Seminar One
Biology, culture and psychology
Microbes

An excerpt from the keynote address to the 2001 convention of the World Pathogen Association

”There are many factors that give us hope. Homo is remarkably hospitable to us. In extraordinary numbers, they are now undernourished and immunodeficient, and they have a penchant for keeping on the move, thereby not only spreading us veterans but providing many new opportunities for the novices among us to join in the feast…

”They misuse antibiotics… blind to our evolutionary capacity to develop resistance. They pay little attention to our important ability to evolve new levels of virulence. And they are recklessly changing the climate, releasing many of us from our historical geographical constraints…

”All in all, my fellow pathogens, Homo is the opportunity that ultimately can benefit us all. Aside from their prevalence in numbers, they show all the weaknesses that maximize our effective potential. Although they themselves deny that there is such a thing as a free lunch, we know better. There is a free lunch, and it is them.”

Drop down in scale to the world of bacteria and viruses, and we find some frightening examples of natural selection—brought about by our antibiotics. Call it human selection, if you will. In any case, the explanation is the same: environmental changes (antibiotics) have brought about a change in the differential survival of bacterial and viral gene variants, or alleles. Nothing could be simpler or more profound.

The drawing below may help clarify the straightforward way antibiotic resistance works:

In any population of bacteria or viruses, a small percentage has a gene variant. When we bathe the population in antibiotics we change the selective pressure on those gene variants, so that different ones now have an advantage. The variants conferring antibiotic resistance (which arose through chance mutations) now survive best, and bacteria or viruses with those alleles come to dominate the population. If it involves an infection of our body, we may die because the antibiotics fail to control the infection.

Why do we worry about this?

Because viral and bacterial infections have plagued us, and all organisms, for the history of life on Earth. Even bacteria have their viral pathogens, called bacteriophages, or phages, for short. It has been said that there are more pathogens and parasites than free -living organisms, and that is probably true. a better generalization would be that there are more hosts than colonists, because many colonists are neutral or even friendly.

Before we leave viral and bacterial infections, we must ask what the pathogens get out of infecting us and their other hosts. Obviously, they get a chance to replicate and spread. But what if their host dies as a result? It may terminate their spread by that host. The word virulence refers to how deadly a pathogen is and how serious an infection it can cause. If a pathogen is too virulent, it may kill off its hosts too quickly. If it is too avirulent (mild in its effects), it may replicate at too low a rate to spread. Somewhere in the middle may be best for a pathogen. B ut this takes time, and the host is co-evolving along with the pathogen, albeit at a slower rate.

The water we drink may spread antibiotic resistance! Antibiotic-resistant bacteria have become so common in aquatic ecosystems that some researchers suggest their genes should be considered environmental contaminants. Water treatment plants and distribution systems may therefore unintentionally serve as incubators for growth of antibiotic-resistant bacteria and selectively increase antibiotic resistance of bacterial communities through horizontal gene transfer.