34.2 Bacterial cell structure is more complex than commonly supposed The bacterial surface Many kinds of bacteria have slender, rigid, helical fla- gella(singular, flagellum)composed of the protein fla- The bacterial cell wall is an important structure because it gellin(figure 34.6). These flagella range from 3 to 12 maintains the shape of the cell and protects the cell from crometers in length and are very thin-only 10 to 20 swelling and rupturing. The cell wall usually consists of nanometers thick. They are anchored in the cell wall and peptidoglycan, a network of polysaccharide molecules spin, pulling the bacteria through the water like a connected by polypeptide cross-links In some bacteria, the propeller peptidoglycan forms a thick, complex network around the Pili(singular, pilus) are other hairlike structures that outer surface of the cell. This network is interlaced with occur on the cells of some bacteria(see figure 34. 4). They peptide chains. In other bacteria a thin layer of peptidogly- are shorter than bacterial flagella, up to several microme can is found sandwiched between two plasma membranes. ters long, and about 7. 5 to 10 nanometers thick. Pili help The outer membrane contains large molecules of lipopoly- the bacterial cells attach saccharide, lipids with polysaccharide chains attached. change genetic information. These two major types of bacteria can be identified using a Some bacteria form thick-walled endospores around staining process called a Gram stain. Gram-positive bac eir chromosome and a small portion of the surrounding teria have the thicker peptidoglycan wall and stain a purple cytoplasm when they are exposed to nutrient-poor condi- color(figure 34.5). The more common gram-negative tions. These endospores are highly resistant to environ- bacteria contain less peptidoglycan and do not retain the mental stress, especially heat, and can germinate to form purple-colored dye. Gram-negative bacteria stain red. The new individuals after decades or even centuries. outer membrane layer makes gram-negative bacteria resis- tant to many antibiotics that interfere with cell wall synthe sis in gram-positive bacteria. In some kinds of bacteria,an Bacteria are encased within a cell wall composed of one additional gelatinous layer, the capsule, surrounds the cell or more polysaccharide layers. They also may contain wall external structures such as flagella and pili Peptide side Plasma membrane Lipopolysaccharides Gram-negative FIGURE 34.5 The Gram stain. The peptidoglycan layer encasing gram-positive bacteria traps crystal violet dye, so the bacteria appear purple in a Gram-stained smear(named after Hans Christian Gram, who developed the technique). Because gram-negative bacteria have much less peptidoglycan(located between the plasma membrane and an outer membrane), they do not retain the crystal violet dye and so exhibit the red background stain(usually a safranin dye) 682 Part IX Viruses and Simple organism
The Bacterial Surface The bacterial cell wall is an important structure because it maintains the shape of the cell and protects the cell from swelling and rupturing. The cell wall usually consists of peptidoglycan, a network of polysaccharide molecules connected by polypeptide cross-links. In some bacteria, the peptidoglycan forms a thick, complex network around the outer surface of the cell. This network is interlaced with peptide chains. In other bacteria a thin layer of peptidoglycan is found sandwiched between two plasma membranes. The outer membrane contains large molecules of lipopolysaccharide, lipids with polysaccharide chains attached. These two major types of bacteria can be identified using a staining process called a Gram stain. Gram-positive bacteria have the thicker peptidoglycan wall and stain a purple color (figure 34.5). The more common gram-negative bacteria contain less peptidoglycan and do not retain the purple-colored dye. Gram-negative bacteria stain red. The outer membrane layer makes gram-negative bacteria resistant to many antibiotics that interfere with cell wall synthesis in gram-positive bacteria. In some kinds of bacteria, an additional gelatinous layer, the capsule, surrounds the cell wall. Many kinds of bacteria have slender, rigid, helical flagella (singular, flagellum) composed of the protein flagellin (figure 34.6). These flagella range from 3 to 12 micrometers in length and are very thin—only 10 to 20 nanometers thick. They are anchored in the cell wall and spin, pulling the bacteria through the water like a propeller. Pili (singular, pilus) are other hairlike structures that occur on the cells of some bacteria (see figure 34.4). They are shorter than bacterial