Does Cold Kill Germs or Just Make Them Inactive?

Cold slows most germs to a crawl and can injure or kill a fraction of them, but it does not reliably sterilize anything. Bacteria, viruses, fungi, and bacterial spores have a range of strategies for riding out freezing temperatures, from reshaping their cell membranes to entering dormant states that can last millennia. The popular idea that a deep freeze “kills germs” is closer to a half-truth: freezing is genuinely lethal for certain parasites and damages some bacterial populations, yet many pathogens emerge from the cold ready to resume business as usual once temperatures rise.

What Freezing Actually Does to a Microbial Cell

When temperatures drop below zero, two things threaten any living cell. First, ice crystals form in and around the cell, and those crystals can physically puncture membranes. Second, as water freezes outside the cell, the remaining liquid becomes increasingly concentrated with salts and other solutes. This draws water out of the cell by osmosis, effectively dehydrating it. Research on lactic acid bacteria found that this osmotic stress is the main source of damage during freezing, sometimes more destructive than the cold temperature itself.¹

The speed of freezing matters, too. Studies on E. coli showed that slow freeze-thaw cycles released more of the cell’s internal components, including proteins and membrane lipids, and killed a larger proportion of cells than rapid freezing did.² Slow freezing gives ice crystals more time to grow and gives the osmotic gradient more time to wring water from the cell. Rapid freezing, by contrast, can flash-freeze intracellular water into tiny crystals that do less structural damage. This is exactly why scientists add cryoprotective agents like glycerol or DMSO when they want to preserve bacteria in a lab freezer: those chemicals prevent the crushing osmotic stress and ice-crystal growth that would otherwise thin the population.³

So freezing is not harmless to microbes. It injures and kills some. But “some” is the key word. A significant fraction of most bacterial populations survives a trip through the freezer, and without chemical protectants the survivors can still number in the millions per milliliter.

How Bacteria Remodel Their Membranes in the Cold

A cell membrane needs to stay fluid to function. At low temperatures, ordinary membrane fats stiffen, which threatens everything from nutrient transport to energy production. Many bacteria counter this by rapidly changing the types of fatty acids in their membranes when they sense a temperature drop. Bacillus subtilis, for instance, shifts toward anteiso-branched fatty acids during cold shock. These branched molecules pack together less tightly, keeping the membrane flexible even in the cold. Strains that cannot make enough of these special fatty acids grow poorly at low temperatures.⁴

E. coli uses a different trick. Its membranes contain cyclopropane fatty acids, which create rigid kinks in the lipid chains. Those kinks prevent the membrane lipids from packing tightly and separating into damaging crystalline phases during cooling. In effect, the cyclopropane rings act like molecular spacers that keep the membrane loose and functional in freezing conditions.⁵

Cold-adapted species go even further. A psychrophilic (cold-loving) Pseudarthrobacter strain isolated from an alpine environment carries extra copies of genes involved in ribosome assembly and the uptake of protective solutes like glycine betaine, giving it an edge at frigid temperatures that would shut down less-equipped bacteria.⁶ And psychrophilic enzymes are structurally looser than their warm-weather counterparts, which keeps them catalytically active at temperatures where typical enzymes would seize up.⁷ The upshot is that cold does not simply “stop” bacteria. Many species have evolved robust biochemical countermeasures that let them keep growing, or at least surviving, well below the temperatures in your refrigerator.

Alive but Undetectable

Perhaps the most unsettling bacterial survival strategy in the cold is the viable-but-nonculturable state, often shortened to VBNC. Bacteria in this state are alive and metabolically active at a low level, but they will not grow on standard lab culture plates. If you swab a surface and try to culture what you find, VBNC cells look like they are dead. They are not.

A temperature downshift is one of the most common triggers for this state. Research on E. coli O157:H7 found that when this dangerous pathogen was held at low temperatures, plate counts dropped to undetectable levels by around 80 days, yet the cells were still viable.⁸ Crucially, when conditions improve, VBNC bacteria can “resuscitate” and become fully infectious again. This has been documented across a wide range of foodborne and environmental pathogens, and the cold-shock response itself appears to push cells into the VBNC state.⁹

For food safety, this is a serious concern. Many food-processing steps, including refrigeration, extreme temperatures, irradiation, and chemical preservatives, have been reported to push pathogens into the VBNC state rather than killing them outright.¹⁰ Standard food-safety tests that rely on culturing may therefore miss viable pathogens lurking in refrigerated or frozen products.

