Bacteria Article Contents: Growth and Adaptation Diversity Bacterial Classification Also known as: Eubacteria From: Encyclopedia of Life Science. The group of organisms referred to as bacteria once encompassed all the prokaryotic organisms, until phylogenetic analysis demonstrated that prokaryotes consisted of two distinct groups, the Bacteria, also called the Eubacteria or true bacteria, and the Archaea. These groups are now recognized as domains or superkingdoms, a classification level higher than kingdom. Bacteria includes the microorganisms with which most people are familiar—the types that scientists use for research, the strains that inhabit the human intestinal tract, the ones used in food production, pathogenic bacteria, photosynthetic bacteria, and the rest of the Bacteria that inhabit moderate environments.
Campylobacter Bacteria
Enlarge Image Archaea are prokaryotes that often live in extreme environments such as those characterized by very high or very low temperatures, extreme acidity, or high solute concentrations and methaneproducing strains. The names of the domains Bacteria and Archaea are capitalized, whereas the terms bacteria and archaean are not capitalized when referring to microorganisms within the domain. Bacteria differ structurally, biochemically, and physiologically from archaeans.
Growth and Adaptation As prokaryotic cells, bacterial cells lack compartmentalization. They reproduce by binary fission, in which one cell divides into two, each containing a copy of the parental chromosome. During optimal growth conditions, replication of this chromosome often limits the generation time, the period required for a population of cells to double. Under optimal conditions, generation times vary between
species from 20 minutes to a few hours. In a natural setting, conditions never remain optimal as nutrients become depleted and metabolic waste accumulates, poisoning the microbes. Some bacteria can form endospores, tough dormant structures that can withstand harsh environmental conditions and then germinate when the conditions improve. Bacterial growth is measured by the number of cells or the size of the population rather than looking at an individual organism. Bacteria growing in a liquid broth culture or in an aquatic environment cloud the liquid as the numbers increase. On a solid growth medium, a single bacterial cell grows and divides repeatedly, piling up to form a raised, circular collection of cells called a colony. All of the cells in a colony contain copies of the same chromosome. Though clonal populations of bacteria all receive identical copies of the chromosome, bacteria do have mechanisms for creating genetic variation that allows evolutionary adaptation to occur. Growth conditions such as temperature, nutrient availability, pH, oxygen concentration, and the presence of certain chemicals can select for spontaneous or induced mutations, causing them to become fixed. Prokaryotes can also obtain new genes by conjugation, a process in which a hollow tubular structure called a pilus forms a bridge between two cells, allowing for the transfer of deoxyribonucleic acid (DNA). In transformation, bacterial cells uptake pieces of DNA from the environment through their cell membranes. Viruses that infect bacteria, called bacteriophages, can also carry genes from one bacterial cell to another, a process called transduction.
Diversity
The domain Bacteria includes tremendously diverse organisms, making them a very successful group. One area of variation is their nutritional mode—how they obtain their energy and what they use as a carbon source. Some organisms are phototrophs, meaning they obtain their energy from sunlight, and others are chemotrophs, meaning they obtain their energy from chemicals. Organisms that use carbon dioxide (CO2) as their sole carbon source are called autotrophs, and organisms that utilize organic molecules as their main carbon source are called heterotrophs. These terms can be combined to give more detailed information about an organism's nutritional requirements. For example, a photoautotroph uses light as its energy source and CO2 as its carbon source. Bacteria exist that represent all the major nutritional modes: photoautotrophs, chemoautotrophs, photoheterotrophs, and chemoheterotrophs. Bacterial requirements for oxygen (O2) also vary: obligate aerobes require oxygen to survive; facultative aerobes use oxygen if it is present but can ferment in its absence; and oxygen is toxic to obligate anaerobes. Nitrogen is a necessary nutrient for synthesizing amino acids of proteins and nitrogenous bases in nucleic acids, but bacteria have different capabilities with respect to nitrogen metabolism. Some can fix nitrogen, meaning they can convert nitrogen gas (N2) to ammonia (NH3), a form that can be readily
incorporated into organic molecules. Bacteria capable of nitrogen fixation play an important role in the cycling of this nutrient in ecosystems. Bacteria with unique metabolic requirements often live in close association with organisms that have complementary metabolisms.