flagella, up to several micrometers long, and about 7.5 to 10 nanometers thick. Pili help the bacterial cells attach to appropriate substrates and exchange genetic information. Some bacteria form thick-walled endospores around their chromosome and a small portion of the surrounding cytoplasm when they are exposed to nutrient-poor conditions. These endospores are highly resistant to environmental stress, especially heat, and can germinate to form new individuals after decades or even centuries. Bacteria are encased within a cell wall composed of one or more polysaccharide layers. They also may contain external structures such as flagella and pili. 682 Part IX Viruses and Simple Organisms 34.2 Bacterial cell structure is more complex than commonly supposed. Peptidoglycan Peptide side chains Cell wall (peptidoglycan) Cell wall Plasma membrane Plasma membrane Protein Outer membrane Gram-positive bacteria Gram-negative bacteria Lipopolysaccharides FIGURE 34.5 The Gram stain. The peptidoglycan layer encasing gram-positive bacteria traps crystal violet dye, so the bacteria appear purple in a Gram-stained smear (named after Hans Christian Gram, who developed the technique). Because gram-negative bacteria have much less peptidoglycan (located between the plasma membrane and an outer membrane), they do not retain the crystal violet dye and so exhibit the red background stain (usually a safranin dye)
Outer membrane Peptidoglycan portion of cell wall Plasma membrane FIGURE 34.6 The flagellar motor of a gram-negative bacterium. A protein filament, composed of the protein flagellin, is attached to a protein shaft that passes through a sleeve in the outer membrane and through a hole in the peptidoglycan layer to rings of protein anchored in the cell wall and plasma membrane, like rings of ballbearings. The shaft rotates when the inner protein ring attached to the shaft turns with respect to the outer ring fixed to the cell wall. The inner ring is an Hion channel, a proton pump that uses the passage of protons into the cell to power the movement of the inner ring past the outer one. The Cell Interior The most fundamental characteristic of bacterial cells is their prokaryotic organization. Bacterial cells lack the ex ensive functional compartmentalization seen within eu Internal membranes. Many bacteria possess invagi- nated regions of the plasma membrane that function in respiration or photosynthesis(figure 34.7) Nucleoid region. Bacteria lack nuclei and do not pos sess the complex chromosomes characteristic of eukary otes. Instead their genes are encoded within a single double-stranded ring of dna that is crammed into one region of the cell known as the nucleoid region. Many acterial cells also possess small, independently replicat ing circles of DNA called plasmidsPlasmids contain FIGURE 34.7 only a few genes, usually not essential for the cell's sur- Bacterial cells often have complex internal membranes. This vival. They are best thought of as an excised portion of robic bacterium(a) exhibits extensive respiratory membranes the bacterial chromosome within its cytoplasm not unlike those seen in mitochondria. This Ribosomes. Bacterial ribosomes are smaller tha cyanobacterium() has thylakoid-like membranes that provide a those of eukaryotes and differ in protein and rNA con- esIs cent. Antibiotics such as tetracycline and chlorampheni col can tell the difference--they bind to bacterial ribo- somes and block protein synthesis, but do not bind to The interior of a bacterial cell may possess internal eukaryotic ribosomes membranes and a nucleoid region Chapter 34 Bacteria 683
The Cell Interior The most fundamental characteristic of bacterial cells is their prokaryotic organization. Bacterial cells lack the extensive functional compartmentalization seen within eukaryotic cells. Internal membranes. Many bacteria possess invaginated regions of the plasma membrane that function in respiration or photosynthesis (figure 34.7). Nucleoid region. Bacteria lack nuclei and do not possess the complex chromosomes characteristic of eukaryotes. Instead, their genes are encoded within a single double-stranded ring of DNA that is crammed into one region of the cell known as the nucleoid region. Many bacterial cells also possess small, independently replicating circles of DNA called plasmids. Plasmids contain only a few genes, usually not essential for the cell’s survival. They are best thought of as an excised portion of the bacterial chromosome. Ribosomes. Bacterial ribosomes are smaller than those of eukaryotes and differ in protein and RNA content. Antibiotics such as tetracycline and chloramphenicol can tell the difference—they bind to bacterial ribosomes