Bacteria are prokaryotes, meaning they don't have organized nuclei or any other membrane-bound organelles. Bacterial DNA floats freely within bacterial cells in a twisted, thread-like mass called the nucleoid. Some also have separate, circular pieces of DNA called plasmids. According to the Microbiology Society , plasmids often contain genes that give bacteria a survival edge, such as genes conveying antibiotic resistance.

Bacteria shouldn't be confused with the other major group of prokaryotes called archaea. Archaea are also single-celled organisms, but the two groups differ in the kinds of molecules they use to build their cell walls and in the metabolic processes they use. Bacteria come in five basic shapes: spherical, cylindrical, comma-shaped, corkscrew and spiral.

The scientific names for these shapes are cocci round , bacilli cylindrical , vibrios comma-shaped , spirochaetes corkscrew and spirilla spiral. The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing Streptococcus pneumoniae are a chain of cocci.

Bacterial cells are generally surrounded by an outer cell wall and an inner cell membrane. Certain bacteria, like the mycoplasmas, do not have a cell wall at all. Some bacteria may even have a third, outermost protective layer, called the capsule.

Whip-like extensions often cover the surfaces of bacteria — long ones, called flagella, or short ones, called pili — and help bacteria move around and attach to a host. Bacteria can be classified by the composition of their cell walls using a test called the Gram stain, according to the Science Education Resource Center at Carleton College.

The test stains Gram-positive bacteria, or bacteria that do not have an outer membrane. Gram-negative bacteria, which do have an outer membrane, don't pick up the stain.

For example, S. Delving beneath the cell wall and membrane, bacteria contain cytoplasm, a solution of mostly water and salts. Within the cytoplasm float the nucleoid, plasmids and tiny protein factories called ribosomes, which are the sites where the cell's genetic instructions are translated into the cell's products.

Some antibiotics, like tetracycline, target bacterial ribosomes to prevent them from synthesizing proteins, thus dooming the cell. The cytoplasm of some bacteria may also have little pockets, called inclusions, where nutrients are stored for lean times. Photosynthetic bacteria, which generate energy from sunlight, may have structures called chromatophores spread throughout their cytoplasm. These chromatophores hold pigments used in photosynthesis.

As some of the oldest life-forms on Earth , bacteria have evolved a dizzying number of ways to survive. Some bacteria are photosynthetic, while others are master decomposers, breaking down rotting and decaying organic material into nutrients. Some enter symbiotic, or mutually beneficial, relationships with a host more on this later. Most bacteria multiply by a process called binary fission , according to the Cornell University College of Agriculture and Life Sciences.

In this process, a single bacterial cell, called the "parent," makes a copy of its DNA and grows larger by doubling its cellular content. The cell then splits apart, pushing the duplicated material out and creating two identical "daughter" cells.

Some types of bacteria, such as cyanobacteria and firmicutes, reproduce via budding. In this case, the daughter cell grows as an offshoot of the parent. It starts off as a small nub, grows until it is the same size as its parent and then splits off.

After binary fission or budding, the DNA found in parents and offspring is exactly the same. Therefore, bacterial cells introduce variation into their genetic material by integrating additional DNA, often from their surroundings, into their genome. The resulting genetic variation ensures that bacteria can adapt and survive as their environment changes, Live Science previously reported.

Cell membrane. Surrounded by cell wall. Cell contents Cytoplasm. Cell organelles include mitochondria, chloroplasts in plants and ribosomes. Ribosomes present. There are no mitochondria or chloroplasts. Genetic material DNA in a nucleus. Plasmids are found in a few simple eukaryotic organisms. DNA is a single molecule, found free in the cytoplasm. Additional DNA is found on one or more rings called plasmids.

Type of cell division Mitosis Binary fission. Chromosomal DNA. The DNA of bacterial cells is found loose in the cytoplasm. Plasmid DNA. Bacteria also have small, closed-circles of DNA called plasmids present in their cytoplasm. They are perfectly good at governing the dynamics of those structures. Here is my hypothesis: eukaryotes enhance the intrinsic assembly features of the helical filament protein systems with two particular kinds of cytoskeleton-associated factors, which have not yet been found in bacteria.

And those two are regulated nucleators - centrioles for example - and linear stepping molecular motor proteins - the eukaryotic myosin and kinesin molecules.

There are other actin nucleators and there are other microtubule nucleators that operate by different mechanisms. But it seems from those two examples that a very reasonable way to regulate the initiation and assembly of helical cytoskeletal polymers is to just make another copy of the gene for the subunit and then allow it to specialize a little bit so that it becomes a regulatable nucleator. Certainly that is the sort of thing that bacteria could do if they wanted.