Bacterial Classification
One means for categorizing bacteria is by evolutionary relationships. Because organisms that are closely related have fewer mutations between them compared with organisms that diverged a long time ago, the molecular differences provide insight into the evolutionary distance between types of organisms. Analysis of the nucleotide sequence of certain ribosomal ribonucleic acid (rRNA) genes divides Bacteria into five major groups (and many more phyla) that share a common evolutionary ancestor: the proteobacteria, chlamydias, spirochetes, cyanobacteria, and grampositive bacteria. Morphologies or physiological characteristics within groups can be extremely variable since the groups are defined by molecular relatedness. The phylum Proteobacteria, the largest of the Bacterial clades, includes more than 1,600 identified species of gramnegative bacteria of all nutritional modes and oxygen requirements. Bacteria are termed gramnegative if they do not retain the crystal violet stain during the Gram staining procedure, and gram positive if they do retain it. The differential reaction depends on the structure of the cell envelope. Some proteobacteria are pathogenic, some are freeliving, and others live in symbiotic relationships. Proteobacteria are divided into five subgroups, named for the first five letters of the Greek alphabet. The class Alphaproteobacteria includes Rhizobium and Agrobacterium species. Rhizobium lives in a symbiotic association with legumes; it resides in nodules of the roots and fixes nitrogen. The plants, belonging to the pea and bean family, benefit from the supply of usable nitrogen and the bacteria benefit from the water and nutrients taken in through the plant's root system.
Root Nodules
Enlarge Image Agrobacterium causes tumor formation in its hosts. Because of its known ability to transfer DNA between itself and the plant, plant geneticists use it to improve crops. Many Agrobacterium species have recently been reclassified as Rhizobium species. The rickettsias, including the human pathogen Rickettsia, are alphaproteobacteria that can only survive as endosymbionts of other cells. Scientists believe the organisms that evolved into eukaryotic mitochondria by endosymbiosis originated from alphaproteobacteria. The class Betaproteobacteria includes many soil and wastewater species, but also some human pathogens such as Neisseria, which causes gonorrhea and a form of meningoencephalitis. Many betaproteobacteria are facultative aerobes, a few are phototrophs, and some have unique metabolisms. The soil microbe Nitrosomonas plays an important role in the cycling of nitrogen by oxidizing ammonium (NH4+) for energy and producing nitrite (NO2) as a waste product.
Gammaproteobacteria includes many medically significant species as well as the sulfur bacteria. Many familiar species belong to this group. Escherichia coli, one wellknown species of gammaproteobacteria, is part of the normal human flora and is used extensively in research. Cluster of E. coli Bacteria
Enlarge Image The causative organisms for cholera (Vibrio cholerae), the foodborne illness salmonellosis (Salmonella enteriditis), typhoid fever (Salmonella typhus), the plague (Yersinia pestis), and an opportunistic human pathogen (Pseudomonas aeruginosa) that often infects the pulmonary tract, the urinary tract, or burns are all members of the class Gammaproteobacteria. The purple sulfur bacteria are capable of photosynthesis and live in hot sulfur springs or stagnant water. They oxidize hydrogen sulfide (H2S) rather than water as plants and algae do and produce granules of elemental sulfur as a product.
