Culturing is the process of growing living organisms in a controlled environment, whether that’s bacteria on an agar plate, mammalian cells in a flask, or wild yeast in a jar of flour and water. The core principles are the same across all types: provide nutrients, control temperature, keep unwanted organisms out, and give your culture time to grow. What changes are the specific tools, media, and conditions you need depending on what you’re growing.
The Basic Steps of Any Culture
Every culturing process follows the same general sequence. First, you prepare a nutrient medium, which is the food source your organisms will grow on or in. Second, you sterilize everything to eliminate competing organisms. Third, you inoculate, meaning you introduce the organism you want to grow into the prepared medium. Fourth, you incubate at the right temperature and conditions for a set period. Finally, you either observe, harvest, or transfer your culture depending on your goal.
The difference between culturing bacteria in a lab and culturing sourdough in your kitchen is really just a difference of scale and precision. A microbiologist uses an autoclave and laminar flow hood. A home baker uses clean jars and warm countertops. Both are managing the same biological variables.
Choosing the Right Growth Medium
The medium is what feeds your culture. In microbiology, solid media typically consist of protein digests (broken-down proteins that microorganisms can absorb) mixed with inorganic salts and hardened with 1.5% agar, a gelatin-like substance derived from seaweed. This gives bacteria a firm surface to grow on inside a petri dish, forming visible colonies you can count and identify.
Liquid media, called broths, contain the same nutrients without the agar. Cells or bacteria grow suspended throughout the liquid, which is useful when you need large quantities or want to measure growth rates. For mammalian cell culture, the medium is more complex and includes sugars, amino acids, vitamins, and often animal serum to supply growth factors.
Some media are designed to be selective, meaning they encourage specific organisms while suppressing others. Mannitol salt agar, for instance, contains a high concentration of sodium chloride that kills most bacteria but lets staphylococci thrive. This is how labs isolate a single species from a mixed sample.
Temperature, pH, and Oxygen
Temperature is the single biggest factor determining whether your culture succeeds or fails. Most bacteria that affect humans are mesophiles, growing best between 15°C and 45°C (59°F to 113°F), with an optimal range around 37°C (98.6°F), which is human body temperature. Organisms that prefer cold are called psychrotrophs and grow between 0°C and 30°C. Heat-loving thermophiles thrive between 40°C and 80°C.
The pH of your medium matters just as much. The majority of bacteria grow best in a neutral range between pH 5.5 and 8.5. Molds and yeasts prefer slightly acidic conditions, typically between pH 4 and 6. This is why fermented foods like yogurt and sourdough naturally become more acidic over time: the organisms producing acid are creating an environment that favors themselves and discourages competitors.
Oxygen requirements vary widely. Some organisms need oxygen to survive, others are killed by it, and many can tolerate either condition. Knowing which category your target organism falls into determines whether you seal your culture vessel, leave it open to air, or use specialized equipment to control gas levels.
Keeping Everything Sterile
Contamination is the most common reason cultures fail. Aseptic technique is the practice of keeping unwanted microorganisms out of your work. In a professional lab, this means sterilizing all tools and media before use, typically through autoclaving, which exposes equipment to high-pressure steam at temperatures above 121°C (250°F) for 15 to 20 minutes. After autoclaving, sterilization indicators like temperature-sensitive tape confirm the process worked.
During the actual work of transferring or inoculating cultures, lab workers use a laminar flow hood that blows filtered air across the workspace, creating a barrier against airborne contaminants. Inoculating loops (thin wire tools used to pick up and transfer organisms) are sterilized by passing them through a flame until they glow red. Surfaces are wiped with 70% ethanol.
If you’re culturing at home for food fermentation, your version of aseptic technique is simpler but still important: use clean jars, clean utensils, and wash your hands. The high acidity of fermented foods provides a natural defense against most harmful bacteria, but starting with clean equipment gives your desired organisms a head start.
Culturing Bacteria and Fungi
To culture bacteria, you typically streak a sample across the surface of an agar plate using an inoculating loop. The streaking pattern gradually dilutes the sample so that individual cells land in separate spots, each growing into a distinct colony over 24 to 48 hours. These colonies can then be picked up and transferred to fresh plates or broth for further study.
