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Saccharomyces boulardii is a totally unique live culture as it's the only probiotic yeast. S. boulardii takes its name from Henri Boulard who discovered the yeast in South East Asia in 1923 upon the skins of lychee fruit. This article looks in more depth at the subsequent taxonomic classification of S. boulardii.

Historically: Saccharomyces boulardii was a species

When Saccharomyces boulardii was first discovered it was identified as a yeast, and was therefore classified under the genus Saccharomyces. Within this genus it was given its own species, as its physiological characteristics were thought to be unlike any other yeast.1 It was therefore named:

Saccharomyces [genus] boulardii [species]

Modern day: Saccharomyces boulardii is most likely a strain of the Saccharomyces cerevisiae species

Since early 2000, with advancements in laboratory testing, Saccharomyces boulardii (S. boulardii) has been shown to have certain genomic and structural similarities to the species Saccharomyces cerevisiae. As such it has been moved from being a species on its own, to being a member of the species S. cerevisiae. Quotes from the following papers confirm this:
‘Whether S. boulardii is different from S. cerevisiae was a matter of debate; this debate is now over. Despite certain phenotypic differences, genotypic and proteomic analyses have definitively recognized S. boulardii as a member of the species S. cerevisiae.'2 ...'Taxonomically however, S. boulardii is not accepted as a distinct species. The protein fingerprint obtained after sodium dodecyl sulfate-polyacrylamide gel electrophoresis was identical for all isolates and therefore confirmed the designation of S. boulardii to the species S. cerevisiae.'3...'The results of the present study strongly indicate a close relatedness of S. boulardii to S. cerevisiae and thereby support the recognition of S. boulardii as a member of S. cerevisiae and not as a separate species'.4
Saccharomyces boulardii, originally extracted from fruit

Therefore, the classification name and nomenclature was changed, and is now Saccharomyces cerevisiae var boulardii, with the breakdown:

Saccharomyces [genus] cerevisiae [species] var boulardii [strain]

The ‘boulardii’ part of the name has now changed from being the species name, to what must be the strain (see the Probiotics Learning Lab for definition) name. A strain is the lowest possible classification of the characteristics of a microorganism. This part of the name gives us the most detailed picture as to the exact qualities of a microorganism. Provided a high quality supplier is being used, from one capsule of S. boulardii to another, the active ingredient and live microorganisms will all have the same genotypic and phenotypic qualities.

The following studies have conducted specific tests on the genomic and molecular characteristics of S. boulardii, and have concluded that it is a strain of S. cerevisiae:

S. boulardii is a strain of S. cerevisiae with some specific genome features including chromosome IX trisomy and different copy numbers of some genes1...The probiotic yeast S. boulardii, classified originally as a separate species, has recently been shown to be a strain of S. cerevisiae, regardless of its strain-specific differences.'1...'Therefore, we conclude that S. boulardii is not a distinct species but is a strain of S. cerevisiae that has lost all intact Ty1/2 elements.'5...'The results of the present study show that these S. boulardii strains are asporogenous strains of the species S. cerevisiae, not representatives of a distinct and separate species, and possess moderate virulence in murine models of systemic infection.'6

b longum
Other probiotics, such as B. longum Rosell-175, pictured above, differ to S. boulardii in shape, size, genetic structure, and health giving properties.

Saccharomyces boulardii as a species or subspecies

Whilst one of the papers above states that ‘the debate is now over’, unfortunately there still remains some inconsistency with regards to classification of this microorganism. This could be in part because S. boulardii is a yeast, whereas probiotic suppliers or manufacturers most often handle bacteria, which is arguably more easily classified. It is worth noting that certain manufacturers seem to refer to S. boulardii as a species, and the taxonomical information which follows it (eg. CNCM-I-1079 or Unique28) as the strain; however we would deem this as incorrect, as the evidence above points to ‘boulardii’ being the strain itself. Saccharomyces boulardii is also sometimes referred to as a ‘sub-species’ of S. cerevisiae, for example by some American supplement suppliers7, though some deem this to be incorrect, and misleading to the consumer. We are unable to find any scientific papers which specifically investigate classification and which describe boulardii as a sub-species. Furthermore referring to S. boulardii as a sub-species this would lead one to believe there is further differentiation into strains. As discussed above S. boulardii cannot be further differentiated, and is therefore a strain in itself.

Can the taxonomy go further than the term ‘Saccharomyces boulardii’?

When it comes to the nomenclature of a probiotic, the name of the strain is regarded as the most salient information for trade and consumers. This is because, even within a single species, there can be large variations between different strains of probiotics in terms of their physical properties. As explored in the previous section, in Saccharomyces boulardii, the term ‘boulardii’ indicates the strain, and therefore the most specific part of the name.

