Pathogenicity is the inherent ability of a microorganism or agent to cause disease in a host. It denotes whether a species or strain can enter, survive and induce pathological effects in a particular host.
Determinants and measurement of pathogenicity
Pathogenicity arises from genetic and phenotypic traits that enable an organism to breach host barriers, evade or manipulate immune responses and damage tissues. Virulence factors such as adhesins, capsules, toxins, secretion systems and enzymes contribute to establishing infection. The presence of these factors, rather than mere presence of the organism, determines pathogenic potential. Pathogenicity is often considered a qualitative attribute—an organism is either capable of causing disease or not—while virulence describes the severity of disease. Scientists measure pathogenicity and virulence using parameters like the infectious dose (ID50), which reflects the number of organisms required to cause infection in 50 % of hosts, or the lethal dose (LD50). Environmental conditions and host susceptibility influence pathogenic outcomes; opportunistic pathogens display low pathogenicity in immunocompetent hosts but can cause disease when defenses are impaired. Understanding these concepts helps in designing vaccines and control strategies.
Examples across microbes and hosts
Pathogenicity varies widely among organisms. Bacillus anthracis, the causative agent of anthrax, possesses a capsule and exotoxins that make it highly pathogenic to mammals. Clostridium botulinum produces a neurotoxin that causes botulism; ingestion of minute amounts leads to severe paralysis. In contrast, most strains of Escherichia coli are nonpathogenic residents of the human gut, but enterohaemorrhagic E. coli O157:H7 acquired genes encoding Shiga toxin, making it pathogenic and capable of causing haemorrhagic colitis. Viruses exhibit different levels of pathogenicity: rabies virus is nearly always fatal once symptoms appear, whereas many rhinoviruses cause mild cold symptoms. Plant pathogenicity can be seen in Ralstonia solanacearum, which causes bacterial wilt, while some fungi such as Aspergillus fumigatus become pathogenic mainly in immunocompromised individuals. These examples illustrate that pathogenicity is not universal and depends on both microbial traits and host context.
Pathogenicity underpins the study of infectious diseases. Distinguishing pathogenic organisms from commensals enables targeted surveillance and informs treatment and prevention efforts.
Related Terms: Pathogen, Virulence, Infectivity, Pathogenesis, Host