack the cytoskeleton and motors of eukaryotic cells, and their cytoplasm has been considered to be purely fluid. New data show that bacterial cytoplasm can solidify to resemble a soft glass, unless enzymatic activity creates motions to fluidize it

Janmey, Paul A.

MacKintosh, Fred C.

Elsevier - Publisher Connector 

Current Biology Vol 24 No 6
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Molecular Cancer Research, Center for
Molecular Medicine, and Cancer Genomics
Netherlands, University Medical Center
Utrecht, 3584 CG, Utrecht, The Netherlands.
E-mail: g.j.p.l.kops@umcutrecht.nlhttp://dx.doi.org/10.1016/j.cub.2014.02.020Cytoplasmic Transport: Bacteria Turn
to Glass Unless KickedBacteria lack the cytoskeleton and motors of eukaryotic cells, and their
cytoplasm has been considered to be purely fluid. New data show that bacterial
cytoplasm can solidify to resemble a soft glass, unless enzymatic activity
creates motions to fluidize it.Paul A. Janmey1,2
and Fred C. MacKintosh1,2
A standard explanation for why
bacteria— unlike eukaryotic cells— do
not express molecular motors, like
myosin, kinesin, or dynein, or have
three-dimensional cytoskeletons is
that they do not need them. The
bacterial cell wall determines their
shape, and the interior volume of
bacteria (w1 mm3) is so small that
thermally-driven transport of
molecules is fast enough not to be the
rate-limiting step of reactions needed
to maintain viability. Only as cells
evolved to become larger and more
irregularly shaped did they need
directed transport to overcome the
slow rate and dilution associated with
molecules diffusing from their source
to their destinations many microns or
evenmeters away, as inmotor neurons.
Directed transport required not only
motors but also cytoskeletal filaments
as tracks, and, as cells developed
cytoskeletons, the viscoelasticity of the
cell interior became much more
complex than that of a solution of
proteins and nucleic acids. Many
studies have modeled the eukaryotic
cell interior as a polymer gel or a soft
colloidal glass rather than a liquid, butthe bacterial cytoplasm was until
recently thought to lack the complex
gel–sol transitions of eukaryotic cells. A
report by Parry et al. [1] now shows that
this simple model is not adequate. The
baseline state of bacterial cytoplasm
shows complex properties reminiscent
of materials such as glasses, and the
motion and fluidity of bacterial
cytoplasm might depend on the
constant activity of enzymes. These
enzymes are not traditional motors, but
they can jostle the cytoplasm more
vigorously than thermal fluctuations
alone [2].
Almost from the time that scientists
first examined living cells using the light
microscope, they noticed the transient
formation of distinct regions of
cytoplasm from which diffusing or
transported intracellular particles were
excluded and in which no Brownian
motion was observed. These solid-like
regions have been described as glassy
or ‘hyaline’ since at least the 19th
century [3]. Other biologists used the
term gel [4] — derived from the Latin
gelare, to freeze — to describe these
non-fluid regions of cytoplasm. The
precise meanings of the terms glass
and gel have been notoriously hard to
define, but the physical properties of
the cell interior drew the attention ofboth biologists and physical chemists
who made analogies between the
cytoplasm and other complex fluids
[5]. The idea that the cell is filled with
an invisible (to contemporary
microscopes) polymer network was,
however, not the ubiquitous image that
it now is in cell biology, and concepts of
colloidal physics, foams, andmolecular
crowding were considered to account
for the glassy appearance of the cell
interior. An enormous amount has now
been learned abut the molecules and
assemblies that endow soft cells with
variably fluid or solid properties, even if
there are still many discussions about
whether this living material has more in
common with polymer networks, soft
glasses, or some other viscoelastic
materials.
The concept of cytoplasmic
glassiness occurs in many different
contexts in animal and plant cells. For
example, a category of malignant cells
termed ‘glassy cells’ is used in
diagnosis of some cancers [6], and
phase transitions similar to inorganic
glass transitions, where even small
molecules have restricted motion,
occurs in the cytoplasm of plants that
can survive desiccation [7,8]. In
contrast, the cytoplasm of bacteria has
generally been considered to be like a
