Bacterial structure, function, and growth. Flashcards
- Describe the major structural features of bacteria and explain the principal function(s) of each feature.
- Bacteria are unicellular organisms with a relatively simple structure. Unlike the cells of higher organisms (eukaryotes), bacteria do not have a nuclear membrane and therefore their genetic material lies within their cytoplasm.
- Bacteria have 70S instead of 80S ribosomes, and they do not have an endoplasmic reticulum.
- In addition, bacteria do not have a mitotic spindle, mitochondria or chloroplasts.
All cells, including bacteria, consist of the same chemical elements and share many basic biochemical units, such as amino acids, nucleotides, fatty acids, vitamins, etc, and they also share a number of closely related biochemical pathways for energy metabolism and biosynthesis.
- Describe the major structural features of bacteria and explain the principal function(s) of each feature.
The bacterial cell wall and cell surface structures.
Most bacteria have a rigid cell wall external to the cytoplasmic membrane that contains peptidoglycan.
Gram-positive bacteria and gram-negative bacteria have different cell wall structures and react differently to the Gram staining procedure that you will perform in the laboratory.
The rigidity of the cell wall is essential for resisting osmotic lysis (the internal osmotic pressure is typically ~5 atmospheres for gram-negative species and ~20 atmospheres for gram-positive species) and for maintaining cell shape.
- Describe the major structural features of bacteria and explain the principal function(s) of each feature.
a) Each bacterial isolate typically has a characteristic, rigid shape.
b) Bacterial shape is determined both by intracellular cytoskeletal elements and by rigid components of the cell wall.
Bacteria have cytoskeletal elements that play essential roles in establishing cell shape.
-The conformation of FtsZ resembles that of tubulin in eukaryotes.
-MreB and ParM resemble actin in eukaryotes
-CreS (crescentin) appears to function like intermediate filament proteins.
The typical intracellular localization of each type of protein is illustrated:
- Describe the major structural features of bacteria and explain the principal function(s) of each feature.
What two hexose sugars make up peptidoglycan?
The _____ residues are linked to tetrapeptide chains tht contain AA only found on bacterial walls.
In gram negative bacteria the tetrapeptides cross-link from one _____ chain to D-ala.
In gram positive bacteria the tetrapeptides cross-link from one _____ chain to D-ala.
Which gram bacteria has more cross-linking?
The peptidoglycan layer forms a rigid mesh that surrounds the cytoplasmic membrane.
Peptidoglycan consists of a polymer with repeating units of two hexose sugars, N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc).
The MurNAc residues are linked to tetrapeptide chains that contain amino acids found only in bacterial cell walls (e.g., meso-diaminopimelic acid [DAP], D-glutamic acid and D-alanine).
-The tetrapeptides are cross-linked from one chain (via DAP in gram-_negative bacteria_ or L-lys in gram-positive bacteria) to **D-ala on another chain, and cross-linking in gram-positive bacteria occurs via an intervening peptide such as pentaglycine in Staphylococcus aureus.
The extent of cross-linking of peptidoglycan chains is typically much greater in gram-positive bacteria than in gram-negative bacteria. The enzyme lysozyme, which is present in many body secretions and which contributes to innate host defenses against bacteria, hydrolyzes peptidoglycan by specific cleavage of the glycosidic bond between MurNAc and GlcNAc*.
- Explain the importance of differences in cell wall structure among bacteria.
What is the diffeence between grams positive and gram negative in terms of peptodyglycan layer?
-Gram-negative bacteria have a thin, sparsely cross-linked peptidoglycan layer and other major components that are located exterior to the peptidoglycan.
- The outer membrane (OM) of gram-negative bacteria is a lipid bilayer that contains lipopolysaccharide (LPS), lipoproteins (which are linked covalently to the peptidoglycan), and porins (which form transmembrane channels permitting diffusion across the membrane of hydrophilic molecules <600 MW), other membrane proteins, and phospholipids. The OM is a barrier to entry of some antibiotics and also protects the cell against the action of detergents and other toxic compounds.