Salmonella enteritidis
Enlarge Image Members of Deltaproteobacteria include the unusual myxobacteria, sulfate and sulfurreducing bacteria, and other anaerobic bacteria. Myxobacteria live in the soil, move by gliding in swarms, and produce fruiting bodies when growth conditions are unfavorable. The fruiting bodies release resistant spores that germinate when conditions improve. Sulfatereducing bacteria use sulfate (and sometimes other oxidized sulfur compounds) as an oxidizing agent, reducing it to sulfide, but do not incorporate it in organic compounds. This anaerobic process is called dissimilatory sulfur reduction. Sulfurreducing bacteria obtain energy by reducing elemental sulfur to H2S with hydrogen or organic compounds. Some deltaproteobacteria reduce other oxidized inorganic compounds, such as ferric iron. Members of the class Epsilon Proteobacteria live in animal digestive tracts and sometimes are pathogenic. Examples include Campylobacter, which can cause gastroenteritis in humans, and Helicobacter pylori, which causes stomach ulcers. Bacteria that belong to the phylum Chlamydiae are nonmotile obligate parasites, meaning they cannot complete their life cycle without an animal or protozoan host. Their metabolic capabilities are extremely limited, and their cell walls lack peptidoglycan. One example is Chlamydia trachomatis, a bacterial species that
causes the common sexually transmitted disease nongonococcal urethritis as well as trachoma, a leading cause of blindness in humans. The phylum Spirochaetes includes bacteria that are characteristically long and helical and possess axial filaments. These flagellalike filaments run the length of the cell between the wall and the cell membrane and twist, causing a bacterium to move by rotating like a corkscrew. Most spirochetes are anaerobic and free living, but a few are parasitic. Treponema pallidum causes the sexually transmitted disease syphilis, Borrelia burgdorferi causes Lyme disease, and Leptospira causes leptospirosis. The cyanobacteria are unique in that they are the only prokaryotic organisms that undergo oxygenic photosynthesis, meaning that water serves as the electron donor and oxygen is produced as a byproduct. Evidence strongly suggests that chloroplasts in plants and algae evolved from endosymbiotic cyanobacteria. Also called bluegreen algae because they were once thought to be algae, these aquatic prokaryotes can be unicellular, filamentous, or colonial. Some cyanobacteria can also fix nitrogen, reducing it to NH 4+, which can be incorporated in cellular metabolism. The fifth group of Bacteria, the grampositive bacteria, includes all the bacteria that retain crystal violet when stained by the Gram procedure. The cell walls contain as many as 40 layers of the carbohydrateprotein complex peptidoglycan, and teichoic acids are present in the cell membrane. Grampositive bacteria include freeliving and parasitic forms and consist of two major phyla: the Firmicutes and the Actinobacteria. The largest phylum of grampositive genera is the Firmicutes, whose members have a low percentage composition of the nucleotides guanine (G) and cytosine (C) in their nucleic acid. Firmicutes include two sporeforming genera— Clostridium, which includes the species that cause gas gangrene and botulism, and Bacillus, which includes the species that causes anthrax—in addition to beneficial species that serve as a source of antibiotics and that are used as natural pesticides. Firmicutes also comprise many nonsporeforming genera: Staphylococcus, a normal inhabitant of human skin that can also be pathogenic; Streptococcus, including Streptococcus pyogenes, which causes strep throat and rheumatic fever; Lactococcus, which produces lactic acid as an end product of fermentation; and Enterococcus, which can cause urinary tract infections, bacterial endocarditis, diverticulitis, and meningitis. The last group of Firmicutes is the mycoplasmas, tiny cell wall–less bacteria that evolved from grampositive bacteria. Mycoplasma pneumoniae causes pneumonia. The other major phylum of grampositive bacteria is Actinobacteria, whose members have a high GC content. Soildwelling actinomycetes play an
ecologically important role in the decomposition of organic matter such as cellulose and chitin and are also the source of numerous antibiotics. Some actinobacteria are filamentous and resemble mold. Another type of actinobacteria, the coryneform bacteria, exhibit unique shapes, sometimes resembling the letter Y or V. Corynebacterium diphtheriae can cause the disease diphtheria. Members of the genera Mycobacterium, characterized by the presence of mycolic acids in the cell membrane, cause tuberculosis and leprosy. Propionibacterium species are used in the production of swiss cheese, and some cause acne.