How fast do cells move? Cell movements are one of the signature features of the living world. Whether we observe the many and varied movements of microbes in a drop of water, the crawling of Dictyostelium cells to form fruiting bodies after starvation, the growth of protrusions in neuronal cells or the synchronized cell movements during gastrulation in the developing embryo, each of these processes paints a lively picture of cells in incessant motion. Back in 1683, Leeuwenhoek wrote to the Royal Society about his observations with his primitive microscope on the plaque between his own teeth, "a little white matter, which is as thick as if 'twere batter." He repeated these observations on two ladies (probably his own wife and daughter), and on two old men who had never cleaned their teeth in their lives. Looking at these samples with his microscope, Leeuwenhoek reported how in his own mouth: "I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-­‐moving. The biggest sort. . . had a very strong and swift motion, and shot through the water (or spittle) like a pike does through the water. The second sort. . . oft-­‐times spun round like a top. . . and these were far more in number.". These excerpts beautifully illustrate both our wonder at movement and the roots of our understanding in careful microscopic observations. As noted by van Leeuwenhoek himself, there are many different types of cell movements. Many microorganisms (and larger organisms too!) make their way hither and yon by swimming. Indeed, swimming is one of the most common mechanisms exploited by motile cells as exemplified by both E. coli and Paramecium, as classic examples. Another classic mechanism is the subject of one of the most famous series of time lapse images in all of biology several snapshots of which are shown in Figure XX. In particular, David Rogers captured the motion of a neutrophil crawling along a surface in hot pursuit of a bacterium. Yet another mode of bacterial motility is known as gliding and refers to a form of motion that is not yet fully understood. Of course, such cell movements are not at all the exclusive prerogative of single-­‐
celled organisms with all sorts of cell movements forming a key part of the life cycles of multicellular organisms as well. One impressive example of such movements are revealed in the developing nervous system in which neurons undergo a kind of pathfinding where protrusions from certain neurons grow outward as part of the process of creating the branched network of connections in the brain. The fascination of watching cells move has left us with a wide variety of data for a number of different cell types. We summarize this data in Table 1. One of the best ways to put all of these movements in perspective is to evaluate how many body lengths a given organism moves every second. Usain Bolt’s amazing performance in the 2008 Beijing Olympic Games saw him travel 100 meters in just under 10 seconds, meaning that he traveled roughly 5 body lengths every second. When undergoing its chemotactic wanderings, an E. coli cell has a mean speed of roughly 30 µm/s, meaning that it travels roughly 15 of its 2 µm body lengths every second. Similarly, amoeba such as Dictyostelium move at a rate of RP, corresponding, with similar speeds seen in the motion of a neutrophil chasing down its prey as shown in the famed Rogers video. What is the underlying molecular basis of these various cell speeds? Of course, the answer to that question will be quite different for the different motility mechanisms considered here. As an example, we consider the role played by actin motility in several different kinds of motion including the development of protrusions in polarized eukaryotic cells and the corresponding motions of bacteria which have hijacked the host cell cytoskeleton. In these examples, the most naïve estimate holds that the rate will reflect the underlying rates of actin polymerization. What can be said about the sources for the diversity in speeds? Some of the fastest bacteria are at high temperatures where rates of nearly everything tend to be higher or in organisms that have to depend on their speediness to make a living such as the case of Bdellovibrio bacteriovorus that has to be faster than the bacteria it preys on. In others, like the record holder Thiovulum majus the pressure to run swiftly is less clear. With respect to most other bacteria the speed range is not very dramatic. Its functional role is usually discussed in terms of the ability of bacteria to perform chemotaxis where they perform a biased random walk using their flagella to environments of higher nutrient concentrations. Different lines of evidence suggest that motility might have important parts to play in the dense communities of bacteria where the survival and growth often depend on more intricate issues of communication, cooperation and relative location, all affected by motility. Table 1: Cell speeds of different cells given in µm per time unit and as body lengths per time unit.
Assuming bacterial length of ≈2 µm and eukaryotic cell length of ≈15 µm unless otherwise stated.
Speeds depend on temperature, experimental conditions etc. Values given here are those
reported in the literature. Most measurements are based on time lapse microscopy.
Organism
Speed
Speed in
body lengths
(bl) per sec
BNID/Comments
Bacteria & Archaea
Thiovulum majus
600 µm/sec
90 bl/sec
Methanocaldococcus
jannaschii
400 µm/sec
200 bl/sec
Bdellovibrio bacteriovorus
160 µm/sec
80 bl/sec
45-100 µm/sec
22 to 50
bl/sec
108083, Sodium ion motor, 1
polar flagellum
Caulobacter crescentus
35-45 µm/sec
18 to 23
bl/sec
108085, Proton motor, 1 polar
flagellum
Spirochete Brachyspira
hyodysenteriae
40±4 µm/sec
8 bl/sec
104904, assuming cell length of
5 µm (wiki)
16-30 µm/sec
8 to 15 bl/sec
Vibrio cholerae
E. coli
S. typhimurium
107652, cell length ≈7 µm
O
107649, measured at ≈80 C
101969, has to catch other
bacteria it preys on
101793, 106819, 108082, proton
motor, 4-8 lateral flagella
28 µm/sec
14 bl/sec
106818
Myxococcus Xanthus
motility system S
>20 µm/min
>10 bl/min
106811
Myxococcus Xanthus
motility systemA
2 to 4 µm/min
1 to 2 bl/min
106811
6 µm/min
3 bl/min
Listeria monocytogenes
106823 in vitro motility assays
Eukaryotes
Green algae
Chlamydomonas Reinhardtii
58 µm/sec
≈5 bl/sec
Ciliate Paramecium
tetraurelia
140 to 470
µm/sec
1-2 bl/sec
Amoeba Dictyostelium
discoideum
10 µm/min
≈1 bl/sec
Fish keratocytes - wound
healing fibroblasts of the
cornea
10-45 µm/min
0.7 to 3
bl/min
Human neutrophil in a tube
108086
108087, assuming cell length of
200 µm (wiki)
106825
106807, 106817
9±1.8 µm/min
≈1 bl/min
106809
Glioma cells
54 µm/hour
4 bl/hour
106810
Mouse fibroblastoid L929
cells
29 µm/hour
2 bl/hour
106808
Human H69 small cell lung
cancer cell
16 µm/hour
1 bl/hour
106815