The Staple Food: Oat Flakes
In laboratories and homes around the world, plain uncooked oat flakes are the standard food for Physarum polycephalum. This is not a random choice. Decades of laboratory experience have shown that oat flakes provide an excellent balance of carbohydrates (mainly starch), protein, and minerals in a convenient, shelf-stable, and inexpensive form.
The oat flake also serves a practical purpose beyond nutrition: it acts as a discrete food point. Because it sits as a solid piece on the substrate, the slime mold must actively extend toward it, creating the beautiful network of veins that makes Physarum so visually striking and scientifically useful for experiments.
Choosing the Right Oat Flakes
| Type of Oats | Suitability | Notes |
|---|---|---|
| Quick oats / instant oat flakes | Best choice | Thin and flat, providing maximum surface area. The slime mold colonizes them quickly. |
| Rolled oats (regular) | Good | Slightly thicker. Still works well, just takes a bit longer to colonize. |
| Steel-cut oats | Poor | Hard and dense. Difficult for the slime mold to colonize efficiently. |
| Organic oats | Good (slight preference) | Theoretically free of pesticide residues, though the practical difference is minimal. |
| Cooked / pre-soaked oats | Avoid | Too wet, break apart, and promote mold contamination. |
| Flavored instant oatmeal | Avoid | Contains sugar, salt, and flavorings that can harm the plasmodium. |
How Much and How Often to Feed
One of the most common beginner mistakes is overfeeding. A slime mold in a standard 9 cm Petri dish needs very little food at each feeding.
Feeding Schedule
| Culture Size | Oat Flakes per Feeding | Feeding Frequency | Notes |
|---|---|---|---|
| Small (just started, 1-2 cm plasmodium) | 1-2 flakes | Every 2-3 days | Place flakes close (1-2 cm) to help the young plasmodium find them. |
| Medium (5-7 cm plasmodium) | 2-4 flakes | Every 2 days | Spread flakes to encourage network expansion. |
| Large (filling most of the dish) | 3-5 flakes | Every 1-2 days | Consider transferring to a larger container or splitting the culture. |
Placement Strategy
Where you place the food matters as much as how much you give:
- Distance: Place new flakes 2-3 cm ahead of the leading edge of the plasmodium. This encourages the slime mold to extend and explore, building new veins.
- Do not pile food directly on the plasmodium. Covering the plasmodium with food can suffocate it and promotes contamination.
- Create a network: For more interesting growth patterns, place flakes at multiple points around the dish rather than in a single spot. The slime mold will build a network connecting all food sources, often optimizing the routes between them.
Removing Old Food
Colonized oat flakes left in the dish for more than 3-4 days become a contamination risk. They darken, dry out, and can develop mold. Remove spent flakes with clean tweezers before adding fresh ones. This single habit prevents the majority of contamination problems.
Alternative Foods
While oat flakes should be your primary food source, experimenting with alternative foods is one of the most enjoyable aspects of keeping slime mold. It also reveals how the organism makes dietary choices.
Foods That Work
| Food | Type | How to Use | Slime Mold Response |
|---|---|---|---|
| Brewer's yeast (dried) | Protein-rich supplement | A small pinch sprinkled near the plasmodium, once a week | Strong attraction. Rich in B vitamins and amino acids. |
| Fresh mushroom pieces | Natural food source | Small cubes (3-5 mm) of shiitake or white mushroom | Good response. Closer to the natural diet of bacteria on decaying wood. |
| Rice grains (uncooked) | Starch source | 1-2 grains, placed like oat flakes | Moderate interest. Colonized more slowly than oat flakes. |
| Sprouted seeds | Experimental | Small pieces of alfalfa or radish sprouts | Variable. Interesting for preference experiments. |
| Plain agar with bacteria | Lab-grade diet | Agar plates inoculated with E. coli or other bacteria | Excellent. This is closest to the natural diet. Requires microbiology skills. |
| Wheat bran | Carbohydrate source | Small pinch on the substrate | Moderate response. Fine particles can be messy. |
Foods to Absolutely Avoid
- Salt: Even trace amounts cause osmotic stress, dehydrating the plasmodium's cells. Table salt, soy sauce, salted snacks, and any food containing sodium should never be placed in the dish. Salt is commonly used in experiments specifically because of its strong repellent effect.
- Citrus fruits and juice: Limonene and other terpenes found in lemon, orange, and grapefruit are highly toxic and repulsive to Physarum. Even handling food after peeling an orange can transfer enough residue to disturb the culture.
- Essential oils: Tea tree, eucalyptus, lavender, and other essential oils are lethal to slime mold at very low concentrations.
- Alcohol: Ethanol, isopropanol, and alcohol-based disinfectants damage the cell membrane.
- Sugar in large quantities: While slime mold does metabolize sugars, concentrated sugar solutions create osmotic stress similar to salt. A tiny amount dissolved in the substrate is tolerated, but pure sugar crystals should be avoided.
- Processed food: Anything cooked, seasoned, or containing preservatives is unsuitable. These products contain combinations of salt, fats, and chemicals that harm the plasmodium.
- Metal particles: Keep metallic objects away from the culture. Some metals are toxic to the organism.
