Our gut is colonized by numerous bacteria throughout our life, and the gut epithelium
is constantly exposed to foreign microbes and antigens derived from digested
foods. Thus, the gut epithelium acts as a physical barrier against microbial invaders
and is equipped with various elements of the innate defense system. Resident
commensal and foreign invading bacteria interact intimately with the gut epithelium
and can impact host cellular and innate immune responses. From the perspective
of many pathogenic bacteria, the gut epithelium serves as an infectious
foothold and port of entry for dissemination into deeper tissues. In some instances,
when the intestinal defense activity and host immune system become compromised,
even commensal and opportunistic pathogenic bacteria can cross the barrier and
initiate local and systematic infectious diseases. Conversely, some highly pathogenic
bacteria, such as those highlighted in this book, are able to colonize or invade
the intestinal epithelium despite the gut barrier function being intact. These pathogenic
bacteria are capable of circumventing the gut defense barriers, leading to
colonization within and beyond the gut. Some pathogenic bacteria can disseminate
to distal tissues and cause severe enteric and systemic diseases. Therefore, the
relationship between the defensive activity of the intestinal epithelium against
microbes and the pathogenesis of infective microbes becomes the basis for maintaining
a healthy life