It is critical, both for the host and for the long-term benefit of the bacteria that colonize the gut, that bacterial overgrowth with subsequent bacterial translocation, which may lead to sepsis and death of the host, be avoided. Secretory IgA (sIgA) is known to be a key factor in this process, agglutinating bacteria and preventing their translocation in a process termed 'immune exclusion'. To determine whether human sIgA might facilitate the growth of normal enteric bacteria under some conditions, the growth of human enteric bacteria on cultured, fixed human epithelial cells was evaluated in the presence of sIgA or various other proteins. Human sIgA was found to facilitate biofilm formation by normal human gut flora and by Escherichia coli on cultured human epithelial cell surfaces under conditions in which non-adherent bacteria were repeatedly washed away. In addition, the presence of sIgA resulted in a 64% increase in adherence of E. coli to live cultured epithelial cells over a 45-min period. Mucin, another defence factor thought to play a key role in immune exclusion, was found to facilitate biofilm formation by E. coli. Our findings suggest that sIgA may contribute to biofilm formation in the gut.

The expression of colonization factors by gut bacteria, the growth rate of gut bacteria, and the rate of plasmid exchange by gut bacteria indicate that biofilms are a normal component of bacterial growth in the large bowel. Further, in vitro experiments demonstrate that growth of normal enteric bacteria in biofilms can be facilitated by secretory IgA (SIgA) and by mucins, 2 major components of the gut milieu. However, biofilms have not been previously observed in the normal gut. In this study, bacterial colonies characteristic of biofilms were observed by electron microscopy in normal rat, baboon, and human gut by electron microscopy. Confirming these results, acridine orange staining of flash-frozen tissues revealed biofilms in the mucus lining along normal gut epithelium. Immunofluorescenct microscopy supported this finding and demonstrated an association between IgA and the biofilms. These findings provide direct evidence that biofilms are present and may play an important role in the commensal relationship between enteric bacteria and their hosts. Hematoxylin and eosin staining of formalin-fixed tissues resulted in dissociation of the luminal contents from the epithelium, suggesting that the association between biofilms and the gut epithelium is sensitive to some conditions used to preserve tissue for histologic evaluation.

The expression of colonization factors by gut bacteria, the growth rate of gut bacteria, and the rate of plasmid exchange by gut bacteria indicate that biofilms are a normal component of bacterial growth in the large bowel. Further, in vitro experiments demonstrate that growth of normal enteric bacteria in biofilms can be facilitated by secretory IgA (SIgA) and by mucins, 2 major components of the gut milieu. However, biofilms have not been previously observed in the normal gut. In this study, bacterial colonies characteristic of biofilms were observed by electron microscopy in normal rat, baboon, and human gut by electron microscopy. Confirming these results, acridine orange staining of flash-frozen tissues revealed biofilms in the mucus lining along normal gut epithelium. Immunofluorescenct microscopy supported this finding and demonstrated an association between IgA and the biofilms. These findings provide direct evidence that biofilms are present and may play an important role in the commensal relationship between enteric bacteria and their hosts. Hematoxylin and eosin staining of formalin-fixed tissues resulted in dissociation of the luminal contents from the epithelium, suggesting that the association between biofilms and the gut epithelium is sensitive to some conditions used to preserve tissue for histologic evaluation.

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