Probiotics in Infant Nutrition

Concept of probiotics
The term probiotic was introduced into the scientific literature in the 1965 by Stillwell and Lilly. A widely accepted definition of probiotics is ‘live microbial food ingredients that are beneficial to health’. However, the scientific basis of this definition has recently been questioned since animal studies suggest that some probiotic effects can be achieved by nonviable bacteria and even by isolated bacterial DNA. Therefore, probiotics have more recently been defined as ‘microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well being of the host’ (Salminen S 1999).

Characteristics and mechanism activity of probiotics
An effective probiotic should: 

1. exert a beneficial effect on the host;
2. be nonpathogenic and nontoxic;
3. contain a large number of viable cells; 
4. be capable of surviving and metabolizing in the gut; 
5. remain viable during storage and use; 
6. have good sensory properties; 
7. be isolated from the same species as its intended host.

The microorganisms most frequently used as probiotic agents are lactic acid bacteria (species of Lactobacillus) and nonpathogenic, antibiotic-resistant, ascosporic yeasts, such as Saccharomyces boulardii. Lactobacillus rhamnosus strain GG, which was originally isolated from human intestinal flora, is the most widely studied probiotic agent for adults and children (Gorbach SL 2000).

The establishment of a normal intestinal bacterial flora has important implications for health and disease. The major function of the gut microbiota, from the host’s point of view, is to prevent colonization of the intestine with pathogenic organisms and to inhibit the proliferation of potentially pathogenic microorganisms, increasing the natural resistance to infectious diseases of the intestinal tract. Probiotics exert this effect by preventing the binding of pathogenic bacteria to the enterocyte, either directly by producing antimicrobial compounds or indirectly by altering the pH of the intestinal lumen through the synthesis of short-chain volatile fatty acids.

In addition, the normal flora might stimulate gastrointestinal immunity, enhance mucosal IgA production, stimulate the local production of anti-inflammatory cytokines (Kliegman RM 2005) and reduce the generation of proinflammatory cytokines characteristic of allergic inflammation. A survey of the most relevant studies concerning the use of probiotics in food allergy, atopic dermatitis, and primary prevention of atopy has shown that probiotic therapy alleviates allergic inflammation as demonstrated by the control of clinical symptoms and the reduction of local and systemic inflammatory markers (Miraglia del Giudice M, 2004).
 
Microbial metabolism also serves as an important source of energy for the gut wall, providing up to fifty percent of the daily energy requirements of colonocytes by fermentation of carbohydrates to organic acids, mainly butyrate and, in the case of breastfed infants, lactate. Other postulated health advantages associated with probiotic intake are the alleviation of symptoms of lactose malabsorption, improved digestion, suppression of carcinogenesis and reduction of serum cholesterol concentrations (Collins MD, 1999).

Use of probiotics
Probiotics are increasingly being studied and used in humans. A range of probiotic strains has been evaluated for their antidiarrhoeal capabilities, with varying degrees of success. Bifidobacterium bifidum, given in conjunction with Streptococcus thermophilus in standard milk formula for preventive purposes, has been shown to reduce the incidence of rotavirus diarrhoea (Saavedra JM 1994).  A meta-analysis indicates clearly that Lactobacillus is safe and effective in reducing the duration of acute infantile diarrhoea (Van Niel C 2002) and a systematic review suggests probiotics are clinically beneficial in the treatment of acute infectious diarrhea in infants and children, particularly in viral gastroenteritis (Szajewska H 2001). Lactobacillus GG has shown the most consistent effect, although other probiotic strains also appear to be effective. 

