lecture 8 Prokaryotic Reproduction and Antibiotic Targets

Plasma membrane and membrane components

• Bacteria are generally beneficial, with their cells performing essential functions in our bodies (e.g., in the mouth microbiome aiding milk digestion in newborns).
• Plasma membrane basics
  • Composed of a phospholipid bilayer: phospholipids with a hydrophilic polar head group and hydrophobic fatty acid tails.
  • Hydrophilic heads face aqueous environments (inside and outside the cell); hydrophobic tails hide away from water.
  • The bilayer forms spontaneously in aqueous environments, creating a selectively permeable barrier.
• Proteins in the membrane
  • Integral (intrinsic) proteins span the membrane; they are not easily removed and often function as channels for selective transport, energy production, and signaling.
  • Peripheral (extrinsic) proteins are loosely attached to the membrane and can be removed depending on cellular needs.
• Other membrane components
  • Glycolipids and oligosaccharides on the cell surface participate in signaling and cell communication.
  • Steroids: bacteria lack cholesterol; instead they have hopanoids that perform a similar stabilizing role in the membrane.
• Learning outcomes reference
  • The slide’s learning outcomes are listed at the end of the lecture for study alignment.
• Targeting membranes for antibiotics
  • A key strategy for selective antibiotic design is to identify bacterial-specific components (e.g., features of the bacterial cell envelope) that differ from mammalian cells.

Flagella and motility

• Flagellum (singular) / flagella (plural)
  • Thread-like structure used for movement; some bacteria have one, others have many.
  • Function: propulsion via rotation like a propeller, enabling movement toward nutrients and away from toxins.
• Additional roles
  • Attachment to surfaces, aiding colonization.
• Energy cost
  • Flagellar expression is energetically costly (ATP-demanding); bacteria regulate expression based on need.

Nucleoid and DNA organization in bacteria

• Nucleoid region (no nucleus in bacteria)
  • A single circular dsDNA molecule occupies a central, irregular nucleoid rather than a defined nucleus.
  • Most bacteria have one circular chromosome; some have linear chromosomes or multiple chromosomes (e.g., Vibrochlo R).
• DNA length and packaging
  • The bacterial genome is extremely compacted; if stretched, DNA length can be ~230–700 times the cell length.
  • Packaging is aided by nucleoid-associated proteins (distinct from eukaryotic histones).
• Transcription-translation coupling
  • In bacteria, transcription and translation can occur simultaneously in the cytoplasm because there is no nuclear envelope separating processes.
  • In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm after mRNA processing.
• DNA topology and organization
  • DNA length and organization require efficient packing to fit within the small cell volume.

Ribosomes in bacteria

• Function and location
  • Ribosomes synthesize proteins; present throughout the cytoplasm and sometimes attached to the plasma membrane for targeted protein synthesis.
• Composition and size
  • Prokaryotic ribosome: 70S, composed of a large subunit (50S) and a small subunit (30S).
  • The 70S notation reflects Svedberg units (sedimentation rate) and is not strictly additive as ordinary masses are; i.e., 70S is not simply 50S + 30S, though they assemble into the functional ribosome.
  • Ribosomal RNA (rRNA) forms part of the ribosome alongside ribosomal proteins.
• Eukaryotic contrast
  • Eukaryotic ribosomes are 80S (60S + 40S) in the cytoplasm.
• Significance for antibiotics
  • Bacterial ribosomes are structurally distinct from mammalian ribosomes, making them a common antibiotic target (e.g., drugs that inhibit bacterial translation with limited effects on human cells).

