Complete Exam Guide Subject-wise practice platform
Microbiology Quiz

The Microbial Cell: Structure and Function

Everything in microbiology begins with the structure of the microbial cell, because cell structure explains staining behaviour, antibiotic targets, virulence and identification. These notes cover the difference between prokaryotic and eukaryotic cells, bacterial shapes and arrangements, the cell wall and the basis of the Gram stain, the cell membrane and cytoplasmic contents, external structures such as capsule, flagella and pili, endospores, growth and binary fission, and the way each structure is exploited by antibiotics and laboratory tests. A 20-question practice set follows.

Prokaryotic Versus Eukaryotic Cells

Bacteria and archaea are prokaryotes; fungi, protozoa and algae are eukaryotes. The single defining difference is the nucleus: prokaryotes have no nuclear membrane, so their DNA lies free in the cytoplasm in a region called the nucleoid. Almost every other difference follows from a simpler internal organisation.

  • Prokaryotes: no membrane-bound nucleus, no mitochondria, no endoplasmic reticulum or Golgi apparatus, single circular chromosome, 70S ribosomes made of 50S and 30S subunits.
  • Eukaryotes: true nucleus with a nuclear membrane, membrane-bound organelles, several linear chromosomes, 80S ribosomes made of 60S and 40S subunits.
  • Prokaryotes divide by binary fission; eukaryotes divide by mitosis and meiosis.
  • The 70S versus 80S difference is the basis of selective toxicity: aminoglycosides and tetracyclines act on the 30S subunit, while macrolides, chloramphenicol and clindamycin act on the 50S subunit, sparing human ribosomes.
  • Bacterial cell size is typically 0.5 to 5 micrometres; a typical eukaryotic cell is 10 to 100 micrometres.
  • Many bacteria also carry plasmids, small circular extrachromosomal DNA molecules that often encode antibiotic resistance and can be transferred between cells.

Shape, Arrangement and the Cell Wall

Bacteria are classified first by shape and arrangement, then by their reaction to the Gram stain. The Gram reaction depends entirely on the thickness of the peptidoglycan layer in the cell wall, so this single structural fact drives most of clinical bacteriology.

  • Shapes: coccus (spherical), bacillus (rod), spirillum and spirochaete (spiral), vibrio (comma-shaped), coccobacillus (short oval rod).
  • Arrangements: diplo- (pairs), strepto- (chains), staphylo- (clusters), tetrad (groups of four), sarcina (cubes of eight).
  • Peptidoglycan (murein) is a mesh of alternating N-acetylglucosamine and N-acetylmuramic acid chains cross-linked by short peptide bridges; the cross-linking is catalysed by transpeptidase, also called penicillin-binding protein.
  • Gram-positive wall: thick peptidoglycan (20 to 80 nanometres) with teichoic and lipoteichoic acids, no outer membrane. It retains crystal violet and stains purple.
  • Gram-negative wall: thin peptidoglycan (2 to 7 nanometres) sandwiched between the inner membrane and an outer membrane containing lipopolysaccharide, with a periplasmic space between. Crystal violet is washed out by alcohol and the safranin counterstain makes the cell pink.
  • Gram stain steps in order: crystal violet (primary stain), Gram's iodine (mordant), alcohol or acetone (decolouriser, the critical step), safranin (counterstain).
  • Lipopolysaccharide has three parts: lipid A (the toxic endotoxin, released on cell lysis and responsible for fever, hypotension and septic shock), the core polysaccharide, and the O antigen used for serotyping.
  • Mycobacteria have a peptidoglycan wall rich in mycolic acid, so they resist the Gram stain and require the Ziehl-Neelsen acid-fast stain, appearing red against a blue background.
  • Mycoplasma species have no cell wall at all; their membrane contains sterols. This is why they are intrinsically resistant to penicillins and are treated with macrolides or tetracyclines.
  • Penicillins and cephalosporins inhibit transpeptidase cross-linking; vancomycin binds the D-alanyl-D-alanine terminus of the peptide precursor and is effective mainly against Gram-positive organisms because it cannot cross the Gram-negative outer membrane. Lysozyme in tears and saliva cleaves the bond between the two sugars.

Membrane, Cytoplasm and Genetic Material

Beneath the wall lies the cytoplasmic membrane, a phospholipid bilayer that is the true permeability barrier of the cell. Because bacteria have no mitochondria, this membrane also carries the electron transport chain and generates the proton motive force used to make adenosine triphosphate.

  • Functions of the cytoplasmic membrane: selective permeability, active transport, energy generation (electron transport and adenosine triphosphate synthesis), and secretion of enzymes and toxins.
  • Bacterial membranes contain no sterols, with the exception of Mycoplasma. This is why antifungal polyenes such as amphotericin B, which bind ergosterol, have no effect on bacteria.
  • Polymyxins are detergents that disrupt the bacterial membrane, acting mainly on Gram-negative organisms.
  • The nucleoid holds a single circular double-stranded DNA chromosome that is not bound by histones in the eukaryotic sense.
  • Plasmids replicate independently and may carry resistance genes (R factors), toxin genes or genes for conjugation.
  • Genetic exchange occurs by three mechanisms: transformation (uptake of naked DNA from the environment), transduction (transfer by a bacteriophage), and conjugation (direct transfer through a sex pilus, the main route for spreading multidrug resistance).
  • Inclusion bodies such as volutin (metachromatic) granules in Corynebacterium diphtheriae store phosphate and are used in identification.
  • Bacteria store energy as glycogen and as poly-beta-hydroxybutyrate granules.