Fungal cultures grow more slowly, often taking several days to a week or more. They tend to prefer slightly acidic media and lower temperatures than many bacteria. Molds grow as fuzzy, spreading colonies on solid media, while yeasts form smaller, smoother colonies that look more like bacterial growth.
Culturing Mammalian Cells
Growing animal or human cells is more demanding. These cells require a precisely controlled environment: 37°C, 5% carbon dioxide in the atmosphere, and a nutrient-rich liquid medium that often includes serum from fetal cows. Cells grow attached to the bottom of plastic flasks or dishes that have been specially treated to help cells stick.
As cells multiply, they eventually cover the entire surface of the flask. When they reach about 90% to 100% coverage (called confluence), they need to be “passaged,” which means detaching them from the surface and splitting them into new flasks at a lower density. This is done by adding an enzyme that dissolves the proteins holding cells to the flask surface. The detached cells are then diluted and transferred to fresh flasks with new medium.
Timing passages correctly is critical. If cells become too crowded, they stop growing normally and can permanently change their behavior, ruining your experiment or production run.
Culturing for Food: Sourdough and Fermentation
If you’re here because you want to culture a sourdough starter, the process is straightforward but requires patience. Mix equal parts flour and water by weight (for example, 50 grams of each) with a small amount of starter culture. Place the mixture in a warm spot between 70°F and 85°F (21°C to 29°C) and wait 12 to 24 hours.
After the first fermentation period, feed the starter again by discarding all but half a cup and mixing in fresh flour and water. Repeat this feeding every 12 hours. After 3 to 7 days of consistent feeding, the starter will begin bubbling regularly within a few hours of each feeding, which means the wild yeast and bacteria colonies are established and active.
Once your starter is active, you have two maintenance options. If you bake frequently, keep it at room temperature and feed it every 12 to 24 hours using a 1:1:1 ratio by weight of starter, water, and flour. Use it 12 to 24 hours after the last feeding, or within 3 to 4 hours for peak activity. If you bake less often, store it in the refrigerator and feed it once a week. After feeding a refrigerated starter, let it sit at room temperature for 1 to 2 hours until it looks light and bubbly, then return it to the fridge.
Storing and Preserving Cultures
Short-term storage for most microbial cultures means refrigeration below 4°C (39°F), kept in the dark. Agar plates sealed with tape or film can last a few weeks this way, and liquid cultures can be refrigerated for days to weeks depending on the organism.
For long-term preservation, two methods dominate: cryopreservation and lyophilization (freeze-drying). Cryopreservation involves mixing your culture with a protective agent, most commonly glycerol, and freezing it at ultra-low temperatures. The glycerol forms hydrogen bonds with water molecules in the storage medium, preventing ice crystals from forming and rupturing cells during freezing. Glycerol-preserved cultures stored at -80°C can remain viable for years.
Lyophilization removes water from frozen cultures through vacuum pressure, leaving behind a dry powder that can be stored at room temperature and shipped easily. Freeze-dried cultures can retain full functionality for months or longer. In one study, freeze-dried gut microbes preserved with skimmed milk retained 100% of their metabolic function after revival. Both methods work well, but glycerol-based cryopreservation tends to preserve the original community structure of mixed cultures more faithfully than other approaches.
Essential Equipment
What you need depends entirely on what you’re culturing. A basic microbiology setup requires petri dishes, an inoculating loop, prepared agar media, an incubator (or a warm spot with stable temperature), and a way to sterilize equipment. At minimum, you need gloves and eye protection.
A mammalian cell culture lab requires significantly more: a laminar flow hood, a CO2 incubator, a microscope, sterile pipettes, culture flasks, and an autoclave. The flow hood alone can cost thousands of dollars, which is why cell culture is almost exclusively done in institutional settings.
For food fermentation, your equipment list is mercifully short: a clean glass jar, a kitchen scale, flour, water, and patience. A thermometer helps you find the warmest spot in your kitchen, but it’s not strictly necessary if your home stays in the 70°F to 85°F range.