Occasionally additional letters and numbers appear after the strain name, for example S. boulardii MTCC 5375; in this case ‘MTCC 5375’ is the reference number for the microbiology bank in which the strain is deposited. It is not necessary to include the bank reference name on the packaging of this product, as it does not provide the consumer with any additional information on the qualities and characteristics of the product. Very few, if any, other companies seem to include the bank reference number on packaging or any consumer-facing materials.

Another type of classification which can follow the strain name is the name given to a particular microorganism by a manufacturer. For example the S. boulardii used by Wren Laboratories Ltd is sometimes called ‘Unique 28’. Again, this additional information is deemed insignificant (and unnecessary for the consumer) in terms of identity and characteristics of the microorganism; it simply denotes the laboratory from which this microorganism originates.

It is fairly common to find S. boulardii with different bank reference numbers or manufacturing names on the market. The first S. boulardii to ever be isolated has the reference number Hansen CBS 5926, and is supplied by Biocodex. Other high quality S. boulardii microorganisms since then have shown a high level of similarity if not complete similarity to this original isolate, using both laboratory techniques to sequence the genome9 and clinical trials to indicate mechanism of actions in the body. Clinically, S. boulardii from Unique Biotech Ltd has been shown to give very similar outcomes to the trials conducted using the named Biocodex S. boulardii10. This clinical trial involved patients with acute diarrhoea, and measured outcomes such as duration of diarrhoea, frequency of defaecation, abdominal pain and consistency of stool. There are over 80 clinical trials on S. boulardii, in most cases it has seemingly been sourced from the Biocodex laboratories. More than 15 of these trials investigate acute diarrhoea, measuring the same parameters to that in the Unique 28 trial, and give very similar outcomes.11,12,13,14,15,16,17,18,19,20,21,22,23,24,25 This provides further assurance that at the genomic, molecular and clinical level, we are dealing with the same microorganism strain.

Also note that occurrences of Saccharomyces boulardii lyo refer to the lyophilisation of the probiotic and are not refering to a specific strain.

Omission of ‘cerevisiae’ in Saccharomyces boulardii

As aforementioned, the full name for S. boulardii is Saccharomyces cerevisiae var boulardii.
S. boulardii is a unique microorganism, by virtue that as a strain it has vastly different properties to other yeasts in the same S. cerevisiae species. For example, baker’s yeast and brewer’s yeast, which are also in the S. cerevisiae species, are known to invoke intolerances in some people.26 Inclusion of the species name in this case could result in consumer confusion – as consumers might understand S. boulardii to be the same as brewer’s yeast, which is not the case. This is perhaps why, in fact, S. boulardii was previously classified as a separate species – as it has such different properties to the S. cerevisiae species. It may be worth noting that certain experts in the field of microbiology still believe S. boulardii should be classified separately to ‘cerevisiae’.27 Furthermore, for the vast majority of S. boulardii on the market, the species name ‘cerevisiae’ is not listed on packaging and information material.28


Having reviewed scientific papers (both regarding classification in particular, as well as clinical trials examining the effect of S. boulardii) it is understood that S. boulardii is a strain of yeast, and that in the full name Saccharomyces cerevisiae var boulardii, the term ‘var’ is interchangeable with ‘strain’.

Healthcare practitioners may be interested to read more about the scientific research behind Saccharomyces boulardii on the Probiotics Database.