Several probiotic strains have been shown to reduce the incidence and duration of antibiotic-associated diarrhoea, the side effects of “triple therapy” with antibiotics used to eradicate elicobacter pylori from the stomach and the incidence of traveller’s diarrohea. Regular doses of probiotics could work by restoring the resident intestinal flora, thus producing a beneficial effect on the immune system and maintaining the patient’s well-being. Displacement of Clostridia from the intestinal tract may be another key mechanism for the favourable effects of ingested probiotics. Two meta-analyses (D’Souza AL 2002; Cremonini F 2002) have suggested a strong benefit of probiotic administration (in particular for S boulardii and lactobacilli) on antibiotic ssociated diarrhea but, once again, ”further data are needed” concerning the costs of probiotic use and the need for routine use of these agents.  
 
Other studies have examined the possible effects of probiotics in preventing common childhood infections of any type. A recent controlled study (Weizman Z, 2005) compared two different species of probiotic microorganisms, Bifidobacterium lactis (BB-12) and Lactobacillus reuteri, for their efficacy in the prevention of common acute infectious illnesses in child care centers. The authors investigated whether probiotic foods containing either B. lactis or L. reuteri decrease the risk of diarrhea, respiratory symptoms, fever and other ”morbidity parameters” compared with a placebo formula. The probiotic groups had fewer and shorter episodes of diarrhea but there was no difference in effect on respiratory illnesses between groups. These effects were more prominent with L. reuteri. Probiotic supplementation is thought to provide a significant microbial stimulus for the immature immune system thereby confering protection gainst atopic disease, promoting the non-atopic mode of immune responsiveness. Accordingly, it has been suggested that the principal microbial provocation resulting in non-atopic immune maturation in infancy may be derived  from the indigenous intestinal microbiota or nvironmental bacterial products rather from pathogens. As a consequence, the early contact with commensal microbes might have a basic role, not only in establishing a non-atopic immune responder phenotype but also in protecting the host also against infectious and autoimmune diseases. 

Other conditions that may benefit from probiotic use include chronic diarrhea (Xiao SD 2003), inflammatory bowel disease (Schultz M 2004), irritable bowel syndrome (Saggioro A 2004) and food allergy (Majamaa H 1997). Conditions thought to be preventable by probiotic use range from travelers’ diarrhea (Hilton E 1997) and necrotizing enterocolitis (Dani C 2002) to urogenital infections (Cadieux P 2002), atopic diseases (Kalliomaki M 2001) and even dental caries (Nase L 2001). Vanderhoof (2001) has proposed future applications in cystic fibrosis, rheumatoid arthritis and cancers, particularly colorectal cancer.

Use of probiotics in neonates at risk for necrotizing enterocolitis (NEC)
An important rationale for the use of probiotics in neonates at risk for NEC is the observation that very low birth weight (VLBW) infants have aberrant faecal colonization compared with healthy, term infants. The predominant facultative species in the faecal flora of preterm infants undergoing intensive care are staphylococci (coagulase negative Staphylococcus spp and Staphylococcus aureus), enterobacteriaceae (such as Klebsiella spp) and enterococci. Clostridia are the most common anaerobes and bifidobacteria are less common than the flora of healthy breast fed term infants in whom bifidobacteria predominate (Millar M, 2003). VLBW infants often have a paucity (oligocolonization) of normal enteric bacterial species (Bifidobacterium and Lactobacillus) and a delayed onset of colonization compared with term infants (Kliegman RM, 2005). Since microbial invasion of the gut wall may be a contributing cause of NEC, altering microbial flora by enteral feeding of probiotics might be beneficial. Early studies have shown either no effects (Dani C, 2002) or positive effects (Hoyos AB, 1999).

Mechanisms by which probiotics may protect high-risk infants from developing NEC include increasing the barrier to translocation of bacteria and bacterial products across mucosa, competitively excluding potential pathogens, modifying host response to microbial products and enhancing enteral nutrition that inhibits the growth of pathogens such as Klebsiella pneumoniae, Escherichia coli and Candida albicans (Millar M, 2003). Recently, a prospective, masked, randomized control trial was conducted to  evaluate the beneficial effects of two probiotics (Lactobacillus acidophilus and Bifidobacterium infantis) in reducing the incidence and severity of NEC among VLBW (<1500 g) infants. The incidence of NEC was significantly lower in the probiotic vs. the control group (2 of 180 [1.1%] vs. 10 of 187 [5.3%]; P=0.04); however, probiotics alone did not eliminate NEC, which confirms the theory that NEC is a multifactorial disease, of which intestinal colonization with unfavorable organisms is only one factor(Lin HC, 2005). 

