Bacterial diseases

Definition: Single-celled organisms lacking a nucleus, found in and on humans and widespread in the environment.

Significance: Bacteria are ubiquitous on Earth, and some species can cause disease in humans. An understanding of the classification of bacteria as well as the ways in which bacterial populations grow and reproduce is useful to the identification, diagnosis, and treatment of bacterial diseases.

The tiny unicellular organisms known as bacteria define the biosphere on Earth—that is, if bacteria do not inhabit a particular environment, no living things reside there. Bacteria are extremely adaptable and have managed to exploit a wide variety of habitats successfully. One niche exploited by bacteria is the human body. Humans support a population of more than two hundred species of bacteria in numbers greater than the cells that make up an individual human host. These members of the normal flora are found on the skin and in the digestive, urinary, reproductive, and upper respiratory tracts of humans.

Although some species of bacteria can cause disease in humans, other animals, and plants, the majority of bacterial species are not pathogenic (disease-causing). Bacteria are key players in the ecology of the Earth, functioning in important roles in global chemical cycles. Perhaps most important, bacteria are the only organisms on Earth that possess the ability to fix nitrogen—that is, to convert the nitrogen gas in the atmosphere to a form that is usable by other organisms.

Disease-causing bacteria have attracted the most interest and study since the confirmation of the germ theory of disease by Louis Pasteur and Robert Koch in the 1870s. It is interesting to note that Koch’s proof that germs cause disease involved the bacterium Bacillus anthracis, which causes anthrax, an organism that has been used as a biological weapon.

The first sixty years of the study of medical bacteriology focused on identification and diagnosis, with little attention to the basic biology of bacteria. The discovery and development of antibiotics led to an overly optimistic view that infectious disease had been conquered. The emergence of antibiotic-resistant strains of bacteria as well as outbreaks of previously unknown pathogens stimulated a renewed interest in bacteriology.

Classification and Taxonomy

Bacteria are classified as prokaryotic cells—that is, the genetic material of a bacterium is not enclosed in a nucleus. This lack of a nucleus distinguishes bacterial cells from the cells that make up plants and animals, which are classified as eukaryotic. Additional differences between bacterial cells and eukaryotic cells include the types of molecules found in the cell walls, organization and expression of genes, and sensitivity to certain antibiotics.

Bacteria themselves have been classified in several ways. In 1923, the first edition of Bergey’s Manual of Determinative Bacteriology offered descriptions of all the species of bacteria then identified, an outline of the taxonomic relationships among bacteria, and keys for diagnosis of diseases caused by bacteria. The ninth edition of Bergey’s Manual, published in 1994, focuses primarily on identification of bacteria and uses taxonomic divisions that do not necessarily reflect evolutionary relationships.

During the 1980s, Bergey’s Manual of Systematic Bacteriology was published in an attempt to organize bacterial species into the type of hierarchical classification schemes that have been applied to eukaryotic organisms. This manual later underwent revision to include new species and to cover the progress that had been made in molecular classification methods.

The International Committee on Systematics of Prokaryotes (ICSP) is the organization that oversees the nomenclature of prokaryotes and issues opinions concerning related taxonomic matters. When a researcher discovers a previously undescribed bacterium, the ICSP must approve the researcher’s proposed name for the newly described species as well as the taxonomic classification of the species.

Clinically, classification of bacteria is performed primarily to diagnose particular diseases. Identification of bacteria in a clinical specimen can be accomplished through direct microscopic examination, isolation and culture of the responsible bacteria, and biochemical and immunological tests. Researchers have developed and marketed a number of automated microbial diagnosis systems that allow rapid diagnosis without the need to isolate the organisms of interest.

Cell and Population Growth

In discussing the growth of living organisms, one can focus on the growth of an individual or the growth of a population. Because bacteria are single-celled organisms, growth of an individual bacterium does not include development of organs or other body parts, but rather just enlargement of the cell itself.

Discussion of the growth of bacterial species is usually concerned with the growth of a population of cells. Because almost all bacteria reproduce through the division of one cell into two, the growth of a population of bacterial cells is geometric—that is, the population doubles in size with each round of cell division. The length of time required for a population of bacterial cells to double varies depending on the species and strain of bacteria as well as on the environmental conditions, including temperature, pH, nutrient availability, and waste accumulation.

Some bacteria, such as Escherichia coli, have a maximum doubling rate of less than thirty minutes. At this rate, a single cell could generate a population of one million cells in less than ten hours. In fact, if the environmental conditions remained optimal, with ready nutrients and regular waste removal, a culture of maximally reproducing E. coli bacteria would equal the mass of the planet Earth within one week. Other bacteria, such as Mycobacterium tuberculosis, divide much more slowly, taking twelve to eighteen hours under optimal conditions for one round of binary fission. The optimal growth rates estimated for many bacteria are merely speculative because the majority of species have not yet been cultured on defined or artificial media.

Even slowly dividing bacteria can reproduce in far less time than nearly every other type of organism. Because of their rapid reproductive rates and omnipresence in the living world, bacteria can rapidly overwhelm any unpreserved biological sample. Unrefrigerated food, blood and tissue samples, and other biological specimens can quickly become host to a diverse, rapidly growing population of bacteria.

Reproduction

Most bacteria reproduce by binary fission. One cell grows by manufacturing more cellular components. The genome is replicated, and the single cell divides into two essentially identical cells. This type of reproduction is termed asexual because it does not involve the recombination of genetic material from two parents. Because the cells that result from binary fission are virtually identical genetically, the individual cells in a group or colony of bacteria all descended from a single ancestral cell could well be clones of the original cell.

The cellular machinery involved in replicating the genetic material does not perform this replication with perfect fidelity. At each round of replication, there is a finite probability of errors occurring. These errors lead to changes in the genetic material known as mutations. These mutations may result in cells with characteristics that are different from those of the other cells in the population. These altered characteristics may lead to cells that are better adapted to a particular environment—perhaps the ability to metabolize a new nutrient or survive in the presence of an antibiotic. Because bacterial cells reproduce by simple cell division, altered characteristics are transmitted to all offspring of the altered cell (barring further mutation).

Although bacteria do not reproduce sexually by recombination of genetic material from two parents, many bacteria are capable of obtaining genetic material from other cells through various methods. Some bacteria can take up DNA (deoxyribonucleic acid) from the environment (probably released from decomposing cells), can receive DNA through viral infections, and can transfer DNA directly from one living cell to another. These genetic recombination processes allow genes (such as those that confer antibiotic resistance) to be spread throughout a bacterial population rapidly.

Bibliography

Betsy, Tom, and James Keogh. Microbiology Demystified. New York: McGraw, 2005. Print.

Cliff, John B., et al. Chemical and Physical Signatures for Microbial Forensics. Humana Press, 2012.

Madigan, Michael T., John M. Martinko, Paul V. Dunlap, and David P. Clark. Brock Biology of Microorganisms. 14th ed. Upper Saddle River: Pearson, 2015. Print.

Nester, Eugene W., Denise G. Anderson, Jr., C. Evans Roberts, and Martha T. Nester. Microbiology: A Human Perspective. 7th ed. New York: McGraw, 2012. Print.

Pommerville, Jeffrey C. Alcamo’s Fundamentals of Microbiology. 10th ed. Sudbury: Jones, 2014. Print.

Willey, Joanne, Linda Sherwood, and Chris Woolverton. Prescott, Harley, and Klein’s Microbiology. 7th ed. New York: McGraw, 2007. Print.