simple fluid filled with many solutes,
but not so concentrated or reticulated
as to form polymer networks or
colloidal solids. This distinction
between prokaryotic and eukaryotic
cytoplasm was evident in early studies,
such as in an analysis of the
cytoplasmic fluidity of different
pathogens in which the eukaryotic
parasite Entameoba histolytica was
Figure 1. Constrained particle motion in
a gel.
The motion of particles embedded in an inho-
mogeneous polymer gel show dynamical
heterogeneity, with highly constrained mo-
tion in denser regions (upper left and right)
and freer motion in regions of lower concen-
tration (lower left). In a cross-linked gel, these
heterogeneities persist in time, thereby
breaking ergodicity.
Dispatch
R227compared with the bacterium
Escherichia coli: ‘‘In E. coli the
ecto- and endoplasm canwith difficulty
be distinguished. and there is never
present in this species the glass-like,
perfectly hyaline appearance of the
ectoplasm invariably observed in
E. histolytica’’ [9].
Parry et al. [1], now armedwith higher
precision imaging methods and
motivated by a recent flurry of activity
in soft matter and biophysics that
describes the cytoplasm as an active
material far from equilibrium, take a
new look at diffusion in the bacterium.
They find some striking effects that
depend on the size of the diffusing
species and the metabolic state of the
cell. By tracking the motions of
individual protein assemblies and other
particles of different sizes, the authors
show that small molecules indeed
move in a manner consistent with
diffusion, but, when chemical energy
stores are depleted by addition of
compounds like dinitrophenol that
prevent the generation of ATP, larger
solutes appear to be trapped in cages
from which they can only infrequently
escape. Even when they can move in a
limited manner, they do so at different
rates in different regions of the
bacterial interior. The need for energy in
order to observe diffusive motion does
not involve traditional molecular
motors or the movement of chromatin
driven by DNA-remodeling enzymes.
Parry et al. [1] conclude that the
constrained and location-dependent
motions of large solutes in
energy-depleted but otherwise intact
bacteria reveal that the basal state of
the bacterial cytoplasm is best
described as having glass-like
properties, which the metabolic energy
of the cell can fluidize.
As evidence for glassy behavior,
Parry et al. [1] show three
characteristics also found in more
standard glasses: non-Gaussian
fluctuations; nonergodic behavior; and
dynamic heterogeneity. Of these,
ergodicity is a particularly fascinating
idea introduced by Boltzmann
approximately 150 years ago, in an
effort to reconcile the second law of
thermodynamics with mechanics and
the atomic theory of matter. As
examples of ergodicity, most materials
are locally homogeneous, when viewed
over sufficiently long time periods. For
instance, in a simple liquid, different
regions of the fluid will appear to be
different on the scale of nanometers,although these differences will vanish
over the course of time. Different
regions of the fluid can be thought of as
different samples, and these will have
the same average properties over time:
the sample or ensemble average is the
same as the time average of a single
sample. This is the essence of
ergodicity.
As appealing as the ergodic
hypothesis may be, however, it is not
always true in practice. When cooled
down, some liquids will undergo
a transition to a glass, in which
differences from point to point in the
sample become frozen, and these
differences do not vanish over time
(even on geological or astrophysical
time scales!), as shown in Figure 1.
Parry et al. [1] observe such a violation
of ergodicity in the bacterial cytoplasm,
along with the closely associated
property of dynamic heterogeneity:
given that not all regions of a sample
have the same average properties,
one can observe significant spatial
variation of the local dynamics, e.g., of
particles within themedium. In addition
to their findings of non-Gaussian
fluctuations, nonergodicity and
dynamical heterogeneities, Parry et al.
[1] also find that metabolic processes
significantly enhance motion in
bacteria and can even appear to
‘uncage’ otherwise constrained or
confined particles. Specifically, the
authors find that when enzymes are
turned off in response to chemical
signals, like nutrient deprivation,
motion in the cytoplasm is strongly
