Virulence can be defined as the quality of being poisonous or injurious to life (i.e., virulent). For an organism to be virulent, it must be able to infect its human host, reproduce, and/or cause a disease. This broad definition of virulence is more inclusive than the narrow definition commonly used by microbiologists (i.e., virulence is solely the severity of the disease produced after exposure and infection). Each of the microbiological attributes that contribute to virulence can in general be linked to specific architectural elements or biochemical compounds within the organism. Together, these elements and compounds can generally be termed “virulence factors,” and the blueprints for them are included in the genetic code of an organism. For this reason, a principal topic of this chapter is the genetic structure of various microorganisms because of its direct relationship to virulence factors.

Owing to recent advances in molecular biology, the genetic structures of many thousands of organisms (especially bacteria and viruses) have been identified, reported, and stored in what are called gene banks. Sophisticated computer programs allow for the sorting and matching of genetic structures and specific genes. The discipline that organizes and studies these genes is known as bioinformatics, while the study of genes and their function is known as genomics. In addition, a growing area of related interest is functional genomics, that is, understanding the specific role of genes in terms of the function of the organism. The ability to use these and related tools to address the microbial contamination of drinking water is illustrated by some of the following observations:

  • The genetic structures of most known waterborne pathogens have been characterized at least partially, with the information stored in gene databanks. The complete genome of several important waterborne pathogens, such as Vibrio cholerae (the agent of cholera), is now known, and many more will be characterized in the near future (Heidelberg et al., 2000).

  • Other related information is accumulating that allows the use of these databanks to determine or predict the ability of a microorganism to produce virulence factors, such as toxins, attachment factors, and other surface proteins, and genes that encode bacterial resistance to antibiotics.

  • On a more basic level, these data can be used to characterize similarities and differences between a microorganism of interest and known pathogens.



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