Making agar for petri dishes is straightforward: dissolve agar powder in water or broth, sterilize the mixture, and pour it into dishes before it sets. The basic ratio is roughly 15 grams of agar per liter of liquid, though the exact concentration depends on what you plan to grow. Whether you are outfitting a teaching lab, running quality-control tests, or exploring microbiology at home, the process follows the same core steps with room for customization at every stage.
What Agar Actually Is
Agar is a polysaccharide extracted from marine red algae. It consists of two main components: agarose, which is a neutral, linear polymer responsible for the gel structure, and agaropectin, a charged, sulfated polymer that contributes less to gelling.1Nano Hybrids and Composites. Electronic, Vibrational, and Structural Study of Polysaccharide Agar-Agar Biopolymer The agarose fraction is what makes agar so useful for microbiology: it melts at around 85°C but does not re-solidify until it cools to about 35–40°C. That wide gap between melting and setting temperatures gives you a comfortable window to add heat-sensitive supplements, pour the liquid into plates, and work without rushing. It is also biologically inert, meaning most microorganisms cannot digest it, so the gel stays solid while bacteria or fungi grow on top.
The Standard Recipe
The most common all-purpose medium for growing bacteria is nutrient agar or tryptic soy agar. For a basic nutrient agar batch that fills about 40 standard petri dishes (90 mm diameter), you need roughly one liter of medium. A typical formulation looks like this:
- Agar powder: 15 g (this is the standard concentration for a firm gel)
- Nutrient broth powder: 8–13 g, depending on the brand (this supplies nitrogen, carbon, and vitamins for bacterial growth)
- Distilled or deionized water: 1 liter
If you are using a pre-mixed dehydrated medium like tryptic soy agar, the agar is already included in the powder and you just add water according to the label. If you are building your medium from scratch, weigh out the agar and nutrient components separately, combine them in an Erlenmeyer flask or a glass bottle, add the water, and swirl to suspend the powder. The agar will not dissolve at room temperature; it needs heat. Use a flask that is at least twice the volume of your liquid, because agar foams vigorously when heated and can boil over.
Sterilization
Before you pour, the medium must be sterilized to kill any contaminating microorganisms already present in the powder or water. The standard method uses an autoclave set to 121°C at 15 PSI for at least 15–30 minutes.2PLOS ONE. Assessment and verification of commercially available pressure cookers for laboratory sterilization This combination of pressure and temperature destroys vegetative bacteria, fungal spores, and even highly resistant endospores.
If you do not have access to a laboratory autoclave, a kitchen pressure cooker can serve as a substitute. Research has confirmed that several commercially available pressure cookers can reach the temperatures and pressures needed for effective sterilization.2PLOS ONE. Assessment and verification of commercially available pressure cookers for laboratory sterilization The key is verifying that your cooker actually reaches 121°C (or at least 115°C for longer cycle times). A cheap maximum-registering thermometer or autoclave indicator tape can help you confirm conditions. If your cooker only reaches 115°C, you can compensate by running the cycle longer, as lower-temperature sterilization at extended times is considered appropriate for inactivating contaminants.
One practical detail: loosen the caps on your bottles or cover your flasks with aluminum foil rather than screwing them tight. Sealed containers can build pressure unevenly and crack. After the cycle is complete, let the pressure drop naturally rather than venting rapidly, since a sudden pressure release can cause your medium to boil over inside the chamber.
Sterilizing Agar Separately From Nutrients
An approach that has gained traction in quality-control labs is autoclaving the agar and the nutrient components in separate containers, then combining them after sterilization. When agar is heated alongside sugars and amino acids, Maillard-type browning reactions can occur, slightly degrading both the nutrients and the gel. Researchers who tested this separation method found that it significantly improved the growth of several common test organisms, including strains of Staphylococcus aureus, Salmonella, Candida albicans, and Saccharomyces cerevisiae, across multiple standard media formulations.3PubMed Central. Improvement of the Pour Plate Method by Separate Sterilization of Agar and Other Medium Components and Reduction of the Agar Concentration The same study found that reducing the agar concentration to 10 g/L (instead of the usual 15 g/L) also enhanced colony recovery without compromising the gel’s usability. For routine home or classroom projects, standard combined autoclaving works fine, but if you are doing any kind of quantitative microbiology where colony counts matter, separate sterilization is worth the extra effort.
Pouring and Setting the Plates
After sterilization, remove your medium and let it cool to around 50–55°C. You should be able to hold the flask comfortably against the inside of your wrist without it feeling scalding. If you are adding any heat-sensitive supplements (antibiotics, indicator dyes, or growth factors), stir them in at this stage. Pouring too hot risks killing the supplements; pouring too cool means the agar starts gelling in the flask and you end up with lumpy, uneven plates.
