THE EARLY DYNAMIC-RESPONSE OF THE CALF ILEAL EPITHELIUM TO SALMONELLA-TYPHIMURIUM

Ileal loops including Peyer's patch were prepared in five 28-day-old c alves and infused with Salmonella typhimurium strain ST4/74. Loops wer e fixed 5 minutes to 2 hours after inoculation, and the mucosa was exa mined by light and electron microscopy. Within 5 minutes, the bacteria were interacting with the follicle-associated epithelium (FAE); the s urface of M cells changed to lamellipodia, engulfing many bacteria. Th is process proceeded rapidly to 30 minutes, involving most M cells abo ve crypt level. Most cells were exfoliated, and many were packed with bacteria, and the domed villi became stunted. There was a rapid migrat ion of neutrophils through the FAE into the lumen by 15 minutes. By 60 minutes, there was no further interaction between the bacteria and th e FAE; at this time bacteria were present in macrophages in the lamina propria. Restitution of the FAE was complete by 2 hours in spite of t he many bacteria in the cell debris overlying the epithelium. Interact ion of bacteria with the absorptive villi was delayed compared with in teraction with the FAE. After 15 minutes, bacteria were seen adhering to some enterocytes of the upper third of the villi; many bacteria wer e adhering to the surface of the enterocytes at 20 and 30 minutes, but few were seen thereafter. Adherence was patchy and largely confined t o cells whose surfaces were depressed relative to others. The microvil lous surface of these enterocytes was extensively remodelled. Tissue r esponse, with uptake of bacteria into vacuoles, exfoliation of enteroc ytes containing bacteria, and subsequent stunting of the villi, began at 30 minutes and was severe and progressive to 2 hours. Following the initial attachment and uptake of the bacteria, loss of enterocytes pr ogressed from these initial sites; bacteria were associated with the l ateral cell membrane of cells adjacent to cells being extruded and not with the microvilli of cells at new sites. In a calf 4 hours after do sing orally with the same strain, M cells were engulfing bacteria and their cell surface was changed as seen in the inoculated loops; absorp tive enterocytes were also taking up bacteria as seen in the ileal loo ps, indicating the process seen in the loops and after oral dosage was similar. For this strain of S. typhimurium, there was an initial conc entration of bacilli around the domed villus epithelium. This distribu tion was not random but may have resulted from a specific attraction t o the FAE.