Abstract : An in vitro assay system, consisting of monolayers of human embryonic intestinal cells (HEI) and Clostridium difficile, was used to observe cell surface and cytoplasmic interactions. Microorganism test conditions include toxin B positive ( 938) and toxin B negative ( 789) strains, each harvested from standard sheep blood agar (SBA), casamino acids agar (CAS), and CAS containing subinhibitory concentrations of clindamycin (CAS-CL). Effects of toxin B were seen when organisms were harvested from SBA. Adherence and internalization were observed with organisms harvested from CAS and CAS-CL. Evidence of adherence and/or movement into HEI cells was observed at 15, 30, 60, and 90 min time periods using the in vitro internalization assay, conventional scanning electron microscopy, transmission electron microscopy, and field emission scanning electron microscopy of freeze fractured preparations. Quantitative organism determinations, and the observed activity of microorganisms harvested from CAS and CAS-CL, revealed their capability to adhere to and/or enter HEI cells. The toxin B producer, 938, harvested from CAS-CL, entered the cytoplasm and remained intact within it.
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