Lactobacillus sanfranciscensis
Introduction
This page was curated by Kay Burrows for Dr. Joan Slonczewski's Microbiology Class, Spring 2018.
Lactobacillus sanfranciscensis is a rod-shaped, Gram positive, lactic acid bacteria, closely related to the Lactobacillus casei-Pediococcus group. It is nonsporulating, nonrespiring, acid- and aero-tolerant. Famous for being an important flavor component of sourdough bread, L. sanfranciscensis is an obligate, heterofermentative bacteria. Its fastidiousness arises from a symbiotic relationship with different yeasts, including Saccharomyces cerevisiae and Candida milleri. This yeast, also useful in the making of sourdough, provides L. sanfranciscensis with amino acids and peptides to aid in growth and fermentation. During fermentation, the microbe consumes maltose and glucose to ultimately release acids that create the sour taste in sourdough.[1]
There are over 180 species of Lactobacillus currently documented. Lactobacillus can be found in the human reproductive system as well as the urinary and digestive tracts. A major contributor to gut flora, Lactobacilli have numerous beneficial roles in the human gastrointestinal tract, including immunomodulation and nutrient breakdown. More than 50 species of Lactobacilli are known as useful non-pathogenic bacteria for the processing of different foods, or for medical application in restoring normal, healthy flora to the skin, vaginal biofilm, and digestive tract. Many species within Lactobacillus have a high hydrogen peroxide tolerance, lending further to their role in inhibiting potential pathogens. Among this genus, of course, is L. sanfranciscensis, containing a very small genome and a relatively high ratio of rRNA operons, crucial for rapid growth. Unlike the majority of homofermentative lactobacilli, L. sanfranciscensis uses heterofermentative metabolism, processing sugars into either lactic acid or alcohol. This unique processing is what contributes to its role in sourdough formation.
Ultimately, we will aim to answer the question: How do we form the ideal sourdough using these microbes?
History
Though humans have been making bread with flour and water for 5,000 years, the science behind this magical process was uncovered much later. In the 1800s, Louis Pasteur uncovered the importance of yeast and fermentation in the famous bread rise. The most robust of these yeast for this purpose, Saccharomyces cerevisiae, has since been purified and mass produced for use in bread products around the world. Microbes that contribute to the flavor and characteristics of different breads, however, are more mysterious. Different starters can contain a multitude of bacteria and yeast in differing proportions to give breads a recognizable flavor. These components were once reliant on the available microbes in the region in which the bread was produced, but are now available worldwide. Lactobacillus sanfranciscensis is one of such bacteria that became popular for its distinctive tang contribution-- perfect for sourdough bread. In fact, in a sourdough culture, L. sanfranciscensis can outnumber its yeast counterpart 100 to 1.
Sourdough production began as a method for processing cereals. Without the addition of yeast, ancient grains such as spelt and barley created flat, sour breads.Many of these breads are still made and consumed today, such as Injera, the flat, brown bread popular in Ethiopia. Variations grew from this flat bread, using different grains depending on the region. Rye bread, requiring inhibition of a-amylase and pentosans, is still popular in Central Europe as a result of the spread of sourdough. Once brewing became more popular, sour bread combined with wheat flour and yeast to create the leavened sourdough bread we know today. Since sourdough stayed fresh longer than other breads, it was popularized in the long trips of the California Gold Rush.
Originally, it was assumed that the famous L. sanfranciscensis, discovered and popularized in San Francisco, was unique to the Bay Area. San Francisco was producing great numbers of phenomenally complex sourdoughs, and an effort was made by several groups to uncover the composition of their starters. The results were in, and Lactobacillus sanfranciscensis seemed to be the defining microbe of this west coast phenomenon. It was first isolated by Kline and Sugihara in 1971, who characterized and named the bacteria. However, it was later discovered by the Sourdough Project that L. sanfranciscensis could be found in nearly 90 other countries-- either spread from other successful sourdough starters, or native to the regions themselves. L. sanfranciscensis is, in fact, identical to separately-identified European strain Lactobacillus brevis var. lindneri. The San Francisco name stuck, despite the bacteria’s lack of specificity to that region. Lactobacillus sanfranciscensis in a classic San Francisco sourdough bread is often used with yeast strain Candida milleri. Since C. milleri is particularly acid tolerant, it can handle increasing lactic acid concentrations and lower pH that result from L. sanfranciscensis growth and metabolism. Furthermore, Candida milleri cannot digest maltose from flour starch. This sugar is luckily necessary for L. sanfranciscensis survival, which makes a pairing of this yeast and bacterium particularly symbiotic. Regardless, most larger bakeries still maintain S. cerevisiae as their primary yeast in sourdough breads, due to its simplicity and availability. Some of such bakeries rely on last minute acetic acid additions to achieve the ultimate tangy taste that would have been produced by a more ideal pairing.