They would have no problem duplicating and modifying the genes for the cytoskeletal proteins, as they have demonstrated with the proliferation of the different flavors of actin and tubulin homologs that are used in such a wide variety of contexts. For example, Bacillus subtilis has three different chromosomally encoded paralogs, each of which is homologous to actin, MreB, Mbl, and MreBH, that appear to have somewhat overlapping functions [ 40 ].

But so far, we do not know of any specialized actin- or tubulin-related proteins in bacteria that are used specifically as regulated nucleators for their main self-assembling subunits MreB and FtsZ. Cytoskeletal filament nucleation by modified subunits. Left: diagram of microtubule templated from a ring complex Copyright from Molecular Biology of the Cell, 5th edition by Alberts et al.

Right, electron micrograph of the end of a microtubule nucleated from a ring complex reprinted by permission from Macmillan Publishers Ltd: Nat Cell Biol 2: — , copyright [ 42 ]. This is the corollary to my argument. If my hypothesis that bacteria do not have regulated cytoskeletal nucleation proteins is true - and I will go through the cell biological evidence that makes me think this is true - then the question is whether they really do not want to have them or whether they just never had the opportunity to develop them.

I think, at least as far as nucleators go, the opportunity to develop them is not a very high barrier. So I think it must be that bacteria simply have a fundamentally different strategy for cytoplasmic organization as compared to eukaryotes. However, at least in the case of actin, there are many different, distinct molecular families of nucleators that can operate by different but equally simple mechanisms.

For example, the actin nucleators Spire [ 45 ] and Cordon-bleu [ 46 ] both appear to nucleate actin by having a series of three or four domains that bind directly or indirectly to actin monomers; these domains can bring the actin subunits into close enough proximity and appropriate enough orientation to get over the kinetic barrier to actin nucleation and start the growth of a filament.

In the particular case of this category of nucleators, I am quite confident that bacteria would be able to develop them if they wanted to, as indeed two bacterial pathogens are known to express secreted virulence factors that act as host cell actin nucleating factors by exactly this mechanism [ 47 , 48 ]. For these virulence factors, it is not clear whether the pathogens picked up their actin nucleators by horizontal gene transfer or by convergent evolution, but in either case it is still striking that bacteria are easily able to nucleate eukaryotic actin filaments but do not seem to have any regulated protein nucleators for their own cytoskeletal filaments.

But the thing that I think is really interesting about cytoskeletal filament nucleation in this context is that classically when we were taught the theory of protein polymerization from Fumio Oosawa [ 49 , 50 ] and Terrell Hill [ 51 , 52 ] and all those giants in the field, their argument was that it is important, kinetically, that nucleation be the rate-limiting step for polymer formation.

And that is indeed observably true for actin and for microtubules and for the bacterial flagellum, the classical examples of helical protein self-assembly that they were trying to describe with their comprehensive theoretical treatments. But when people started doing very careful kinetic studies on the bacterial cytoskeletal proteins - and this I think has been done best for FtsZ [ 53 ] and for ParM [ 54 ] - it became clear that nucleation for the bacterial cytoskeletal proteins is actually very, very fast.

The way bacterial cells regulate where they have their filaments is not by regulating the site of nucleation, but rather by regulating the sites of stabilization and destabilization of spontaneously nucleating filaments. For those of us who have been raised on the thermodynamic theory of protein polymerization in the context of cell biology, this is deeply shocking.

Spatial localization of cytoskeletal components in bacteria simply appears to use a fundamentally different mode of organization from the one we see for all of the organized cytoskeletal assemblies in eukaryotes, and frankly we as cell biologists are justified in being a little bit freaked out.

The dynamic cytoskeletal polymers found in bacteria seem to be just as important to the bacterial cells as they are to us eukaryotes, and they are involved in similarly crucial cell biological processes. Also the bacterial cytoskeletal proteins are very widely distributed among bacteria and even archaea [ 55 , 56 ].

I absolutely do not mean to disparage the many very interesting things that bacteria do and have done in their evolutionary history. The cyanobacteria invented oxygenic photosynthesis for which I am very grateful, and in general bacteria have much more interesting twists on metabolism than do us chemically unimaginative eukaryotes. But I do realistically claim organismal size, morphological complexity, and true multicellularity as eukaryote-specific features that deserve explaining.