Studies in VLBW infants treated with different probiotic preparations have universally emonstrated safety. Nonetheless, one must be cautious in the introduction of any new and living (potentially invasive) microrganism as therapy for immunologically immature VLBW infants. Probiotic therapies occasionally have been associated with adverse effects, such as bacteremia, neonatal sepsis, meningitis or endocarditis, in selected subsets of adult patients and children. Conditions which predispose to invasive disease include acquired immunodeficiency syndrome, immunosuppression after bone marrow transplantation for treatment of aplastic anemia, treatment with central catheters and dental abscesses. Two pediatric cases of probiotic-related invasive disease have been described by Land MH et al. (2005). They were both attributable to Lactobacillus GG, probably passing into the bloodstream across the gastrointestinal mucosa. One was a 6-week-old male infant with double-outlet right ventricle and pulmonic stenosis, that, postoperatively, developed bacteremia and sepsis. He had received Lactobacillus GG through the gastrostomy tube for 20 days before presentation.

The second reported case involved a 6-year-old female patient with cerebral palsy, icrocephaly, mental retardation and seizures, who required feeding through a gastrojejunostomy tube. She received Lactobacillus GG for 45 days before a  peripheral blood culture yielded greater than 100 colony-forming units of Lactobacillus GG while the patient was febrile. These isolated cases should not discourage continued evaluation of  Lactobacillus or other probiotics, but it must be remembered that these agents may cause invasive disease in special, high-risk patients.

Probiotics for formulae and infant foods
The Scientific Committee on Food of the European Commission has recommended that infant formulae with microorganisms regarded as probiotics should be introduced into the market only if their benefit and safety have been evaluated according to the principles outlined by the same Committee. The Committee, on the other hand, did not object to the addition of bacteria regarded as probiotic to follow-on formulae. However, the Committee stated that only bacterial strains with identity and genetic stability demonstrated by cultural and molecular methods should be used and that the identity of the probiotic strain determined in this way should be available to the food control authorities. The content of viable bacteria in the formulae throughout the shelf-life of the formulae should be  106 to 108 colony forming units (CFU) per gram of formula prepared as ready for consumption. Unfortunately, there are only limited published data on the safety and clinical effects of infant or follow-up formulae and of infant foods for special medical purposes supplemented with a limited number of probiotic preparations. There is no published evidence of any long term clinical benefit of supplementation of infant formulae with probiotic bacteria. Further, no data are available concerning possible long-term effects on intestinal colonization and its potential effects on long-term gastrointestinal and immune unctions. Such data are highly desirable since some data suggest that bacteria ingested during early infancy are more likely to permanently colonize the intestine than those ingested during later life. The content of viable bacteria in a dietetic product must be of a dose shown to be safe and effective with regard to defined outcomes in clinical trials. Further evaluation of the safety and efficacy of adding probiotic bacteria to dietetic products for infants is necessary. Each strain must be evaluated within ranges of doses intended for use to define both the minimal and optimal effective doses. Specific safety questions that should be addressed concern the possible effects on nutrient utilization, the exclusion of transfer of antibiotic resistance, and the short and long term effects on intestinal colonization, immune response, and infections.

Conclusions
There are indications for possible short term benefits of some probiotic strains in treatment of infants and young children with infectious diarrhea, mainly of viral etiology, as well as in prevention of antibiotic-associated diarrhea. All the other postulated efects of probiotics, even if promising, remain to be convincingly demonstrated. Safety issues are a major priority when adding probiotics to dietetic products for infants.
 
References

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This material has been prepared by a member of the IFM's Advisory Committee on Child Health and Nutrition, May 2005.