Why Do Bacteria Filament?

It launch'd forth filament, filament, filament, out of itself…
         Walt Whitman

Streptomyces coelicolor (false colored).
Source: Nora Ausmees, University of Uppsala

Some bacteria naturally grow as filaments, e.g., members of the actinomycetes. Many others, e.g., E. coli and B. subtilis, make filaments only when under stress─a fact that has been known for about one hundred years but is still a bit of a mystery. Many kinds of stress can prompt this response, including DNA damage that elicits the SOS response, partial inhibition of cell wall synthesis by antibiotics, and the expression of certain thermosensitive mutations affecting cell division (called fts mutants for "filament forming temperature sensitive"). So general is this phenomenon that, over 30 years ago, we commented in a review: …it seems possible that any chemical at some concentration, whether attainable in the laboratory or not, can cause filament formation. (When the great geneticist, Rollin Hotchkiss, heard of this, he muttered: How depressing!) Be that as it may, filamentation is to bacteria what fever is to children.

Ultrathin section of an E. coli temperature-
sensitive mutant grown at 42° for 45 minutes.
Scale bar: 1mm. Source: Burdett, I. D. J.
and R. G. E. Murray. 1974. Septum Forma-
tion in E. coli. J. Bacteriol. 119: 303-324

Filaments, in both bacteria and fungi, result when rod-shaped cells cease to divide but continue to grow. In many cases, growth can continue for quite a while and at a rapid speed, resulting in long and often healthy-looking filaments. Nucleoids continue to segregate and are spaced normally along the filament. (Some folks like to call this polyploidy, but multinucleate seems more appropriate.) Apparently, under some circumstances, cell division is a dispensable process─at least for a while.

Schematic drawing of a mutant defective in
decatenation of progeny chromosomes. The
DNA does not segregate but remains as a
mass in the center of the cell. Source:
Schaechter, M., J. L. Ingraham, and F. C.
Neidhardt. 2006. Microbe p.182.

To show how indifferent cell growth can be to whether a cell divides or not, cells also become filamentous when decatenation of their intertwined progeny chromosomes is inhibited by mutation or by drugs. Given the pleiotropic nature of the response, it has proven difficult to figure out why cell division is so much more delicate than the rest of the cell's functions.

For now, let's leave questions of mechanism aside and ask instead, how this phenomenon matters in the ecology of these organisms. This question has recently been examined in an article from Scott Hultgren's lab. The article goes a long way towards making sense of why bacteria might have developed such a strategy. It is pleasurable reading, illustrated with many exciting instances. Their examples suggest that filamentation can confer protection against grazing predators (including phagocytes in mammalian hosts), resistance to intracellular killing, swarming motility to evade immune cells, and insensitivity to some antibiotics and other inimical agents.

Making filaments to avoid grazing by predatory protists is often seen in marine and other environments. In general, bacteria longer than 7 μm are inedible by many protists, and filamentation occurs in direct response to effectors produced by the predators. In other cases, e.g., in some Proteus, filamentation is part of their life cycle. These organisms "swarm" intermittently on agar as well as on the surface of catheters. The pathogenic E. coli that cause urinary tract infections invade the epithelial cells of the bladder, and there they transform into filaments some 50 times the normal length. This strategy enables these filamenting bacteria (and others) to survive engulfment by phagocytic cells. Also, at body temperatures, Legionella make phagocytosis-resistant biolfilms composed of filamentous cells.

Filamentous E. coli on
infected mouse bladder
cells. The bacteria were
stained with a red fluores-
cent nucleic acid dye
(ToPro3) and examined
under a laser scanning
confocal fluorescent
microscope. Scale bar:
30 μm. Source: Justice,
S. S., D. A. Hunstad, P.
C. Seed, and S. J. Hult-
gren. 2006. Filamenta-
tion by E. coli subverts
innate defenses during
urinary tract infection.
PNAS 103(52) 19884-
19889.

After completing their extensive survey, the authors conclude that filamentation is a survival tactic employed by diverse bacteria under a variety of conditions. Considering the reliance of some pathogens on filamentation, they suggest that drugs blocking filament formation may be useful against specific pathogens. We thank the authors for calling attention to the broad ecological aspects of this distinctive bacterial (and fungal) talent.