As we delve into the details of my argument I will delineate a few of the many biological examples of well-understood systems that have convinced me that bacteria simply do not have cytoskeletal nucleators or cytoskeletal motor proteins as we understand them in eukaryotes. The diagram in Figure 2 shows - given some reasonable assumptions about the universality and fundamental nature of helical protein filament assembly - what larger-scale structures you can get with and without nucleators and motors.

In particular these drawings show structures that can be formed by polarized cytoskeletal filaments, where the subunits assemble in a head-to-tail fashion so that the two ends of the filaments are structurally distinct.

According to the basic theories of protein polymerization, this is expected to give a polymer where the kinetics of subunit addition and loss at the two ends are also distinct, where one end grows and shrinks more quickly than the other [ 51 ]. In microtubules, the fast-growing end is called the plus end and the slow-growing end is called the minus end.

In actin filaments, the fast-growing end is called the barbed end and the slow-growing end is called the pointed end. Types of cytoskeletal filament arrays. Type A : simple filament arrays that can self-assemble in the absence of spatially regulated nucleators or molecular motor protiens.

Shading indicates the orientation of filament polarity. Type B : complex filament arrays that require either nucleation or motor protein activity, or both. Dark circles represent nucleators. The simple structures that can be made from polarized filaments I will call type A structures. In the absence of nucleators you can obviously make a single filament of essentially any length and that single filament can have many protofilaments. A microtubule is a single filament with 13 protofilaments that can be arbitrarily long.

A bacterial flagellum is also a single filament that happens to have 11 protofilaments, and flagella can also be very long - 10 microns long in vivo. Both of these structures self-assemble quite nicely from solutions of purified protein monomers; indeed these were the examples that have formed much of the basis of our understanding of the fundamental thermodynamics of protein polymerization [ 50 ].

So those kinds of structures you can make regardless of whether you are a bacterium or a eukaryote and regardless of the presence of nucleators or motors. The other kind of structure that is very easy to make is a mixed polarity bundle. In crowded solutions, such as in the cytoplasm of a living cell, colloidal rods will tend to align with one another simply because of entropy and excluded volume effects [ 57 ].

When the rods happen to be cytoskeletal filaments, they can easily form bundles either by interacting with one another laterally, or else by having cross-linking proteins that help pull them together. For the bacterial cytoskeleton, the clearest example of a mixed polarity bundle is the plasmid-segregating actin homolog ParM, which can assemble into mixed polarity bundles on its own [ 58 ].

It is also very likely that the FtsZ ring in bacterial cytokinesis is essentially a mixed polarity bundle, formed with the help of cross-linking proteins [ 59 ].

The kinds of structures for which I think, theoretically, you need to have either localized nucleation or motor activity, or both, the type B structures, are structures like asters, where many cytoskeletal filaments with the same polarity emanate from a single location, or parallel bundles of filaments, where all of the filaments are pointing in the same direction.

If filaments form spontaneously and then come together through purely entropic effects, there is no intrinsic reason for them to assemble in a particular orientation. So if you want to have a parallel bundle, such as in a muscle sarcomere, you have to control the assembly or orientation of the filaments, for example by having them all nucleated from the same site.

And of course a great example of all of these properties is the mitotic spindle, where you have parallel bundling and anti-parallel bundling of microtubules, and also their nucleation from particular sites at the spindle poles.

There are plenty of examples of single polarized filaments in bacteria. There are plenty of examples of mixed polarity filament bundles in bacteria. But the type B structures are critical I think to making eukaryotes what we are today, by allowing the elaboration of the microtubule cytoskeleton to give complex organelle dynamics and fabulously flexible DNA segregation capacity, and elaboration of the actin cytoskeleton to give us the possibility of amoeboid motion and phagocytosis, which allow us to run around and eat all those pesky bacterial biofilms and tame endosymbionts.

And then once we have those kinds of structures and mechanisms, we are able to overcome the diffusion barrier and the increase in size and complexity of eukaryotic cells follows naturally from that. The supporting details can be discussed from three different perspectives. The first focuses on self-assembly dynamics, and the rules about the kinetics and thermodynamics of self-assembly that come from the intrinsic properties of proteins - can these really be different between bacteria and eukaryotes?

And if not, why not? And beyond that, there are also other possible explanations besides the cytoskeletal hypothesis for why eukaryotes and bacteria are different; this is a fourth level, even more general and more speculative, but one that I think helps tie this whole story together. The first thing to think about is the question of protein self-assembly, because classically, when we think about the cytoskeleton, we imagine lots of little subunits that are able to assemble in an oriented fashion, to make larger structures.

