This process is repeated with as many stages as desired, and then the final product is studied, often in comparison with the original virus.
This sort of facilitated transmission is often conducted in a laboratory setting, because it is of scientific interest to observe how the virus or bacterium that is being passed evolves over the course of the experiment.
[1] This can also create strains that are more transmissible in addition to lower virulence, as demonstrated by A/H5N1 passage in ferrets.
In the in vitro method, a virus or a strain of bacteria will be isolated and allowed to grow for a certain time.
After the sample has grown for that time, part of it will be transferred to a new environment and allowed to grow for the same period.
When serial passage is performed either in vitro or in vivo, the virus or bacterium may evolve by mutating repeatedly.
Accordingly, after serial passage has been performed it can be valuable to compare the resulting virus or sample of bacteria to the original, noting any mutations that have occurred and their collective effects.
In particular, Pasteur worked with cholera and found that if he cultured bacteria for long periods of time, he could create an effective vaccine.
[7][8] Pasteur thought that there was something special about oxygen and this was why he was able to attenuate (create a less virulent version of) the bacteria.
[8] Ultimately, to create his vaccine for rabies, Pasteur used a simple method that involved drying out tissue.
[12] Another study by Kanta Subbaro involved a serial passage experiment in which mice were infected with SARS.
[13] This principle has public health implications, because it suggests that, in very densely populated or overcrowded areas, such as slums or group quarantine facilities, natural selection may favour more virulent viruses.
Good hygiene selects against highly virulent viruses by lowering the ability of pathogens to transmit.
Several serial passage experiments have been conducted to determine the feasibility of the virus becoming transmissible in humans.
[13][16] Similarly, researcher Yoshihiro Kawaoka found that a single mutation is necessary to make the virus transmissible in ferrets.
Accordingly, the likelihood that the H5N1 virus would actually mutate to become transmissible in humans is unknown; however, researcher Derek Smith created an evolutionary model to show that this is possible.