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What is a key characteristic of transmissible plasmids?
What is a key characteristic of transmissible plasmids?
What is the molecular weight range of non-transmissible plasmids?
What is the molecular weight range of non-transmissible plasmids?
What function is NOT associated with the genes carried by plasmids?
What function is NOT associated with the genes carried by plasmids?
Which enzyme is involved in the resistance to heavy metals mediated by plasmids?
Which enzyme is involved in the resistance to heavy metals mediated by plasmids?
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What ability do transposons possess?
What ability do transposons possess?
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Which structure is covered by a gelatinous layer in bacteria?
Which structure is covered by a gelatinous layer in bacteria?
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Which among the following is a function of pili in bacteria?
Which among the following is a function of pili in bacteria?
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How many copies of transmissible plasmids are usually present per cell?
How many copies of transmissible plasmids are usually present per cell?
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What are the three basic shapes into which bacteria are classified?
What are the three basic shapes into which bacteria are classified?
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Which structure is common to all bacteria except Mycoplasma?
Which structure is common to all bacteria except Mycoplasma?
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What provides structural support and maintains the characteristic shape of bacterial cells?
What provides structural support and maintains the characteristic shape of bacterial cells?
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What type of bacteria typically have a thick peptidoglycan layer in their cell wall?
What type of bacteria typically have a thick peptidoglycan layer in their cell wall?
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Which of the following structures is external to the bacterial cell wall?
Which of the following structures is external to the bacterial cell wall?
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What type of bacteria is characterized by having a relaxed coil shape?
What type of bacteria is characterized by having a relaxed coil shape?
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What is the primary function of peptidoglycan in bacteria?
What is the primary function of peptidoglycan in bacteria?
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Which of the following does NOT describe features of cocci bacteria?
Which of the following does NOT describe features of cocci bacteria?
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What is the primary role of mesosomes during cell division?
What is the primary role of mesosomes during cell division?
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Which of the following correctly describes the structure of bacterial ribosomes?
Which of the following correctly describes the structure of bacterial ribosomes?
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What is the key difference between bacterial DNA and eukaryotic DNA?
What is the key difference between bacterial DNA and eukaryotic DNA?
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What characterizes the nucleoid region of prokaryotic cells?
What characterizes the nucleoid region of prokaryotic cells?
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What role do plasmids play in bacteria?
What role do plasmids play in bacteria?
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Which statement accurately describes the cytoplasm of prokaryotic cells?
Which statement accurately describes the cytoplasm of prokaryotic cells?
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What is a notable characteristic of bacterial granules?
What is a notable characteristic of bacterial granules?
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How do antibiotics selectively inhibit bacterial protein synthesis?
How do antibiotics selectively inhibit bacterial protein synthesis?
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What is the main function of penicillin binding proteins (PBP)?
What is the main function of penicillin binding proteins (PBP)?
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What effect does penicillin have on the transpeptidase enzyme?
What effect does penicillin have on the transpeptidase enzyme?
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Which of the following statements is true regarding teichoic acids?
Which of the following statements is true regarding teichoic acids?
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Which component of the gram-negative cell wall is responsible for its endotoxin properties?
Which component of the gram-negative cell wall is responsible for its endotoxin properties?
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What is the role of lipopolysaccharides in gram-negative bacteria?
What is the role of lipopolysaccharides in gram-negative bacteria?
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What distinguishes exotoxins from endotoxins in bacteria?
What distinguishes exotoxins from endotoxins in bacteria?
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What is the primary composition of teichoic acids?
What is the primary composition of teichoic acids?
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How do penicillin and other beta-lactam antibiotics affect gram-positive bacteria?
How do penicillin and other beta-lactam antibiotics affect gram-positive bacteria?
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What is the main role of pili in bacteria?
What is the main role of pili in bacteria?
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Which of the following bacteria is known to form pili and has nonpathogenic mutants?
Which of the following bacteria is known to form pili and has nonpathogenic mutants?
