By Alex Peterson
Observing the interaction between phage and Mycobacterium smegmatis has helped me better understand why humans, multi-celled organisms, have become the dominant species on the planet.
My initial problem with our success stemmed from the fact that single-celled organisms are able to evolve so much faster than we can. They have a genetic pool of billions upon billions to draw upon; we only have six billion. They can reproduce asexually to quickly propagate an adaptation; we cannot–at least without ethically objectionable cloning. And the amount of genetic shuffling that happens on the microscopic level- through cycles of sexual reproduction that require only a few minute & through the exchange of specific genetic information during conjugation (imagine being able to touch someone and absorb only those qualities you most liked about him or her) far outstrips our own. All in all, humanity’s ability to stumble upon favorable mutations and propagate them is inferior to that of even the lowliest amoeba.
Why, then, are we not driven to extinction by these evolutionary superior microorganisms? And why would single-celled organisms give rise to multi-celled organisms in the first place? The answer lies in the plaques phage form in smeg. Asexual reproduction and constant genetic sharing has the potential to create vast colonies of nearly-identical bacteria, leaving them frightfully vulnerable to specialized predatory microorganisms. When phages, bacterial viruses, are introduced to colonies of bacteria, almost all of them die. My first few serial dilutions of phage cleared the entire plates. That is the norm–according to our laboratory manual–which offers instructions for diluting phage by factors of up to 10,000 parts buffer per part phage sample and then refrigerating the samples to further retard the phage’s natural ability to kill. Even then, many small plaques–on the microscopic level, vast killing fields overflowing with dead bacteria–form on the plates.
Our body composition protects us from such a fate. Think of the human body as a colony of cells, similar to the colony of bacteria we see on a plate. The difference is that our colonies consist of many different cells–neurons, muscle cells, skin cells, red blood cells, white blood cells,…, and even symbiotic bacteria–while the smeg plate contains only smeg. Therefore, the communities in which our cells gather, our bodies, are better at resisting specialized microbial attack than colonies of microbes. Our body’s inferior genetic diversity is compensated for by its superior ability to express physiological diversity on a cellular level.
Phage hunting helped provide an answer to the question of how we multi-cellular organisms can successfully compete against single-celled organisms: superior viral resistance. Indeed, without viruses, the multi-celled structure that makes the contemplation of this question possible, the brain, might never have evolved.