Lecture # 46. T. Slieman, Ph.D. (PPT)-1

1. Organisms Studied in Microbiology

Microbiology is the study of microscopic organisms. The five main types of organisms studied are:

• Bacteria

• Archaea

• Fungi (e.g., yeasts, molds)

• Protozoa

• Viruses

1. Gram Stain and its Clinical Application

The Gram stain is a differential staining technique used to classify bacteria based on their cell wall composition. It involves four main steps:

• Primary stain (Crystal Violet): Stains all bacterial cells purple.

• Mordant (Iodine): Forms large crystal violet-iodine complexes within the cell.

• Decolorizer (Alcohol or Acetone): Washes away the crystal violet-iodine complex from Gram-negative cells (due to their thin peptidoglycan and outer membrane).

• Counterstain (Safranin): Stains decolorized Gram-negative cells pink or red.

Gram-positive bacteria: Appear purple because their thick peptidoglycan layer retains the crystal violet-iodine complex.
Gram-negative bacteria: Appear pink/red because the decolorizer washes out the primary stain, and they are then counterstained by safranin.

Clinical Application: Gram staining is a crucial first step in identifying bacterial pathogens. It helps clinicians rapidly narrow down the possible causative agents of an infection and guide the initial choice of appropriate antibiotics, as Gram-positive and Gram-negative bacteria often respond differently to various antimicrobial drugs.

3. Acid-Fast Stain and its Clinical Application

The Acid-Fast stain is another differential staining technique used to identify bacteria that have a waxy substance called mycolic acid in their cell walls, such as members of the genus Mycobacterium (e.g., Mycobacterium tuberculosis).

• Primary stain (Carbolfuchsin): Stains cells red, enhanced by heating.

• Decolorizer (Acid-alcohol): Removes the red stain from non-acid-fast cells. Mycolic acid in acid-fast cells prevents decolorization.

• Counterstain (Methylene Blue): Stains decolorized non-acid-fast cells blue.

Acid-fast bacteria: Appear red because the mycolic acid in their cell walls retains the carbolfuchsin, even after washing with acid-alcohol.
Non-acid-fast bacteria: Appear blue because the acid-alcohol removes the primary stain, and they are then counterstained with methylene blue.

Clinical Application: This stain is vital for diagnosing infections caused by Mycobacterium species, particularly tuberculosis and leprosy, where rapid identification is crucial for treatment and public health measures.

4. Bacterial Envelope and its Components

The bacterial envelope refers to the layers that physically enclose the bacterial cytoplasm. Its components vary slightly between Gram-positive and Gram-negative bacteria.

Components of the Bacterial Envelope:

• Glycocalyx: (External layer, if present)

  • Capsule: A well-organized, distinct, and firmly attached layer, usually made of polysaccharides or sometimes polypeptides.

  • Slime layer: An unorganized, loose, and easily removed layer.

• Cell Wall: A rigid layer made primarily of peptidoglycan, providing structural support and shape.

• Cell Membrane (Cytoplasmic Membrane): Inner membrane composed of a phospholipid bilayer with embedded proteins, regulating transport in and out of the cell.

• Outer Membrane: (Present only in Gram-negative bacteria) An additional outer lipid bilayer containing lipopolysaccharide (LPS), phospholipids, and porin proteins.

1. Cell Wall Structure and Differences Between Gram-Positive and Gram-Negative Bacteria

Cell Wall Structure: The bacterial cell wall is primarily composed of peptidoglycan (also known as murein), a polymer of NN-acetylglucosamine (NAG) and NN-acetylmuramic acid (NAM) subunits, cross-linked by short peptide chains. It provides structural integrity, maintains cell shape, and protects against osmotic lysis.

Differences Between Gram-Positive and Gram-Negative Cell Walls:

• Gram-Positive Bacteria:

  • Peptidoglycan Layer: Very thick (20-80 nm), multiple layers of peptidoglycan.

  • Teichoic Acids: Contains teichoic acids and lipoteichoic acids, which anchor the cell wall to the cytoplasmic membrane and contribute to pathogenicity.

  • Outer Membrane: Absent.

  • Periplasmic Space: Minimal or absent.

  • Vulnerability: More susceptible to antibiotics that target peptidoglycan synthesis (e.g., penicillin).

• Gram-Negative Bacteria:

  • Peptidoglycan Layer: Thin (5-10 nm), usually one or two layers of peptidoglycan.

  • Outer Membrane: Presents an outer membrane external to the peptidoglycan layer.

  • Composed of lipopolysaccharide (LPS), phospholipids, and porin proteins.

  • LPS acts as an endotoxin (Lipid A component), a major virulence factor.

  • Porins allow passage of small molecules into the periplasmic space.

  • Periplasmic Space: Prominent space between the cytoplasmic membrane and the outer membrane, containing various enzymes (e.g., hydrolytic enzymes, antibiotic-inactivating enzymes).