and block protein synthesis, but do not bind to eukaryotic ribosomes. The interior of a bacterial cell may possess internal membranes and a nucleoid region. Chapter 34 Bacteria 683 Flagellum Filament Sleeve Hook Outer membrane Peptidoglycan portion of cell wall Rod H+ Plasma membrane Outer protein ring Inner protein ring H+ FIGURE 34.6 The flagellar motor of a gram-negative bacterium. A protein filament, composed of the protein flagellin, is attached to a protein shaft that passes through a sleeve in the outer membrane and through a hole in the peptidoglycan layer to rings of protein anchored in the cell wall and plasma membrane, like rings of ballbearings. The shaft rotates when the inner protein ring attached to the shaft turns with respect to the outer ring fixed to the cell wall. The inner ring is an H+ ion channel, a proton pump that uses the passage of protons into the cell to power the movement of the inner ring past the outer one. FIGURE 34.7 Bacterial cells often have complex internal membranes. This aerobic bacterium (a) exhibits extensive respiratory membranes within its cytoplasm not unlike those seen in mitochondria. This cyanobacterium (b) has thylakoid-like membranes that provide a site for photosynthesis. (a) (b)
34.3 Bacteria exhibit considerable diversity in both structure and metabolism Bacterial Diversity niques. We clearly have only scraped the surface of bacte- Bacteria are not easily classified according to thei As we learned in chapter 32, bacteria split into two forms, and only recently has enough been learned about lines early in the history of life, so different in structure their biochemical and metabolic characteristics to de- and metabolism that they are as different from each other velop a satisfactory overall classification comparable to as either is from eukaryotes. The differences are so fun that used for other organisms. Early systems for classify- damental that biologists assign the two groups of bacteria ing bacteria relied on differential stains such as the to separate domains. One domain, the Archaea, consists Gram stain. Key bacterial characteristics used in classify- of the archaebacteria ("ancient bacteria"although they Ing are actually not as ancient as the other bacterial domain). It was once thought that survivors of this group were 1. Photosynthetic or nonphotosynthetic confined to extreme environments that may resemble 2. Motile or nonmotile 3. Unicellular or multicellular habitats on the early earth. However, the use of genetic screening has revealed that these "ancient" bacteria live 4. Formation of spores or dividing by tr in nonextreme environments as well. The other more an- binary fission cient domain, the Bacteria, consists of the eubacteria With the development of genetic and molecular ap (true bacteria"). It includes nearly all of the named proaches, bacterial classifications can at last reflect true species of bacteria evolutionary relatedness. Molecular approaches include (1)the analysis of the amino acid sequences of key pro- Comparing Archaebacteria and Eubacteria teins;(2)the analysis of nucleic acid base sequences by establishing the percent of guanine( G)and cytosine(C); Archaebacteria and eubacteria are similar in that they both ave a prokaryotic cellular but they vary considerably at the (3)nucleic acid hybridization, which is essenti ally the biochemical and molecular level. There are four key areas nixing of single-stranded DNA from two species and determining the amount of base-pairing(closely related pecies will have more bases pairing); and (4)nucleic 1. Cell wall. Both kinds of bacteria typically have acid sequencing especially looking at ribosomal RNA. cell walls covering the plasma membrane that Lynn Margulis and Karlene Schwartz proposed a useful strengthen the cell. The cell walls of eubacteria are classification system that divides bacteria into 16 phyla construct red of carbohydrate-protein complexes according to their most significant features. Table 34.1 alled peptidoglycan, which link together to cr utlines some of the major features of the phyla we a strong mesh that gives the eubacterial cell wall describe great strength. The cell walls of archaebacteria lack can 2. Plasma membranes. All bacteria have plasma Kinds of bacteria membranes with a lipid-bilayer architecture(as de Although they lack the structural complexity of eukar scribed in chapter 6). The plasma membranes of eu otes, bacteria have diverse internal chemistries. metabo- bacteria and archaebacteria. however. are made of lisms and unique functions. Bacteria have adapted to ery different kinds of lipids many kinds of environments, including some you might 3. Gene translation