Answer: b, d The composition of the gut microflora is established in neonates after ingestion of microbes that are acquired during contamination from the birth canal and during initial feeding, and remain relatively constant thereafter. Although this flora acts to promote development of the immune system, the specific interactions that produce this effect have not been fully elucidated. The microflora also contributes to physical and chemical barriers at the mucus membrane level, in that many autochthonous microbes possess adhesion proteins by which they can bind to certain areas of the mucosal cell or to specific types of bacteria, occupying potential binding sites for pathogenic organisms and producing a substantial physical mucobacterial layer. The oropharynx contains a number of aerobic and anaerobic microorganisms, however, these microbial inhabitants do not usually pass into the intestine, because the stomach itself represents a significant barrier to invading microorganisms by virtue of its low pH, which kills most microbes. The upper small intestine contains few organisms, mainly gram-positive aerobes and lactobacilli. Conversely, the lower small intestine contains a large number of aerobes and anaerobic forms, especially in patients in whom the ileocecal valve allows free backwash of cecal contents into the terminal ileum. Within the colon, a wide diversity and a large number of facultative and strict anaerobic isolates are present. Only a small number of aerobes are present, these microbes being outnumbered 100–300 to 1 by anaerobes