The probiotic concept
Since probiotics do not permanently colonise the host, they need to be ingested regularly for any health promoting properties to persist. Most studies on probiosis have been observational rather than mechanistic, and thus the processes responsible for many probiotic phenomena are seldom explained. Some probiotics are members of the normal colonic microflora and are not viewed as being overtly pathogenic. However, these organisms have occasionally caused infections in people whose health is compromised in other ways.3,4
Commercial probiotic preparations are usually mixtures of lactobacilli and bifidobacteria, although yeasts such as saccharomyces have also been used (box). Bifidobacteria are of particular interest. These are anaerobic pleomorphic rods or club shaped organisms (fig (fig2)2) which normally have an important role in breaking down dietary carbohydrate and interact directly with the host metabolism.5 Bifidobacteria also synthesise and excrete water soluble vitamins, but there are considerable differences in species and strains.6 These organisms predominate in the colons of breastfed babies; they account for up to 95% of all culturable bacteria and protect against infection.7 Bifidobacteria do not occur in such high numbers in adults.