Work near a flame (a Bunsen burner in a lab, or a candle in a pinch for home projects) to create an updraft that pushes airborne contaminants away from your open plates. Lift each petri dish lid just enough to pour, fill the dish to roughly a third of its depth (about 20–25 mL for a standard 90 mm dish), and replace the lid immediately. Swirl the plate gently on the bench surface if you see any bubbles, but avoid sloshing agar onto the lid.
Let the plates sit undisturbed on a flat surface for 15–30 minutes until the agar has fully solidified. Once set, the gel will appear translucent and slightly firm to the touch if you tilt the plate. If you are preparing a large batch, stack the plates and leave them at room temperature for a day or two before use; this lets excess surface moisture evaporate without drying out the gel.
Dealing With Condensation
Those water droplets that collect on the inside of petri dish lids are more than a nuisance. Condensation can drip onto the agar surface and cause colonies to spread into each other, ruining any work that depends on isolated colonies. The droplets form when the lid is slightly cooler than the agar surface beneath it, and the temperature difference does not have to be large. Research on stacked plates found that condensation appeared whenever the upper dish was warmer than the lower dish by as little as 0.13°C to 0.46°C, but that stacking the plates horizontally (rather than in tall vertical towers) effectively prevented detectable condensation.4Measurement Science and Technology. Preventing moisture condensation on Petri dish lids
A common lab trick is to store finished plates upside down (agar on top, lid on the bottom). This way, any condensation that does form drips onto the lid rather than pooling on the agar surface. If you are incubating plates at 30–37°C, keep them inverted and avoid stacking them too high, since the dishes in the middle of a tall stack tend to develop the steepest temperature gradients.
Storing Your Plates
Prepared agar plates lose moisture over time. Signs of desiccation include cracks in the gel, visible shrinkage away from the sides of the dish, and a noticeable drop in volume.5EJPPS EUROPEAN JOURNAL OF PARENTERAL AND PHARMACEUTICAL SCIENCES. Agar Desiccation – The Causes and How to Address Them To slow this down, seal your plates in plastic bags or resealable zipper bags and store them in the refrigerator at about 4°C. Most nutrient agar plates stay usable for four to six weeks stored this way. Pull them out and let them warm to room temperature for an hour or so before inoculating; cold plates can inhibit bacterial growth and encourage extra condensation once placed in a warm incubator.
If you notice your plates are consistently drying out faster than expected, check the humidity level of your storage environment. Refrigerators tend to be very dry, and plates stored unwrapped can lose significant moisture in just a week. Double-bagging helps, and some labs place a damp paper towel inside the outer bag to buffer humidity.
Food-Grade Agar as a Budget Alternative
Bacteriological-grade agar is purified to minimize impurities that could inhibit microbial growth, but it is expensive, often several times the price of the agar sold in grocery stores for cooking. A study comparing food-grade agar to bacteriological-grade agar found no difference in colony morphology, growth rate, or viability of bacteria on solid media prepared with the cheaper product, and the switch reduced the cost of common solid media by 80% or more.6PubMed. Back to the kitchen: food-grade agar is a low-cost alternative to bacteriological agar For educational settings, citizen science projects, or any application where you are growing common lab strains, food-grade agar from an Asian grocery store or health-food shop works well. Just make sure you are buying pure agar powder or agar flakes, not a flavored dessert mix.
The one caveat is that food-grade agar can vary more between brands and batches. Some brands set firmer than others, and a few contain trace additives. If your plates seem too soft or too firm, adjust the concentration up or down by a gram or two per liter and test a small batch first.
Selective and Differential Additives
Plain nutrient agar supports a wide range of bacteria, which is the point for general-purpose culturing. But sometimes you want to grow only certain organisms or tell different species apart by sight. That is where selective and differential additives come in.
Selective agents suppress unwanted organisms. Antibiotics are the most common: adding ampicillin at 100 µg/mL, for example, kills off any bacteria that lack the ampicillin-resistance gene, so only your engineered strain survives. High salt concentrations (7.5% NaCl in mannitol salt agar) select for salt-tolerant species like staphylococci while inhibiting most others. Bile salts or crystal violet in MacConkey agar inhibit gram-positive bacteria, letting you focus on gram-negatives.
Differential agents do not kill anything but change the appearance of colonies depending on their metabolic activity. A pH indicator dye shifts color when an organism ferments a sugar, so lactose fermenters turn pink on MacConkey agar while non-fermenters stay colorless. Some formulations combine both strategies. A modified MRS agar designed for isolating a specific probiotic strain, for instance, included the antibiotic vancomycin to suppress competing bacteria and the pH indicator bromophenol blue to distinguish colonies by their acid production.7PubMed. Culture media for differential isolation of Lactobacillus casei Shirota from oral samples
If you are making selective plates at home, add any antibiotic or heat-sensitive supplement after autoclaving, once the medium has cooled to around 50°C. Most antibiotic stock solutions can be filter-sterilized and stored frozen in small aliquots so you only thaw what you need for each batch.