The ability of proteins to form homo-oligomers is very prevalent and, in fact, I would say it is almost the default thing for proteins to be able to do.

Structural biologists have done a very nice job of breaking down the kinds of symmetries you can get in these homo-oligomers into different kinds of classifications. Really making a helix is just one particular phylogenetic group, if you will, of the kinds of structures that proteins can make by self-assembly.

Now there are two really nice things about helices. One is that a helix enables you to make structures of variable length, while most other oligomer types make a closed structure with a defined size, such as a viral capsid.

But a helix that grows by addition of subunits onto the end can in principle be tuned over a very wide size or length range. They used protein structural arguments to explain that when you allow many copies of the same protein to aggregate together you can hardly help but make a helix Figure 3 a. If you allow a protein to self-assemble, a helix of some kind is going to be the default.

Helical protein filaments formed by self-assembly. For any globular protein of arbitrary shape, as shown at the top, considered as interacting with a second copy of itself in all possible orientations, there will be some pair of surface patches that result in optimal binding energy.

It is highly unlikely that those two interface patches will happen to reflect any specific geometrical symmetry. When many copies of the same subunit self-associate by binding to one another through these surface interactions, a one-start helix with a single protofilament is the default structure formed, as shown in the middle. At bottom, if weaker interactions can also form laterally between subunits, multi-start helices may be stabilized adapted with permission from the Royal Society of Chemistry [ 62 ].

Yes, hemoglobin is a terrific example. In sickle-cell disease, a single point mutation in hemoglobin changes one charged residue on the surface to a neutral residue [ 64 ], and now in this dense cellular bag of the erythrocyte, filled almost entirely with one protein, you have a condition where the oxygen-depleted form of hemoglobin is able to self-assemble into a spectacularly beautiful helical structure with 14 protofilaments that looks absolutely classically like a microtubule or some other cytoskeletal filament [ 63 ] Figure 3 b.

Sickle-cell hemoglobin is, of course, a very famous example of many principles of protein structure and function, but in this particular case it clearly shows that when you take a very soluble protein and create a condition in which it is not quite soluble, a helix is what you get.

If any old protein will assemble into a helix, then what is special about the cytoskeletal proteins? There are several possible answers, but one that I find compelling is that the common feature of the universally conserved cytoskeletal proteins - the actin superfamily, the tubulin superfamily - is that both of them are nucleotide hydrolases.

They use the energy of nucleotide hydrolysis to switch between at least two distinct conformations. One of those conformations has a lower energy barrier to forming a filament than the other one. What this means is that if you can couple nucleotide hydrolysis kinetics to the interactions that the protein can form when it is in a helix, you can use the energy of nucleotide hydrolysis to regulate stability [ 65 ].

You can have the filaments assemble when the subunits have the ATP or GTP bound, and then after hydrolysis takes place, the energy released by hydrolysis is stored in the lattice in such a way that now disassembly becomes favorable. And this means that within a cytoplasm, where you have a good supply of ATP and GTP, you could have constantly dynamic filaments without having to change the concentration of anything.

Absolutely not. And in fact bacteria use the cycle of nucleotide hydrolysis to modulate the assembly of their cytoskeletal filaments quite nicely. This is not the difference between bacteria and eukaryotes. If you look at the dynamics of, for example, FtsZ, it turns over very fast, even in the cytokinetic ring. You can see a beautiful ring that persists stably for some minutes before cytokinesis and before the cells separate [ 66 ], and yet there are very convincing photobleaching studies showing that the filaments within that ring are continuously turning over just like the microtubules in a mitotic spindle, or the actin filaments in a lamellipodium.

Indeed it has been shown that mutants in FtsZ that have slowed GTP hydrolysis kinetics also have a slower turnover rate inside the living cell [ 67 ]. ParM, which is the very well characterized actin homolog that is used to segregate plasmids in bacteria [ 31 ], even shows dynamic instability [ 54 ], which is one of the classic outcomes of the coupling of assembly to nucleotide hydrolysis for eukaryotic cytoskeletal filaments [ 65 , 68 — 70 ]. I think it is very clear that those intrinsic, dynamic properties of the self-assembling filaments - the coupling to nucleotide hydrolysis, the rapid turnover, kinetic properties like dynamic instability - those things are universal in cellular cytoskeletons Figure 4.