suppressed. They interpret the effect
of metabolism as a sort of fluidization
of the otherwise solid-like cytoplasm
by enzymatic activity. Interestingly,
studies of plant cell cytoplasm show
the converse relationship: when
cytoplasmic fluidity is lost by removal
of water in desiccation-tolerant plants,
enzymatic activity ceases even without
chemical modification of the
enzyme [10].
While the suggested analogy
between the bacterial cytoplasm and
glassy systems is a very interesting
one, other interpretations are possible.
Polymer and colloidal gels also
exhibit nonergodicity and dynamical
heterogeneity [11], while molecular
motor activity is known to lead to
non-Gaussian effects in biopolymer
systems [12]. Nevertheless, with the
results reported by Parry et al. [1],
bacteria join eukaryotic cells as
having cytoplasms that cannot becharacterized as solutions of freely
diffusing molecules. These findings
have implications for how rate
constants and biochemical reaction
pathways that are defined in dilute
solution in vitro can be applied to the
intracellular setting. As was recognized
by earlier generations of cell biologists,
the concepts and methods of physical
chemistry have a deep, yet only partly
appreciated, relevance to biological
function.
References
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O’Hern, C.S., Dufresne, E.R., and Jacobs-
Wagner, C. (2014). The bacterial cytoplasm has
glass-like properties and is fluidized by
metabolic activity. Cell 156, 183–194.
2. Radmacher, M., Fritz, M., Hansma, H.G., and
Hansma, P.K. (1994). Direct observation of
enzyme activity with the atomic force
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3. Bu¨tschli, O. (1894). Investigations on
Microscopic Foams and on Protoplasm
(London: Adam and Charles Black).
4. Hyman, L.H. (1917). Metabolic gradients in
Amoeba and their relation to the mechanism of
amoeboid movement. J. Exp. Zool. 24, 55–99.
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and of inorganic gels: An analogy. J. Exp. Biol.
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Wang, C.C., Robboy, S.J., Taft, P.D.,
Ulfelder, H., and Scully, R.E. (1976). Glassy
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7. Buitink, J., van den Dries, I.J., Hoekstra, F.A.,
Alberda, M., and Hemminga, M.A. (2000). High
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(2009). Relationship between desiccation
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Current Biology Vol 24 No 6
R2289. Craig, C. (1908). Studies upon the amebae in
the intestine of man. J. Infect. Dis. 5, 324–377.
10. Fernandez-Marin, B., Kranner, I.,
Sebastian, M.S., Artetxe, U., Laza, J.M.,
Vilas, J.L., Pritchard, H.W., Nadajaran, J.,
Miguez, F., Becerril, J.M., et al. (2013). Evidence
for the absence of enzymatic reactions in the
glassy state. A case study of xanthophyll
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moss Syntrichia ruralis. J. Exp. Bot. 64,
3033–3043.11. Pusey, P.N., and Van Megen, W. (1989).
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1Departments of Physiology and Physics &
Astronomy, Institute for Medicine andEngineering, University of Pennsylvania,
Philadelphia, PA 19104, USA. 2Department of
Physics and Astronomy, Vrije Universiteit,
1081 HV Amsterdam, The Netherlands.
E-mail: janmey@mail.med.upenn.edu (P.A.J.),
fcm@nat.vu.nl (F.C.M.)http://dx.doi.org/10.1016/j.cub.2014.02.018Pheromones: The Scent of a MaleThere has been an enduring fascination with discovering biological odours
which can evoke behavioral and physiological responses in mammals. New
findings in goats have now identified a key molecule involved in the effect male
odours have on female reproductive cycles.Keith M. Kendrick
Despite extensive evidence for
biologically active pheromones in
invertebrate species the search for
them in mammals has met with less
success. The term ‘pheromone’ was
first introduced over 50 years ago [1] to
describe airborne communication
molecules by which individual animals
could evoke robust and specific
physiological/behavioral effects in
recipient individuals of the same
species. Pheromones can either have a
‘releaser’ function by promoting rapid
behavioral effects, such as sexual or
aggression responses, or act as
‘primers’ by facilitating longer term
physiological changes, which may also
ultimately influence behavior.
While there is reasonable evidence
that biological odours can influence
both physiology and behavior in a