Bacterial cell division: binary fission vs. mitosis (contrast)

• Binary fission (bacteria)
  • A rapid asexual division process producing two genetically identical daughter cells.
  • No mitotic spindle or chromosomes metaphase/anaphase/telophase; no nucleus.
• Generation time and growth rate
  • Bacteria typically divide much faster than mammalian cells.
  • Example given: bacteria divide roughly every 20 minutes; with 12 hours, this leads to rapid exponential growth (the lecture notes estimate ~1,000,000 cells in 12 hours for a culture starting from a certain number—an illustrative figure; a precise calculation with 20-minute divisions yields 2^ generations where generations ≈ (12 h)/(0.333 h) ≈ 36, giving very large numbers, e.g., starting from 1,000 cells yields ≈ 1,000 × 2^36 ≈ 6.9 × 10^13 cells; the lecture also noted the “million” figure as an approximate takeaway).
• Key steps in binary fission
  • Growth in size and volume.
  • DNA replication of the single circular chromosome; the origin of replication initiates duplication and replication forks proceed bidirectionally until termination.
  • Duplication of other cellular components (e.g., ribosomes and organelles) to provide for two daughter cells.
  • Segregation of DNA and cellular components toward opposite ends of the cell.
  • Formation of the septum (cross-wall) at mid-cell (via the divisome) and eventual cell wall synthesis to separate the two daughter cells.
  • Some cells may remain attached briefly after division to exchange nutrients or signals, but most detach to become independent cells.
• Septum formation and the divisome
  • FtsZ protein forms a ring (the “Z-ring”) at the future division site (center of the cell).
  • The ring recruits other division proteins (collectively called the divisome).
  • Key early players referenced: FtsZ (formation of the ring), and proteins SIPA and FtsA (anchor/attach the ring to the membrane).
  • Autolysins are enzymes that locally break down existing peptidoglycan to allow insertion of new cell wall during septation; their activity is tightly regulated to occur only when division is imminent.
• Visual aid concepts
  • Imagine a 3D balloon representing the cell; the divisome and FtsZ ring form a constricting ring around the middle to drive septum formation and ultimately self-separation.

Bacterial genome and plasmids

• Chromosome basics
  • Bacteria typically carry a single circular dsDNA molecule containing all essential genetic information.
  • The chromosome is highly compacted via supercoiling with bacterial proteins (different from histones in eukaryotes).
  • The origin of replication marks the start; replication proceeds bidirectionally and terminates opposite the origin at the termination sequence.
• Plasmids
  • Small, circular DNA molecules separate from the main chromosome.
  • Often carry advantageous genes (e.g., antibiotic resistance, virulence factors, metabolic traits).
  • Plasmid replication is independent of chromosomal replication; daughter cells typically inherit plasmids from the parent during division.
  • Bacteria can carry multiple plasmids, though typically one is common; plasmids can be stably inherited.
• DNA packaging and comparison to eukaryotes
  • Bacteria lack a nucleus; DNA in the nucleoid region is highly compacted but not enclosed by a membrane.
  • Eukaryotes package DNA with histones into chromatin inside a nucleus; in bacteria, other proteins take on the packaging role.

Telomeres, aging, and the cancer connection (conceptual contrast)

• Telomeres and replication limits in eukaryotes
  • Eukaryotic chromosomes have telomeres at ends; telomere shortening with replication limits contributes to cellular aging.
  • Telomerase enzyme can extend telomeres, enabling continued division; about ~85% of cancers express telomerase, which helps cancer cells bypass normal aging limits.
  • Normal cells largely lack telomerase activity after development, limiting their replicative lifespan and contributing to aging.
• Relevance to bacteria
  • Bacterial chromosomes are circular and do not have telomeres; they do not face the same end-replication problem as linear eukaryotic chromosomes, avoiding telomere shortening with each division. However, bacteria still die from other stresses (nutrient depletion, DNA damage, cell wall failure).
• Practical implications
  • Telomerase is a potential anti-cancer drug target due to its role in enabling limitless replication in cancer cells.
  • Conceptually, understanding replication limits in bacteria highlights fundamental differences in how cells age and divide across life forms.