External Structures, Endospores and Growth

Structures outside the cell wall are the main determinants of virulence and of a bacterium's ability to survive outside the host. Capsules resist phagocytosis, pili allow attachment, flagella allow movement toward nutrients, and endospores allow survival in extreme conditions.

  • Capsule: an organised polysaccharide layer (polypeptide in Bacillus anthracis) outside the wall. It is antiphagocytic, so it is a major virulence factor, and it is the antigen used in polysaccharide vaccines against Streptococcus pneumoniae, Haemophilus influenzae type b and Neisseria meningitidis. It is demonstrated by negative staining, for example India ink, or by the quellung reaction.
  • Glycocalyx or slime layer: a loose sugar coat that helps form biofilms on catheters and prosthetic material, characteristic of Staphylococcus epidermidis and Pseudomonas aeruginosa.
  • Flagella: made of the protein flagellin, they provide motility and carry the H antigen. Arrangements include monotrichous (one), lophotrichous (a tuft at one pole), amphitrichous (at both poles) and peritrichous (all over, as in Escherichia coli and Salmonella).
  • Fimbriae (common pili) are short and numerous and mediate adhesion to host cells, a first step in infection. The sex pilus is longer, present in a single copy or a few, and mediates conjugation.
  • Endospores are dormant, highly resistant structures formed by the Gram-positive genera Bacillus and Clostridium when nutrients run out. They contain dipicolinic acid and calcium and very little water, which explains their resistance to heat, drying, radiation and disinfectants.
  • Endospores are not killed by boiling; they require autoclaving at 121 degrees Celsius and 15 pounds per square inch for 15 to 20 minutes. They are demonstrated by the Schaeffer-Fulton stain using malachite green.
  • Binary fission: the chromosome replicates, the cell elongates, a septum forms and the cell divides into two identical daughter cells. Generation time is about 20 minutes for Escherichia coli and about 20 hours for Mycobacterium tuberculosis.
  • The bacterial growth curve has four phases: lag (adaptation, no division), log or exponential (maximum division, the phase in which cells are most susceptible to antibiotics), stationary (division balanced by death, nutrient depletion, spore formation and toxin production), and death or decline.
  • Classification by oxygen requirement: obligate aerobe, obligate anaerobe, facultative anaerobe, microaerophile and aerotolerant anaerobe. Obligate anaerobes lack catalase and superoxide dismutase and are killed by oxygen.

Key Terms

Peptidoglycan
The rigid mesh of N-acetylglucosamine and N-acetylmuramic acid cross-linked by peptide bridges that forms the bacterial cell wall and gives it shape and osmotic protection.
Lipopolysaccharide
The outer membrane molecule of Gram-negative bacteria whose lipid A component is endotoxin, responsible for fever, hypotension and septic shock.
Endospore
A dehydrated dormant body containing calcium dipicolinate, formed by Bacillus and Clostridium, that survives boiling and requires autoclaving to destroy.
Conjugation
Direct transfer of DNA, usually a plasmid, from one bacterium to another through a sex pilus; the principal route by which multidrug resistance spreads.

Practice Quiz — 20 Questions

correct out of 20

  1. The single feature that defines a prokaryotic cell is:

    • A.Absence of ribosomes
    • B.Absence of a membrane-bound nucleus
    • C.Absence of DNA
    • D.Absence of a cell membrane
    B. Absence of a membrane-bound nucleus — Prokaryotes have no nuclear membrane; their circular chromosome lies free in the cytoplasm in the nucleoid region.

  2. Bacterial ribosomes are:

    • A.80S with 60S and 40S subunits
    • B.70S with 50S and 30S subunits
    • C.70S with 60S and 30S subunits
    • D.60S with 40S and 20S subunits
    B. 70S with 50S and 30S subunits — The 70S bacterial ribosome differs from the human 80S ribosome, allowing selective antibiotic action.

  3. Aminoglycosides such as gentamicin act on which ribosomal subunit?

    • A.30S
    • B.50S
    • C.40S
    • D.60S
    A. 30S — Aminoglycosides and tetracyclines bind the 30S subunit; macrolides, clindamycin and chloramphenicol bind the 50S subunit.

  4. In the Gram stain, which step is the critical differentiating step?

    • A.Applying crystal violet
    • B.Applying iodine
    • C.Decolourisation with alcohol
    • D.Applying safranin
    C. Decolourisation with alcohol — Alcohol washes the crystal violet-iodine complex out of the thin-walled Gram-negative cell but not out of the thick Gram-positive wall.

  5. Gram-positive bacteria appear purple because their cell wall contains:

    • A.An outer membrane with lipopolysaccharide
    • B.A thick layer of peptidoglycan
    • C.Mycolic acid
    • D.No cell wall at all
    B. A thick layer of peptidoglycan — The thick peptidoglycan traps the crystal violet-iodine complex during decolourisation.

