Serial passage

Serial passage is the process of growing bacteria or a virus in iterations. For instance, a virus may be grown in one environment, and then a portion of that virus population can be removed and put into a new environment. 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 (experimental evolution). In particular, serial passage can be quite useful in studies that seek to alter the virulence of a virus or other pathogen. One consequence of this is that serial passage can be useful in creating vaccines, since scientists can apply serial passage and create a strain of a pathogen that has low virulence, yet has comparable immunogenicity to the original strain. This can also create strains that are more transmissible in addition to lower virulence, as demonstrated by A/H5N1 passage in ferrets.

Mechanism

Serial passage can either be performed in vitro or in vivo. 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. This process will be repeated as many times as desired.

Alternatively, an in vivo experiment can be performed where an animal is infected with a pathogen, and this pathogen allowed time to grow in that host before a sample of it is removed from the host and passed to another host. This process is repeated for a certain number of hosts; the individual experiment determines this number.

When serial passage is performed either in vitro or in vivo, the virus or bacterium may evolve by mutating repeatedly. Identifying and studying mutations that occur often reveals information about the virus or bacterium being studied. 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. Various significant outcomes may occur. The virulence of the virus may be changed, or a virus could evolve to become adapted to a different host environment than that in which it is typically found.

History

The technique of serial passage has been around since the 1800s. In particular, Louis Pasteur's work with the rabies vaccine in the late 1800s exemplifies this method.

Pasteur created several vaccines over the course of his lifetime. His work prior to rabies involved attenuation of pathogens, but not through serial passage. In particular, Pasteur worked with cholera and found that if he cultured bacteria for long periods of time, he could create an effective vaccine. 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. Pasteur also tried to apply this method to create a vaccine for anthrax, although with less success.

To solve this problem, Pasteur worked with the rabies virus in vivo. However, just as the virus evolved to become attenuated, it may reverse-evolve in the host, leading to infection.

Experiments

Researchers have conducted many experiments using serial passage. Some of the experimental uses for serial passage include changing the virulence of a virus, to study the adaptive evolution or potential evolution of zoonotic diseases to new hosts, and studying antibiotic resistance.

Increasing virulence for use in animal modeling

When developing vaccines for viruses, the emphasis is on attenuating the virus, or decreasing its virulence, in a given host. Sometimes it is useful to employ serial passage to increase the virulence of a virus. Usually, when serial passage is performed in a species, the result is a virus that is more virulent to that species. used serial passage in baboons to create a strain of HIV-2 that is particularly virulent to baboons. Typical strains of HIV-2 only infect baboons slowly. SARS usually does not make mice particularly sick, however, after the virus had undergone serial passage in the mice, it had become lethal.

Influenza

The H5N1 virus is a particularly lethal strain of influenza. Currently, it can infect humans, but it is not contagious between humans. Still, over 600 people worldwide are known to have died from animal-transmitted H5N1 virus, Similarly, researcher Yoshihiro Kawaoka found that a single mutation is necessary to make the virus transmissible in ferrets. delineated fundamental differences between continuous and serial passage ecosystems and work by Hal Smith explored the applicability of the competitive exclusion principle to serial passage. Later, Erez and Lopez et al. predicted that serial passage ecosystems are strongly structured by an Early-bird effect in which otherwise poor competitors can leverage early advantage to outcompete other organisms. Experimental work has demonstrated the existence of this effect empirically.