9 - The host–parasite relationship

Abstract

Historically, the study of the host–pathogen (parasite) interrelationship has relied on information gained from the study of specific organisms examined under laboratory conditions. However, advances in molecular biology and DNA sequencing have revealed the existence of microorganisms in the host which cannot be cultured or directly observed. This has led to a quest to understand more completely the full range of microorganisms present in the host, collectively referred to as the microbiota and its genetic content – the microbiome. Analysis of the microbiome is an aspect of what is generally referred to as metagenomics: the study of genomic content and diversity in a given environment. In 2007 a concerted large-scale effort in this regard began as The Human Microbiome Project, a 5-year initiative of the United States National Institutes of Health (NIH). As a result, the terms microbiota and microbiome are largely replacing the phrase ‘normal flora’ although the latter will still be used on occasion in this book. Preceding book chapters have focused primarily on organisms that are disease agents. Small numbers may be found in healthy individuals, but their presence in large numbers is usually associated with pathological changes. The first section of this chapter considers members of the microbiota found in the normal healthy individual, in some cases necessary for normal functioning of the human body but able to cause disease under certain circumstances (e.g. in the newborn or in stressed, traumatized or immunocompromised individuals). Their relationship with the host makes an interesting comparison with that of species that are considered as true parasites or pathogens discussed later in this chapter in the broader context of symbiotic relationships and the evolution of host–parasite relationships.