CHAPTER 21
The Nature of Bacteria
Bacteria are the smallest and most versatile independently living cells known. In the
hierarchy of infectious agents bacteria are the first and smallest capable of independent
existence. In the wider microbial world their prokaryotic cell plan is still considered to
provide the minimum possible size for an independently reproducing organism. This
chapter examines the structural, metabolic, and genetic features that contribute to the
ubiquity and diversity of this large group of organisms. Discussion focuses particularly
on the characteristics of bacteria which enable them to cause disease in humans.
Bacterial Structure
1. Bacteria are in the range of 1–10 μm
2. Bacteria exhibit sphere, rod and spiral shapes
3. Prokaryotic design includes envelope, appendages, cytosol, and nucleoid
4. Chemical nature is similar to eukaryotic cells plus unique components
5. Design facilitates rapid growth
I. ENVELOPE AND APPENDAGES
1. Bacteria have a very plain interior but a complex exterior
2. Envelope and appendages carry out multiple functions
A. Capsule
1. Hydrophilic capsules are usually polysaccharides
2. Capsules protect from immune system
3. Capsule synthesis depends on growth conditions
B. Cell Wall
1. Unique wall structure prevents osmotic lysis, determines shape
2. Gram stain distinguishes two major envelope structures
3. Poorly staining bacteria still have a Gram category
a. Gram-Positive Cell Wall
1. Major components of Gram-positive walls are peptidoglycan and
teichoic acid
2. Peptidoglycan comprises glycan chains cross-linked by peptide chains
3. Scaffold-like sac surrounds cell
4. Components of peptidoglycan provide resistance to most mammalian
enzymes
5. Loss of cell wall leads to lysis or production of protoplasts
6. Teichoic and lipoteichoic acids promote adhesion and anchor wall to
membrane
7. Other cell wall components related to species
b. Gram-Negative Cell Wall
1. Thin peptidoglycan sac is imbedded in periplasmic gel
2. Periplasmic proteins have transport, chemotactic, and hydrolytic roles
3. Gram-negative outer membrane is phospholipoprotein bilayer, of which
the outer leaflet is LPS endotoxin
4. Lipid A is toxic moiety of LPS; polysaccharides are antigenic
determinants
5. Impermeability of outer membrane is overcome by porins
6. Outer membrane has many functions
C. Cell Membrane
1. Phospholipid–protein bilayer lacking sterols
2. Roles in synthetic, homeostatic, secretory, and electron transport processes
3. Functional equivalent of many eukaryotic organelles
D. Flagella
1. Flagella are rotating helical protein structures responsible for locomotion
2. Flagella have bushing rings in cell envelope
3. Flagellar filament is composed of the protein flagellin
E. Pili
1. Pili are tubular hair-like projections
2. Pili have adherence roles and can “twitch”
3. Specialized pili mediate selective attachment or genetic transfer
II. CORE
A. Cytosol
1. Cytosol contains densely packed 70 S ribosomes
2. Number of ribosomes varies with growth rate
B. Nucleoid
1. Circular chromosome of supercoiled double-stranded DNA
2. Attached to cell membrane and central structures
3. Cell may contain 2–4 nucleoids depending on growth rate
C. Plasmids
1. Plasmids are small, usually circular, double-stranded DNA molecules separate
from the chromosome
III. SPORES
1. Endospores are hardy, quiescent forms of some Gram-positives
2. Spore-forming allows survival under adverse conditions
3. Resistance of spore is due to dehydrated state, calcium dipicolinate, and
specialized coats
4. Germination reproduces cell identical to that which sporulated
Bacterial Growth and Metabolism
1. Growth requires metabolism, regulation, and division by binary fission
I. Bacterial Metabolism
A. Differences between bacteria and human eukaryotic cells
1. Speed. Bacteria metabolize at a rate 10 to 100 times faster.
2. Versatility. Bacteria use more varied compounds as energy sources and
are much more diverse in their nutritional requirements.
3. Simplicity. The prokaryotic body plan makes it possible for bacteria to
synthesize macromolecules in a streamlined way.
4. Uniqueness. Some biosynthetic processes, such as those producing
peptidoglycan, lipopolysaccharide, and toxins, are unique to bacteria.
B. Fueling Reactions
1.
2.
3.
4.
5.
6.
7.