Last updated 26/4/18.
Notes & References:
1. Edwards-Ingram L, Gitsham P, Burton N, Warhurst G, Clarke I, Hoyle D, Oliver SG, Stateva L (2007) Genotypic and physiological characterization of Saccharomyces boulardii, the probiotic strain of Saccharomyces cerevisiae. Appl Environ Microbiol 73 8 2458-67
2. Herbrecht R, Nivoix Y (2005) Saccharomyces cerevisiae fungemia: an adverse effect of Saccharomyces boulardii probiotic administration. Clin Infect Dis 40 11 1635-7
3. Mitterdorfer G, Mayer HK, Kneifel W, Viernstein H (2002) Protein fingerprinting of Saccharomyces isolates with therapeutic relevance using one- and two-dimensional electrophoresis. Proteomics 2 1532-8
4. van der Aa KA, Jespersen L (2003) The taxonomic position of Saccharomyces boulardii as evaluated by sequence analysis of the D1/D2 domain of 26S rDNA, the ITS1-5.8S rDNA-ITS2 region and the mitochondrial cytochrome-c oxidase II gene. Syst Appl Microbiol 26 564-71
5. Edwards-Ingram L, Gent M, Hoyle D, Hayes A, Stateva L, and Oliver S (2004) Comparative genomic hybridization provides new insights into the molecular taxonomy of the Saccharomyces sensu stricto complex. Genome Res 14 1043-1051
6. McCullough M, Clemons K, McCusker J, and Stevens D (1998) Species identification and virulence attributes of Saccharomyces boulardii (nom. inval.). J Clin Microbiol 36 2613-2617
7. Now Foods; Jarrow Formulas; BIOCeuticals
8. Leading supplements brands which do not include the name of their strains on packaging or marketing materials: Viridian, Higher Nature, Terra nova and New Chapter. Solgar, perhaps the largest supplement company selling in the UK, has very recently starting naming the strains that they use.
9. Unique Biotech Ltd, ‘Saccharomyces boulardii – Info & molecular characterisation’ Internal Report, received April 2010
10. Sudha MR, Bhonagiri S, Kumar MA (2012) Oral consumption of potential probiotic Saccharomyces boulardii strain Unique 28 in patients with acute diarrhoea: a clinical report. Benef Microbes 3 2 145-50
11. Htwe K, Yee KS, Tin M, Vandenplas Y (2008) Effect of Saccharomyces boulardii in the treatment of acute watery diarrhea in Myanmar children: a randomized controlled study. Am J Trop Med Hyg 78 2 214-6.
12. Urganci N, Polat T, Uysalol M, Çetinkaya F (2001) Evaluation of the efficacy of Saccharomyces boulardii in children with acute diarrhoea. Arch Gastroenterhepatol 20 3–4
13. Villarruel G, Rubio DM, Lopez F, Cintioni J, Gurevech R, Romero G, Vandenplas Y (2007) Saccharomyces boulardii in acute childhood diarrhoea: a randomized, placebo-controlled study. Acta Paediatr 96 4 538-41
14. Hochter W et al. (1990) Saccharomyces boulardii in acute adult diarrhoea. Efficacy and tolerance of treatment. Münchner Medizinische Wochenschrift 132 12 188-192
15. Cetina-Sauri G, Basto GS (1994) Evaluation thérapeutique de Saccharomyces boulardii chez des enfants souffrant de diarrhée aiguë [Evaluation of Saccharomyces boulardii for the treatment of acute diarrhoea in pediatric patients] Ann Pediatr 41 397-400
16. Kurugöl Z, Koturoğlu G (2005) Effects of Saccharomyces boulardii in children with acute diarrhoea Acta Paediatr 94 1 44-7
17. Hafeez A, Tariq P, Ali S, Kundi ZU, Khan A, Hassan M (2002)The efficacy of Saccharomyces boulardii in the treatment of acute watery diarrhoea in children: a multicentre randomized controlled trial. J Coll Phys Surg Pakist 12 432–4
18. Dinleyici EC, Eren M, Yargic ZA, Dogan N, Vandenplas Y (2009) Clinical efficacy of Saccharomyces boulardii and met*******ole compared to met*******ole alone in children with acute bloody diarrhea caused by amebiasis: a prospective, randomized, open label study. Am J Trop Med Hyg 80 6 953-5
19. Canani RB, Cirillo P, Terrin G, Cesarano L, Spagnuolo MI, De Vincenzo A, Albano F, Passariello A, De Marco G, Manquso F, Guarino A (2007) Probiotics for treatment of acute diarrhoea in children: randomised clinical trial of five different preparations. BMJ 335 7615 340
20. Riaz M, Alam S, Malik A, Ali SM (2012) Efficacy and safety of Saccharomyces boulardii in acute childhood diarrhea: a double blind randomised controlled trial. Indian J Pediatr 79 4 478-82
21. Corrêa NB, Penna FJ, Lima FM, Nicoli JR, Filho LA (2011) Treatment of acute diarrhea with Saccharomyces boulardii in infants. J Pediatr Gastroenterol Nutr 53 5 497-501
22. Le Luyer B, Makhoul G, Duhamel JF (2010) [A multicentric study of a lactose free formula supplemented with Saccharomyces boulardii in children with acute diarrhea]. [Article in French] Arch Pediatr 17 5 459-65.
23. Eren M, Dinleyici EC, Vandenplas Y (2010) Clinical efficacy comparison of Saccharomyces boulardii and yogurt fluid in acute non-bloody diarrhea in children: a randomized, controlled, open label study. Am J Trop Med Hyg 82 3 488-91
24. Chapoy P (1985) [Treatment of acute infantile diarrhea: controlled trial of Saccharomyces boulardii]. [Article in French] Ann Pediatr (Paris) 32 6 561-3
25. Hernandez CL, Pineda EE, Jimenez MIR, Lucena MS (1998) Clinical therapeutic affect of Saccharomyces boulardii on children with acute diarrhea. Revista de Enfermedades Infecciosas en Pediatria 11 43 87-9
26. Allergy UK:
27. McFarland LV (1996) Saccharomyces boulardii is not Saccharomyces cerevisiae. Clin Infect Dis 22 1 200-1. This author is determined that S. boulardii is a completely different species of yeast from baker's, brewer's, or wine yeast. (Note the date 1996 – this has since been overturned)
28. Florastor, Ultra-levure, Cytoplan & Metagenics.

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