Fructilactobacillus sanfranciscensis is a heterofermentative species of lactic acid bacteria which, through the production mainly of lactic and acetic acids, helps give sourdough bread its characteristic taste. It is named after San Francisco, where sourdough was found to contain the variety, though it is dominant in Type I sourdoughs globally.[2][3] In fact, F. sanfranciscensis has been used in sourdough breads for thousands of years, and is used in 3 million tons of sourdough goods yearly.[4] For commercial use, specific strains of F. sanfranciscensis are grown on defined media, freeze-dried, and shipped to bakeries worldwide.
| Fructilactobacillus sanfranciscensis | |
|---|---|
Scientific classification 
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| Domain: | Bacteria |
| Phylum: | Bacillota |
| Class: | Bacilli |
| Order: | Lactobacillales |
| Family: | Lactobacillaceae |
| Genus: | Fructilactobacillus |
| Species: | F. sanfranciscensis
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| Binomial name | |
| Fructilactobacillus sanfranciscensis (Weiss and Schillinger 1984) Zheng et al. 2020
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| Synonyms[1] | |
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Overview
editFructilactobacillus sanfranciscensis was first known to be isolated in 1971 by Kline and Sugihara. As lactic acid bacteria, the strains are Gram-positive, slender, rod-shaped, nonsporulating, and non-motile.[5] They are also obligately heterofermentative, meaning that they can convert hexose sugars into not just lactic acid, but also ethanol, CO2, and/or acetic acid.[6] This heterofermentative ability is key for this species’ role in creating the unique flavor of sourdough bread.
Sourdough starters are leavened by a mixture of yeast and lactobacilli in a ratio of about 1:100. The yeast is most commonly Kasachstania humilis (formerly Candida humilis or C. milleri). This yeast cannot metabolize the maltose found in the dough, while the Fructilactobacillus requires maltose.[7] They therefore act without conflict for substrate, with lactobacilli utilizing maltose and the yeast utilizing the other sugars, including the glucose produced by the F. sanfranciscensis.
Growth Conditions
editExternal conditions such as acidity and temperature affect the growth rates of F. sanfranciscensis. A temperature of 33 °C (91 °F) leads to maximum growth rates, whereas temperatures over 41 °C (105 °F) completely inhibit the bacteria growth. And in terms of pH, most strains can tolerate levels as low as 3.6, but the optimal range for growth is slightly higher (around 4-5) as it is also the optimum pH for some of the key proteins involved—for example, those involved in maltose transport function optimally at 5.2-5.6.[6] However, there is lots of intraspecies diversity within Fructilactobacillus sanfranciscensis, so the optimal temperature and pH for growth will vary from strain to strain, and depend on a variety of factors—namely, the type of carbon source for metabolism, and the resulting proteins involved. For instance, a common yeast in sourdough, K. humilis, prefers 27 °C (81 °F) and will not grow above 36 °C (97 °F).[8]
Genome
editThe genomes of Fructilactobacillus sanfranciscensis strains are often quite small—in fact, they are suggested to be the smallest of all the lactobacilli.[9] It is even thought that many genes within F. sanfranciscensis (that are also present in other heterofermentative lactobacilli) were lost or deleted via mutation. However, despite this loss of genes and overall smaller genome size, the F. sanfranciscensis genomes are relatively dense in ribosomal RNA (rRNA) operons, which contributes to more rapid growth and protein production. Additionally, the smaller genome allows for a significant amount of metabolic energy to be conserved.[10] Overall, the genome length can vary from strain to strain; some can have more plasmids than others, some have slightly longer circular chromosomes, etc. But most strains share this characteristically small genome with a high density of rRNA operons, which allow for relatively fast growth rates.
References
edit- ^ Zheng, Jinshui; Wittouck, Stijn; Salvetti, Elisa; Franz, Charles M.A.P.; Harris, Hugh M.B.; Mattarelli, Paola; O’Toole, Paul W.; Pot, Bruno; Vandamme, Peter; Walter, Jens; Watanabe, Koichi (2020). "A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae". International Journal of Systematic and Evolutionary Microbiology. 70 (4): 2782–2858. doi:10.1099/ijsem.0.004107. hdl:10067/1738330151162165141. ISSN 1466-5026. PMID 32293557.
- ^ Gänzle, Michael G.; Zheng, Jinshui (2019-08-02). "Lifestyles of sourdough lactobacilli - Do they matter for microbial ecology and bread quality?". International Journal of Food Microbiology. 302: 15–23. doi:10.1016/j.ijfoodmicro.2018.08.019. ISSN 1879-3460. PMID 30172443. S2CID 52143236.
- ^ De Vuyst, Luc; Van Kerrebroeck, Simon; Leroy, Frédéric (2017). "Microbial Ecology and Process Technology of Sourdough Fermentation". Advances in Applied Microbiology. 100: 49–160. doi:10.1016/bs.aambs.2017.02.003. ISBN 9780128120484. ISSN 0065-2164. PMID 28732554.
- ^ Rudi F. Vogel; Melanie Pavlovic; Matthias A. Ehrmann; Arnim Wiezer; Heiko Liesegang; Stefanie Offschanka; Sonja Voget; Angel Angelov; Georg Bocker; Wolfgang Liebl (1 September 2011). "Genomic analysis reveals Lactobacillus sanfranciscensis as stable element in traditional sourdoughs". Microbial Cell Factories. 10 (Suppl 1): S6. doi:10.1186/1475-2859-10-S1-S6. PMC 3231932. PMID 21995419.
- ^ Kline, L.; Sugihara, T. F. (March 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. doi:10.1128/am.21.3.459-465.1971. ISSN 0003-6919. PMC 377203. PMID 5553285.
- ^ a b Gobbetti, M.; Corsetti, A. (April 1997). "Lactobacillus sanfrancisco, a key sourdough lactic acid bacterium: a review". Food Microbiology. 14 (2): 175–187. doi:10.1006/fmic.1996.0083 – via Elsevier Science Direct.
- ^ Neubauer H, Glaasker E, Hammes WP, Poolman B, Konings WN (1994). "Mechanism of maltose uptake and glucose excretion in Lactobacillus sanfrancisco". J Bacteriol. 176 (10): 3007–12. doi:10.1128/jb.176.10.3007-3012.1994. PMC 205458. PMID 8188601.
- ^ Ganzle MG, Ehmann M, Hammes WP (1998). "Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri in Response to Process Parameters of Sourdough Fermentation". Appl Environ Microbiol. 64 (7): 2616–2623. Bibcode:1998ApEnM..64.2616G. doi:10.1128/AEM.64.7.2616-2623.1998. PMC 106434. PMID 9647838.
- ^ Vogel, R.F.; Pavlovic, M.; Ehrmann, M.A.; Wiezer, A.; Liesegang, H.; Offschanka, S.; Voget, S.; Angelov, A.; Böcker, G.; Liebl, W. (September 2011). "Genomic analysis reveals Lactobacillus sanfranciscensis as stable element in traditional sourdoughs". Microbial Cell Factories. 10 (Supplementary 1): S6. doi:10.1186/1475-2859-10-S1-S6. PMC 3231932. PMID 21995419.
- ^ Rogalski, E.; Ehrmann, M.A.; Vogel, R.F. (February 2021). "Intraspecies diversity and genome-phenotype-associations in Fructilactobacillus sanfranciscensis". Microbiological Research. 243. doi:10.1016/j.micres.2020.126625. PMID 33129664 – via Elsevier Science Direct.