Last updated: 13 July 2026 by Maria Williams
When bakers talk about the elusive, complex tang of a truly great sourdough loaf, they are — knowingly or not — talking about a bacterium. Its name is Lactobacillus sanfranciscensis, and it is the single most important microbial character in the story of San Francisco sourdough. Understanding what it is and why it matters is the fastest way to raise your baking game.
What is Lactobacillus sanfranciscensis?
Lab-verified Fructilactobacillus sanfranciscensis in our starter
We had our own San Francisco-style sourdough starter tested at an independent UK laboratory. The results confirmed the presence of F. sanfranciscensis. If you want the strain in your kitchen without the years of maintenance, try our live starter here.
Lactobacillus sanfranciscensis (recently reclassified as Fructilactobacillus sanfranciscensis) is a species of lactic acid bacteria (LAB) that lives in symbiosis with wild yeasts inside long-maintained sourdough starters. It was first isolated and characterised in the 1970s by microbiologists studying the century-old cultures at the famous Boudin Bakery in San Francisco — where it had been shaping the city's iconic sourdough since the Gold Rush.
Unlike other LAB commonly found in sourdough starters around the world, L. sanfranciscensis is metabolically distinctive. It ferments maltose extremely efficiently and produces both lactic and acetic acid — but in a ratio that skews sharper than most European starters. That specific chemistry is what gives San Francisco sourdough its characteristic bright, clean tang and complex mouthfeel.
Why It Produces a Tangier Flavour
Every sourdough starter is essentially a community — wild yeast for leavening, and lactic acid bacteria for flavour. The ratio of acetic acid (sharper, vinegar-like) to lactic acid (softer, yoghurt-like) largely determines how tangy a loaf tastes. L. sanfranciscensis is unusually good at producing acetic acid alongside lactic acid, giving finished loaves that hallmark sharp bite that mild European starters simply cannot replicate.
The result: a loaf with more depth, more length on the palate, and a flavour that people immediately recognise as San Francisco style — even if they have never set foot in California.
The History Behind the Bacteria
San Francisco's association with sourdough goes back to the Gold Rush of 1849, when miners baked their bread from cultures they carried in pouches — hence the nickname "sourdoughs" for the miners themselves. The city's mild, foggy climate and coastal microbiome created near-perfect conditions for a very particular microbial community to thrive. Over 170 years later, that community — dominated by L. sanfranciscensis — is still baking the world's most famous sourdough bread.
When Boudin Bakery had its culture scientifically analysed in the 1970s, the microbiologists gave the species its name in tribute to the city where it was first identified. Today it is recognised as the defining LAB of authentic San Francisco sourdough, prized by artisan bakers worldwide.
Why Most Sourdough Starters Are Unverified
Here is the uncomfortable truth about most starters sold in the UK: no one has ever tested what is actually inside them. Sellers ferment a batch of flour and water, decide it looks lively, and ship it. The microbial community may contain L. sanfranciscensis. It may contain a completely different LAB species. It may vary batch to batch. You have no way of knowing.
For a home baker chasing a specific flavour profile, that is a real problem. You might buy a "San Francisco style" starter and end up with something milder, funkier, or simply inconsistent — because the label is a marketing claim rather than a verified specification.
How Dough Dough Verified Its Culture
We took a different approach. Dough Dough commissioned independent genetic sequencing of our starter culture through an accredited laboratory. The sequencing confirmed that Lactobacillus sanfranciscensis is the dominant lactic acid bacteria in our culture — the same organism found in the great San Francisco bakeries. We have the lab report to prove it.
That is why Dough Dough is now the only UK sourdough starter you can buy with confidence that you are getting authentic San Francisco microbiology. Combined with our proprietary preservation process that captures the culture at its biological peak, you get bakeable, verified sourdough authenticity — activated in just 2 hours, at home.
Ready to Taste the Difference?
Shop the lab-verified starter and start baking real San Francisco sourdough today. Prefer the deeper dive first? Read about our culture and lab testing to see exactly what our sequencing found.
Related Reading
Ready to buy? See our companion guide: Where to Buy an Authentic San Francisco Sourdough Starter in the UK
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Frequently asked questions about Lactobacillus sanfranciscensis
What is Lactobacillus sanfranciscensis?
