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Probiotics: Their Tiny Worlds Are Under Scrutiny
| In the lab, in the yogurt cup, in animal feed-- these microbes are making their huge numbers felt
by Bob Beale

gut reaction
GUT REACTION: Researchers have found that Lactobacillus GR-1 and RC-14 can penetrate Escherichia coli biofilms, multiply, and survive.

Image: Courtesy of Mark Neysmith, © Gregor Reid

The human body plays host to a complex and thriving microbial ecosystem of vast numbers of tiny creatures. Some of these species, already well studied, can cause disease. But a renewed appreciation is growing for many lesser-known species called probiotics that help maintain health and may have the potential to prevent disease. A steady stream of research papers on this topic is making its way into scientific and medical journals. The food industry is especially active in studying probiotics because the gastrointestinal tract is one of the richest zones of biodiversity within the body, with at least 400 known species of bacteria commonly found there.

Apart from their possible inclusion in foods or their development as food additives, many medical, dental, and veterinary researchers are looking at a wide range of therapeutic possibilities for probiotics. They include the treatment of ear, intestinal, and urinary tract infections; reduction of blood cholesterol levels; and prevention of skin and surgical wound infections, tooth decay and even some cancers.[1-4]

The modern concept of probiotics has been around for almost a century, yet the science behind it is still sparse. The validity of much of the research into the efficacy of probiotics has been questioned (or goes unpublished because of commercial secrecy); their impact on human nutrition is still only vaguely understood; the selection of desirable microbial strains is an uncertain process and often based on commercial or technical factors; and the necessary tools to monitor their performance in vivo are still being developed. "There does seem to be lots of potential there, but it's a very complicated area," says independent nutritionist Rosemary Stanton, in Sydney, Australia, "I think it sounds good and I want to be a believer. But when you read the studies they don't match the headlines. The food industry is pushing to be able to make more health claims for these products, but, based on what we know so far, I think it would be foolish to allow that."


An adult body has about 10 trillion cells comprising 200 different types, but bacteria greatly outnumber them. Gerald Tannock, University of Otago, New Zealand, says the typical adult body, inside and out, has 100 trillion bacterial cells consisting of at least 500 species, not to mention billions of viruses and fungi.[5] Most microbiota, also known as microflora, live in relative peace and harmony with the body. Some aid digestion, act as a first line of defense against invading pathogens, or help to keep the immune system primed, says Tannock. Humans may have ancient symbiotic relationships with many of them, and they seem to operate as members of any ecosystem do, competing with each other for nutrients, energy, and habitats and actively repelling or deterring invaders, he says.

Gregor Reid and colleagues at the Lawson Health Research Institute, University of Western Ontario, Canada, reported that competition between microbes and their mammalian hosts has been evolving for millions of years. "The fact that we have survived is in no small part owing to the ability of commensal bacteria to protect the host from microbial-induced disease processes," they argued in a recent paper.[6]

Experiments with germ-free animals have shown, paradoxically, that they are often sickly. Absence of intestinal microbiota leaves the animals' immune systems underdeveloped and can disrupt their intestinal morphology, problems that can be reversed to varying degrees by experimentally introducing probiotic species. Mahnaz Banasaz and colleagues at the Karolinska Institute in Sweden recently published experiments showing that one of the most widely studied probiotics, Lactobacillus rhamnosus GG, is easily established in germ-free rats and can markedly alter their gut morphology.[7] After three days of exposure, the rate of mitosis increases in the cells of the upper small intestine, significantly increasing the number of cells in the villi lining the intestinal wall, thus aiding absorption of soluble food. Probiotic strains are now used routinely in livestock nutrition, and some certainly seem to have potential for human use against a variety of pathogens.[8]

Probiotics may have potential to boost disease resistance, says Tannock. It is well known, for example, that normal mammalian commensal microbes can increase circulating specific and natural antibodies, and thus reduce antibiotic use. Future clinical applications for probiotics might include treating food allergies, reducing hypertension, or using them as vectors for oral vaccines. A recent paper hypothesizes that probiotics might even help detoxification in cases of mercury poisoning.[9] Proponents also note that probiotics have aided in restoring intestinal flora after antibiotic therapy, reducing the duration of rotaviral diarrhea and gastroenteritis in infants, and preventing traveler's diarrhea.[3]

probiotics exposed
PROBIOTICS EXPOSED: Bifidobacteria isolated from the intestine.