Specialized Media for Unusual Organisms
Standard nutrient agar is designed for fast-growing, well-characterized lab bacteria. If you are trying to culture something less conventional, the medium itself may need to change. Marine microbiologists, for example, often use seawater-based agar and experiment with different gelling agents and nitrogen sources. Research on culturing marine bacteria found that varying the medium composition, including swapping standard agar for alternative gelling agents and adding ammonium to seawater-based formulations, increased the proportion of less commonly cultured groups like Alphaproteobacteria and Actinobacteria.8PubMed Central. Culturing marine bacteria – an essential prerequisite for biodiscovery
For fungi, Sabouraud dextrose agar (SDA) is the go-to: it has a low pH (around 5.6) and high sugar content, both of which favor fungal growth over bacteria. Potato dextrose agar, made by boiling potatoes and adding dextrose and agar, is another popular choice for molds. If you are trying to culture environmental samples from soil or water, consider dilute nutrient media (a quarter or a tenth of the standard concentration), since many environmental bacteria grow better on lean media and are actually inhibited by the rich nutrient levels that lab-adapted species prefer.
Common Mistakes and How to Avoid Them
A few pitfalls trip up beginners reliably. The most common is contamination: fuzzy mold colonies or spreading bacterial lawns showing up on plates that should have been sterile. Usually this means the medium was not sterilized long enough, the plates were poured in a drafty area, or the dishes were left open for too long. Work quickly, keep lids on whenever possible, and run your autoclave cycle for the full duration rather than cutting it short.
Another frequent issue is agar that fails to set properly. If your plates are still squishy after cooling, you likely did not add enough agar powder. Confirm your weighing, and remember that agar concentration is measured before autoclaving (the powder absorbs water, so the final volume stays roughly the same). Some cheap agar products have lower gel strength, so you may need to bump the concentration up to 18–20 g/L.
Bubbles in the set agar are mostly cosmetic, but they can interfere with colony counting. Swirling plates gently right after pouring helps, and briefly passing a flame from a lighter or Bunsen burner across the surface of the liquid agar pops surface bubbles. Do not overdo the flame, though; you can scorch the agar or melt plastic dishes.
Finally, watch for browning. If your medium comes out of the autoclave noticeably darker than the powder suggested, you may be autoclaving too long or at too high a temperature. Excessive heating caramelizes sugars in the medium and can produce compounds that inhibit the growth of sensitive organisms. Keep your cycle at 121°C for 15 minutes for one liter; scale up the time only for larger volumes.
Safe Disposal of Used Plates
Once you have grown bacteria on your plates, those cultures need to be treated as potentially hazardous. Professional labs autoclave used plates before discarding them, which kills everything on the agar. If you are working at home without an autoclave, a chemical deactivation method is an option. One published approach allows you to deactivate cultivated microorganisms on plates using low-cost household sanitizing reagents without even opening the petri dish, reducing the risk of exposure.9PubMed Central. Growsafe: A Chemical Method To Deactivate Cultivated Microorganisms Using Low-Cost Kitchen Supplies The general idea is to inject or pour a concentrated bleach or disinfectant solution into the sealed plate and let it sit for at least 24 hours before disposal.
Never throw used agar plates directly into household trash without deactivation. Even if you only cultured organisms from a kitchen sponge or a doorknob, the plate has amplified whatever was there into dense colonies. Treat every culture as if it could contain something you would rather not release into the environment, seal your waste, and disinfect it thoroughly.
Making Plates Without a Lab
You do not need professional equipment to make functional agar plates. Here is a minimal home setup: food-grade agar powder, bouillon cubes or beef broth as a nutrient source, a kitchen pressure cooker, clean glass jars or mason jars, and disposable petri dishes (available cheaply online in sleeves of 20). Dissolve one bouillon cube and 15 grams of agar in a liter of water, transfer to a mason jar with a loosely fitted lid, pressure-cook for 30 minutes at full pressure, let the pressure drop naturally, and pour the medium into petri dishes near a candle flame. It will not match the sterility of a proper lab, but for science-fair projects, growing environmental samples, or just satisfying curiosity, it works surprisingly well.
For an even simpler approach, some teachers use gelatin instead of agar as a gelling agent. Gelatin sets at lower concentrations and does not require any special equipment to melt. The downside is that many bacteria produce enzymes that digest gelatin, so your plates may literally dissolve underneath the colonies after a few days. Agar is resistant to this digestion, which is a big part of why it became the standard for microbiology in the first place.