This lesson explains how scientists connect a microorganism to a disease. It introduces Koch’s original postulates, their limitations, and the molecular postulates that examine the role of specific genes.
In 1884, physician Robert Koch proposed four criteria for establishing that a particular microorganism causes a particular disease. Together, the criteria describe a process involving observations in diseased and healthy hosts, laboratory culture, infection of a test host, and recovery of the pathogen.
All flashcards
What are Koch’s original postulates?
Four criteria linking a pathogen to disease: it is associated with disease, can be isolated and cultured, causes the same disease in a healthy susceptible host, and can be re-isolated as the same pathogen.
Why can an asymptomatic carrier challenge Koch’s first postulate?
The first postulate expects the pathogen to be absent from healthy individuals, but carriers can harbor and sometimes transmit pathogens without showing symptoms.
Why can differences between hosts complicate disease experiments?
Hosts vary in immune status, age, health, and microbiota, so they may not be equally susceptible or develop the same symptoms.
What do molecular Koch’s postulates investigate?
They investigate whether genes contribute to pathogenicity by associating a gene with disease, disabling it to test for loss of pathogenicity, and restoring it to test for recovery of the disease phenotype.
What is one limitation shared by the original and molecular postulates?
Both can be difficult to apply when no suitable animal model exists; molecular postulates are also limited when a pathogen cannot be genetically manipulated.
Quiz questions
What does Koch’s fourth postulate require after a test subject develops the disease?Answer: Re-isolate the same pathogen from the newly diseased host.
Why can the first original postulate fail for some pathogens?Answer: Some healthy hosts carry a pathogen without symptoms.
Why are viruses difficult to test using the pure-culture requirement?Answer: They require living host cells to reproduce.
What is the main focus of molecular Koch’s postulates?Answer: Whether specific genes contribute to disease-associated traits.
A suspected virulence gene is disabled and disease-causing ability decreases. What further result would support the gene’s role?Answer: Restoring the gene restores the disease-associated phenotype.
Based on Microbiology by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.