Viruses Thrive in the Cold

Viruses are not cells. They have no metabolism to slow down and no membranes to remodel (though enveloped viruses do have a lipid coat). Cold temperatures tend to preserve viruses rather than damage them. At 4 °C, both enveloped and non-enveloped viruses have been shown to remain stable for at least eight weeks in liquid formulation, with some non-enveloped viruses staying stable for over 30 weeks.¹¹ On dry surfaces, enveloped viruses generally persist for less than five days, while non-enveloped viruses can linger for weeks.¹²

This is why scientists routinely store virus samples at ultra-low temperatures. Far from killing viruses, a freezer is one of the best places to keep them intact. Heat, UV light, and chemical disinfectants are far more effective at destroying viral particles than cold ever is.

Spores Are Nearly Indestructible

Certain bacteria, particularly species of Clostridium and Bacillus, form endospores when conditions turn hostile. These spores are among the most resilient biological structures known, shrugging off heat, radiation, desiccation, and cold alike.

Clostridium difficile spores, responsible for serious hospital-acquired infections, survived ten consecutive freeze-thaw cycles in laboratory experiments. Viability dropped but remained high, with millions of colony-forming units per milliliter still present after the final cycle.¹³ Thermophilic (heat-loving) endospores from marine sediments also survived pretreatment at both −20 °C and −80 °C, then germinated and grew normally when returned to warm conditions. The colder pretreatment slowed their recovery somewhat but did not eliminate them: 21 of 22 spore types detected in the study were observed after one or both freezing treatments.¹⁴

If your question is whether sticking a contaminated cutting board in the freezer will sterilize it, the answer for spore-forming bacteria is an emphatic no. Cooking to high internal temperatures and proper chemical disinfection are the only reliable ways to destroy spores.

Parasites Are a Different Story

While bacteria and viruses tend to ride out the cold, many parasites are genuinely vulnerable to freezing. This is because parasites are often larger, multicellular organisms whose complex tissues cannot withstand ice-crystal damage the way a single-celled microbe can.

Anisakid worms, a common concern in raw fish, are a well-studied example. Freezing cod fillets at −15 °C or lower killed all Anisakis simplex and Pseudoterranova larvae. However, the same treatment was not always sufficient for whole herring: some larvae in intact herring showed spontaneous movement even after 24 hours at −15 °C, −18 °C, and −20 °C, because the insulation of surrounding tissue slowed the temperature drop at the core.¹⁵ This is why food-safety regulations for raw fish typically specify both the target temperature and the minimum holding time: the parasite must actually reach the lethal temperature throughout the product, not just at the surface.

Trematode parasites show a similar vulnerability. Cryptocotyle lingua metacercariae in cod flesh were killed at temperatures between −20 °C and −80 °C in less than two hours, while controls held at a cool but above-freezing 5 °C survived for nine days.¹⁶ Freezing is, in fact, one of the primary recommended methods for making raw or lightly cooked fish safe from parasites. It is one of the few cases where cold genuinely and reliably kills a class of pathogens.

Frozen Food Is Not Sterile Food

One persistent myth is that freezing food makes it safe in the same way cooking does. It does not. Freezing holds bacterial populations roughly in place; it does not eliminate them. When researchers incubated common foodborne bacteria at −18 °C (standard home-freezer temperature) for 20 days, E. coli O157:H7 and Pseudomonas aeruginosa remained at high levels around 10,000 colony-forming units per milliliter, while the more cold-sensitive Campylobacter jejuni died off considerably.¹⁷ So while some species are more susceptible than others, many dangerous bacteria survive standard freezer conditions just fine.

Thawing is arguably the more dangerous phase. As frozen food warms back through the “danger zone” between roughly 4 °C and 60 °C, surviving bacteria can resume multiplying. Adequate temperature control and good hygiene during thawing are important for retaining food safety.¹⁸ Leaving frozen chicken on the counter for hours creates conditions where bacteria that survived the freezer can multiply to dangerous levels before the food ever reaches the pan.

Life Locked in Permafrost

The most dramatic evidence that cold does not kill germs comes from permafrost, the permanently frozen ground found in arctic and subarctic regions. Microorganisms have been found alive in permafrost thousands and even hundreds of thousands of years old.