machinery. Eubacteria possess consider harsh. They have successfully invaded very salt ribosomal proteins and an RNa polymerase that waters,very acidic or alkaline environments, and very hot are distinctly different from those of eukaryotes or cold areas. They are found in hot springs where the However, the ribosomal proteins and RNA of temperatures exceed 78C(172 F)and have been recov archaebacteria are very similar to those of ered living beneath 435 meters of ice in Antarctica Much of what we know of bacteria we have learned 4. Gene architecture. The genes of eubacteria are from studies in the laboratory. It is important to under not interrupted by introns, while at least some of the stand the limits this has placed on our knowledge: we have genes of archaebacteria do possess introns. only been able to study those bacteria that can be cultured in laboratories. Field studies suggest that these represent While superficially similar, bacteria differ from one but a small fraction of the kinds of bacteria that occur in another in a wide variety of characteristics soil, most of which cannot be cultured with existing tech 684 Part IX Viruses and Simple organism
Bacterial Diversity Bacteria are not easily classified according to their forms, and only recently has enough been learned about their biochemical and metabolic characteristics to develop a satisfactory overall classification comparable to that used for other organisms. Early systems for classifying bacteria relied on differential stains such as the Gram stain. Key bacterial characteristics used in classifying bacteria were: 1. Photosynthetic or nonphotosynthetic 2. Motile or nonmotile 3. Unicellular or multicellular 4. Formation of spores or dividing by transverse binary fission With the development of genetic and molecular approaches, bacterial classifications can at last reflect true evolutionary relatedness. Molecular approaches include: (1) the analysis of the amino acid sequences of key proteins; (2) the analysis of nucleic acid base sequences by establishing the percent of guanine (G) and cytosine (C); (3) nucleic acid hybridization, which is essentially the mixing of single-stranded DNA from two species and determining the amount of base-pairing (closely related species will have more bases pairing); and (4) nucleic acid sequencing especially looking at ribosomal RNA. Lynn Margulis and Karlene Schwartz proposed a useful classification system that divides bacteria into 16 phyla, according to their most significant features. Table 34.1 outlines some of the major features of the phyla we describe. Kinds of Bacteria Although they lack the structural complexity of eukaryotes, bacteria have diverse internal chemistries, metabolisms and unique functions. Bacteria have adapted to many kinds of environments, including some you might consider harsh. They have successfully invaded very salty waters, very acidic or alkaline environments, and very hot or cold areas. They are found in hot springs where the temperatures exceed 78°C (172°F) and have been recovered living beneath 435 meters of ice in Antarctica! Much of what we know of bacteria we have learned from studies in the laboratory. It is important to understand the limits this has placed on our knowledge: we have only been able to study those bacteria that can be cultured in laboratories. Field studies suggest that these represent but a small fraction of the kinds of bacteria that occur in soil, most of which cannot be cultured with existing techniques. We clearly have only scraped the surface of bacterial diversity. As we learned in chapter 32, bacteria split into two lines early in the history of life, so different in structure and metabolism that they are as different from each other as either is from eukaryotes. The differences are so fundamental that biologists assign the two groups of bacteria to separate domains. One domain, the Archaea, consists of the archaebacteria (“ancient bacteria”—although they are actually not as ancient as the other bacterial domain). It was once thought that survivors of this group were confined to extreme environments that may resemble habitats on the early earth. However, the use of genetic screening has revealed that these “ancient” bacteria live in nonextreme environments as well. The other more ancient domain, the Bacteria, consists of the eubacteria (“true bacteria”). It includes nearly all of the named species of bacteria. Comparing Archaebacteria and Eubacteria Archaebacteria and eubacteria are similar in that they both have a prokaryotic cellular but they vary considerably at the biochemical and molecular level. There are four key areas in which they differ: 1. Cell wall. Both kinds of bacteria typically have cell walls covering the plasma membrane that strengthen the cell. The cell walls of eubacteria are constructed of carbohydrate-protein complexes called peptidoglycan, which link together to create a strong mesh that gives the eubacterial cell wall great strength. The cell walls of archaebacteria lack peptidoglycan. 