Microbial evolution refers to the genetically driven changes that occur in microorganisms and that are retained over time. Some microbial changes can be in response to a selective pressure. The best examples of this are the various changes that can occur in bacteria in response to the presence of antibiotics. These changes can make an individual bacterium less susceptible or completely resistant to the killing action of one or more antibiotics.
Other microbial changes can occur randomly in the absence of any selective pressure. These changes, which often are due to a change in the sequence of the units (nucleotides) that comprise an organism's genetic material, can confer an advantage to the organism, as compared to unaltered organisms. In the classic scenario of evolution, such as advantageous trait will be retained and can be passed on to future generations of the organism.
Gene transfer between bacteria can occur even between species that are not related to one another. This so-called horizontal gene transfer is an important form of microbial evolution that occurs in nature, and it can be important in infectious disease, for example in the acquisition of a gene that determines antibiotic resistance.
In contrast to Darwinian evolution, which takes place over millions of years, microbial evolution can occur within hours. This is because some bacteria are capable of growing and dividing in about 20 minutes under ideal growth conditions. A bacterium containing an altered gene that confers a survival advantage can, over 24 hours, give rise to thousands of progeny that carry the same gene. Each new bacterium can in turn give rise to thousands of progeny by the next day. Thus, a mutation can rapidly spread in a bacterial population and, because the trait is capable of being transferred to unrelated bacteria, to other bacterial populations as well.
Human-imposed selective pressures, such as the overuse or misuse of antibiotics, factory-farm types of agriculture that crowd animals in a small space, and the encroachment of humans on previously undisturbed territory are influencing microbial evolution and the emergence or re-emergence of infectious diseases.
Darwinian evolution can be depicted as a tree, with the original organism at the base of the trunk and the myriad evolutionary changes that occur over time generating the branches and even smaller twigs at their tips. Put another way, this route of evolution is vertical, with genetic changes transferred from one generation of a species to succeeding generations.
In contrast, evidence that has been accumulating since the 1970s has firmly established that microbial evolution occurs differently. The tree analogy is inaccurate when describing microbial evolution. Rather, microbial evolution is considered to be more like a web or a net, with the transfer of genetic information occurring between many different species simultaneously, rather than between succeeding generations of one particular type of microbe.
This wider, interspecies transfer is called horizontal transfer. It is one route by which a bacterium can become resistant to one or more antibiotics. A bacterium that carries the genetic determinants for resistance to an antibiotic may be able to transfer the gene to another, unrelated bacterium, which then also becomes resistant to the antibiotic.
The transfer of genes between bacteria can occur in several ways. A gene that resides in the deoxyribonucleic acid (DNA) of a donor bacterium can be transferred to the recipient bacterium through a tube that transiently connects the two cells. Once inside the recipient, the inserted DNA can become part of the recipient's genome (its hereditary information encoded in its DNA) and express its encoded product.
Bacterial genes can also reside on more mobile genetic elements known as plasmids. Plasmids are more easily transferable between bacteria. Genes that code for products that render a cell resistant to particular anti-biotics can be located on plasmids. If a bacterium that possesses an antibiotic-resistance gene is adjacent to another bacterium (not necessarily the same type of bacterium), a copy of the plasmid can move to the recipient bacterium, which then becomes resistant to the antibiotic(s).
WORDS TO KNOW
ANTIBIOTIC RESISTANCE: The ability of bacteria to resist the actions of antibiotic drugs.
BACTERIOPHAGE: A virus that infects bacteria. When a bacteriophage that carries the diphtheria toxin gene infects diphtheria bacteria, the bacteria produce diphtheria toxin.
HORIZONTAL GENE TRANSFER: Horizontal gene transfer is a major mechanism by which anti-biotic resistance genes get passed between bacteria and accounts for many hospital-acquired infections.
MUTATION: A mutation is a change in an organism's DNA that occurs over time and may render it less sensitive to drugs which are used against it.
PLASMID: A circular piece of DNA that exists outside of the bacterial chromosome and copies itself independently. Scientists often use bacterial plasmids in genetic engineering to carry genes into other organisms.
SELECTIVE PRESSURE: Selective pressure refers to the tendency of an organism that has a certain characteristic to be eliminated from an environment or to increase in numbers. An example is the increased prevalence of bacteria that are resistant to multiple kinds of antibiotics.
