In microbiological media the case for replacing agar is chemical, not commercial. Agar is a polysaccharide extracted from red seaweed, and a fair number of microorganisms produce agarase and eat it. When they do, the plate liquefies. Low acyl gellan is produced by bacterial fermentation, is not a substrate for agarase, and holds its set at temperatures where agar softens. Those are the two reasons a laboratory makes this change, and they are the only two that reliably justify it — which is why you should be sceptical of anyone presenting this substitution as a saving.
For comparison across all three agar applications, see agar substitutes for industrial and laboratory use. For the plant side of the same substitution, see replacing agar in plant tissue culture media.
Why agar actually gets replaced here
| Reason | What agar does | What gellan does instead |
|---|---|---|
| Agarolytic organisms | Some bacteria and fungi secrete agarase and liquefy the plate | Not a substrate for agarase; plate stays intact |
| Thermophilic culture | Softens appreciably above about 50 °C | Holds set at incubation temperatures that soften agar |
| Defined composition | Seaweed extract, lot-to-lot variation includes trace impurities | Fermentation product with a specification |
| Clarity | Translucent | Clearer, useful for colony morphology and motility work |
| Purity for sensitive work | Trace metals and sulphates vary by lot | Consistent, which matters for defined media |
If your reason is not on this list, agar is usually the right answer. It has a century of validated protocols behind it and it is forgiving in a way gellan is not.
The substitution table
| Property | Agar | Low acyl gellan | Practical consequence |
|---|---|---|---|
| Working dose | 12–15 g/L | 8–10 g/L | Roughly two thirds of the weight |
| Susceptible to agarase | Yes | No | The main technical reason to change |
| Set behaviour at 50–60 °C | Softens | Holds | Enables thermophile work |
| Ion sensitivity | Low | High | Water quality becomes a variable |
| Hydration | Boil to dissolve | Heat above about 80 °C | Similar handling |
| Clarity | Translucent | Clear | Better colony observation |
| Forgiving dose window | Broad | Narrower | Weigh accurately |
The dose arithmetic
| Line | Agar medium | Gellan medium |
|---|---|---|
| Gelling agent | 12–15 g/L | 8–10 g/L |
| Powder per 1,000 L | 12–15 kg | 8–10 kg |
| Weight change | baseline | roughly −30% |
| What decides it | — | agarase susceptibility and incubation temperature |
A reduction of about a third in weight is real, and so is the higher unit price. This is not an application where the two comfortably meet in the middle, which is why laboratories that switch do so for a technical reason and accept the material difference. Put your own two prices against the weights above if you need the answer for a procurement file, but do not expect it to be the deciding argument.
Four cases where you should not switch
- Routine culture at mesophilic temperatures. If nothing is digesting the agar and nothing is molten, agar is fine and better documented.
- Organisms with documented altered colony morphology on gellan. Colony appearance, motility and sporulation can differ. If your identification workflow depends on colony morphology, validate before changing anything.
- Facilities without control of make-up water. Gellan sets by ionic crosslinking; variable water gives variable plates. Without RO or deionised water on the media line, this will frustrate you.
- Teaching labs and anything cost-critical. The technical advantages do not apply, and agar's tolerance of imprecise technique does.
Migration in four steps
- Confirm the reason, in writing. "Agarolytic strain" or "incubation above 50 °C" are reasons. "Cheaper" is not, and starting from the wrong reason produces the wrong protocol.
- Fix the water and the weighing first. Move to deionised or RO water, and weigh to the stated precision before changing the gelling agent. Most variability blamed on gellan lives in these two steps.
- Run the dose within the stated window and compare colony morphology side by side. Include type strains you recognise. If colonies look different, note it before anything else, because downstream identification may depend on it.
- Validate with the organisms you actually work on. Do not borrow another lab's protocol. Agarase susceptibility and morphology are strain-specific.
Failure modes
| Symptom | Likely cause | First check |
|---|---|---|
| Medium did not set | Insufficient cations, pH too low | Water conductivity, then pH before sterilising |
| Set much firmer than expected | Higher Ca²⁺/Mg²⁺ than assumed | Water analysis before reducing dose |
| Precipitate after autoclaving | Cations contacted partially hydrated gum | Dissolve fully before adding salt stock |
| Batch-to-batch variation | Water source variation | Conductivity logged every batch |
| Colony appearance different | Genuine matrix effect | Compare against a recognised reference strain |
| Plates dry out quickly | Different water retention | Incubation humidity, or seal the plates |
Tools
- Dosage calculator — includes microbiological media at 0.8–1.0% and tissue culture at 0.15–0.30%.
- Formula pre-check — flags hydration, pH and ion combinations before you pour plates.
Frequently asked
How much gellan replaces agar in microbiological media?
About two thirds of the weight — 8–10 g/L of low acyl gellan against 12–15 g/L of agar. The relationship is not a straight potency ratio, because the two set by different mechanisms.
Is this about saving money?
No. If saving money is the goal, agar will usually win. The reasons to change are agarase digestion, incubation temperature above about 50 °C, and lot-to-lot compositional consistency.
Can I use the same autoclave cycle?
Generally yes. Both survive standard autoclaving. The process difference is in water quality and dispersion order rather than in the sterilising step.
Is Gelrite the same material?
Yes. Gelrite and Phytagel are trade names for low acyl gellan sold into the laboratory channel. The polymer is the same; what differs is specification, documentation and price.