number of mammalian species, it is
clear that their effects aremore variable
and generally less specific than in
invertebrates [2,3]. As such their
actions do not really conform to the
strict definition of pheromones,
particularly in terms of being single
molecules having both species and
functional specificity and whose
effects are not influenced by learning.
Nevertheless, the term has been
adopted widely in mammalian
olfactory research despite this
caveat. In mammals the main focus
has been on identifying pheromones
influencing reproductive physiology
and behavior, notably the control of
puberty, ovulation and sexual
attraction and receptivity. Putative
signaling pheromones have alsobeen localized to a variety of different
biological sources such as urine,
tears, skin glands, wool, saliva
and vaginal secretions. In mice, for
example, the major urinary protein
pheromones inmale urine, 2-sec-butyl-
4,5-dihydrothiazole, 3,4-dehydro-exo-
brevicomin, a- and ß-farnesene, and
6-hydroxy-6-methyl-3-heptanone
from the preputial gland all appear
to have an influence on oestrus
synchronization and acceleration of
puberty in females [4]. The peptide
exocrine gland-secreting peptide 1
(ESP1) released in the tear fluid of male
mice also enhances female sexual
receptivity [5]. In hamsters, female
vaginal secretions contain dimethyl
disulfide which in association with a
carrier molecule, aphrodisin, facilitates
male sexual responses [6]. In pigs,
androstenone (5a-androst-16-en-3-one)
and androstenol (5a-androst-16-en-
3-ol) in boar saliva facilitate sexual
receptivity in females [7].
One of the areas where identification
of pheromones has proved elusive has
been in relation to the impact that male
odours have on female reproductive
cycles in seasonally breeding species
such as sheep and goats. Odours from
the wool of rams and from the head and
sebaceous glands of bucks can
stimulate ovulatory cycles in
seasonally anoestrus females and
thereby prolong the breeding season.
This has become known as the ‘male
effect’ and is testosterone dependent.
In fact this primer pheromone effect is
not restricted to restoring ovulatory
cycles but can also influence their
duration and synchronicity (see [8]). In
keeping with a less rigorous definitionof pheromone specificity in mammals
relevant odours from the two species
can influence each other to some
extent. Also other non-olfactory
sociosexual stimuli from themale, such
as sexual vigour, are influential.
Further, the number of females already
ovulating modulates the strength of the
male effect [8] so female-produced
pheromones may act synergistically
with those produced by males to
induce/synchronize ovulation.
A key problem which has hindered
identification of the ‘male-effect
pheromone’, and indeed other
mammalian pheromones, has been the
lack of robust biomarkers to test the
efficacy of the many different potential
candidate molecules. In this issue of
Current Biology, Murata et al. [9] have
utilized such a biomarker to identify a
key novel odourant molecule from the
skin and sebaceous glands of the male
goat responsible for influencing female
reproductive cycles. This group has
previously established this biomarker
using recordings of the electrical
activity from the hypothalamic region
containing the small population of
gonadotrophin releasing hormone
(GnRH) neurons responsible for
stimulating pulsatile release of
luteinizing hormone (LH) from the
anterior pituitary and subsequently
ovarian function [10,11].
By a systematic exploration of
different extracts from samples
collected from the head of the male
using gas chromatography and mass
spectroscopy they have identified a
small number of ethyl-branched
aldehydes and ketones which reliably
increased electrophysiological activity.
In particular, 4-ethyloctanol proved to
be the most influential single molecule,
although a cocktail of 18 compounds
including this was the most effective,
suggesting that a number of other
components also contribute. The
authors have yet to demonstrate
directly whether 4-ethyloctanol alone


Cytoplasmic Transport: Bacteria Turn to Glass Unless Kicked 

Paul A. Janmey

Fred C. MacKintosh

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Current Biology

Cytoplasmic Transport: Bacteria Turn to Glass Unless Kicked

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Cytoplasmic Transport: Bacteria Turn to Glass Unless Kicked

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