- The LPS of gram-negative bacteria is located exclusively in the outer leaflet of the outer membrane, and the inner leaflet consists of phospholipids. LPS contains Lipid A (the toxic component of endotoxin), core polysaccharide, and O side chain oligosaccharides that function as somatic antigens (O antigen),
- Explain the importance of differences in cell wall structure among bacteria.
Gram positive bacteria
- Gram-positive bacteria have a thick, extensively cross-linked peptidoglycan layer that also contains teichoic acids.
- Teichoic acids of gram-positive bacteria have a repeating polyglycerol-P or polyribitol-P backbone substituted with other molecules (sugars, aminosugars, D-alanine), and they are covalently attached to the peptidoglycan layer.
**Lipoteichoic acids are attached to the underlying cytoplasmic membrane and help to anchor the cell wall to the membrane
What are capsules?
They allow bacteria to resist what?
Capsules are firmly attached, gelatinous outer surface layers that usually consist of complex polysaccharides (although the capsule of **Bacillus anthracis is a polymer of D-glutamic acid), and coats the surface of individual cells.
Capsules often enhance virulence by enabling the encapsulated bacteria to resist phagocytosis. Most capsular polysaccharies are antigenic, and some are used as components of vaccines to prevent specific bacterial infections (e.g., in the protein-polysaccharice conjugate vaccines used to immunize against Streptococcus pneumoniae or Hemophilus influenzae type b).
Glycocalyx
An extracellular glycocalyx, responsible for the formation of microbial biofilms is relatively common feature of pathogenic and environmental bacteria.
Many (perhaps most) bacteria exist in their natural ecosystems (e.g., soil, intestinal, urinary tract) in communities called “biofilms”. Within the biofilm, the community bacterial cells are embedded within an adherent, irregular, diffuse polysaccharide/glycoprotein matrix or “slime layer”.
Biofilms allows the bacteria to adhere to inert surfaces (e.g., rocks, teeth, indwelling catheters), to host cells (e.g. Streptococcus pneumoniae attachment to lung cells) or to other bacteria (e.g., within complex, multi-species communities). It also protects cells from phagocytosis, host defensive proteins (complement, antibody, defensins), and serves to limit access of many antimicrobial agents to the embedded cells
Flagella
What does peritrichous refers to?
What does polar refers to?
Counterclockwise rotation results in_______.
Clockwise rotation results in________.
What antigen in flagella is used to classify bacteria?
- Flagella are appendages originating in the cytoplasmic membrane that function as organs of motility. Some bacteria have flagella distributed over their surface (peritrichous); others may have one or several flagella at one end of the cell (polar).
- Bacterial chemotaxis (movement toward attractive nutrients or away from toxic substances) involves the control of flagellar rotation (counterclockwise results in swimming; clockwise results in tumbling).
Motile bacteria that exhibit chemotaxis spend more time swimming and less time tumbling when attractants or repellents are present, resulting in directed motion. Most flagella are antigenic, and the H antigens used for classification of enteric bacteria are flagellar antigens.
What are Pilli?
The cytoplasmic membrane:
How is it different from plasma membranes of animal cells?
It is permeable to what?
Where is the electron transport system located at?
The Cytoplasmic membrane (also called the inner membrane in gram-negative bacteria) is the anatomical and physiological barrier between the inside and outside of the bacterial cell. It is a lipid bilayer made up primarily of phospholipids and proteins, but unlike plasma membranes of animal cells it usually contains no sterols and has a much higher content (60-70%) of proteins.
- The cytoplasmic membrane exhibits selective permeability. It is essentially impermeable to all charged substances, even H+. Only hydrophobic molecules or uncharged molecules no larger than glycerol* can diffuse through it. Thus, essential metabolites are not readily lost from the cytoplasm.