Image: Courtesy of Glenn Gibson200

Establishing new species within a body is normally difficult because the resident species are thought to exclude invaders through a process known as colonization resistance. But antibiotic drugs can kill many bacteria other than pathogens, and may, for example, disrupt the digestive ecology enough to let antibiotic-resistant strains gain a foothold, leading to diarrhea. Because the prospects for new drugs that overpower resistant strains are few and probably distant, researchers are turning to probiotics to help. "The worldwide emergence of bacterial resistance to antibacterial agents has produced a need for new methods of combating bacterial infections," argued Pentti Huovinen, chief physician at the Antimicrobial Research Laboratory, National Public Health Institute, in Finland.[10] "Bacteriotherapy using harmless bacteria to displace pathogenic organisms is an alternative and promising way of combating infections."


• Researchers in Sweden are using a streptococcus-laced nasal spray to reduce the recurrence of otitis media, a middle-ear infection. The bacteria apparently deter pathogens from spreading from the nose into the middle ear.[4]

• Reid's group found encouraging evidence after they simultaneously added the probiotic Lactobacillus fermentum and potentially deadly Staphylococcus aureus (commonly known as a golden staph) simultaneously to simulated surgical wounds in rats which had a strong protective effect against the disease-causing S. aureus.[6]

Rolien Free and colleagues, University Hospital of Groningen, Netherlands, used two strains of probiotic streptococci to reduce the incidence of unwanted bacteria and fungi on prosthetic voice boxes.[11]

• Likewise, Richard Hull of Baylor College of Medicine in Houston recently reported using probiotics to achieve significant reductions in recurrent bladder infections among catheterized patients with spinal-cord injuries.[11]

Jeffrey D. Hillman, University of Florida College of Dentistry in Gainesville, is inoculating rats with a territorially aggressive but harmless, modified strain of Streptococcus mutans.[11] That strain displaces the harmful variant of the bacterium that metabolizes lactic acid in the mouth and causes enamel damage to teeth. Hillman predicted that dentists might one day use probiotic sprays during routine cleaning procedures in children.

Andrew S. Neish and colleagues from Emory University School of Medicine in Atlanta, Ga., have suggested that orally administered probiotics might also be used to treat inflammatory disorders of the intestinal tract. They reported experimental evidence[12] that nonpathogenic salmonellae interfere with an inflammatory signaling pathway in intestinal mucosa, suggesting a possible mechanism by which bacteria diminish the host's immune responses to their presence. In an editorial accompanying publication of that report, Ramnik J. Xavier and Daniel K. Podolsky, of Massachusetts General Hospital in Boston, noted that probiotic therapy is already showing some success in the treatment of inflammatory bowel disease. "A better understanding of how the normal gut microflora remains largely invisible to the host intestinal epithelium may provide a clearer picture of the molecular pathways of chronic inflammation," they suggested.

David Gaon and colleagues at the University of Buenos Aires also recently found evidence that some lactobacillus are effective for treating chronic diarrhea linked to the bacterial overgrowth that often accompanies some anatomic disorders and partial small-bowel obstructions.[13]

Probiotics have a well established place in Japan, where hundreds of companies make and sell such products in Europe and increasingly in the United States. Food companies are developing chemical compounds, known as prebiotics, to selectively foster the growth of specific commensal bacteria in the gut. "These compounds are often part of a normal diet and are mainly nondigestible sugars, such as inulin and its derivatives, as found in vegetables such as leeks, onions, and garlic," says Glenn Gibson, University of Reading, UK.

bacterial diversity

BACTERIAL DIVERSITY: This scanning electron micrograph of human feces shows the diversity of bacterial types in the intestinal ecosystem, and how many bacterial inhabitants live in the gut. About 50% of fecal mass consists of bacterial cells.

