This study grew out of an interest in the so-called" secondary" or complicating infections which develop in the oropharynx or bowel in patients undergoing treatment with antibiotics. 1'2, 3 Such infections may be caused by a variety of microorganisms, mostly rather non-virulent ones like Pseudomonas or Monilia4 or other yeast-like fungi, but occasionally by highly virulent strains of staphylococci. 5'6 Since these microorganisms are all common contaminants of the oropharynx and bowel, and since, with rare exceptions, they produce infection only during antibiotic therapy, it seems reasonable to suppose that they are able to do so only when the normal bacterial flora of these areas has been altered by an antibiotic. 7 8 9 Or, to put the explanation the other way around, these common contaminants are unable to initiate infection in the oropharynx or bowel when the normal microflora is intact.

This explanation raises several questions: May not the normal microflora of the mouth and the rest of the alimentary tract play a significant role in the body's defense against bacterial invasion? And, if so, is it the whole flora which is responsible or only certain of its constituents? Lastly, what are the mechanisms involved?