- The electron transport system, which is the principal source for generating the proton motive force during respiration in bacteria, is located in the cytoplasmic membrane. Other functions of the cytoplasmic membrane include transport of metabolites into the cytoplasm, biosynthesis of lipids and other cell envelope components, certain aspects of DNA replication, and flagellar rotation.
The cytoplasm
Contains:
What size are the ribosomes?
What does polycistronic mean?
Where is the DNA of bacteria located on?
1. Ribosomes. Bacterial 70S ribosomes are closely related to the 70S ribosomes of mitochondria from eukaryotes, but they are less closely related to the 80S cytoplasmic ribosomes from eukaryotes. Protein synthesis occurs on the ribosomes. Polyribosomes are formed by the interaction of several ribosomes with a single messenger RNA. Bacterial mRNAs may by polycistronic* (e.g., encode more than one protein product).
2. The Nucleoid. The DNA of bacteria is located within a distinct region of the cytoplasm known as the nucleoid or nuclear body. The DNA is tightly packed and supercoiled, and there is no nuclear membrane surrounding the nucleoid. The older name prokaryote referred to this primitive nuclear structure. The name prokaryote is outdated as a taxonomic term because members of the bacteria and the archea, which constitute different biological kingdoms, both lack nuclear membranes. Because there is no nuclear membrane, transcription and translation can occur as coupled processes in bacteria. Several different genetic elements can contribute to the bacterial genome, including:
Several different genetic elements can contribute to the bacterial genome, including:
a) The chromosome often consists of a single, double-stranded, circular DNA molecule with a contour length hundreds to thousands of times greater than the longest dimension of the bacterium. Some bacterial chromosomes are linear, and some bacteria have more than one chromosome. Cytoskeletal components appear to function as a primitive mitotic apparatus during bacterial cell division.
b. Plasmids are extra-chromosomal, self-replicating DNA molecules, much smaller than bacterial chromosomes, and they are usually not essential for bacterial viability. Plasmids in pathogenic bacteria often encode virulence factors. Plasmids called R factors carry genes that determine resistance to antibiotics in many pathogenic bacteria.
c. Bacteriophages (phages) are viruses that infect bacteria. The DNA genomes of temperate bacteriophages can integrate into bacterial chromosomes and replicate as part of those chromosome. Temperate bacteriophages often carry genes that encode bacterial toxins, other bacterial virulence factors or resistance to antibiotics. *Phage conversion is defined as a change in the phenotype of a host bacterium as a consequence of expression of a gene that is encoded by a bacteriophage within the host bacterium (e.g., production of diphtheria toxin by isolates of Corynebacterium diphtheriae harboring a prophage that carries a gene encoding the toxin).
3- Draw a typical bacterial growth curve and explain the characteristics of each growth phase.
Do bacteria age?
Bacteria divide via _____.
In a nutritionally complete medium, a bacterial cell grows larger and eventually divides by binary fission to form two cells of nearly equal size. Each of these grows to about twice its original size or mass, and divides again.
–The products of the division are physiologically the same—each is equally “young”. Unlike the cells of higher life forms, there is no aging process in bacteria that leads inevitably to senescence and cell death.
Therefore, the concept of age for a bacterium applies only to its progression through the cycle of cell division. If the environment remains favorable, bacterial cells are capable of unlimited growth and division
- Draw a typical bacterial growth curve and explain the characteristics of each growth phase
In which phase do bacteria spend most of their time?
The bacterial growth curve typically reveals at least three—and sometimes four— distinct phases.
- An initial lag phase is a period of _physiologic adjustmen_t for the starting cells, or inoculum, involving the induction of new enzymes and the establishment of a proper intracellular environment for optimal growth in the new medium.
- During the exponential (logarithmic) phase of growth, the rate of increase in cell number/cell mass is proportional to the cell number/cell mass already present. A constant interval of time (ranging from about 20 minutes up to about 1 day) is required for doubling of cell number/cell mass, and this interval is termed the generation time (doubling time). During exponential growth, the rate of cell division is maximal for the available nutritional conditions.