2. Plasma membranes. All bacteria have plasma membranes with a lipid-bilayer architecture (as described in chapter 6). The plasma membranes of eubacteria and archaebacteria, however, are made of very different kinds of lipids. 3. Gene translation machinery. Eubacteria possess ribosomal proteins and an RNA polymerase that are distinctly different from those of eukaryotes. However, the ribosomal proteins and RNA of archaebacteria are very similar to those of eukaryotes. 4. Gene architecture. The genes of eubacteria are not interrupted by introns, while at least some of the genes of archaebacteria do possess introns. While superficially similar, bacteria differ from one another in a wide variety of characteristics. 684 Part IX Viruses and Simple Organisms 34.3 Bacteria exhibit considerable diversity in both structure and metabolism
Table 34.1 Bacteria Major Group Typical Examples Key Characteristic ARCHAEBACTERIA Archaebacteria Methanogens Bacteria that are not members of the kingdom eubacteria. 修多 Mostly anaerobic with unusual cell walls. Some produce methane. Others reduce sulfu EUBACTERIA Actinomycetes Streptomyces, Gram-positive bacteria. Form branching filaments and produce spores; often mistaken for fungi. Produce many commonly used antibiotics, inemdiog strea bactera; also common in dental in and tetracycline. One of the most common types of soil plaque Chemoautotrophs Sulfur bacteria, Bacteria able to obtain their energy from inorganic chemicals Nitrobacter Most extract chemical energy from reduced gases such as H2S Nitrosomonas (hydrogen sulfide), NH3(ammonia), and CH4(methane). Play a key role in th Cyanobacteria A form of photosynt bacteria common in both marine and freshwater environments. Deeply pigmented; often responsible for"blooms"in polluted waters. Enterobacteria Gram-negative, rod-shaped bacteria. Do not form spores; usually aerobic heterotrophs; cause many important diseases, including bubonic plague and cholera. Gliding and Myxobacteria Gram-negative bacteria Exhibit gliding motility by secreting budding bacteria groups form upright multicelluar structures carrying spores called fruiting bodi Pseudomonads Pseudomona gative heterotrophic rods with polar flagella. Very form of soil bacteria; also contain many important plant Rickettsia and Rickettsia Small, gram-negative intracelluar parasites. Rickettsia life cycle chlamydia Chlamydi 066 involves both mammals and arthropods such as fleas and ticks: Rickettsia are responsible for fatal human disease including typhus(Rickettsia prowazeki) and Rocky Mountain spotted fever. Chlamydial infections are one of the most common sexually transmitted disease Spirochaetes Long, coil-shaped cells. Common in aquatic environments; a parasitic form is responsible for the disease syphilis Chapter 34 Bacteria 685
Chapter 34 Bacteria 685 Table 34.1 Bacteria Major Group Typical Examples Key Characteristics ARCHAEBACTERIA Archaebacteria Methanogens, thermophiles, halophiles EUBACTERIA Actinomycetes Streptomyces, Actinomyces Chemoautotrophs Sulfur bacteria, Nitrobacter, Nitrosomonas Cyanobacteria Anabaena, Nostoc Enterobacteria Escherichia coli, Salmonella, Vibrio Gliding and Myxobacteria, budding bacteria Chondromyces Pseudomonads Pseudomonas Rickettsias and Rickettsia, chlamydias Chlamydia Spirochaetes Treponema Bacteria that are not members of the kingdom Eubacteria. Mostly anaerobic with unusual cell walls. Some produce methane. Others reduce sulfur. Gram-positive bacteria. Form branching filaments and produce spores; often mistaken for fungi. Produce many commonly used antibiotics, including streptomycin and tetracycline. One of the most common types of soil bacteria; also common in dental plaque. Bacteria able to obtain their energy from inorganic chemicals. Most extract chemical energy from reduced gases such as H2S (hydrogen sulfide), NH3 (ammonia), and CH4 (methane). Play a key role in the nitrogen cycle. A form of photosynthetic bacteria common in both marine and freshwater environments. Deeply pigmented; often responsible for “blooms” in polluted waters. Gram-negative, rod-shaped bacteria. Do not form spores; usually aerobic heterotrophs; cause many important diseases, including bubonic plague and cholera. Gram-negative bacteria. Exhibit gliding motility by secreting slimy polysaccharides over which masses of cells glide; some groups form upright multicelluar structures carrying spores called fruiting bodies. Gram-negative heterotrophic rods with polar flagella. Very common form of soil bacteria; also contain many important plant pathogens. Small, gram-negative intracelluar parasites. Rickettsia life cycle involves both mammals and arthropods such as fleas and ticks; Rickettsia are responsible for many fatal human diseases, including typhus (Rickettsia prowazekii) and Rocky Mountain spotted fever. Chlamydial infections are one of the most common sexually transmitted diseases. Long, coil-shaped cells. Common in aquatic environments; a parasitic form is responsible for the disease syphilis