Lactobacillus sanfranciscensis (recently reclassified as Fructilactobacillus sanfranciscensis) is a lactic acid bacterium first identified in San Francisco sourdough starters in 1971. It ferments the maltose in wheat and rye flour, producing the lactic and acetic acids that give San Francisco sourdough its distinctive tangy flavour.
How is Lactobacillus sanfranciscensis different from other Lactobacillus?
Most Lactobacillus species prefer glucose, but L. sanfranciscensis is specialised for maltose - the sugar released when flour enzymes break down starch. This gives it a competitive advantage in sourdough starters, where maltose is the dominant sugar. It also thrives at the slightly acidic pH range of a mature starter (3.8-4.5).
Is Lactobacillus sanfranciscensis only found in San Francisco?
No. Despite the name, the species is found in mature sourdough starters worldwide. It was first characterised in San Francisco because that's where researchers Kline and Sugihara isolated and studied it, but modern DNA sequencing has confirmed its presence in traditional bakeries across Europe, Asia and the Americas.
How can I tell if my sourdough contains Lactobacillus sanfranciscensis?
The only way to know for certain is laboratory analysis - typically 16S rRNA sequencing or targeted PCR. You cannot identify it by taste or appearance alone, though a distinctive sharp, tangy flavour with mild acidity is often associated with its presence. Commercial suppliers offering "lab-verified" starters should be able to show you the sequencing report.
What flavour does Lactobacillus sanfranciscensis produce?
L. sanfranciscensis is heterofermentative - it produces both lactic acid (mild, yoghurt-like tang) and acetic acid (sharper, vinegar-like tang). The ratio depends on hydration, temperature and feeding schedule. Cooler, stiffer starters tend to produce more acetic acid; warmer, wetter ones favour lactic acid.
Can Lactobacillus sanfranciscensis survive long-term storage?
Under the right conditions, yes. The species survives dehydration and freezing when protected by sugars and cell-wall polysaccharides from the flour matrix. Shelf-stable formats (below 15% water activity) can preserve viability for months. Reactivation requires patient re-feeding with a maltose-rich flour (typically wholemeal or rye).
What temperature does Lactobacillus sanfranciscensis prefer?
The optimal growth range is 20-30°C, with peak activity around 27°C. Below 15°C the bacteria slow dramatically; above 40°C they begin to die. This is why most bakers keep their active starter on the counter rather than in the fridge, and why cold retard slows sourdough fermentation.
How was Lactobacillus sanfranciscensis first identified?
Leo Kline and Tomoya Sugihara at the USDA Western Regional Research Center published the first characterisation in 1971 (Applied Microbiology 21:459-465). They isolated the strain from commercial San Francisco sourdough starters. It was formally named Lactobacillus sanfrancisco, then reclassified as L. sanfranciscensis, and most recently placed in the new genus Fructilactobacillus in 2020.
Do all sourdough starters contain Lactobacillus sanfranciscensis?
No. Many home-made starters develop different Lactobacillus species depending on the flour, water, temperature and starter environment. L. sanfranciscensis is typical of mature, established starters that have been fed regularly for months or years, but younger starters or those fed exclusively on refined white flour often host different dominant species.
What flour supports Lactobacillus sanfranciscensis best?
The bacteria thrive on maltose, which is released by flour enzymes (alpha and beta amylase) breaking down starch. Wholemeal wheat and rye flours contain more of these enzymes than white flour, so they support faster L. sanfranciscensis growth. Feeding a starter a mix of wholemeal and white flour tends to encourage this species to dominate.
References
- Kline, L. & Sugihara, T. F. (1971). Microorganisms of the San Francisco sour dough bread process. II. Isolation and characterization of undescribed bacterial species responsible for the souring activity. Applied Microbiology, 21(3), 459-465.
- Gänzle, M. G. (2014). Enzymatic and bacterial conversions during sourdough fermentation. Food Microbiology, 37, 2-10.
- Zheng, J. et al. (2020). A taxonomic note on the genus Lactobacillus: Description of 23 novel genera. International Journal of Systematic and Evolutionary Microbiology, 70(4), 2782-2858.
- Foschino, R. et al. (1999). Comparison of Lactobacillus sanfranciscensis strains isolated from different sourdoughs. Systematic and Applied Microbiology, 22(4), 656-664.
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