- The stationary phase occurs as essential **nutrients are consumed and toxic products of metabolism accumulate. Cell growth may slow dramatically or cease, and growth that occurs is balanced by cell death. Such non-growing or slow-growing cells may exhibit markedly increased resistance to antibiotics such as **penicillin or other β-lactam antibiotics that act on growing cells. In nature, bacteria probably spend most of their time instationary phase.
- Some bacterial species remain viable for long periods of time in stationary phase, but others are less hardy. If a death phase occurs, the number of viable bacteria will decrease over time. If spontaneous cell lysis (autolysis) occurs, the mass of intact bacteria in the culture will also decrease.
- Describe how bacteria are classified according to their nutritional requirements
C. Bacterial Nutrition. Nutrition may be broadly defined as the provision of proper environmental conditions for promoting the growth of bacteria. This includes factors such as nutrients, pH, temperature, aeration (O2 tension), salt concentration, and osmotic pressure.
- Minimal requirements for growth. Most bacteria require a nutrient medium that contains several inorganic ions (NH4+, PO4=, SO4=, K+, Mg++, Fe++, etc.) plus sources of carbon and energy.
–>Bacteria that require an _organic carbon sourc_e (including most bacterial pathogens) are heterotrophic.
–>Bacteria that obtain their carbon exclusively from CO2 are autotrophic.
Many bacterial pathogens are deficient in one or more biosynthetic pathways. Such bacteria (often called “fastidious” bacteria) require, in addition to sources of carbon and energy, a number of essential growth factors such as amino acids, vitamins, purines, pyrimidines and inorganic ions. They are typically grown in rich, complex growth media.
–>Some bacterial pathogens are obligate intracellular bacteria that can grow within eukaryotic cells but cannot be cultivated on artificial media.
Organisms that grow in the presence of oxygen produce toxic oxygen metabolites, such as hydrogen peroxide and superoxide. Professional human phagocytes such as neutrophils and macrophages use reactive oxygen species as defense mechanisms against ingested bacterial pathogens.
–>Bacteria that can grow in the presence of oxygen usually produce catalase (or peroxidase) and superoxide dismutase (SOD) that protect them against toxic reactive oxygen species. Anaerobes that are frequently associated with disease tend to be more aeroterant than most strict anaerobes, and they may possess small amounts of catalase or SOD.
- Define respiration and fermentation and explain how metabolic “energy currency” is generated
What does ATP drive?
What do electrochemical gradients drive?
What is fermentation?
What organisms obtain energy via fermentation in either anaerobic or aerobic conditions due to being incapable of respiration?
There are two forms of “energy currency” in bacteria and higher cells: ATP and electrochemical gradients (the proton motive force). –
–> ATP drives many biosynthetic reactions.
–> Electrochemical gradients drive other functions like flagellar rotation and certain substrate transport systems.
-These two types of potential energy are interconvertible by the membrane ATPase. Bacteria also require reducing power in the form of NADH and NADPH to drive various metabolic interconversions.
Heterotrophic bacteria obtain both energy and reducing power by subjecting nutrients to fermentation or respiration.
b. In fermentation, organic compounds serve as both electron donors and electron acceptors, and no net oxidation of substrates occurs. Both anaerobic and facultative or indifferent bacteria grown under anaerobic conditions obtain energy by fermenting organic substrates.
***Indifferent organisms (aerotolerant anaerobes, see table above), obtain energy by fermentation under either anaerobic or aerobic conditions, because they are incapable of respiration.
c. In respiration, many bacterial species, like the mitochondria of higher organisms, generate ATP through electron transport and use molecular oxygen as the final electron acceptor.
–In anaerobic respiration, certain bacteria may use inorganic substrates such as nitrate or nitrite as terminal electron acceptors instead of O2.
Inhibitors of protein synthesis at ribosomal levels:
Inhibitors of nucleic acid synthesis:
Metabolic inhibitory antimicrobials