  • Vulnerability: Outer membrane provides a barrier against many antibiotics and lysozyme, making them generally more resistant to certain drugs.

1. Structure of Capsules and their Role in Virulence

Structure of Capsules: A capsule is a well-organized, distinct, and firmly attached layer of glycocalyx, located external to the cell wall. It is typically composed of polysaccharides (complex carbohydrates), though some are made of polypeptides. Capsules are generally hydrated and difficult to wash off.

Role in Virulence: Capsules play a significant role in bacterial virulence (the ability of a pathogen to cause disease) through several mechanisms:

• Antiphagocytic: The smooth, hydrophilic surface of the capsule makes it difficult for phagocytic cells (like macrophages and neutrophils) to engulf the bacterium, allowing it to evade the host immune system. This is a primary virulence factor for many pathogenic bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae).

• Adherence: Capsules can help bacteria adhere to host tissues and surfaces, facilitating colonization and biofilm formation (e.g., adherence to teeth by Streptococcus mutans).

• Protection against Desiccation: The high water content of the capsule can protect the bacterial cell from drying out in harsh environments.

• Protection against Antimicrobials: Can offer some protection against certain antimicrobial agents or detergents.

1. Structure of Endospores

Endospores are highly resistant, dormant, and non-reproductive structures produced by certain Gram-positive bacteria (notably Bacillus and Clostridium species) in response to unfavorable environmental conditions. They are designed for survival, not for reproduction. When conditions become favorable, endospores can germinate back into vegetative cells.

Structure of an Endospore (from outermost to innermost):

• Exosporium: A thin, delicate outer layer composed of protein, not present in all endospores.

• Spore Coat: Several layers of spore-specific proteins that provide significant chemical and enzymatic resistance.

• Cortex: A thick layer of peptidoglycan, less cross-linked than the vegetative cell wall. Dehydration of the cortex is crucial for endospore resistance.

• Core Wall: A normal peptidoglycan layer that surrounds the core protoplast.

• Core (Protoplast): Contains the essential components of the bacterial cell, including:

  • Cytoplasm: Dehydrated, contributing to heat resistance.

  • DNA: Compacted and protected by small acid-soluble spore proteins (SASPs).

  • Ribosomes: Essential for protein synthesis upon germination.

  • Dipicolinic Acid: Complexed with calcium, contributes to dehydration and heat resistance by stabilizing DNA and proteins.

1. Three Mechanisms of Horizontal Gene Transfer in Bacteria

Horizontal gene transfer (HGT) is the process by which genetic material is transferred between bacterial cells that are not parent and offspring. This is a major factor in bacterial evolution, especially in the spread of antibiotic resistance and virulence factors.

• Transformation: The uptake of naked DNA from the environment by a bacterial cell. This DNA can originate from lysed (dead) bacterial cells. The recipient cell must be in a state of 'competence,' meaning it is able to take up external DNA. Once taken up, the foreign DNA can be integrated into the host chromosome via homologous recombination or exist as a plasmid.

• Conjugation: The direct transfer of genetic material from one bacterial cell (donor) to another (recipient) through physical contact, often involving a pilus (sex pilus). This typically involves the transfer of plasmids (e.g., F plasmid for fertility, R plasmids for antibiotic resistance). The donor cell contains a conjugative plasmid, which encodes the genes required for pilus formation and DNA transfer.

• Transduction: The transfer of bacterial DNA from one bacterium to another via a bacteriophage (a virus that infects bacteria).

  • Generalized transduction: Occurs when a bacteriophage mistakenly packages bacterial chromosomal DNA fragments instead of viral DNA into its capsid. When this phage infects a new bacterium, it injects the bacterial DNA, which can then be incorporated into the recipient's genome.

  • Specialized transduction: Occurs when a temperate bacteriophage (e.g., lambda phage) excises imperfectly from the bacterial chromosome, carrying a small piece of adjacent bacterial DNA along with its viral DNA. This hybrid DNA is then replicated and packaged into new phage particles, which can subsequently transfer both viral and bacterial genes to other bacteria.

1. Processes of Replication, Transcription, and Translation

• Replication: The process by which a cell makes an exact copy of its DNA. This is a semiconservative process, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. It involves unwinding the double helix, synthesizing new strands using DNA polymerase, and ensuring accurate duplication of genetic information before cell division.

• Transcription: The process of synthesizing RNA from a DNA template. An enzyme called RNA polymerase binds to a promoter region on the DNA, unwinds a portion of the DNA, and then synthesizes a complementary RNA molecule (mRNA, tRNA, or rRNA) using the DNA as a template. This process typically ends at a terminator sequence on the DNA.