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What is the primary function of the glycocalyx in bacteria?
What is the primary function of the glycocalyx in bacteria?
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Which type of bacterial structure is specifically formed in response to adverse conditions?
Which type of bacterial structure is specifically formed in response to adverse conditions?
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From which genera do medically important gram-positive rods that form spores originate?
From which genera do medically important gram-positive rods that form spores originate?
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What does the endospore primarily contain?
What does the endospore primarily contain?
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Which bacteria are primarily responsible for dental plaque formation?
Which bacteria are primarily responsible for dental plaque formation?
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What is unique about the sex pilus?
What is unique about the sex pilus?
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What is the primary composition of the capsule in most bacteria?
What is the primary composition of the capsule in most bacteria?
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How many different serologic types of Streptococcus pneumoniae are recognized?
How many different serologic types of Streptococcus pneumoniae are recognized?
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What effect does the capsule have on phagocyte activity?
What effect does the capsule have on phagocyte activity?
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What is the purpose of the Quellung reaction?
What is the purpose of the Quellung reaction?
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What type of structures are flagella classified as?
What type of structures are flagella classified as?
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Which type of bacteria is more likely to have flagella?
Which type of bacteria is more likely to have flagella?
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What term describes the movement of bacteria towards nutrients using flagella?
What term describes the movement of bacteria towards nutrients using flagella?
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In what way can flagella contribute to pathogenicity for certain bacteria?
In what way can flagella contribute to pathogenicity for certain bacteria?
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Study Notes
Bacterial Structure
- Bacteria are classified by shape into three basic groups: cocci, bacilli, and spirochetes.
- Cocci are round, bacilli are rod-shaped, and spirochetes are spiral-shaped.
- Some bacteria are variable in shape and are said to be pleomorphic.
Learning Objectives
- Students should be able to define typical bacteria.
- Bacterial morphology (appearance)
- Cell wall structure, peptidoglycans.
- Cell wall in gram-positive and gram-negative bacteria.
- Cell membrane, mesosome, ribosome, genetic material.
- Specialized structures outside the cell wall: capsule, pili, flagella, glycocalyx
Structure of Bacterial Cells
- Bacteria are classified by three basic shapes: cocci, bacilli, and spirochetes.
- Cocci are round; bacilli are rod-shaped; spirochetes are spiral-shaped.
- Some bacteria are variable in shape and are said to be pleomorphic (many-shaped).
Bacterial Morphology
- Cocci: grouped in clusters (Staphylococcus), chains (Streptococcus), pairs (Streptococcus pneumoniae), pairs with pointed ends (Neisseria - kidney bean shape).
- Rods (bacilli): square ends (Bacillus), rounded ends (Salmonella), club-shaped (Corynebacterium), fusiform (Fusobacterium), comma-shaped (Vibrio).
- Spirochetes: relaxed coil Borrelia; tightly coiled Treponema.
Structure
- Cytoplasm
- Ribosomes
- Cell membrane
- Cell wall
- Capsule
- Flagella
- Attachment pili
- Plasmid
- Sex pilus
Cell Wall
- The cell wall is the outermost component of bacteria (except Mycoplasma).
- Some bacteria have surface features (capsule, flagella, pili) external to the cell wall.
- The cell wall is composed of an inner layer of peptidoglycan and an outer membrane (whose thickness and composition vary between bacteria types).
- Peptidoglycan provides structural support and maintains the shape of the cell.
- The cell wall protects from osmotic pressure.
Peptidoglycan
- Also called murein or mucopeptide.
- Found only in bacterial walls.
- Provides rigid support to the cell.
- Important in maintaining the characteristic shape of the cell.
- Composed of glycan chains connected by peptide bonds.
- Repeating sugars: N-acetyl-muramic acid (NAM) and N-acetyl-glucosamine (NAG) connected by beta 1,4-glycoside bonds.
- Target region of lysozyme.
Gram-positive Cell Wall
- Thick peptidoglycan layer.