Photo: Courtesy of Gerald Tannock

The concept of edible germs has been already tried and tested in the marketplace and well accepted by many consumers in dairy products such as yogurt, which owes its character to live Lactobacillus cultures and now often incorporates additional probiotic species, such as bifidobacteria. Ironically, the supposed healthful qualities associated with the long and widespread use of lactobacilli in yogurt may be based on a misperception, Tannock says, "because they are not numerically dominant in the intestinal tract and are absent from the microflora of about 25% of human subjects."[5]

Bifidobacteria are common, but Tannock says that "it may be difficult to achieve probiotic colonization of the adult digestive tract even when strains of intestinal origin are utilized." The already established microflora would defend against the introduced strain, just as they would against a pathogen entering the ecosystem. In studies reported so far, it appears that large numbers of microbial cells need to be ingested on a daily basis to achieve persistence of the probiotic strain in the digestive tract.[6] That need for repeated ingestion is, of course, a major attraction to food manufacturers. Gibson says this industry undoubtedly has bigger plans for probiotics: "We are likely to see [probiotics] ... included soon in everything from cheese to ice cream and even some preserved meats, such as salami," he says.

He predicts, for example, that prebiotic additives aimed at fostering the growth of certain intestinal microbes in bottlefed babies will find their way into infant milk formulas within the next two years. The thinking behind that proposal is that a breast-fed baby seems to get fast, dual protection from intestinal disease by being rapidly colonized with bifidobacteria from its mother, the growth of which is selectively favored by a bifidus factor in her breast milk. Such babies seem to get a better head start in life: There is evidence that their intestinal tracts are colonized faster than those of Caesarian-born infants, and they have fewer gastrointestinal infections than bottle-fed babies.[14]

The beneficial microbiota overwhelmingly dominate the intestinal ecosystems of healthy babies. At weaning, the bifidobacteria lay first claim to the gut habitat and alter its acidity, making it hostile to potentially harmful species. Intriguingly, Reid's team has suggested the future possibility of inoculating newborn infants with probiotics (because it is difficult to artificially introduce new organisms into an established microbial ecosystem) — the child might then enjoy lifelong protection with no need for external replenishment of the probiotic strain.

Probiotic research and development is poised to make great advances during the next five years, says Tannock. "A detailed understanding of the intestinal microflora will quickly emerge as molecular technologies are applied increasingly to analysis of intestinal communities, in conjunction with the use of biochemical and bacteriological methodologies. It is an exciting time to be involved in intestinal microflora research."

Bob Beale ( is a freelance writer in Sydney, Australia

1. I.P. Kaur et al., "Probiotics: Potential pharmaceutical applications," European Journal of Pharmaceutical Science, 15:1-9, February 2002.

2. J.A. Vanderhoof, "Probiotics: Future directions," American Journal of Clinical Nutrition, 73:1152S-5S, June 2001.

3. A.L. D'Souza et al., "Probiotics in prevention of antibiotic associated diarrhoea: Meta-analysis," British Medical Journal, 324:1361, June 8, 2002.

4. K. Roos et al., "Effect of recolonization with 'interfering' streptococci on recurrences of acute and secretory otitis media in children: Randomized placebo controlled trial," British Medical Journal, 322:210, 2001.

5. G.W. Tannock, Normal Microflora, New York: Chapman and Hall, 1995.

6. G. Reid et al., "Can bacterial interference prevent infection?" Trends in Microbiology, 9:424-8, September 2001.

7. M. Banasaz et al., "Increased enterocyte production in gnotobiotic rats mono-associated with Lactobacillus rhamnosus GG," Applied Environmental Microbiology, 68:3031-4, June 2002.

8. G. Reuter, "Probiotics: Possibilities and limitations of their application in food, animal feed, and in pharmaceutical preparations for men and animals," Berliner Und Münchener Tierärztliche Wochenschrift, 114:410-9, Nov.-Dec. 2001.

9. M.A. Brudnak, "Probiotics as an adjuvant to detoxification protocols," Medical Hypotheses, 58:382-5, May 2002.

10. P. Huovinen, "Bacteriotherapy: The time has come," British Medical Journal, 323:353-4, Aug. 18, 2001.

11. B. Harder, "Germs that do a body good," Science News, 161:72-4, Feb. 2, 2002.

12. A. Neish et al., "Prokaryotic regulation of epithelial responses by inhibition of IkB-a ubiquitination," Science, 289:1560-3, Sept. 1, 2000.

13. D. Gaon et al., "Effect of Lactobacillus strains (L. casei and L. acidophilus Strains cerela) on bacterial overgrowth-related chronic diarrhea," Medicina (B Aires), 62:159-63, 2002.

14. C. Liepke et al., "Human milk provides peptides highly stimulating the growth of bifidobacteria," European Journal of Biochemistry, 269:712-8, February 2002.


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