Metagenomic studies of Alaskan permafrost dated to 19,000–33,000 years old found that roughly a fifth of microbial cells still had intact membranes when stained, meaning they were classified as alive. Total cell counts reached tens of millions per gram of dry sediment, even in the oldest samples.¹⁹ Even more strikingly, research on ancient Siberian permafrost pushed the timeline back by orders of magnitude. Spore-forming bacteria dominated the metabolically active communities in sediments up to 1.1 million years old, extending the known survivability of microbes in frozen ground from tens of thousands of years to the million-year range.²⁰

This raises real public-health questions as climate change thaws permafrost at accelerating rates. Permafrost is a reservoir of largely uncharacterized microorganisms and viruses, many of which could still be viable.²¹ Whether these ancient organisms pose any meaningful threat to modern populations is still debated, but their very existence is a powerful reminder that cold, by itself, does not sterilize.

Why Cold Weather Brings More Infections

If cold does not kill germs, it makes sense that winter is peak season for many infectious diseases. But the relationship between cold weather and infection rates is not as straightforward as “cold preserves viruses, therefore more people get sick.”

Influenza seasonality in temperate regions is strongly linked to both temperature and humidity. Experimental studies in animal models demonstrated that influenza virus transmits more efficiently in cold, dry air, and epidemiological analyses have confirmed robust associations between flu incidence and local conditions of low humidity and temperature.²² Cold, dry air also appears to favor the airborne survival of many respiratory viruses and bacteria more broadly.²³

But the weather itself is only part of the story. People spend more time indoors in winter, in closer contact and in less-ventilated spaces. Cold air dries out nasal mucous membranes, potentially weakening the body’s first line of defense. Shorter days mean less UV radiation, which otherwise helps inactivate pathogens on surfaces and in aerosols. The seasonality of respiratory infections is driven by all of these factors working together, not by cold alone.

Cold Water Handwashing Works Just Fine

A related misconception is that you need hot water to wash germs off your hands. Multiple studies have tested this directly and found that water temperature makes no meaningful difference in how many bacteria are removed during handwashing. In one controlled trial, washing with water as cool as 15 °C (60 °F) reduced E. coli just as effectively as washing at 38 °C (100 °F).²⁴ An earlier study similarly found no effect of water temperature on the reduction of either resident or transient bacteria during normal handwashing with plain soap.²⁵

What actually matters is the mechanical action: lathering with soap, rubbing all surfaces of the hands, and spending enough time doing it. The soap lifts microbes and oils from the skin, and the rinsing carries them away. Temperature is basically irrelevant to this process at the range of temperatures people use at a sink. If cold water is all you have, it works.

Disinfectants Can Fail in the Cold

An underappreciated wrinkle in cold-weather hygiene is that some chemical disinfectants lose effectiveness at sub-zero temperatures. A study testing several disinfectants against African swine fever virus found that while most performed adequately in cold conditions, benzalkonium chloride and acetic acid failed to achieve the required level of virus reduction at −10 °C and −20 °C.²⁶

This has real-world implications for anyone cleaning equipment or surfaces outdoors in winter, whether on a farm, at a slaughterhouse, or in any setting where disinfection happens in unheated environments. The chemical reactions that disinfectants rely on slow down in the cold, just like biological reactions do. If you are decontaminating surfaces in freezing weather, choosing the right disinfectant and possibly allowing longer contact times becomes more important than it would be indoors at room temperature.

Cold-Adapted Microbes in Everyday Life

Psychrophilic and psychrotolerant organisms are not confined to glaciers and permafrost. They live in your refrigerator. Species of Pseudomonas, Listeria, and various molds grow comfortably at standard fridge temperatures around 4 °C. Listeria monocytogenes is infamous precisely because it can multiply in refrigerated foods that other pathogens cannot exploit as effectively. This is why refrigeration extends the shelf life of food but does not make it last forever: cold-tolerant spoilage organisms and pathogens continue to grow, just more slowly.

The enzymes these organisms use are structurally adapted for the cold, with more flexible molecular architecture that maintains activity at temperatures where typical enzymes stiffen and slow down.⁷ This molecular flexibility is a trade-off: psychrophilic enzymes tend to be less heat-stable, falling apart at temperatures that conventional enzymes handle easily. But in a cold fridge or a glacial stream, these enzymes give their owners a decisive competitive advantage. Your refrigerator is not a dead zone. It is a selective environment that favors microbes equipped to function in the cold while suppressing those that are not.