- Glycan strings are tightened with teichoic acid strings.
Gram-negative Cell Wall
- Thin peptidoglycan layer.
- Outer membrane (with lipopolysaccharide (LPS)).
- Cytoplasmic membrane.
- Periplasmic space.
Teichoic Acid
- Present only in gram-positive bacteria.
- Activates the same pathways as lipopolysaccharide (LPS) in gram-negative bacteria.
- Phage receptors in gram-negative bacteria.
- Composed of polymers of glycerol phosphate or ribitol phosphate.
Medical Importance of Teichoic Acid
- Ability to induce septic shock.
- Gram-positive bacteria activate the same pathways as does endotoxin (LPS) in gram-negative bacteria.
- Mediates attachment of staphylococci to mucosal cells.
Gram-negative Cell Wall
- Composed of an outer membrane.
- Thin peptidoglycan layer is between the cytoplasmic membrane and outer membrane.
- Contains lipopolysaccharide (LPS).
- Contains porin proteins (OMPs).
Outer Membrane
- Inner and outer layers form a bilayered membrane that are different from each other.
- Outermost phospholipid layer contains a lot of lipopolysaccharide molecules.
Exotoxin and Endotoxin
- Lipopolysaccharide (LPS) of gram-negative bacteria is endotoxin.
- Responsible for disease features like fever and shock.
- Endotoxin is part of the cell wall.
- Exotoxin is released from bacteria.
- Pathologic effects of endotoxin are similar regardless of the organism.
Composition of LPS
- Composed of three molecules: lipid A (toxin), core polysaccharide, an outer polysaccharide layer.
- Core polysaccharide is 5 sugars linked to lipid A.
- Outer polysaccharide is 25 repeating units of 3-5 sugars; this outer polymer is the somatic (O antigen).
Porin Proteins
- Outer membrane contains protein structures (porins).
- Porins are channels for substances.
- OMPs (outer membrane porins).
- Phage receptors in gram-negative bacteria.
Acid-Fast and Associated Cell Wall
- Cell walls of mycobacteria and Nocardia are thick and rich in lipids (mycolic acid).
- These bacteria resist decolorization with acid-alcohol after being stained with carbolfuchsin.
- Resistance is due to high concentration of mycolic acids in the cell wall.
L-form Bacteria
- Cell wall-deficient (CWD) bacteria.
- Strains of bacteria lack cell walls.
- Bacteria may survive if cell wall structures are lost when exposed to lysozyme or antibiotics.
Transpeptidation
- Transpeptidase, endopeptidase, and carboxypeptidase create cell walls.
- Penicillin binding proteins (PBPs) are targets for beta-lactam antibiotics.
Effect on Cell Wall
- Beta-lactam antibiotics (penicillin) show molecular similarity and bind with PBP.
- Irreversibly binds to transpeptidase enzyme active site.
- Penicillin prevents enzyme from binding with D-ala.
- Peptidoglycan synthesis is inhibited.
Cytoplasmic Membrane
- Just inside the peptidoglycan layer.
- Phospholipid bilayer.
- Contains sterols in eukaryotic cells, but prokaryotes generally do not (except for Mycoplasma).
- Active transport of molecules.
- Energy generation by oxidative phosphorylation.
- Synthesis of precursors of the cell wall.
- Secretion of enzymes and toxins.
Bacterial Cytoplasmic Membrane
- Glycolipid and hopanoid chains.
- Integral and peripheral proteins.
- Only on one side.
Mesosome
- Invagination of the cytoplasmic membrane.
- Important during cell division.
- Origin of transverse septum that divides the cell in half.
- Binding site for DNA, forms genetic material for daughter cells.
Cytoplasm
- Two areas when viewed using electron microscope:
- Amorphous matrix containing ribosomes, nutrient granules, metabolites, and plasmids.
- Inner, nucleoid region composed of DNA.
Ribosomes
- Site of protein synthesis.
- Differ from eukaryotic ribosomes in size and composition.