I finish with an aside: the opposite of filamentation, i.e., division without growth, also occurs. In the lab, bacterial cells in stationary phase are generally smaller than growing ones. Likewise, most bacteria making a living in oligotrophic environments are on the small side, some so small as to merit the label nanobacteria. I recall once observing under the microscope the "growth" of E. coli on purified agar containing only phosphate buffer. Each cell divided three or four times, resulting in an average cell size 1/8 to 1/16 of the starting one! Filaments count, but so do numbers.

Comments

[Neumann]

does these e. coli filaments having also different colored filaments and having also melanin?

Elio replies: Not that I know fo

[Joshua Sanchez]

I am currently conducting experiments on the LPS transport system of pseudomonas aeurginosa and I have observed this strange phenomenon after knocking out genes encoding proteins vital to LPS transport. I was using O.D. to measure growth initially but with filamenting it seems that O.D. would be misleading as to calculate the number of cells. Any ideas on how to get a count on bacteria that filament? I was thinking of using a grid and counting manually but the filamenting cells cluster in such a way as to make it difficult to do so. Thanks for any help.

-Josh

Elio replies:
Your problem is not easy to solve. You could try staining the cells with DAPI and counting how many nucleoids there are per filament. You could divide the number of colony-forming units by that number, whcih would give you the # nucleoids/ml.

If you have access to a flow cytometer, that could direclty give you the numbers you want.

Good luck,

Elio

[Miguel Prazeres]

Dear Dr Schaechter,

I recently came across with filamentation of E. coli when reviewing some aspects of the large scale production of plasmid DNA in fermentors (this is gaining some industrial important due to the advent of plasmid biopharmaceuticals).

Apparently, and according to one report (Carnes et al, Biotechnology and Applied Biochemistry, 2006, 45, pp 155), when plasmid-bearing E. coli cells are grown to high cell density at elevated temperatures (37 and 42 Celsius), cell filamentation ensues and growth is arrested. This does not occur when cells are grown at 30 Celsius. In another paper (Journal of Microbiology and Biotechnology 19(2009)1408-1414.) where filamentation of plasmid-containing E. coli cells was described, the authors reported that the phenomena did not occur when plasmid-free cells were grown under exactly the same conditions.

I guess that the phenomenum is triggered by the metabolic burden associated with both plasmid replication and survival at higher temperatures...could the presence of antibiotic in the media account for filamentation also?

[Heather Shepard]

I thought this article was very interesting. Filamentation seems like an important yet unexplained phenomena that scientists have just recently been discussing. I especially liked learning why filamentation is so senstitive. If this is a process that has been happening for decades, the cause must be still affecting the bacteria.

[Peter Kemp]

Dear Dr Schaechter,

I am an amateur so please excuse me if this question is nonsense.

I was wondering if filamenting bacteria could explain a strange phenomenon of the spirochete borrelia burgdorferi (agent of Lyme disease).

Bb is described as being capable of producing multiple new individuals from tiny coccoids which occur along its length - sometimes called a 'string of pearls'.

Could this process be similar to a filamented bacteria reproducing within the filament?

Many Thanks,
Peter Kemp

Peter,

Your question is entirely appropriate. From what I gather, B.b. "strings of pearls" consist of cells that have divided but not separated. They would be different than a non-septate filament. I hope this helps.

Elio

[William Margolin]

These are interesting ideas. I would add another, at least for fast growing cells like E. coli. Some types of filamentation are transient, such as cell division inhibition after DNA damage. If you're E. coli, and something goes wrong with your cell cycle, you can either arrest the cell cycle like many eukaryotes, or keep doing everything else (replicate and segregate chromosomes, elongate), except divide. If you arrest growth, then once the problem is solved, you're just one cell. But if you have been forming filaments during the time of stress, then once you solve the problem, you can divide into multiple viable cells all at once, and therefore have many more progeny than the cell that arrested growth. For bacterial like E. coli that succeed by competing with other bacteria by fast growth, this would seem like the best strategy.

[Paul Orwin]

Terrific discussion of a common but fascinating feature of microbial physiology. One additional function of filamentation may be to enhance attachment to surfaces during biofilm formation (this was discussed in Kevin Young's review on bacterial cell shapes). It seems to be a response that microbes undertake at the least provocation, as you point out. I wonder too about the value to the microbe of being able to adopt several different filamentous forms, such as septate filaments and monofilaments with no internal divisions? I think this has been overlooked in favor of "sexier" topics in microbiology, but maybe we can learn a lot about how bacteria respond to their natural environments by how they make this decision.