Contribution to Sourdough Starter
In sourdough bread making, the starter determines quite a bit of the bread’s flavor, texture, and aroma. Different starters containing unique combinations of bacteria and yeast are responsible for the many of the differences in bread types. In sourdough bread, as we know, the main flavor contributing microbes are lactic acid bacteria, contributing a sharp sour taste from high acidity. Other contributions to the bread’s texture and flavor include the types of flour used, the conditions of the bread rise, and a slew of additional ingredients optional to the breadmaker.
Among sourdough starters, as little as 30% and as many as 95% of the lactic acid bacteria have been recorded to be Lactobacillus sanfranciscensis. This high percentage of L. sanfranciscensis may be due in part to the species’ antimicrobial activity. Though homofermentative bacteria generally have greater antimicrobial properties against coliforms than heterofermentative bacteria, L. sanfranciscensis produces a bacteriocin-like inhibitory substance (BLIS C57) that has bacteriocidal and bacteriolytic properties. All known lactic acid bacteria strains in sourdough, other than Lactobacillus fructivorans, are sensitive to this compound. Therefore, production of BLIS C57 by L. sanfranciscensis likely contributes to its ability to dominate a sourdough starter.
L. sanfranciscensis also contributes to sourdough volatile compounds, important for both flavor and aroma. Though the flavor of the original wheat, temperature during fermentation, and baking contribute the largest amount of aroma to a bread, microbes cannot be underrated. In a study of 87 species of lactic acid bacteria, Gobbetti et al (1995) found that certain types of bacteria contributed different volatile compounds. Without lactic acid bacteria, the breads presented aroma compounds of “acetaldehyde, hexanal, octanal, nonanal and ethylacetate”. These compounds were also found in sourdoughs with lactic acid bacteria, but in significantly higher amounts. Yeast fermentation itself contributes iso-alcohols as well. Table 1 (to be posted) demonstrates the different compounds contributes by specific species of lactic acid bacteria. In general, increased concentrations of ethylacetate, alcohols, and aldehydes were due to contribution of heterofermentative lactic acid bacteria. On the other hand, homofermentative lactic acid bacteria contributes diacetyl and other carbonyls. The study highlights two specific strains, L.breviss and L. plantarum, as two bacteria with particularly extensive contributions to volatile compound profile. Furthermore, both heterofermentative and homofermentative lactic acid bacteria helped enhance the volatile compounds released by yeast alone. This supports that a starter rich in different varieties of lactic acid bacteria can help contribute unique compounds to the bread’s aroma and flavor.
Sourdough starters are often considered like an heirloom-- bakers will start their own starter as a gift or purchase of a population of microbes from another baker. However, research from Ripari (2016) has been conducted to determine the ability to organically start a sourdough starter from other sources. The use of fruits, flowers, or other plants have been used to select for certain microorganisms and encourage microbial evolution in sourdough starters. In their study, the group found that plant material frequently resulted in rapidly stable microbial communities in sourdough starters. Furthermore, in four of the seven organically-started inoculates, L. sanfranciscensis quickly dominated the microbial scene. One interesting addition was that sourdoughs started with apple flowers or pulp saw significant contribution from Acetobacter, not found in any other plant inoculates. Others saw higher populations of L. plantarum, graminis, and rossiae, along with the L. sanfranciscensis. S. cerevisiae was organically cultured in every inoculate.
Section 3
Include some current research, with at least one figure showing data.
Section 4
Conclusion
References
Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2018, Kenyon College.
