OVERVIEW OF PROBIOTIC CONCEPT
15.3 OVERVIEW OF PROBIOTIC CONCEPT
Current definition restricts probiotic classification to live microorganisms that, upon ingestion, elicit beneficial effects on the intestinal balance of the host. 3 Currently, most of the microorganisms corresponding to the definition of probiotics are from bifidobacteria and lactobacilli genera (Table 15.1).
Enteroccocus spp., Escherichia coli, and yeast such as Saccharomyces boulardii and Saccharomyces cerevisiae have also reported probiotic effects, but their positive effects are strongly counterbalanced by safety risks; therefore, there is much debate as to their compliance to the definition of probiotic stricto sensu.
Evidence supporting prophylactic action of characterized probiotics strains such as Lactobacillus rhamnosus and Lactobacillus fermentum in the vagina 18 may lead to extension of the probiotic definition to their use for topical application on tissues other than the digestive tract. It has been suggested that strains only be classified as
probiotic once their beneficial effects have been established in vivo. 19 This accounts for the fact that probiotic traits are generally specific to a particular strain and may not be applicable to all probiotics. 20
Functional Food Carbohydrates
TABLE 15.1 Probiotic Bacteria with Documented Clinical Effects
Bacterial Group Reference Bifidobacteria
Bifidobacterium bifidum Marteau et al. 4 Bifidobacterium breve Yakult strain
Shimakawa et al. 5 Bifidobacterium lactis Bb-12
Isolauri et al. 6 Bifidobacterium longum
Kiessling et al. 7
Lactobacilli
Lactobacillus acidophilus NCFM Sui et al. 8 Lactobacillus casei immunitass DN114001
Faure et al. 9 Lactobacillus casei Shirota (YIT 0918)
Aso and Akazan 10 Lactobacillus gasseri
Pedrosa et al. 11 Lactobacillus johnsonii
Marteau et al. 12 Lactobacillus LA-1
Bernet et al. 13 Lactobacillus LB
Xiao et al. 14 Lactobacillus plantarum 299v
Molin 15
Lactobacillus reuteri Casas and Dobrogosz 16 Lactobacillus rhamnosus GG (ATCC 53103)
Goldin et al. 17
15.3.1 T YPES OF P ROBIOTIC P RODUCTS
In the design of new product or dietary management strategy, the array of probiotic strains currently available is a notable advantage. Several strains can be combined in the same product. Food vehicles range from fermented milks, dairy products, and fruit juices to various forms of freeze-dried supplements (capsule, pills). Survival of live bacteria is, however, an issue. Probiotics are usually anaerobic bacteria and do not survive well during temperature changes. Fermented milks, for example, need refrigeration, which is a burden to their distribution in developing countries. Strain efficacy after freeze drying is debatable. 21
15.3.2 S URVIVAL OF P ROBIOTICS S TRAINS
A point that is sometimes argued is the necessity for the probiotic to be alive not only at the time of ingestion, but also at the site of action, namely, the large bowel. This implies a resistance of the probiotic strains to chemical changes and secretions occurring during transit through the stomach and upper intestine. Some studies have shown that a stimulation of the immune system could be displayed by dead probiotic cells. 22,23 This passive mechanism, although desirable, is less potent than active health-promoting effects elicited by live microorganisms. For example, live probi-
otics strains may adhere more strongly to the intestinal mucosa 24 or produce bacte- riocins against potential pathogens. 25 Determination of survivability in vivo still requires clarification. There is often confusion between the transient, persistent, and colonizing effects of studied strains. Gastric acid, bile salts, and pancreatic secretions are all barriers toward long-term persistence of probiotics. Most commercial probi-
Probiotics, Prebiotics, and Synbiotics
otics have been tested in vitro for their resistance to gastric acidity and bile salts. 26,27 Fewer data are available relative to the survivability in situ (Table 15.2). Survival varies considerably between strains belonging to the same genus. Lactobacillus strains, for example, generally survive well in vitro in the presence of acidic pH and
bile acid, confirming their strong potential as probiotics. 27 These specific traits may, however, not be sufficient to determine survivability in situ. Additional properties may be needed for effective competition of probiotic strains against the diversity of the indigenous microflora. Cell attachment and antimicrobial activities of the pro- biotic candidate may contribute greatly to the bacterial survival in vivo. The envi- ronment from which the probiotic strain originates may also affect chances of survival. In a study in vitro comparing the survival of 47 strains of Lactobacillus spp., strains isolated from the human gut were found to display a better survivability