• Translation: The process by which genetic information encoded in messenger RNA (mRNA) is decoded to produce a specific sequence of amino acids, forming a polypeptide chain (protein). Ribosomes read the mRNA codons, transfer RNA (tRNA) molecules bring the corresponding amino acids to the ribosome, and peptide bonds are formed between amino acids, building the protein.

1. Compare and Contrast Replication, Transcription, and Translation Between Prokaryotes and Eukaryotes

11. Classify Antimicrobials According to their Mechanism of Action

Antimicrobials can be classified based on how they selectively target and inhibit microbial growth or kill microbes:

• Inhibition of Cell Wall Synthesis: These drugs prevent the formation of peptidoglycan, leading to osmotic lysis of the bacterial cell. They are typically bactericidal and target actively growing cells.

  • Examples: Penicillins, Cephalosporins, Vancomycin.

• Inhibition of Protein Synthesis: These drugs target bacterial ribosomes or associated molecules, interfering with translation. Since bacterial ribosomes (70S70S) are different from eukaryotic ribosomes (80S80S), selective toxicity is possible.

  • Examples: Aminoglycosides (e.g., streptomycin, gentamicin), Tetracyclines, Macrolides (e.g., erythromycin), Chloramphenicol, Lincosamides (e.g., clindamycin).

• Disruption of Cell Membrane Function: These drugs alter the permeability of the bacterial cell membrane, leading to leakage of cellular components.

  • Examples: Polymyxins (e.g., colistin), Daptomycin.

• Inhibition of Nucleic Acid Synthesis: These drugs interfere with DNA replication or RNA transcription.

  • DNA Replication: Fluoroquinolones (e.g., ciprofloxacin, levofloxacin) inhibit DNA gyrase.

  • RNA Transcription: Rifamycins (e.g., rifampin) inhibit bacterial RNA polymerase.

• Inhibition of Essential Metabolite Synthesis (Antimetabolites): These drugs block metabolic pathways by acting as competitive inhibitors for enzymes or by being incorporated into essential molecules.

  • Examples: Sulfonamides (inhibit folic acid synthesis), Trimethoprim (inhibits dihydrofolate reductase in folic acid synthesis).

1. List Several Antimicrobials and their Target Molecules

• Penicillins: Target the cell wall by inhibiting transpeptidases (penicillin-binding proteins, PBPs) involved in peptidoglycan cross-linking.

• Cephalosporins: Similar to penicillins, they are beta-lactam antibiotics that inhibit cell wall synthesis by binding to PBPs.

• Vancomycin: Targets the cell wall by binding to the DD-Ala-$D−Alaterminusofpeptidoglycanprecursors,preventingcross−linkingandelongation.Aminoglycosides(e.g.,Streptomycin,Gentamicin):Targetthe−Alaterminusofpeptidoglycanprecursors,preventingcross−linkingandelongation.Aminoglycosides(e.g.,Streptomycin,Gentamicin):Targetthe30Ssubunitofthebacterialribosome,causingmistranslationofmRNAandprematureterminationofproteinsynthesis.Tetracyclines:Targetthesubunitofthebacterialribosome,causingmistranslationofmRNAandprematureterminationofproteinsynthesis.Tetracyclines:Targetthe30Ssubunitofthebacterialribosome,preventingthebindingoftRNAtotheA−site,therebyinhibitingproteinsynthesis.Macrolides(e.g.,Erythromycin,Azithromycin):Targetthesubunitofthebacterialribosome,preventingthebindingoftRNAtotheA−site,therebyinhibitingproteinsynthesis.Macrolides(e.g.,Erythromycin,Azithromycin):Targetthe50Ssubunitofthebacterialribosome,inhibitingtranslocationoftheribosomealongmRNAorblockingthepolypeptideexittunnel.Chloramphenicol:Targetsthesubunitofthebacterialribosome,inhibitingtranslocationoftheribosomealongmRNAorblockingthepolypeptideexittunnel.Chloramphenicol:Targetsthe50S$$ subunit of the bacterial ribosome, inhibiting peptidyl transferase activity and thus peptide bond formation.

• Polymyxins (e.g., Colistin): Target the outer and inner membranes of Gram-negative bacteria, disrupting their integrity and causing cell leakage.

• Fluoroquinolones (e.g., Ciprofloxacin): Target bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, enzymes essential for DNA replication, repair, and transcription.

• Rifampin: Targets bacterial RNA polymerase, preventing the initiation of RNA synthesis.

• Sulfonamides: Target the metabolic pathway for folic acid synthesis by competitively inhibiting dihydropteroate synthase, an enzyme involved in synthesizing dihydrofolic acid from para-aminobenzoic acid (PABA).

• Trimethoprim: Targets the metabolic pathway for folic acid synthesis by inhibiting dihydrofolate reductase, an enzyme that converts dihydrofolic acid to tetrahydrofolic acid.