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Chapter 1 : Microbial Flora of Humans and Microbial Virulence Factors
The terms normal microbial flora, normal commensal flora, and indigenous flora are synonymous and are used to describe microorganisms that are frequently found in particular anatomic sites in healthy individuals. Microbial flora is associated with the skin and mucous membranes of every human from shortly after birth until death and represents an extremely large and diverse population of microorganisms. The normal microbial flora for various anatomic sites is reviewed in this chapter. The microbial factors that contribute to the virulence of a microorganism can be divided into three major categories: (i) those that promote colonization of host surfaces, (ii) those that evade the host’s immune system and promote tissue invasion, and (iii) those that produce toxins that result in tissue damage in the human host. Most infections are initiated by the attachment or adherence of the microbe to host tissue, followed by microbial replication in order to establish colonization. Organisms within biofilms are more resistant to antibiotics than individual bacteria and are partially protected from phagocytes as well. Some mucosal surfaces, such as the mouth, stomach, and small intestine, are protected from microbial colonization because they are constantly being washed with fluids. Toxins produced by certain microorganisms during growth may alter the normal metabolism of human cells with damaging and sometimes deleterious effects on the host. Many pathogenic organisms produce extracellular enzymes such as hyaluronidase, proteases, DNases, collagenase, elastinase, and phospholipases which are capable of hydrolyzing host tissues and disrupting cellular structure.