Nutritional Geometry: The Science of Slime Mold Diet
Some of the most fascinating research on slime mold feeding behavior comes from the field of nutritional geometry, developed by researchers Stephen Simpson and David Raubenheimer and applied to Physarum by Audrey Dussutour and her team at CNRS.
What Is Nutritional Geometry?
Nutritional geometry is a framework for understanding how organisms balance their intake of different nutrients. Instead of looking at calories alone, it maps food choices along multiple nutritional axes, typically protein versus carbohydrate.
Key Findings in Slime Mold
Dussutour's research revealed that Physarum polycephalum does not simply eat whatever is in front of it. Instead, it actively regulates its nutritional intake:
- Protein-carbohydrate balance: When given a choice between foods with different protein-to-carbohydrate ratios, Physarum consistently selects a diet with roughly two parts protein to one part carbohydrate (a 2:1 ratio). This is its "intake target."
- Active regulation: If placed on a substrate where only one type of food is available (say, pure carbohydrate), the slime mold will adjust its intake to get as close to its target ratio as possible. When a protein source becomes available later, it will preferentially consume protein to compensate.
- No-choice dilemma: When forced to eat a food with the wrong ratio and given no alternative, the slime mold prioritizes protein intake, even if this means over-consuming carbohydrates. This mirrors a pattern seen in many animals, from insects to primates.
- Speed versus accuracy: In maze-like setups where the slime mold must choose between a nearby but nutritionally poor food and a distant but nutritionally ideal food, it often takes the longer path to reach the better food source.
A Diet Without a Brain
What makes these findings remarkable is that Physarum polycephalum is a single cell with no brain, no digestive system, and no sense of taste as we understand it. Yet it makes dietary decisions that are functionally identical to those made by animals with complex nervous systems. The mechanisms behind this are still being studied, but they appear to involve chemical sensing at the cell membrane level combined with memory of previous nutritional states.
How Slime Mold Eats: The Mechanics
Understanding how slime mold actually consumes food helps explain why certain foods work better than others.
Physarum feeds primarily through two mechanisms:
- Phagocytosis: The plasmodium engulfs bacteria and small food particles by wrapping its cell membrane around them, pulling them inside, and digesting them in vacuoles (internal pockets). This is the same mechanism used by human white blood cells.
- Extracellular digestion: For larger food items like oat flakes, the plasmodium secretes digestive enzymes onto the food surface, breaking it down externally before absorbing the resulting nutrients. This is why you see the plasmodium spreading across and eventually "consuming" an oat flake.
The digested nutrients are then distributed throughout the entire organism via cytoplasmic streaming, the rhythmic, back-and-forth flow of fluid through the plasmodium's vein network.
Signs Your Slime Mold Is Well-Fed
- Bright yellow to orange color: A well-nourished plasmodium has vivid pigmentation.
- Active exploration: The leading edge extends steadily, forming new branches.
- Thick veins: The transport network is clearly visible with well-defined, pulsing veins.
- Growth over food sources: The plasmodium fully envelops oat flakes within 12-24 hours of contact.
Signs of Nutritional Problems
| Symptom | Likely Cause | Solution |
|---|---|---|
| Pale, almost white color | Underfeeding or wrong food | Add fresh oat flakes, check food quality |
| Retreating from food | Food contaminated or repulsive substance present | Replace food, check for mold on the flakes |
| Very slow growth despite good conditions | Insufficient nutrition | Feed more frequently, try brewer's yeast as supplement |
| Sporulation (forming fruiting bodies) | Starvation triggering reproduction | This means the organism has decided food is permanently gone. Add food to an active area if any remains |
| Entering sclerotium (drying out) | Multiple stress factors including hunger | Check all environmental conditions. See environment guide |
Feeding and Experiments
Food placement is the primary tool for most slime mold experiments. By controlling where you place food, you can:
- Guide the plasmodium through a maze
- Test food preferences by offering choices at equal distances
- Observe network optimization by placing food at the vertices of geometric shapes
- Study habituation and memory using foods paired with mild repellents
What Slime Mold Eats in Nature
In the wild, Physarum polycephalum and other plasmodial slime molds have a diet very different from the oat flakes we offer in captivity. Their natural food sources include:
- Bacteria: The primary natural food. Billions of bacteria live on decaying logs, leaf litter, and soil. The plasmodium moves across these surfaces, engulfing bacteria as it goes.
- Fungal spores: Wild slime molds consume spores from fungi growing on the same substrates. This contributes to their ecological role in regulating fungal populations.
- Yeasts: Wild yeasts growing on decaying organic matter are a nutrient-rich food source, similar to the brewer's yeast used as a supplement in captive cultures.
- Microalgae: In wet environments, small algae growing on bark or rock surfaces can be consumed by the advancing plasmodium.
- Organic particles: Fine particles of decomposing plant material, pollen grains, and other organic debris are absorbed as the plasmodium spreads across surfaces.
This diverse natural diet is part of why oat flakes work so well in captivity: they provide a broad nutritional profile that roughly approximates the mix of carbohydrates, proteins, and minerals the organism would encounter across many different bacterial and organic food sources in the wild.
For the full guide on getting started with feeding and care, see our beginner's growing guide.