Critical pathways in microbial virulence.

The virulence of a microbe represents a combination of complex factors including the agent's transmissibility and the severity of the disease associated with infection and is also significantly influenced by the susceptibility of the colonized host. Virulence factors may be defined as those products of the organism which are required to complete the various stages of the life cycle leading to pathology in the host. In this review, we examine some of the approaches which have been adopted in other fields of infectious disease in order to categorically identify virulence factors using a classical genetics approach with relevant models or human subjects. The absence of an accurate experimental model for periodontal disease means that our understanding of the microbial virulence determinants and pathways in this disease remains hypothetical and based largely on observations in vitro. However, factors which enable the organism to persist in spite of the elevated immune and inflammatory pressure at sites of disease are liable to be critical. Periodontal bacterial genomics is liable to make a significant impact on the field through an increased appreciation of the role of gene acquisition and gene loss in the evolution of periodontal bacteria and of the consequences of strain variation in gene content on virulence potential.