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In a question over on Skeptics Stack Exchange, I half-jokingly listed "blunt force trauma" as a means by which a bacterium could be killed. That makes me wonder if it really is only a joke or whether such an assault is plausible. The webcomic xkcd also did an episode on the possibility of physically attacking a cell (though not a bacterial one).

There are plenty of resources on how bacteria can die from biological processes (e.g. infection with a bacteriophage, biological effects of antibiotics, etc.), chemical attacks (e.g. membranes getting lysed with bleach), temperature (i.e. cooking), and radiation, but have bacteria ever been shown to be vulnerable to a direct physical assault such as being hit with a hammer, cut with a knife, thrown onto a hard surface, twisted until they burst, or being drawn and quartered?

I do know that many common weapons that are generally effective against animal life (swords, axes, clubs, assault rifles, etc.) do not have the exact same properties at the microscopic scale, but does that mean that physically beating a bacterium to death is absurd or is there a known way to kill a bacterium using only physical force?

To be clear, I recognize that our normal means of killing unwanted bacteria - cooking food to prevent food poisoning, treating bacterial infections with antibiotics, cleaning hard surfaces with bleach, applying rubbing alcohol to wounds, etc., are probably effective enough that most people don't bother pulling out that old Wild West six-shooter they have in the closet or the antique sword hanging on their wall the way they might consider them if attacked by a bear or wolf, but that's not the question.

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    $\begingroup$ What's the limit of what you'd consider physical trauma? Bacteria can be killed with sufficient centrifugation, which is reverse blunt-trauma in that they are smashed against a comparably-large and comparably-blunt tube bottom $\endgroup$ – Punintended Sep 30 '20 at 20:02
  • $\begingroup$ Does a diamond knife count? $\endgroup$ – Joshua Oct 1 '20 at 3:42
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    $\begingroup$ @Joshua Only if it's a blunt knife. $\endgroup$ – Mast Oct 1 '20 at 4:28
  • $\begingroup$ @Punintended That counts. Blunt force trauma isn't really a vector quantity, it's the resulting state produced by the application of a strong force. Humpty Dumpty doesn't really care which reference frame you pick - either way he's broken. $\endgroup$ – J... Oct 1 '20 at 12:40
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    $\begingroup$ Through google found a similar question on Skeptics.SE Is it possible to crush bacteria? $\endgroup$ – Bernhard Döbler Oct 1 '20 at 20:57
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My (limited) understanding is that it is quite hard to avoid killing some bacteria even with very gentle physical manipulation. On the other hand, it is quite hard to use physical force to achieve reasonable level of sterilization. Let's bring some examples with a few (hastily found) references.

Pressure

My guess is that most examples the OP mentions (hit with a hammer, knife) will exert their biggest effect on bacteria through a peak in pressure - bacteria are comparable in size to width of a moderately sharp knife edge and thus there will be more "crushing" than "cutting" even with a knife (Science of Sharp discusses that Chosera 1k - a very good stone - will sharpen a knife to 0.5 μm, Wiki has bacteria at 0.5 - 5 μm).

I recently collaborated on a project where initial interpretation of results was wrong simply because putting a glass slip over a sample to inspect it under a microscope caused around 10% of bacterial (B. subtilis) cells to die. Putting a small weight on the slip resulted in roughly 80kPa additional pressure applied and ~ 80% of the cells dying within 25 minutes. But 80% death rate won't do much to e.g. reduce risk of food poisoning - as long as there are nutrients, bacteria will regrow quickly.

As m4rio mentions in a comment to another answer, and in line with The effect of high pressure processing on the microbial, physical and chemical properties of Valencia and Navel orange juice you need to ramp up the pressure to 100s of MPa to achieve high efficacy. And even that may not work so cleanly - e.g., the linked paper mentions that after applying 600 MPa, bacteria regrew to detectable levels after 4 weeks of storage at 4°C. Killing all (or almost all) bacteria is just really hard.

Because it was fun, I'll also link to study showing that High pressure-processed guacamole (4 cycles, 689 MPa, 5 min each) is clean enough to not spoil for 30 days in 25°C.

Note: the two sources above were linked from a commercial vendor of high-pressure products and may thus be biased. While the exact numbers might be problematic, I think the main point is unlikely to be affected.

A review High-pressure processing – effects on microbial food safety and food quality mentions some limitations, e.g.:

Endospores tend to be extremely HPP resistant compared with vegetative cells, withstanding treatments of more than 1000 MPa

ascospores of heat-resistant moulds such as Byssochlamys, Neosartorya and Talaromyces are generally considered to be extremely HPP resistant

It should however be noted that it seems that high-pressure treatment is good enough to be semi-commonly used as the main sterilization for suitable foodstuffs - Current status and future trends of high-pressure processing in food industry reports high-pressure treatment to be FDA approved and USD 10 billion worth of food being treated as of 2015.

Pressure for killing cells previously briefly discussed at Can bacteria or other microorganisms be killed by applying pressure?

Ultrasound

While frequently used for cell lysis in lab, it also doesn't kill all cells. For example The Sterilization of Suspensions Contaminated with MicroorganismsUsing Ultrasound Irradiation reports that roughly 20-30% of bacteria (Bacillus) survived 60 minutes of ultrasound treatment.

Ball mill

This is probably closest to actual "physical trauma" - you put small beads and a suspension with microorganisms into a cylinder and roll it hard. A low energy process for the recovery of bioproducts from cyanobacteria using a ball mill discusses using this process to extract proteins from cyanobacteria. They don't report absolute number of cells killed, just that it varies considerably with the rotation speed, size of beads, .... My interpretation is that in most cases a non-negligible proportion of the cells survive while majority is killed (and their contents extracted for further use).