Substrates enter despite permeability barriers
Facilitated diffusion involves shuttling by carrier protein
Active transport involves binding proteins and ATP or proton gradient energy
Group translocation involves chemical conversion of transported molecule
Bacterial siderophores chelate iron and are actively transported into cell
Central fueling pathways produce biosynthetic precursors
Fermentation and respiration pathways each regenerate ATP and NAD+
a. Fermentation
1. Fermentation involves direct transfer of proton and electron to organic
receptor acceptor
2. ATP-generating efficiency is low
b. Respiration
1. Respiration uses electron chain for which oxygen is usually the terminal
acceptor
2. Respiration is efficient energy producer
C. Aerobes and Anaerobes
1. Bacteria exhibit different characteristic responses to oxygen
2. Aerobic metabolism produces peroxide and toxic oxygen radicals
3. Superoxide dismutase and peroxidase allow growth in air; their absence requires
strict anaerobiosis
4. Organisms growing in air may or may not have a respiratory pathway
5. Aerobes require oxygen and anaerobes are killed by it
6. Facultative bacteria grow either way
7. Pathogenic anaerobes tolerate brief oxygen exposures.
C. Biosynthesis
1. Biosynthesis requires precursor metabolites, energy, amino nitrogen, sulfur, and
reducing power
2. Nutritional requirements differ depending on synthetic ability
3. Few pathways are unique to bacteria
4. ADP-ribosylation is the action of multiple toxins
D. Polymerization Reactions
1.
2.
3.
4.
5.
6.
7.
8.
Polymerization of DNA is called replication
Bidirectional, semiconservative replication occurs at replication forks
DNA gyrase inhibitors are selectively toxic for bacteria
Transcription is the synthesis of RNA
A single RNA polymerase makes all forms of bacterial RNA
Rifampin inhibits RNA polymerase
Translation is the name given to protein synthesis
Amino acid residues are polymerized from specific tRNAs at the direction of
mRNA
9. Many antibiotics act on bacterial translation machinery
10. Translation of mRNA occurs simultaneously with transcription
a. Peptidoglycan Synthesis
1. In the cytosol a series of reactions leads to the synthesis, on a nucleotide carrier
(UDP), of an N-acetylmuramic acid (NAM) residue bearing a pentapeptide.
2. Precursor is added to bactoprenol carrier
3. N-acetylglucosamine (NAG) and bridge amino acids are added
4. Outside the cell membrane, this disaccharide subunit is attached to the end of a
growing glycan chain
5. Chain cross-links are formed by transpeptidases (PBPs)
E. Protein Secretion
1.
2.
3.
4.
Proteins are transported to locations in the cell structure or the exterior
In Gram-negatives the periplasm and outer membrane are additional barriers
General secretory pathway (GSP) uses signal peptide and chaperone proteins
5 systems transport across the outer membrane
5. Injection systems also use a syringe to penetrate host cells
II. Cell Division and Growth
1.
2.
3.
4.
Growth of a liquid culture can be monitored by colony counts or turbidimetrically
Growth rate is dependent on nutrient availability, pH, and temperature
Following a lag period, liquid cultures exhibit exponential growth
Nutrient depletion or waste product accumulation terminates growth
A. Regulation and Adaptation
a. Control of Enzyme Activity
1. Most metabolic pathways are controlled by allosteric enzymes
2. Feedback inhibition provides economy and efficiency
b. Control of Gene Expression
1. Changes in transcription can rapidly change enzyme synthesis because of
mRNA degradation
2. Most regulation operates at initiation of transcription
3. Genes are organized as transcriptional units called operons
4. RNA polymerase binds to the promoter of an operon and transcribes until it
meets the terminator
5. Activator and repressor proteins regulate transcription by binding to the
operator region of operons
6. Regulons are groups of unlinked operons controlled by a common regulator
7. 2-component systems link environmental sensing with regulation
B. CELL SURVIVAL
a. Cell Stress Regulons
1. Bacteria have regulons that help cope with nutritional, injury, and heat stress
2. In the heat-shock response up to 20 genes may be transcriptionally activated
b. Endospores
1. Encased spore, though metabolically inert is capable of germinating into a
growing (vegetative) cell.
