Introduction

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The term "probiotic" was first used to describe "a live microbial supplement, which beneficially affects the host by improving its microbial balance."¹ Since then, research has looked at possible clinical uses for these agents. In 1995, when a greater understanding of their properties had developed, the term "biotherapeutic agents" was proposed to describe micro-organisms with specific therapeutic properties that also inhibit the growth of pathogenic bacteria.²

A number of agents have been isolated and studied with a view to clinical use. Streptococcus thermophilus and Lactobacillus bulgaricus, commonly used in the dairy food industry, were among the first to be studied. Other strains that have been used are Bifidobacterium bifidum, B longum, Enterococcus faecium, Saccharomyces boulardii, L acidophilus, L casei, and Lactobacillus GG. However, doctors are still reluctant to use these agents in clinical practice.

In this paper, we review the results from various trials carried out to study their benefits. We also look at the properties of biotherapeutic agents and options for further research.

	Materials and methods

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Literature search
We searched Medline between 1966 to 2000 with the terms "probiotics," "biotherapeutic agents," "lactobacilli," "antibiotic associated diarrhoea," and "Clostridium difficile." We also searched the Cochrane Controlled Trials Register and the Cochrane Database of Systematic Reviews. We included all randomised double blind trials that compared the effects of probiotic therapy and placebo (both given in combination with antibiotics).

Ten double blind placebo controlled trials were relevant to our area of interest (nine published in English and one in French).^3-12 Our meta-analysis included nine that looked at prevention of diarrhoea. We excluded the other trial, which looked at treatment of diarrhoea.¹²

Meta-analysis and data abstraction
The meta-analysis was carried out according to the recommendations of the QUOROM statement.¹³ The key outcome data taken for analysis included the sample size, treatment regimens, and numbers of patients in both arms of the study who had an absence of diarrhoea (table 1).

	Results

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Nine trials were included in the final analysis (fig 1). The study regimens used probiotics combined with one antibiotic or a variety of antibiotics (table 1). All trials studied the efficacy of a probiotic in the prevention of antibiotic associated diarrhoea. The numbers of patients and the duration of follow up varied greatly from study to study, but the patients' characteristics were similar for the active treatment and placebo groups within each study.

View larger version (33K): [in this window] [in a new window]   Fig 1.   Meta-analysis profile summarising trial flow. RCT=randomised controlled trial

We calculated the odds ratio on the basis of the proportion of patients free of diarrhoea on treatment compared with that in control groups. After tests of homogeneity, summary odds ratios and 95% confidence interval limits were provided for the combined data of the four trials that used S boulardii (yeast trials), the five non-yeast trials, and all nine trials together. The combined odds ratios for the four yeast trials and for the five non-yeast trials were similar (0.39 (95% confidence interval 0.25 to 0.62) and 0.34 (0.19 to 0.61), respectively); both favoured active treatment over placebo in the prevention of antibiotic associated diarrhoea. The odds ratio for pooled data from all nine trials was in favour of active treatment over placebo in the prevention of antibiotic associated diarrhoea (0.37; 0.26 to 0.53). Six studies showed a significant benefit of probiotic treatment compared with placebo (P<0.05) (fig 2). ^{3 5 6 8 9 11} One study showed benefit for only a subgroup of patients who did not receive non-antibiotic drugs likely to induce diarrhoea, such as magnesium hydroxide (for constipation), lactulose, and bisacodyl (for hepatic encephalopathy).⁴

View larger version (28K): [in this window] [in a new window]   Fig 2.   Plot of the log of odds ratios for the proportion of patients free of diarrhoea in treatment groups compared with control groups

	Discussion

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Our meta-analysis of trials that used live organisms to prevent diarrhoea associated with antibiotics shows that probiotics may be effective in preventing antibiotic associated diarrhoea. We had only a small number of trials in our meta-analysis, and it should be noted that the different antibiotics used in the trials may have altered the risk of patients getting diarrhoea and their response to the probiotics. Although probiotics have been used to prevent or treat diarrhoea of other causesnamely traveller's diarrhoea and infantile infectious diarrhoeawe did not include trials that investigated probiotics in these indications; however, most of these studies showed positive results, and some reviews have been encouraging.¹⁴

The way in which probiotics affect the gut has drawn much interest. To combat the problems of gastrointestinal infection, probiotics must be non-pathogenic and must act against pathogens by different mechanisms from antibioticsfor example, by competition. More importantly, they should have a fairly rapid onset of action and survive the challenges of gastric acid, bile, or concurrent antibiotics. It is also desirable that they modify immune processes to destroy the invading organism. Saccharomyces boulardii and lactobacilli display these common properties.

A few live organisms have been used in many trials. S boulardii, a non-pathogenic yeast, is one such organism. It has a growth temperature of 37°C, rapidly colonises the bowel, does not alter the normal gut flora, and is cleared from the colon after treatment is discontinued.¹⁵ Of the four yeast trials, three studies individually showed significant benefit, ^{3 5 9} but one did not¹⁰; differences in the dose and duration of treatment with S boulardii and variations in the period of follow up may explain this disparity. Interestingly, S boulardii can also destroy the receptor site for C difficile toxin A and B by producing a protease¹⁶; this could explain how S boulardii was noted to reduce the frequency of toxin B positivity.¹²

The other probiotic agent used widely in clinical trials is the Lactobacillus species. The mechanism of action of lactobacilli may be through multiple means: Lactobacillus GG has shown beneficial effects on intestinal immunity; it increases the numbers of cells that secrete immunoglobulin A and other immunoglobulins in the intestinal mucosa, and it stimulates the local release of interferon.¹⁷ It also facilitates antigen transport to underlying lymphoid cells, and shows increased uptake in Peyer's patches.¹⁷ Lactobacillus GG has also been shown to produce an antimicrobial substance that inhibits the growth of Escherichia coli, streptococci, Clostridium difficile, Bacteroides fragilis, and Salmonella.¹⁸ L casei shirota also showed good survival in the gut in separate studies, and mucosal antibody titres (specific to lactobacilli) were increased in the presence of this agent. ^{19 20} Although there was no discernible change to the numbers of clostridia or enterococci, there was an increase in the numbers of excreted bifidobacteriaa normal bowel anaerobe. ^{19 20} It is possible that this increase in bifidobacteria interferes with the pathogenic potential of C difficile.

Probiotics are a possible solution in the prevention of antibiotic-associated diarrhoea. Clostridium difficile infection is increasingly prevalent in today's hospital setting, particularly in elderly patients, in whom 10-20% of such cases occur.²¹ The incidence of antibiotic associated diarrhoea depends on the antibiotic used and each individual patient's risk factors. The standard regimens to treat colitis associated with Clostridium difficile are metronidazole and vancomycin; although these drugs are successful in 80% of cases, about 20% of patients suffer from recurrence.²² In light of the need to control costs in these days of managed health care, we must re-examine the benefits of using live organisms. Whether the use of probiotics can actually reduce the length of hospital stay by reducing the incidence of infection with C difficile and the need to use antibiotics such as metronidazole and vancomycin are issues that need to be addressed in a clinical trial.

Conclusion
Our meta-analysis of nine trials shows that biotherapeutic agents may be useful in preventing antibiotic associated diarrhoea.

The increasing availability, lower costs, and relative lack of side effects of probiotics contrast with the problems associated with current antibiotic regimens. Commercially available strains are being marketed in capsules and yoghurt based drinks, but their potential benefit needs further investigation. Data from trials have provided us with clear evidence on the efficacy of some strains in the gut, but we still need to see confirmation of their clinical benefit.