Aims

The principal aims of this chapter are:

• to explain what is meant by the term ‘invasion’

• to describe the various mechanisms underlying bacterial invasion of host cells

• to emphasise that invasion involves bacterial manipulation of the signalling pathways and cytoskeleton of the host cell

• to provide examples of invasive organisms and the diseases associated with them

• to indicate the consequences of invasion for both the bacterium and the host cell

• to describe the different ways in which bacteria can survive within the host cell

Introduction

We have seen in Chapters 1 and 7 that the ability of a bacterium to adhere to a body surface (the mucosa or skin) or structure (e.g. a tooth) is an attribute of many organisms and is not limited to those species that induce disease. Hence, colonisation of epithelial surfaces by members of the normal microflora is, ultimately, of benefit to the host as it can protect against the adhesion of pathogenic species. The adhesion of such benign organisms rarely results in a pathological process, unless the host defences are defective in some way, for example in an immunocompromised individual. However, for some organisms, adhesion is only the first step in a pathological process that involves invasion of the host. Invasion of the epithelium may result in penetration of the organism to only the superficial layers of tissue, as in the case of cholera and gonorrhoea or it may, ultimately, involve deeper penetration, resulting in dissemination of the organism throughout the body.