That is not a problem for bacteria, and that is not the difference between bacteria and eukaryotes. Dynamic instability of cytoskeletal filaments from eukaryotes and bacteria. Left: direct observation using dark-field microscopy of a microtubule undergoing dynamic instability. Middle: graph showing position of plus ends top and minus ends bottom for two dynamically unstable microtubules, with repeated cycles of growth and shrinkage. Numbered points correspond to individual video frames as labeled on the left reprinted by permission from Macmillan Publishers Ltd: Nature — , copyright [ 69 ].

Right, schematic diagram showing the connection between nucleotide hydrolysis and dynamic instability Copyright from Molecular Biology of the Cell, 5th edition by Alberts et al. Left: fluorescence time-lapse images of a single ParM filament over time. Blue arrowhead shows position of initial filament appearance; red arrowheads mark the most extreme positions of the two tips. Right, traces of filament length over time for six different ParM filaments, showing a phase of growth followed by catastrophic shrinking.

Science , — Reprinted with permission from AAAS [ 54 ]. Moving on to the second perspective for my argument, if helical protein self-assembly regulated by nucleotide hydrolysis is universal, then what can we say about the role of regulated nucleation of cytoskeletal filaments in determining the difference between bacterial and eukaryotic cell organizational strategies? Here I think we are digging into much richer soil. As a cell, you would really have to put a lot of effort into not nucleating them.

For ParM, the filaments undergo very rapid dynamic instability and shrink back to nothingness unless they are stabilized by encountering cognate segments of DNA bound by the correct protein partner, both of which are normally found on the plasmid that is using ParM for segregation [ 71 ]. This mechanism rather neatly ensures that ParM filaments forming in a cell will be stabilized to push the plasmids apart only when there are two copies of the plasmid present, one to stabilize each end of the normally unstable filament.

For FtsZ, its major regulator is a destabilizing factor, MinC [ 72 ], which undergoes its own very fascinating form of spatial regulation, but the short version is that the FtsZ ring that initiates bacterial cell division can form only where MinC is not; that is, FtsZ nucleation is spontaneous, but filament stability is regulated.

MinD self-assembles on the bacterial membrane, and the MinD filaments are then destabilized by another protein factor, MinE. The kinetic interaction between MinD assembly and MinE destabilization results in spectacular oscillatory positioning of the MinC inhibitor inside of cells [ 74 ] and self-propagating waves when reconstituted in vitro [ 75 ]. In brief, this impressively dynamic and very precise system that the bacterial cell uses to choose the site of division depends on the spontaneous nucleation of one filamentous structure MinD that is destabilized by a regulator MinE.

The biological purpose of MinD and MinE is to regulate the localization of MinC, which acts to destabilize the spontaneously nucleating tubulin homolog FtsZ. Over and over for bacterial cytoskeletal and cytoskeletal-like elements, we are seeing spontaneous nucleation followed by spatially localized stabilization or destabilization as the general organizing principle.

Again the really surprising thing here is that, for the cases that we understand well, nucleation plays no obvious part in the spatial regulation of cytoskeletal assembly for bacteria; everything where we understand the molecular details of spatial regulation regards filament stabilization and destabilization. My examples here are the best-characterized systems that we know in bacteria.

For most of the other examples of bacterial cytoskeletal filaments, too little is known about their dynamics to enable us to guess how the nucleation versus stabilization equation will play out. I think it will be very, very interesting in the next few years to see if this is really a universal, decisive difference between the eukaryotes and the bacteria, or just an intriguing feature of the first few well understood systems.

Honestly, I really think bacteria could do that if they wanted to. But so far we do not know of any bacterial proteins that are specifically dedicated to nucleation of bacterial cytoskeletal filaments. There are many cases where having localized nucleators has been shown to be sufficient to give you really very interesting kinds of self-organized systems. A famous example I really like comes from experiments on dropping centrosomes or beads covered with microtubule nucleators into little microfabricated wells - you can grow up asters of microtubules and these will push the bead or the centrosome into the center of that well [ 76 ] Figure 5 a.

Each growing microtubule end pushes against the wall of the well, generating a few picoNewtons of force [ 77 ], and the forces are equally balanced when the nucleating bead is near the middle. Because the microtubules are dynamic, and specifically because they are undergoing dynamic instability and occasionally shrinking back to their origin, the system does not get stuck and the centering can be maintained.

This mechanism of self-centering by having centrally nucleated microtubules nudging at walls appears to be the way that the fission yeast Schizosaccharomyces pombe maintains the mid-cell location of its nucleus [ 78 ].