2. Sporulation involves sequential activation of interrelated regulons
c. Stationary Phase Cells
1. Stationary phase cell is different morphologically from an endospore but is
metabolically quiescent
2. Formation of a stationary phase cell involves activation of many regulons in a
coordinated cascade
d. Motility and Chemotaxis
1. Chemotaxis is directed movement toward chemical attractants and away from
chemical repellents
2. Direction of flagellar rotation determines a run or a tumble
3. Changes in duration of runs and tumbles determine chemotactic response
4. Chemotaxis serves survival, growth-promoting, and pathogenic roles
Bacterial Genetics
I. Mutation
1. Because bacteria are haploid, the consequences of a mutation, even a recessive
one, are immediately evident in the mutant cell
2. Mutations are rapidly expressed and predominate under selective conditions
A. Kinds of Mutations
1. Mutations are heritable changes in the structure of genes
2. Replacements involve the substitution of one base for another
3. Microdeletions and microinsertions involve the removal and addition,
respectively, of a single nucleotide
4. Insertions involve the addition of many base pairs of nucleotides at a single site
5. Deletions remove a contiguous segment of many base pairs
6. Inversions change the direction of a segment of DNA
7. Duplications produce a redundant segment of DNA
8. Missense mutation changes the mRNA transcript to a different amino acid
9. Nonsense mutation changes a codon specifying an amino acid to one specifying
none
10. Frame shift mutations cause changes in the reading frame by which the ribosomes
translate the mRNA from the mutated
11. Mutations may affect neighboring genes by termination of transcription
12. Mutagens increase the natural frequency of mutation
II. Recombination
1. Recombination is the process in which nucleic acid molecules from different
sources are combined or rearranged to produce a new nucleotide sequence
2. Source of recombinant DNA may be another part of the same chromosome
(endogenote) or from outside the cell (exogenote)
3. If successful, a new hybrid chromosome is formed
A. Homologous Recombination
1. Donor DNA must possess reasonably large regions of nucleotide sequence
identity or similarity to segments of the host chromosome
2. Recipient cell must possess the genetic ability to make a set of enzymes that can
bring about the covalent substitution of a segment of the donor DNA
3. A protein known as RecA (recombination) controls the entire process
4. Crossover event results in the substitution of the donor segment between the two
crossovers for the homologous segment of the host
B. Site-Specific Recombination
1. Site-specific operates only on unique sequences
2. Enzymes are usually encoded by exogenote genes
C. Recombination and Antigenic Variation
1. Antigenic variation can be brought about by a recombinational event
2. Invertible elements can act as a genetic switch
III. Transposition
1. Transposition involves transposable elements which are genetic units that are
capable of mediating their own transfer
2. Genetic units move within and between chromosomes and plasmids
A. Insertion Sequences (IS)
1. IS elements encode only proteins for their own transposition
2. Insertion of IS elements into a gene causes mutation
B. Transposons
1.
2.
3.
4.
Transposons encode functions beyond those needed for their own transposition
Direct transposition moves the transposon to a new site
Replicative transposition leaves a copy behind
Transposons promote many changes in DNA
IV. Genetic Exchange
1. One-way passage of DNA from a donor to a recipient adds an exogenote to the
recipient endogenote
2. Transformation, transduction, and conjugation are the major processes of DNA
transfer
3. Transformation, transduction, and conjugation are mediated by chromosomal,
viral, and plasmid genes, respectively
A. Transformation
1. Competence is the ability to take DNA from the environment
2. Internalized DNA either recombines or is degraded
3. Artificial transformation involves treatment of cells
B. Transduction
1. Transduction is the transfer of genetic information from donor to recipient cell by
viruses of bacteria called bacteriophages
2. Lytic phages produce new virions in the host bacterial cell
3. Temperate phages either lyse the bacterial host cell or lysogenize it as a prophage
4. Generalized transduction can transfer any bacterial DNA
5. Specialized transduction is limited to certain sites
6. Transduction is a source of genes for bacterial toxins
C. Conjugation
1. Conjugation is the transfer of genetic information from donor to recipient
bacterial
2. Conjugation is plasmid-encoded and requires cell contact
3. Plasmids are small circular DNA molecules
4. Conjugative plasmids contain the genes for transfer
5. Conjugation may cross species lines
6. Nonconjugative plasmids transferred by plasmid mobilization
7. Many plasmid genes promote survival and pathogenesis
8. In absence of selection pressure plasmids may be lost
a. Conjugation in Gram-Negative Species
1. F factor is a conjugative plasmid that can transfer bacterial chromosome genes
2. Secretion systems or sex pili form bridges between cells
3. Replication or recombination follows transfer
b. Conjugation in Gram-Positive Species
1. Plasmids carrying genes encoding antimicrobic resistance, adhesins, and
exotoxins are readily transferred by conjugation
2. Gram-positive species may involve chromosomal genes in the process
3. Coupling results from adhesin–receptor interaction
4. Adhesin is produced in response to recipient pheromone
c. R Plasmids
1.
2.
3.
4.
R plasmids can encode and transfer multiresistance
Resistance genes acquired by plasmids from transposons
Spread is facilitated by plasmid-chromosome transposon hopping
Widespread antimicrobial use selects spread R plasmids