A third genetic mechanism of bacterial evolution involves bacteriophages—viruses that specifically infect a particular type of bacteria (for example, various types of coliphages infect various strains of Escherichia coli). When a bacteriophage infects a bacterium, the viral genetic material can insert into the host's genetic material. When the viral material is excised, some of the host's genetic material can be removed as well, to become part of the genome of the bacteriophage. A subsequent infection by the bacteriophage of another bacterium can transfer genes from the first bacterium to the second bacterial host. If the new gene confers an advantage to the second bacterium, it will be retained and passed on to subsequent generations of that bacterium.
The processes described above are directed in the sense that a genetic trait that changes an organism is transferred from one organism to another. In contrast, a final mechanism of microbial evolution—mutation— can occur randomly. A change in the arrangement of nucleotides that makes up a gene can occur by chance during the replication of the DNA. For example, one nucleotide can be substituted for another. Alternatively, additional nucleotides may be accidentally inserted or may be deleted. If the genetic change is not drastic enough to completely disable the gene's action, then the protein produced will be different. Sometimes this difference can be advantageous to the microbe. For example, the altered protein may produce enhanced activity of an enzyme that degrades antibiotics, or it may produce a membrane protein that adopts a different three-dimensional configuration that makes the microbial surface more resistant to antimicrobial compounds. Once again, such an advantageous mutation will be retained and can be passed to subsequent generations.
The ability of bacteria to evolve via horizontal gene transfer has been exploited in genetic engineering that involves the deliberate insertion of a certain gene into a recipient bacterium and the expression of the gene product by the recipient. Indeed, this aspect of biotechnology is essentially a faster version of the natural pace of microbial evolution.
The acquisition of a gene by a microorganism can be tracked. Similar genes can be isolated from various organisms and the sequence of nucleotides that makes up the gene can be deduced. By comparing the gene sequences, researchers can determine how precisely the sequences match. Sequences from different organisms that match exactly provide strong evidence that the gene arose in a single organism and was passed on to another organism. Since changes in a genetic sequence will occur randomly over time, the degree of gene difference can be used as an indication of how recently a gene was acquired by one microbe, relative to another. In this way it is possible to generate a sort of map of the movement of a gene among microbes over a long period of time.
The ability of disease-causing microorganisms, particularly bacteria and viruses, to evolve is a fundamentally important factor in infectious diseases. For example, the horizontal acquisition of a gene that encodes for the production of a potent and destructive toxin created Escherichia coli O157:H7, which can cause a serious and even lethal infection in humans. Without the gene, E. coli is a normal and harmless resident of the intestinal tract of warm-blooded creatures, including man. In another example, genetic changes have also spawned a variety of Mycobacterium tuberculosis that is resistant to all antibacterial agents currently used to treat tuberculosis. The fact that the bacterium is also easily passed from person-to-person is a cause for concern.
The emergence of avian influenza (caused by an influenza virus designated H5N1) is one example of how human agricultural practices can influence microbial evolution. The tremendous crowding together of poultry that is done to optimize the income generated by a poultry farm made it easier for viral disease to spread in a flock. Then, the ability of many viruses to rapidly mutate allowed the avian influenza virus to spread, first, from bird-to-human and now from human-to-human. While the latter is still rare, the number of cases of human transmitted H5N1 infection is growing and the geographical range is expanding. The possibility that this serious and sometimes fatal disease will develop into a global epidemic is real and has spurred efforts by agencies, including the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC), to monitor the disease and participate in efforts to develop a vaccine.
The fact that microbial evolution can be manipulated in the laboratory has implications for bioterrorism. In the aftermath of World War II (1939–1945), a number of countries, including the United States, engaged in research aimed at designing more potently infectious bacterial and viral diseases. While this research was discontinued, the advent of molecular biology in the 1970s has created legitimate fears that rogue nations or organizations could design and deploy a deadly version of a contagious microorganism.
Harvey B. Simon is a physician and an associate professor of medicine at Harvard Medical School. He also serves as a consultant in infectious disease at Massachusetts General Hospital in Boston, Massachusetts. In this article appearing in Newsweek magazine in December 2006, Simon discusses the evolution of some increasingly troublesome microorganisms.
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Seifert, H. Steven, and Victor J. Dirta, eds. Evolution of Microbial Pathogens. Washington, DC: ASM Press, 2006.