MRS Broth (de Man, Rogosa, Sharpe)
An original two-tube bench visual for reading gas production in MRS broth, differentiating gas-producing Leuconostoc sp. from non-gas-producing Lactobacillus sp.
MRS broth tubes A-B
Confirm growth by turbidity first, then examine the Durham tube for trapped gas.
Growth present (turbid broth) and a gas bubble is trapped at the closed top of the Durham tube, indicating gas production during glucose fermentation.
Growth present (turbid broth) but the Durham tube is completely filled with broth and contains no gas bubble; organism ferments glucose homofermentatively.
Growth present (turbid broth) and a gas bubble is trapped at the closed top of the Durham tube, indicating gas production during glucose fermentation.
Growth present (turbid broth) but the Durham tube is completely filled with broth and contains no gas bubble; organism ferments glucose homofermentatively.
What to examine
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Turbid amber broth + gas bubble in Durham tube | Organism produced CO2 during heterofermentative glucose fermentation; gas displaced broth from the inverted tube. | Positive with gas — Leuconostoc |
| Turbid amber broth + no gas in Durham tube | Growth occurred but fermentation was homofermentative; no CO2 produced. | Positive, no gas — Lactobacillus |
| Clear broth, no growth | Organism does not grow in MRS medium at all. | Negative — not a lactic acid bacterium |
| Tiny bubbles vs. large bubble | Only a bubble displacing at least 10% of the Durham tube volume is a confirmed positive gas result. | Small wisps are not reportable |
MRS broth study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Turbid + gas (Durham) | Heterofermentative; produces CO2, ethanol, and lactic acid from glucose | Leuconostoc sp.; also some heterofermentative Lactobacillus species |
| Turbid + no gas | Homofermentative; produces only lactic acid from glucose | Lactobacillus sp. (e.g., L. acidophilus, L. lactis) |
| Clear, no growth | Not a lactic acid bacterium; or inhibited by selective agents | E. coli and other non-lactobacilli are inhibited |
- MRS broth selects for lactic acid bacteria (lactobacilli, leuconostocs) by providing rich nutrients and acetate/citrate as selective inhibitors.
- The Durham tube inside the broth tube traps gas produced during fermentation at the closed (inverted) end.
- Gas in the Durham tube = heterofermentative (Leuconostoc pattern); no gas = homofermentative (Lactobacillus pattern).
Best mental model: If the Durham tube has a clear space at its sealed top, the organism is a heterofermentor and made CO2; if it is fully submerged and filled, the organism ferments without making gas.