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    $\begingroup$ In microbiology, the "ball mill" method of bacterial cell lysis is called bead-beating. $\endgroup$ – acvill Oct 1 '20 at 17:32
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    $\begingroup$ @user1136 I am not primarily a biologist - I did statistics on the project, so hope I am not messing up. What my colleagues did was use SYTOX green staining which "penetrates into the cells with compromised/permeabilized membranes where it stains nucleic acids and indicates cell death", so I think the cells were not completely broken (which is presumably why all cells were considered alive in previous work on the subject). You are welcome to check the methods and details at doi.org/10.1038/s41467-020-18800-2, primarily Figure 2. $\endgroup$ – Martin Modrák Oct 2 '20 at 13:00
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There are plenty of physical or mechanical methods of killing bacteria, but most are used in conjunction with other agents and probably don't qualify as "blunt force trauma". For example, beer distributers might snake a brush through a keg line to disrupt any bacterial biofilms before flushing them out with disinfectant. Similar logic applies to rubbing your hands together when you wash them or using a brush to scrub biofilms off your teeth. We might not think of it as traumatic force, but it can be to bacteria.

Since you mentioned it, there is something that might resemble them being "twisted until they burst" (more like crushed to death). A device called a French pressure cell press (or just French press) used to lyse bacterial cells with purely mechanical forces. It basically compacts a culture of cells under high pressure, causing them the burst and release the cellular contents. Of course, it would require somehow gathering all of them together into a containment vessel and having an expensive laboratory instrument to put them in.

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    $\begingroup$ My understanding of how a French Press works is that the disruption is due to forces generated on the cells as they pass through the valve rather than the high pressure crushing the cells. $\endgroup$ – tyersome Sep 30 '20 at 20:51
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    $\begingroup$ @tyersome Yeah, that's probably a more accurate description of the mechanism, though there are food safety applications that rely on pressure alone for bacterial killing. $\endgroup$ – MikeyC Sep 30 '20 at 21:11
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    $\begingroup$ The French pressure cell press idea is used today commercially. It doesn't have that name though. It's called HPP or high pressure processing. Hiperbaric is the company that sells and makes the machines. It uses up to 10,000 bar of water pressure in a container in order to kill bacteria. It's effective as long as the water activity of the food being processed is at a certain level. Also the pH matters as HPP cannot kill spores from sporoforming bacteria and has difficulty with molds. However it can kill E. coli, Salmonella, and Listeria in juices easily (I work with these machines). $\endgroup$ – m4rio Oct 1 '20 at 1:59
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    $\begingroup$ @m4rio so to be clear, it’s used as an alternative to pasteurisation for liquid consumables!? $\endgroup$ – Tim Oct 1 '20 at 22:45
  • $\begingroup$ @Tim Yes. Most common liquid (or juice) pasteurizations include heat, UV, or HPP. HPP is the newest among the three and for companies who do not like to heat their product or UV it due to whatever reasons, be it they want it more organic, or it changes the flavor of the juice, HPP can be an alternative. It is the most expensive of the three but can give a good ROI if done in big bulk. As far as I know only big or big-ish companies use it. If you're doing HPP and you get a good CFU/ml reduction of bacteria as the FDA requires you can use HPP as your sole method of pasteurization. $\endgroup$ – m4rio Oct 2 '20 at 17:50
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From a biochemical point of view, it is surprisingly difficult to break open a bacterial cell. A molecular biologist, for example, might want to break E.coli cells expressing a protein of interest, in order to purify the protien

Some of the methods employed are agitation with glass beads, griding with glass beads, sonication, and high-pressure cell disruption with the French Press.

In contrast, liver cells, or brain cells, may be broken by simple homogenization (for example in a Waring blender)

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Yes of course. While you can't swing a blunt object comparable in size to humans at something many orders of magnitude smaller to kill it. You can use something like an ultra sonicator, which swings a small blunt object at --you guessed it-- very high speeds to generate lots of shear forces in a liquid culture. Usually these instruments are not meant for killing cells, but even with my most gentle use for breaking up cell clumps, a look under the microscope would show ruptured cells with their interior spilled out.

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    $\begingroup$ I use a sonicator for making (mammalian) cell lysate all the time. But, I think you might be confused about how a sonicator works. It's not the force of the moving tip itself that does it - the extremely rapid motion of the tip produces very tiny vacuum bubbles in the fluid that almost immediately collapse, sending a strong shearing force through the liquid and disrupting whatever it comes into contact with. The principle is called ultra-cavitation. $\endgroup$ – MattDMo Sep 30 '20 at 20:33
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    $\begingroup$ @MattDMo Didn't I say shearing forces in liquid? $\endgroup$ – Cell Sep 30 '20 at 20:48
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    $\begingroup$ @Cell — The way you wrote your answer does sound like direct action by the tip — I encourage you to change your wording and ideally add the type of information mentioned by MattDMo. Your answer would also be improved by including references. $\endgroup$ – tyersome Sep 30 '20 at 21:00
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...cut with a knife...

Yes, freeze them.... slowly. <evil laugh>

Freezing causes ice crystals to form inside the cells. These act like little knives. The slower the freezing happens, the larger these ice crystals get. The larger the ice crystals, the more damage they will do by piercing the cell wall and other important membranes.

This is why if you freeze fruit in a home freezer it thaws out as a goopy mess; the cell walls have been punctured. This is also a key problem in cryogenics, the freezing and thawing process damages cells.

Flash freezing uses very rapid freezing to very low temperatures to prevent large ice crystals from forming and minimizing damage. Food which is flash frozen is less goopy.

Note that when done properly freezing can be used to reliably kill some microbes, freezing at home is not a reliable disinfectant.

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