Bacterial variation Velveteen Bacteria reproduce rapidly, allowing Cells lifted genetic variations to spread quickly Mutagen-treated through a population. Two processes bacteria are added create variation among bacteria: mu- tation and genetic recombination Mutation Mutations can arise spontaneously bacteria as errors in DNA replica tion occur. Certain factors tend to B increase the likelihood of errors oc- Incubate incubate curring such as radiation, ultraviolet light, and various chemicals. In typical bacterium such as Escbericbia coli there are about 5000 genes. It is Bacterial cells highly probably that one mutation are spread will occur by chance in one out of every million copies of a gene. With FIGURE 34.8 5000 genes in a bacterium, the laws A mutant hunt in bacteria. Mutations in bacteria can be detected by a technique called of probability predict that 1 out of replica plating, which allows the genetic characteristics of the colonies to be investigated every 200 bacteria will have a muta- hout destroying them. The bacterial colonies, growing on a semisolid agar medium, are tion(figure 34.8). A spoonful of soil transferred from a to B using a sterile velveteen disc pressed on the plate. Plate A has a typically contains over a billion bac- medium that includes special growth factors, while B has a medium that lacks some of these teria and therefore should contain growth factors. Bacteria that are not mutated can produce their own growth factors and do something on the order of 5 million not require them to be added to the medium. The colonies absent in B were unable to grow mutant individuals on the deficient medium and were thus mutant colonies; they were already present but undetected in A With adequate food and nutri ents, a population of E. coli can dou ble in under 20 minutes. because bacteria multiply so rapidly, mutations can spread rapidly Genetic Recombination in a population and can change the characteristics of that ulation Another source of genetic variation in populations of bac- The ability of bacteria to change rapidly in response to teria is recombination, discussed in detail in chapter 18 new challenges often has adverse effects on humans. Re- Bacterial recombination occurs by the transfer of genes cently a number of strains of Staphylococcus aureus associated from one cell to another by viruses, or through conjuga- with serious infections in hospitalized patients have ap- tion. The rapid transfer of newly produced, antibiotic eared, some of them with alarming frequency. Unfortu- resistant genes by plasmids has been an important factor nately, these strains have acquired resistance to penicillin and in the appearance of the resistant strains of Staphylococcus a wide variety of other antibiotics, so that infections caused aureus discussed earlier. An even more important example by them are very difficult to treat. Staphylococcus infections provide an excellent example of the way in which mutation the family of bacteria to which the common intestinal and intensive selection can bring about rapid change in bac- terial populations. Such changes have serious medical impli- are many important pathogenic bacteria, including the or cations when, as in the case of Staphylococcus, strains of bacte- ganisms that cause dysentery, typhoid, and other major ria emerge that are resistant to a variety of antibiotics diseases. At times, some of the genetic material from these Recently, concern has arisen over the prevalence of an pathogenic species is exchanged with or transferred to E. tibacterial soaps in the marketplace. They are marketed as a coli by plasmids. Because of its abundance in the human means of protecting your family from harmful bacteria; digestive tract, E. coli poses a special threat if it acquires however, it is likely that their routine use will favor bacteria harmful traits that have mutations making them immune to the antibi otics contained in them. Ultimately, extensive use of an Because of the short generation time of bacteria, tibacterial soaps could have an adverse effect on our ability mutation and recombination play an important role in to treat common bacterial infections generating genetic diversity. 686 Part IX Viruses and Simple organism
