Molecular Koch's postulates are a set of experimental criteria that must be satisfied to show that a gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 found in a pathogenic microorganism

Microorganism

A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

 encodes a product that contributes to the disease caused by the pathogen. Genes that satisfy molecular Koch's postulates are often referred to as virulence

Virulence

Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

 factors. The postulates were formulated by the microbiologist Stanley Falkow

Stanley Falkow

Stanley Falkow, PhD, is microbiologist and a professor of microbiology and immunology at Stanford University School of Medicine. He is sometimes referred to as the father of molecular microbial pathogenesis, which is the study of how infectious microbes and host cells interact to cause disease at...

 in 1988 and are based on Koch's postulates

Koch's postulates

Koch's postulates are four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890...

.

The postulates as originally described by Dr. Falkow are as follows:

1. "The phenotype
   Phenotype
   A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
   
    or property under investigation should be associated with pathogenic members of a genus or pathogenic strains of a species
   Species
   In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
   
   ." Additionally, the gene in question should be found in all pathogenic strains of the genus or species but be absent from nonpathogenic strains.
2. "Specific inactivation of the gene(s) associated with the suspected virulence trait should lead to a measurable loss in pathogenicity or virulence
   Virulence
   Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
   
   ." Virulence of the microorganism with the inactivated gene must be less than that of the unaltered microorganism in an appropriate animal model.
3. "Reversion or allelic replacement of the mutated gene should lead to restoration of pathogenicity." In other words, reintroduction of the gene into the microbe should restore virulence in the animal model.
4. "The gene, which causes virulence, must be expressed during infection."
5. "Immunity must be protective."

For many pathogenic microorganisms, it is not currently possible to apply molecular Koch's postulates to a gene in question. Testing a candidate virulence gene requires a relevant animal model of the disease being examined and the ability to genetically manipulate the microorganism that causes the disease. Suitable animal models are lacking for many important human diseases. Additionally, many pathogens cannot be manipulated genetically.