The bacterial cell wall and peptidoglycan synthesis

• Why the cell wall matters
  • The cell wall provides structural integrity, shape, and some resistance to osmotic pressure.
  • It protects against lysis by osmotic shock and contributes to selective diffusion barriers, though not as selective as the plasma membrane.
• Peptidoglycan (PG) structure
  • PG is made of alternating sugars: N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).
  • NAM carries a short peptide stem; cross-links between peptide stems reinforce the mesh.
• NAM/NAG and amino acids
  • Each PG unit comprises NAM linked to customized amino acids; among these, d-amino acids (e.g., D-alanine) are present in the bacterial PG precursor; L-amino acids are the standard form used in mammalian proteins.
  • The presence of D-amino acids is a hallmark of bacterial cell wall synthesis and a target for selective antibiotics.
• Transport of PG precursors to the periplasm
  • Precursors are synthesized in the cytoplasm and must be moved to the periplasm where the growing cell wall is assembled.
  • Bactoprenol (a lipid carrier) binds PG precursors and shuttles them across the plasma membrane to the periplasm.
• PG synthesis machinery
  • Penicillin-binding proteins (PBPs) catalyze the linking of PG precursors into long glycan strands and then cross-link the strands to form a rigid mesh.
  • Cross-linking creates a strong covalent network, giving the cell wall its mechanical strength while retaining elasticity to handle osmotic stress.
• Autolysins and septum remodeling
  • Autolysins locally cleave bonds in existing PG to allow insertion of new PG during cell division.
  • Their activity is tightly regulated and activated when the divisome is ready to divide the cell.

Gram-positive vs. Gram-negative bacteria; Gram staining rationale

• Gram-positive features
  • Thick, multi-layered peptidoglycan cell wall with little or no outer membrane.
  • Retains crystal violet dye during the Gram stain, appearing purple under the microscope.
• Gram-negative features
  • Thinner peptidoglycan layer and an outer membrane containing lipopolysaccharides and proteins.
  • Has an outer membrane that complicates drug access; after decolorization, these bacteria appear colorless until counterstaining.
• Why the distinction matters for antibiotics
  • Many antibiotics target PG synthesis; the outer membrane in Gram-negatives can restrict drug entry, and the thinner PG layer is a different target landscape compared to Gram-positives.
• Gram stain procedure (classic steps)
  • Step 1: Crystal violet (purple) stains both Gram-positive and Gram-negative bacteria.
  • Step 2: Iodine forms a complex with crystal violet, increasing dye retention.
  • Step 3: Alcohol decolorization separates the two groups: Gram-positive stay purple; Gram-negative become colorless as the outer membrane is disrupted.
  • Step 4: Safranin counterstain recolors Gram-negative bacteria pink/red, while Gram-positive remain purple.
• Clinical use
  • Gram staining helps guide antibiotic selection and interpretation of suspected bacterial infection.

Antibiotics targeting the bacterial cell wall; resistance mechanisms

• Penicillin and related beta-lactams
  • Mechanism: Inhibit the transpeptidation (cross-linking) enzyme step in PG synthesis, preventing proper cross-link formation and cell wall assembly, leading to cell death.
  • Beta-lactam ring is essential for activity; beta-lactamases produced by some bacteria can hydrolyze the ring and confer resistance.
• Antibiotic resistance plasmids
  • Resistance genes (e.g., beta-lactamase) can be carried on plasmids and spread between bacteria.
• Vancomycin
  • Inhibits PG cross-linking by binding to amino acid residues in PG precursors, blocking cross-link enzyme access and halting cell wall synthesis.
• Target selection rationale
  • Cell wall synthesis is an attractive target because eukaryotic cells (humans) lack peptidoglycan, enabling selective toxicity.
• Resistance and stewardship considerations
  • Overuse and incomplete dosing contribute to resistance development.
  • Bacteria can acquire resistance by plasmid-borne enzymes (e.g., beta-lactamases) or alterations in PBPs or permeability barriers.
  • Patients should complete prescribed antibiotic courses to minimize selection of resistant strains.
• Practical diagnostic tool: antibiotic susceptibility testing
  • E-test strips (ET strips) on spread plates generate an inhibition ellipse; the point where the ellipse intersects the strip indicates the minimum inhibitory concentration (MIC).
  • MIC is the lowest drug concentration that inhibits visible growth; guides selection and dosing to minimize resistance pressure.

