[Review] Probiotics: sorting the evidence from the myths

[Michaelangelica]

The mucosa of the gastrointestinal tract is constantly presented with antigens from microorganisms or ingested foods. Over 500 bacterial species are resident in the adult gastrointestinal tract, principally the colon.

This community of microbiota not only lives in peaceful coexistence with the human host but also plays a significant role in the host’s wellbeing.1

There is a constant and complex interaction among these commensal bacteria, the intestinal epithelial cells, and the immune system.2

 

Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a beneficial effect on the health of the host (Box 1).3

The most commonly used probiotic agents are bacteria from the Lactobacillus and Bifidobacterium genera, which form part of the normal healthy intestinal microbiota. Other probiotics include the yeast Saccharomyces boulardii, which is now regarded as a separate cluster located within the species Saccharomyces cerevisiae.4

 

Probiotics exhibit strain-specific differences in their resistance to acid and bile, ability to colonise the gastrointestinal tract, and clinical efficacy.5

The effects of probiotics may be due to various mechanisms of action, including suppressing growth of pathogenic bacteria, blocking epithelial attachment by pathogens, enhancing mucosal function, and modulating host immune response.6

 

Probiotics are now widely marketed in the form of capsules, powder and functional foods such as fermented milks and yoghurts.

Here, we review the evidence for the role of probiotics in treating both gastrointestinal and extragastrointestinal diseases in children and adults.

 

Levels of evidence designated here as E1, E2, E3 and E4 are based on the National Health and Medical Research Council (NHMRC) levels of evidence I, II, III (including III-1, III-2, III-3) and IV, respectively.7

 

Drug profile of probiotics

 

Action: Probiotics are thought to suppress the growth of pathogenic bacteria, block epithelial attachment or invasion by pathogens, enhance mucosal function and modulate host immune response.

 

Preparation: Available in capsule and powder form, and as fermented dairy foods. Stability is an issue with non-yeast preparations and therefore most probiotics require refrigeration.

 

Dosing: There is significant variation in the number of bacteria between different preparations, as production is not standardised. There has been considerable variability between probiotic strains and doses used in clinical trials for a range of gastrointestinal and extraintestinal disorders. Appropriate doses of specific probiotics for specific clinical uses have not been established.

 

Metabolism: Probiotics exhibit strain-specific differences in their resistance to acid and bile, and ability to colonise the colonic mucosa. They are not metabolised.

 

Adverse effects: Probiotics are generally safe and well tolerated. They are contraindicated in patients with severe underlying illnesses or those who are immunocompromised; these patients are at risk of bacteraemia and fungaemia.

 

Regulation: In Australia, probiotics that are marketed for a specified health benefit require review by the Therapeutic Goods Administration and are regulated as complementary medicines.

eMJA: Probiotics: sorting the evidence from the myths