Aims

The principal aims of this chapter are:

• to describe the normal microflora of humans

• to discuss the role of the normal microflora in health and disease

• to describe the types of interaction that exist between humans and bacteria

• to outline some of the diseases caused by members of the normal microflora

• to give examples of diseases caused by exogenous pathogens

• to describe a classification system for infectious diseases based on the presence or absence of key virulence factors of the causative organism

• to introduce the paradigm organisms – Streptococcus pyogenes and Escherichia coli

Introduction

Human beings display a tremendous range of attitudes to bacteria, ranging from a paranoid loathing accompanied by an ultimately futile determination to extinguish all ‘germs’ from the immediate environment to an admiring appreciation of their tremendously important role in the functioning of the biosphere. While a well-educated biologist will, one hopes, tend towards the latter attitude, when it comes to a consideration of infectious diseases, even s/he will often exhibit a response (bacteriophobia) more characteristic of the former. This retrenchment to a ‘them’ against ‘us’ confrontational attitude, while understandable, is not particularly helpful in trying to unravel the complexities of the origin of, and the mechanisms underlying, infectious diseases. What many people fail to realise is that, when it comes to viewing the bacterial world, it is not a case of ‘us’ and ‘them’ because we are, in fact, more ‘them’ than ‘us’.