By Sebastian Amyes
Bacteria have achieved many firsts; they were the first cellular life-forms on the planet, they are the primary biomass on the planet; they are the most prevalent cell type in and on the human body outnumbering our own cells; they are responsible for more human deaths than any other infectious agents; and, in some parts of the world, they are the premier cause of all deaths. How did these small, single-cell organisms, that are invisible to the naked eye become so successful? Essentially this has been through rapid evolution leading to adaptability. All living organisms evolve. The speed at which they can do this is dependent on the generation time; for humans this is about 25 years whereas for bacteria it is often measured in minutes, sometimes as little as 20 minutes. It is believed that 99% of the species that have lived on the planet are now extinct; this is often because their generation time was too long for the necessary evolutionary adjustments needed to survive changes in their environment. Every year yet more species of animals and plants become extinct because they have been too specialised to adapt.
The rapid division of bacteria means that they can adapt overnight to changes in their surroundings. A prime example has been the development of antibiotic resistance in clinical bacteria. It has often been reported that the US Surgeon General indicated in the 1960s that the discovery, first of penicillin, and then of the rest of antibiotics heralded the end of clinical bacterial infections. It is now common knowledge that such a view was fatally flawed. Simple mutations in key genes during cell division provided the bacteria with a means of escaping the action of the antibiotic (resistance). Once learned and part of the bacterial DNA, these genes could then be passed on to other bacteria by the process known as conjugation (bacterial sex) so that these new bacteria benefited from the resistance “learnt” in earlier bacteria. Seventy years ago, almost all clinical bacteria were sensitive to all antibiotics; now many bacteria are resistant to some, some bacteria are resistant to most, and a few bacteria are resistant to all antibiotics. Within one human lifetime, clinical bacteria have evolved the means of overcoming all the antibiotics we can produce.
Witnessing this remarkable adaptive ability, it is hardly surprising that bacteria have been able to inhabit all parts of the planet, from hot springs to the Antarctic, from mountain tops to the bottom of the ocean. The demise of any species is often dependent on the loss of its food supply. Bacteria evolve so quickly that they can adapt to use different nutritional sources. They have evolved so that they can live off virtually any organic matter, they can even adapt to use crude oil. Like some insect populations, bacteria form colonies. Many bacterial colonies comprise one billion individual cells or more. Total eradication of that number of bacteria is difficult and often impossible. Unlike bees, ants, and wasps, for example, the survival of that colony is more egalitarian and is not dependent on a single individual, the queen. If there is a catastrophe, any one of the individual bacterial cells in a bacterial colony can go on to form a new colony if it can survive the eradication of the previous colony. When it has formed a new colony and the next threat comes, the same survival tactic is engaged.
Bacteria preceded mammals by nearly four billion years. It is almost certain that they will be predominant long after humans and other mammals are extinct. There have been suggestions that bacteria arrived on Earth on meteorites; this may be true but it is more likely that they evolved here. However, we have already sent our bacteria into space on satellites and these may, at some time, colonise other planets. Here on Earth, our own bacteria will continue to thrive. As we have unearthed the fossil record, we have classified different eras in geological time, which are often colloquially rephrased as epochs such as the “Age of the Dinosaurs” or the “Age of the Fish”. As they have always been the largest biomass, the truth is that from the Precambrian era, four billion years ago, the Earth has always been in the “Age of the Bacteria” and probably will be forever.
Sebastian Amyes is Professor of Microbial Chemotherapy at the University of Edinburgh. He has specialised on the development of antibiotic resistance in bacteria. He has published more than 500 papers on bacteria and written a number of books on the subject, including Bacteria: A Very Short Introduction.
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