Intestinal mucosal cells, originating in the crypts, migrate up the villi of the small intestine and slough off into the lumen (1, 2). Studies of the kinetics of these events have revealed a differential rate of cellular renewal between the intestinal mucosal cells of germfree animals and those of conventional animals (3, 4). The time for migration of the mucosal cells from the crypt to villus tip is twice as long in the germfree animal (3, 4). The absence of intestinal bacterial flora in the germfree animal may account for this difference in dynamic morphology.Recent advances in intestinal and fecal culturing techniques have provided information about the intestinal bacterial flora under normal and experimental conditions (5-7). The changing patterns of intestinal microorganisms have been followed both in newborn animals (8-11) and after contamination in germfree animals (12).The present study was designed to follow the changes in intestinal cellular renewal and microbial flora as a function of time in going from a germfree system to a conventional one. Experiments involving both gerrafree and conventional animals have utilized conventionalized (formerly germfree) animals as the control subjects; however, the possibility of altering cellular kinetics of the intestinal epithelium and the span necessary for the alteration have not previously been investigated. Materials and MethodsAnimals.--35 male and 35 female CFW mice (Carworth Farms, New City, N. Y.) born and reared under standard germfree conditions were randomly divided into two groups,

The administration of the antibiotic neomycin has been associated with a reversible intestinal malabsorption syndrome (1-5); shorter, broader villi and decreased levels of intestinal enzymes have been reported both in humans (6-11 ) and in experimental animals (12)(13)(14). The severity of the intestinal morphologic and physiologic alterations appears to increase with dose and time (15)(16)(17).Neomycin also reduces the quantity of intestinal bacteria, the intestinal environment becoming analogous to that in the germfree animal. The morphology of the gut of the germfree animal also differs from that of the conventional animal (18-21), but with longer, thinner villi and enhanced intestinal absorption of monosaccharides and amino acids (22, 23). The absence of bacterial flora may account for the differences in gastrointestinal structure and function of the germfree animal from that of its conventional counterpart. Neomycin, on the other hand, may directly affect the epithelial cells of the small bowel, counteracting any villus alteration or enhanced absorptive capacity secondary to microflora reduction.The present study was designed to assess the effect of an antibiotic regimen on fecal microflora and small intestinal function and morphology. Mice were fed neomycin and penicillin in order to evaluate small intestinal cell population kinetics, intestinal transport of an amino acid, and intestinal enzymes by disaccharidase assay of the brush border and histochemical staining reactions of epithelial cell enzyme systems.