Practical diagnostic technique: MIC and antibiotic testing in labs

• E-test strips overview
  • Strips contain a gradient of antibiotic concentrations; after incubation, the inhibition zone provides the MIC.
  • The size and clarity of the inhibition zone reflect drug efficacy against the particular bacterial isolate.
• Interpreting results
  • A larger clearance around the strip indicates higher susceptibility; a smaller clearing indicates reduced susceptibility.
  • Clinicians use MIC values to tailor antibiotic choice and dosing to achieve effective concentrations at the infection site while minimizing toxicity and resistance.

Connections to broader themes and exam-focused takeaways

• Core differences to remember
  • Prokaryotes (bacteria) lack a nucleus; DNA is in the nucleoid and transcription/translation can be coupled.
  • Eukaryotes have linear chromosomes with telomeres and use a nucleus; telomerase activity is a cancer-related phenomenon in humans.
• Key bacterial targets for antibiotics
  • Cell wall synthesis (peptidoglycan) is a primary and historically successful target due to the absence of PG in human cells.
  • Other potential targets include non-PG cell envelope features, specific bacterial enzymes, and unique metabolic pathways; these are active areas of research to combat resistance.
• Ethical and practical implications
  • Antibiotic stewardship is crucial to limit resistance development and preserve drug efficacy for future patients.
  • Overuse, incomplete courses, and environmental exposure to antibiotics contribute to resistance trends—public health considerations are essential.
• Quick recap of numerical and structural highlights
  • Ribosome sizes: 70S (composed of 50S + 30S subunits). Note: 70S is not simply additive in Svedberg units; the assembly yields a functional 70S ribosome.
  • Bacteria divide roughly every 20 minutes under ideal conditions; very rapid growth can yield huge populations in hours.
  • Bacterial DNA length when stretched is ~230–700× the cell length, requiring efficient packaging.
  • Approximately 85% of cancers express telomerase, enabling extended cell division; this is relevant to discussions of aging, cancer biology, and potential drug targets.
• Exam-oriented pointers
  • Be able to explain why the cell wall is a prime antibiotic target and how beta-lactamases confer resistance.
  • Describe the Gram staining steps and how Gram status influences antibiotic choice.
  • Understand the basic architecture of the bacterial envelope (plasma membrane, cell wall, outer membrane in Gram-negatives) and how this affects drug access.
  • Recognize the roles of FtsZ (divisome), FtsA/SIP A, and autolysins in septum formation and cell separation.
  • Distinguish prokaryotic transcription/translation coupling from the eukaryotic process and understand implications for antibiotic targeting of bacterial ribosomes.

Quick glossary of major terms (for exam comfort)

• 70S ribosome: prokaryotic ribosome (50S large subunit + 30S small subunit) with Svedberg units; not strictly additive in a simple mass sense.
• PBPs: penicillin-binding proteins involved in linking PG strands and cross-linking during cell wall synthesis.
• FtsZ: scaffold protein that forms the Z-ring at the division site; initiates divisome assembly.
• SIPA, FtsA: proteins that anchor the Z-ring to the plasma membrane and help recruit divisome components.
• Autolysins: enzymes that locally break PG bonds to allow remodeling during division.
• Bactoprenol: lipid carrier that transports PG precursors across the plasma membrane to the periplasm.
• NAM/NAG: repeating sugars in PG; NAM carries the amino acid stem; NAG is the alternate sugar unit.
• D-amino acids: forms of amino acids used in bacterial PG precursors, not commonly used in human proteins.
• MIC (Minimum Inhibitory Concentration): lowest antibiotic concentration that inhibits visible growth of a bacterium.
• E-test strips: diagnostic tool to determine MIC by showing an inhibition ellipse on an agar plate.
• Gram-positive vs Gram-negative: differences in PG thickness and outer membrane presence; critical for antibiotic choice.
• β-lactamases: enzymes that hydrolyze the β-lactam ring of many penicillins, leading to resistance.
• Vancomycin: antibiotic that inhibits PG cross-linking by binding to PG precursor amino acids.
• Telomerase and cancer (context): enzyme that extends telomeres, enabling sustained cell division in many cancers (≈85%).