Serial passage refers to the process of growing bacteria or a virus in iterations. For instance, a virus may be grown in one environment, and then part of that virus 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 interesting to observe how the virus or bacterium that is being passed evolves over the course of an experiment. In particular, serial passage can be quite useful in experiments that seek to change the virulence of a virus or other pathogen. One consequence of this is that serial passage is useful in creating vaccines, since scientists can apply serial passage and create a strain of virus that has low virulence.
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 period of time. After the sample has grown for some time, part of it will be transferred to a new environment and allowed to grow for the same period of time. 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; this number is determined by the individual experiment.
When serial passage is performed either in vitro or in vivo, the virus or bacterium that is being manipulated may evolve by mutating repeatedly.
Identifying and studying mutations that occur through serial passage 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 what the collective effect of these mutations is. A variety of significant outcomes are possible. The virulence of the virus may be changed, for example, or a virus could evolve to become adapted to a different host environment than that in which it is typically found. Notice that relatively few passages are necessary to produce a noticeable change in a virus; for instance, a virus can typically adapt to a new host within ten or so passages.
In fact, exactly because serial passage allows for rapid evolution of a virus to its host, it can be used to study the evolution of antibiotic resistance; specifically, for determining what mutations could lead to the development of antibiotic resistance.
Pasteur created several vaccines over the course of his lifetime. His work prior to rabies involved virus attenuation, 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 virus. Pasteur also tried to apply this method to create a vaccine for anthrax, although with less success.
Next, Pasteur wanted to apply this method to create a vaccine for rabies. However, unknown to him, rabies is a virus, and so unlike cholera and anthrax, which are bacterial pathogens, rabies is invisible[clarification needed] and thus Pasteur found that it could not be cultured in the same way that cholera and anthrax could be. Methods for serial passage for viruses in vitro were not developed until the 1940s, when John Enders, Thomas Weller, and Frederick Robbins developed a technique for this. These three scientists subsequently won the Nobel Prize for their major advancement.
To solve this problem, Pasteur worked with the rabies virus in vivo. In particular, he took brain tissue from an infected dog and transplanted it into another dog, repeating this process multiple times, and thus performing serial passage in dogs. These attempts increased the virulence of the virus. Then, he realized that he could put dog tissue into a monkey to infect it and then perform serial passage in monkeys. After completing this process and infecting a dog with the resulting virus, Pasteur realized that the virus was less virulent. Mostly, Pasteur worked with the rabies virus in rabbits. Ultimately, to create his vaccine for rabies, Pasteur used a simple method that involved drying out tissue. As is described in his notebook:
In a series of flasks in which air is maintained in a dry state…each day one suspends a thickness of fresh rabbit spinal tissue taken from a rabbit dead of rabies. Each day as well, one inoculates under the skin of a dog 1 mL of sterilized bouillion, in which has dispersed a small fragment of one of these desiccated spinal pieces, beginning with a piece most distant in time from when it was worked upon, in order to be sure that it is not at all virulent.
So, Pasteur mostly used other techniques besides serial passage to create his vaccines. However, the idea of attenuating a virus through serial passage still holds.
Use in vaccines
One way to attenuate a virus to a host is by passing the virus in a different species. The idea is that, as a strain of a virus becomes more adapted to a different species, that strain will become less adapted to the original host, thus decreasing in virulence with respect to the original host. This is the implicit principle that Louis Pasteur was unknowingly making use of when he passed the rabies virus in monkeys and ended up with a virus that was less dangerous to dogs, for example.
The process of serial passage yields a live vaccine. There are both advantages and disadvantages to this. Most notably, live vaccines are sometimes more effective and more long-lasting than inactivated or other types of vaccines. However, just as the virus evolved to become attenuated, it may reverse-evolve in the host, leading to infection.
There have been a large number of experiments conducted that use the technique of serial passage. Some of the experimental uses for serial passage include changing the virulence of a virus, to study the 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, too, 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.
For example, one study 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. This makes it challenging for scientists to use HIV-2 in animal models of HIV-1, since the animals in the model will only show symptoms slowly. However, the more virulent strain of HIV-2 could be practical for use in animal models.
Another study by Kanta Subbaro involved a serial passage experiment by infecting mice with SARS. SARS usually does not make mice particularly sick, however, after the virus had undergone serial passage in the mice, it had become lethal.
Changing the virulence of SARS in this way was important, because without a virulent form of SARS to infect laboratory animals, scientists would have been unable to test the effects of SARS in an animal model.
However, more generally, this experiment also reflects a general medicinal principle: The virulence of a virus is mediated by the difficulty of its transmission.
Generally, if a virus kills its host too quickly, the host will not have a chance to come in contact with other hosts and transmit the virus before dying. However, in serial passage, when a virus was being transmitted from host to host regardless of its virulence, such as Subbaro’s experiment, the viruses that grow the fastest (and are therefore the most virulent) are selected for.
This principle has public health implications, because it suggests that, in very densely populated or overcrowded areas, such as slums, natural selection may favour more virulent viruses. However, more generally, this experiment also reflects a general medicinal principle: The virulence of a virus is mediated by the difficulty of its transmission.
This also helps to explain why good hygiene is so important. Good hygiene selects against highly virulent viruses by lowering the ability of pathogens to transmit.
The H5N1 virus is a particularly lethal strain of influenza. Currently, it can infect humans, but it is not contagious. Still, over 600 people have died from the H5N1 virus, and so the transmissibility of the virus is of major concern to scientists.
Several serial passage experiments have been conducted to determine the feasibility of the virus becoming transmissible in humans. In particular, Ron Fouchier and his colleagues did a 10-step serial passage experiment in ferrets. In doing so, they created a strain of influenza that not only infected ferrets, but was transmissible between ferrets. Notably, this strain was very similar to the original strain with which they had infected the first ferret—in other words, there were only a few mutations that were necessary for the virus to become transmissible from ferret to ferret. Similarly, researcher Yoshihiro Kawaoka found that a single mutation is necessary to make the virus transmissible in ferrets.
Serial passage is an artificial technique that is used in a laboratory setting, rather than a natural process. 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.
Understanding how viruses jump between species
Another use of serial passage is in understanding how pathogens adapt to new species. By introducing a pathogen into a new host species and performing serial passage, scientists can observe as the pathogen adapts to its new host and pinpoint the mutations that allow for this adaption.
Alternatively, serial passage can be used to help determine whether a species could act as an intermediate host species and help a pathogen jump from one species to another. In 2011, Sun et al. performed an experiment to determine the dangers posed by quail as a possible intermediate host for H5N1. The researchers performed serial passage on two strains of H5N1 that are quite pathogenic to quail. They then infected mice with the two resulting strains of H5N1 and compared their effects to the effects of the original strains. One of the strains did not significantly impact virulence, but the other strain was more pathogenic to mice than the original had been. The researchers concluded that quail infected with H5N1 could act as a sort of "stepping stone", making it easier for H5N1 to transition from infecting birds to infecting mammals:
The findings showed that quails might play an important role in the adaptation of H5N1 avian influenza viruses to mammals. Therefore, researchers should enhance surveillance of H5N1 HPAI viruses in the quail population especially in live-bird markets.
Monitoring quail could allow researchers to prevent people from coming into contact with infected quail, and hopefully prevent quail from acting as an intermediate host for H5N1.
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