Ubiquity of Bacteriophages
I’m hitting a milestone today; it’s the 100th posting here on YCPMicro! I found an interesting, and topical article via Science Daily. It summarizes this article from PLOSone, an online journal at the National Institutes of Health. Freshmen undergraduate students at Washington University in St. Louis can take a fascinating course in Biotechnology. In the Fall semester, students isolate novel bacteriophages from the soil in the region, the genetic sequence of the phages are determined during Winter break, and in the Spring semester the student analyze the genomes of their “pet” phages. Students get naming rights to their phage, which has resulted in these classic names: Angelica, Uncle Howie, Corndog, Fruitloop, Tweety, Predator, and Gumball.
Bacteriophages are likely the most common biological entities on the planet, with an estimated 1031 virus particles contributing to 1023 infections every second. To put that into perspective, there are roughly that number of atoms in 6 gram hunk of charcoal. That’s a lot of phages to find!
The students used the bacterium Mycobacterium smegmatis, which is related to the acid-fast organisms responsible for tuberculosis, but doesn’t cause human disease. Soil samples were brought into the lab, and bacteriophages were isolated using the plaque forming assay. As we have learned in lecture, bacteriophage infection can lead to rapid destruction of the bacterial host as the enzymes and ribosomes of the bacterium make new phage particles. This process leads to lysis of the bacterial host, and release of new phages, which then go on to infect neighboring bacteria, beginning the cycle all over again. The cycle of infection and lysis will go on until the phages run out of bacteria that they can infect.
This process can be followed using a technique called the plaque assay. If a sample of bacteria is spread onto a plate of nutrient agar and incubated, the bacteria will grow until they form a lawn of bacteria. If a sample of bacteria are mixed with a small number of bacteriophages and then plated onto nutrient agar, those phages will infect single bacterial cells, and initiate the process resulting in lysis of the host and release of many phage progeny. The progeny then initiate the cycle of infection and release. On the macroscopic level, this can be observed by the appearance of clear zones on the bacterial lawn called plaques, which represent areas where phage lysis has destroyed all of the bacteria present. These plaques will continue to slowly grow, until the bacteria reach stationary phase. If this experiment were done in broth culture, the increase in phage progeny can quickly result in complete lysis of all bacteria present. Each plaque on the petri dish represents progeny phage from a single bacteriophage infection, and therefore a single original phage. This process can then be used to count the number of viable virus particles in a mixture.
So back to the PLOSone article linked above. If you click through to the link, you’ll notice that there is an impressive list of authors for that article. Most of those authors (with the exception of about a half dozen names there) are undergraduate students who worked on that project. Genetic analysis indicates that there is a huge amount of diversity in the phages that infect Mycobacteria, and that the majority of them were essentially undescribed by science to date. Two take-home messages from this posting: first, there is an incredible amount of biological diversity everywhere we look, and second, undergraduate research is an important part of the scientific process.
BONUS: List what you would name your phage, and why you would call it that!