Pathogenic refers to an organism, agent or process capable of causing disease in a host. A pathogenic microbe has the ability to invade, damage or disrupt normal physiological functions.
Factors that determine pathogenic behaviour
Pathogenicity is not an inherent property of a taxonomic group but arises from specific traits that enable disease. Microbes considered pathogenic often possess factors such as adhesins that allow them to attach to host tissues, enzymes and toxins that facilitate invasion or damage, and mechanisms to evade immune responses. The outcome of infection also depends on host factors such as immune competence and microbiota composition. Many bacteria have both nonpathogenic and pathogenic strains: Escherichia coli is a common gut commensal, yet certain strains carry virulence genes that cause urinary tract infections or diarrhoea. Viruses such as influenza and HIV are inherently pathogenic because they replicate inside host cells and cause injury. Fungi and protozoa are typically opportunistic pathogens, causing disease primarily in hosts with compromised defenses. Understanding what makes an organism pathogenic guides research into diagnostics, vaccines and treatments.
Contrasting examples and host interactions
Some microbes are pathogenic across a wide range of hosts, while others cause disease only under specific conditions. Yersinia pestis, the causative agent of plague, expresses virulence factors enabling it to infect rodents and humans. Staphylococcus aureus colonizes human skin and nasal passages but can become pathogenic when introduced into wounds or the bloodstream, leading to abscesses or sepsis. Streptococcus pyogenes is responsible for diseases like scarlet fever and rheumatic fever due to the production of toxins and immune modulation. In contrast, Lactobacillus species are nonpathogenic residents of mucosal surfaces that contribute to health. Plants also encounter pathogenic organisms; for example, Phytophthora infestans causes late blight in potatoes. Identifying the conditions under which organisms become pathogenic is essential for controlling infectious diseases.
The term pathogenic underscores the relationship between biological agents and disease. By characterizing pathogenic factors and host responses, researchers develop strategies to prevent, detect and treat infections.
Related Terms: Pathogenicity, Pathogen, Virulence, Opportunistic pathogen, Nonpathogenic