than probiotic strains isolated from food or dairy products. 20 Subsequently, five strains selected from the screening in vitro were studied in a feeding experiment. Findings confirmed that intestinal strains Lactobacillus rhamnosus, Lactobacillus reuterii , and Lactobacillus GG were persisting in the intestinal tract in higher numbers than dairy strains Lactobacillus casei subsp. alactus and Lactobacillus delbrueckii subsp. lactis. 20
15.3.3 P ROBIOTIC P ERSISTENCE :D OSE E FFECT
The maximum amount of probiotics that can be ingested at each dose has not yet been Concentrated doses of 10 10 colony-forming units (CFU) per day of Lactoba- cillus rhamnosus GG and Lactobacillus johnsonii La1 have been administrated to healthy volunteers with no adverse effect. 30,31
A dose–response study using Lacto- bacillus johnsonii La1 showed that a minimum amount of 10 10 CFU per day was required to observe an immune response. 32 A highly concentrated probiotic prepa- ration (VSL#3) containing eight strains of probiotics is currently gaining interest. 33 When a dose of 3*10 12 per day of VSL#3 was administered to irritable bowel disease (IBD) patients in clinical trials, recorded side effects were deemed minor to nonex- istent. 34–36 The administration of lower doses of probiotics in the range of 10 6 to 10 9 CFU per day may limit the viability of the probiotic in the colonic environment unless an appropriate delivery vehicle is administered. 37
15.3.4 T RANSFER OF A NTIMICROBIAL R ESISTANCE
There is some evidence that Enterococcus spp. could be applied as a probiotic therapy. 38 However, caution must be maintained because some strains of enterococci have been associated with nosocomial and antibiotic infections. One example is the emergence of a pathogenic vancomycin resistance enterococcus, which is known to cause severe infections in patients who have catheters, intravenous devices, or are
undergoing dialysis. 39 The use of enterococci as a probiotic may raise health con- cerns. Such antibiotic resistance genes could be spread widely throughout the gut microbiota to susceptible recipient commensal bacteria. 40 It has been suggested that strains used as probiotics should be susceptible to at least two of the common molecules used in human antibiotherapies. 41
484 TABLE 15.2
Studies of Probiotic Survival In Situ
Days of
Experimental
Survival in Feces and Effect on
Probiotic Medium
Feeding
Design
Gut Microflora
Reference
Lactobacillus casei Shirota
125 ml of fermented milk
3 days
Randomized Placebo
Recovery of 10 7 CFU/g feces
Yuki et al. 28
(10 8 CFU/ml)
controlled, n = 8
Bifidobacterium breve strain Yakult
500 ml of fermented soy
14 days
Randomized,
Recovery of 10 9 CFU/g feces;
Shimakawa et al. 5
milk (10 9 CFU/ml)
double-blind,
increase in bifidobacteria count
placebo-controlled trial, n = 15
Lactobacillus casei subsp.
De Champs et al. 29 rhamnosus Lcr 35
10 8 –10 10 CFU/day
7 days
Randomized trial, 3
Recovery up to 3 weeks after
concentrations
trial; increase in Lactobacilli
tested, n = 12
count; greatest variation in individuals with lower base level of Lactobacilli count
Lactobacillus acidophilus NCFM
240 ml of skimmed milk
14 days
Randomized trial,
Recovery during feeding period;
Sui et al. 8
Functional F Lactobacillus reuteri + Lactobacillus
(10 10 CFU/day)
n = 10
rapid decrease in L. acidophilus during washout period
Jacobsen et al. 20 rhamnosus
Recovery of 10 5 –10 8 CFU/g for
granulates/day
per mix
crossover, n = 12
all strains during feeding;
Lactobacillus GG + Lactobacillus
(10 10 CFU/granulate)
recovery during washout period
casei subsp. alactus + Lactobacillus
ood Carboh delbrueckii
only for L. reuteri, L.
rhamnosus , and L. GG
Saxelin et al. 30 (ATCC 53103)
Lactobacillus GG Capsule
7 days
2 concentrations
Recovery up to 3 days after trial;
(10 8 –10 10 CFU/day)
tested in 2 groups,
no change in Lactobacilli count
n = 20
ydrates
Probiotics, Prebiotics, and Synbiotics
15.3.5 S AFETY OF P ROBIOTICS
As live microorganisms, probiotics other than enterococci and yeasts have a proven safety record of use. Probiotic strains traditionally used in fermented milk have been granted GRAS (generally regarded as safe) status. With the emergence of less well known probiotic microorganisms, potential risks may need more careful investigations. The theoretical risk is bacteremia developing from overgrowth of probiotic organisms or indigenous colonic microbiota. Bifidobacteria and lacto-
bacilli are not closely related to recognized human pathogenic bacteria. 40 Cases of infections by lactobacillus and bifidobacteria are extremely rare. 41 Seldom have
cases of translocation of lactobacilli been reported as a result or complication of existing severe medical conditions. 41 Given their generally safe record of use throughout the world, a recent committee of experts has deemed the risk of death by probiotics of lactobacilli or bifidobacteria as negligible. 41