The individual bacterial members of our indigeneous microbiota are actively engaged in an on-going battle to prevent colonisation and overgrowth of their terrain by competing microbes, some of which might have pathogenic potential for the host. Humans have long attempted to intervene in these bacterial interactions. Ingestion of probiotic bacteria, particularly lactobacilli, is commonly practiced to promote well-balanced intestinal microflora. As bacterial resistance to antimicrobials has increased, so too has research into colonisation of human tissues with specific effector strains capable of out-competing known bacterial pathogens. Recent progress is particularly evident in the application of avirulent Streptococcus mutans to the control of dental caries, alpha hemolytic streptococci to reduction of otitis media recurrences and Streptococcus salivarius to streptococcal pharyngitis prevention.