Antigenic variation is common among pathogenic organisms. Cas9 proteins are highly enriched in pathogenic and commensal bacteria. It reduces the density of biofilm and overgrowth of potential pathogenic bacteria resulting in disease. Immunocompromised individuals are more susceptible to pathogenic bacteria. Endophytes may benefit host plants by preventing other pathogenic or parasitic organisms from colonizing them. Identification and characterizing of bacteria being associated to diseases led to advances in pathogenic bacteriology. Plants have evolved efficient defense systems against pathogenic microbes. The low pH prohibits growth of many strains of pathogenic microbes. Tissue damage associated with pathogenic flu virus infection can ultimately result in death. One such experiment was the cloning of recombinant DNAs derived from highly pathogenic organisms. Porins are also an important factor for complement inhibition for both pathogenic and commensal species. This article deals with human pathogenic bacteria. It is commonly considered a gold standard in epidemiological studies of pathogenic organisms. This is due to immunity that develops with constant, repeated exposure to pathogenic organisms. The skin acts as a barrier to deter the invasion of pathogenic microbes.