  6. Which component of lipopolysaccharide is responsible for endotoxic shock?

    • A.O antigen
    • B.Core polysaccharide
    • C.Lipid A
    • D.Teichoic acid
    C. Lipid A — Lipid A anchors lipopolysaccharide in the outer membrane and triggers the cytokine cascade causing fever and hypotension.

  7. Teichoic acid is found in the cell wall of:

    • A.Gram-negative bacteria
    • B.Gram-positive bacteria
    • C.Mycoplasma
    • D.All viruses
    B. Gram-positive bacteria — Teichoic and lipoteichoic acids run through the thick peptidoglycan of Gram-positive organisms and contribute to adherence and immune activation.

  8. Mycobacterium tuberculosis requires the Ziehl-Neelsen stain because its wall is rich in:

    • A.Teichoic acid
    • B.Mycolic acid
    • C.Lipopolysaccharide
    • D.Chitin
    B. Mycolic acid — The waxy mycolic acid layer resists the Gram stain and makes the organism acid-fast.

  9. Mycoplasma species are naturally resistant to penicillin because they:

    • A.Produce beta-lactamase only
    • B.Lack a cell wall
    • C.Have an extra outer membrane
    • D.Form endospores
    B. Lack a cell wall — Penicillins act on peptidoglycan synthesis, and Mycoplasma has no peptidoglycan wall at all.

  10. Penicillin kills bacteria by inhibiting:

    • A.DNA gyrase
    • B.Transpeptidase cross-linking of peptidoglycan
    • C.The 50S ribosomal subunit
    • D.Folate synthesis
    B. Transpeptidase cross-linking of peptidoglycan — Penicillins bind penicillin-binding proteins (transpeptidases), preventing cross-linking so the wall weakens and the cell lyses.

  11. Vancomycin acts by binding to:

    • A.The D-alanyl-D-alanine terminus of peptidoglycan precursors
    • B.Lipid A
    • C.The 30S ribosome
    • D.RNA polymerase
    A. The D-alanyl-D-alanine terminus of peptidoglycan precursors — By capping the D-ala-D-ala terminus, vancomycin blocks the transglycosylation and cross-linking steps of wall synthesis.

  12. The main virulence function of a bacterial capsule is to:

    • A.Enable motility
    • B.Resist phagocytosis
    • C.Produce endotoxin
    • D.Enable conjugation
    B. Resist phagocytosis — The polysaccharide capsule prevents phagocytes from engaging the bacterial surface, which is why capsular vaccines are protective.

  13. Bacterial motility is provided by:

    • A.Fimbriae
    • B.Flagella
    • C.Capsule
    • D.Plasmids
    B. Flagella — Flagella, composed of flagellin, rotate to propel the cell and carry the H antigen.

  14. A bacterium with flagella distributed over its whole surface is described as:

    • A.Monotrichous
    • B.Lophotrichous
    • C.Amphitrichous
    • D.Peritrichous
    D. Peritrichous — Peritrichous flagellation is seen in Escherichia coli and Salmonella species.

  15. Transfer of antibiotic resistance genes through a sex pilus is called:

    • A.Transformation
    • B.Transduction
    • C.Conjugation
    • D.Transposition
    C. Conjugation — Conjugation is direct cell-to-cell plasmid transfer and is the main mechanism spreading multidrug resistance.

  16. Uptake of naked DNA from the surrounding environment is called:

    • A.Transformation
    • B.Transduction
    • C.Conjugation
    • D.Translation
    A. Transformation — Transformation was demonstrated by Griffith in pneumococci and requires the recipient to be competent.

  17. Endospores are formed by which two genera?

    • A.Staphylococcus and Streptococcus
    • B.Bacillus and Clostridium
    • C.Escherichia and Salmonella
    • D.Neisseria and Haemophilus
    B. Bacillus and Clostridium — Only these Gram-positive genera form endospores, in response to nutrient depletion.

  18. The substance responsible for the heat resistance of endospores is:

    • A.Mycolic acid
    • B.Calcium dipicolinate
    • C.Teichoic acid
    • D.Flagellin
    B. Calcium dipicolinate — Dipicolinic acid complexed with calcium, together with very low water content, makes the spore core extraordinarily heat resistant.

  19. Reliable destruction of endospores requires:

    • A.Boiling at 100 degrees Celsius for 10 minutes
    • B.Autoclaving at 121 degrees Celsius for 15 to 20 minutes
    • C.70 percent alcohol for 5 minutes
    • D.Refrigeration at 4 degrees Celsius
    B. Autoclaving at 121 degrees Celsius for 15 to 20 minutes — Moist heat under pressure at 121 degrees Celsius and 15 pounds per square inch is the standard sporicidal method.

  20. Bacteria are most susceptible to cell-wall-active antibiotics during which growth phase?

    • A.Lag phase
    • B.Log (exponential) phase
    • C.Stationary phase
    • D.Death phase
    B. Log (exponential) phase — Actively dividing cells are synthesising new peptidoglycan, which is the target of these drugs.

References