Bacterial Variation Bacteria reproduce rapidly, allowing genetic variations to spread quickly through a population. Two processes create variation among bacteria: mutation and genetic recombination. Mutation Mutations can arise spontaneously in bacteria as errors in DNA replication occur. Certain factors tend to increase the likelihood of errors occurring such as radiation, ultraviolet light, and various chemicals. In a typical bacterium such as Escherichia coli there are about 5000 genes. It is highly probably that one mutation will occur by chance in one out of every million copies of a gene. With 5000 genes in a bacterium, the laws of probability predict that 1 out of every 200 bacteria will have a mutation (figure 34.8). A spoonful of soil typically contains over a billion bacteria and therefore should contain something on the order of 5 million mutant individuals! With adequate food and nutrients, a population of E. coli can double in under 20 minutes. Because bacteria multiply so rapidly, mutations can spread rapidly in a population and can change the characteristics of that population. The ability of bacteria to change rapidly in response to new challenges often has adverse effects on humans. Recently a number of strains of Staphylococcus aureus associated with serious infections in hospitalized patients have appeared, some of them with alarming frequency. Unfortunately, these strains have acquired resistance to penicillin and a wide variety of other antibiotics, so that infections caused by them are very difficult to treat. Staphylococcus infections provide an excellent example of the way in which mutation and intensive selection can bring about rapid change in bacterial populations. Such changes have serious medical implications when, as in the case of Staphylococcus, strains of bacteria emerge that are resistant to a variety of antibiotics. Recently, concern has arisen over the prevalence of antibacterial soaps in the marketplace. They are marketed as a means of protecting your family from harmful bacteria; however, it is likely that their routine use will favor bacteria that have mutations making them immune to the antibiotics contained in them. Ultimately, extensive use of antibacterial soaps could have an adverse effect on our ability to treat common bacterial infections. Genetic Recombination Another source of genetic variation in populations of bacteria is recombination, discussed in detail in chapter 18. Bacterial recombination occurs by the transfer of genes from one cell to another by viruses, or through conjugation. The rapid transfer of newly produced, antibioticresistant genes by plasmids has been an important factor in the appearance of the resistant strains of Staphylococcus aureus discussed earlier. An even more important example in terms of human health involves the Enterobacteriaceae, the family of bacteria to which the common intestinal bacterium, Escherichia coli, belongs. In this family, there are many important pathogenic bacteria, including the organisms that cause dysentery, typhoid, and other major diseases. At times, some of the genetic material from these pathogenic species is exchanged with or transferred to E. coli by plasmids. Because of its abundance in the human digestive tract, E. coli poses a special threat if it acquires harmful traits. Because of the short generation time of bacteria, mutation and recombination play an important role in generating genetic diversity. 686 Part IX Viruses and Simple Organisms Incubate Incubate Velveteen Cells lifted from colonies Colonies absent Medium lacking growth factor Mutagen-treated bacteria are added Supplemented medium Bacterial colony A A A B Bacterial cells are spread B FIGURE 34.8 A mutant hunt in bacteria. Mutations in bacteria can be detected by a technique called replica plating, which allows the genetic characteristics of the colonies to be investigated without destroying them. The bacterial colonies, growing on a semisolid agar medium, are transferred from A to B using a sterile velveteen disc pressed on the plate. Plate A has a medium that includes special growth factors, while B has a medium that lacks some of these growth factors. Bacteria that are not mutated can produce their own growth factors and do not require them to be added to the medium. The colonies absent in B were unable to grow on the deficient medium and were thus mutant colonies; they were already present but undetected in A