- Bacterial ribosomes are 70S (50S + 30S subunits).
- Eukaryotic ribosomes are 80S (60S + 40S subunits).
Important Note
- Differences in ribosomal RNAs and proteins are the basis of selective antibiotic action (in bacteria, not humans).
Granules
- Cytoplasm contains various granules for nutrient storage.
- Granules stain characteristically with certain dyes.
Nucleoid
- Area of cytoplasm containing DNA.
- DNA of prokaryotes is single, circular molecule.
- Its molecular weight is approximately 2 x 10⁹ and contains approximately 2000 genes.
- No nuclear membrane, nucleolus, mitotic spindle, or histones.
- No resemblance to eukaryotic nucleus.
- Bacterial (prokaryotic) DNA has no introns.
Plasmids
- Extrachromosomal, double-stranded, circular DNA molecules.
- Replicate independently from the bacterial chromosome.
- Can be integrated into the bacterial chromosome.
- Occur in both gram-positive and gram-negative bacteria.
- Several types can exist in one cell.
Transmissible Plasmids
- Can be transferred from one cell to another via conjugation.
- Large (MW 40-100 million), containing many genes for sex pilus and transfer enzymes.
- Typically present in one to three copies per cell.
Non-transmissible Plasmids
- Small (MW 3–20 million).
- Do not contain transfer genes.
- Present in many copies (10–60) per cell.
Important Plasmid Genes
- Carry genes for antibiotic resistance (mediated by various enzymes).
- Resistance to heavy metals (e.g., mercury).
- Resistance to ultraviolet light (mediated by DNA repair enzymes).
- Pilus formation (mediates adherence to epithelial cells).
- Production of exotoxins (e.g., enterotoxins).
Transposons
- Pieces of DNA that move readily within or between bacterial DNA, plasmids, and bacteriophages.
- May code for drug-resistance enzymes, toxins, or metabolic enzymes.
- Not capable of independent replication
Specialized Structures
- Capsule
- Flagella
- Pili
- Glycocalyx
Capsule
- Gelatinous layer surrounding entire bacteria, except anthrax bacillus, which has a polymerized D-glutamic acid capsule.
- Sugar components vary between species and help determine the serologic type within a species.
- Important determinant of virulence: limits phagocyte engulfment.
- Used in microbial identification (e.g., quellung reaction).
- Component of some vaccines (e.g., Streptococcus pneumoniae).
- Aids in bacterial adherence to human tissues (initial step in infection).
Flagella
- Long, whip-like appendages.
- Move bacteria toward nutrients and attractants (chemotaxis).
- Varying numbers and locations (monotrichous, lophotrichous, amphitrichous, peritrichous).
- Important in pathogenesis (e.g., urinary tract infections; propelling bacteria up the urethra into the bladder).
- Identification of bacteria in laboratory using H antigens.
Pili (Fimbriae)
- Hair-like filaments extending from the cell surface.
- Shorter and straighter than flagella.
- Commonly found on gram-negative bacteria.
- Mediate attachment of bacteria to human cell surfaces (e.g., in Neisseria gonorrhoeae).
- Pilus formation important for bacterial pathogenicity.
Glycocalyx (Slime Layer)
- Polysaccharide coating secreted by bacteria.
- Allows bacteria to adhere to surfaces (e.g., skin, heart valves, catheters).
- Plays a role in formation of plaque (precursor to dental caries), especially in Streptococcus mutans.
Spores
- Highly resistant structures formed in response to adverse conditions (nutrient depletion and chemicals).
- Present in two medically important genera of gram-positive rods: Bacillus and Clostridium.
- Contain bacterial DNA, cytoplasm, cell membrane, and peptidoglycan.
- Contain very little water and a thick, keratin-like layer.
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Description
Test your knowledge on the key characteristics of bacterial structures, plasmids, and their functions. This quiz covers essential concepts such as plasmid types, bacterial shapes, and cell wall composition. Perfect for students studying microbiology or related fields.
