Bacterial cell structure, function, and classification

What are the three main characteristics that distinguish prokaryotes from eukaryotes?

Small size, absence of a nuclear membrane, and absence of complex organelles in the cytoplasm.

What does "Prokaryote" mean?

"Primitive nucleus"; lacks a true nucleus and is unicellular.

What does "Eukaryote" mean?

"True nucleus"; has a distinct nucleus with a nuclear membrane, may be unicellular or multicellular.

List the essential (obligatory) components of a bacterial cell.

Cytoplasm, nucleoid, ribosomes, cytoplasmic membrane, and cell wall.

List the non-essential (accessory) components of a bacterial cell.

Plasmids, pili/fimbriae, flagella, capsules/slime layer, and spores.

What is the function of the cytoplasm (cytosol) in bacteria?

Serves as the site for most biochemical reactions; mainly composed of water.

What is the nucleoid?

A single circular double-stranded DNA molecule without a nuclear membrane.

What are plasmids and what do they carry?

Small, circular, extrachromosomal DNA molecules that replicate independently; may carry genes for virulence or antibiotic resistance.

What are ribosomes and why are they important targets for antibiotics?

Sites of protein synthesis; bacterial 70S ribosomes differ from eukaryotic 80S, allowing selective antibiotic targeting.

Describe the structure of the bacterial cytoplasmic membrane.

Phospholipid bilayer with integral and peripheral proteins.

List the main functions of the cytoplasmic membrane.

Acts as a permeability barrier, generates energy, synthesizes and exports cell wall components, and secretes enzymes/toxins.

What are mesosomes and their functions?

Infoldings of the cytoplasmic membrane; involved in respiration and chromosomal separation.

What is the bacterial cell wall composed of?

Peptidoglycan—a polymer of glycan chains (NAG and NAM) cross-linked with peptides.

List five functions of the bacterial cell wall.

Provides structural support, protects from osmotic lysis, determines shape, aids in cell division, and helps in adhesion.

What are the main differences between Gram-positive and Gram-negative cell walls?

Gram-positive: thick peptidoglycan, teichoic acids, no outer membrane. Gram-negative: thin peptidoglycan, outer membrane with LPS, periplasmic space.

What molecules are found in Gram-positive bacterial cell walls?

Lipoteichoic acid and teichoic acid.

What molecules are found in Gram-negative bacterial outer membranes?

Lipopolysaccharides (LPS) and porins.

What is the function of lipopolysaccharide (LPS)?

Acts as an endotoxin that can trigger fever and septic shock.

What are acid-fast cell walls and which genus has them?

Contain mycolic acid (waxy lipid) over peptidoglycan; found in Mycobacterium species.

Why do acid-fast bacteria grow slowly?

Nutrients cannot easily cross the waxy cell wall.

What are fimbriae and pili, and how do they differ?

Fimbriae: short, numerous, for attachment. Pili: longer, fewer, for DNA transfer (sex pili).

What are flagella and their functions?

Long protein filaments for motility; rotate to move bacteria toward or away from stimuli (chemotaxis).

What is the antigenic significance of flagella?

Flagella are antigenic (H antigens), used in bacterial serotyping (e.g., E. coli O157:H7).

What are periplasmic flagella (axial filaments) and where are they found?

Flagella located between membranes in spirochetes (e.g., Treponema pallidum); enable corkscrew motion.

What is the glycocalyx and its types?

Gelatinous polysaccharide layer external to the cell wall; includes capsules (organized) and slime layers (unorganized).

List functions of the glycocalyx.

Prevents phagocytosis, blocks toxins, prevents drying, aids in attachment and biofilm formation.

What are biofilms?

Communities of bacteria adhering to surfaces and to each other via glycocalyx.

What are endospores and which genera form them?

Dormant, highly resistant structures formed by Gram-positive bacteria such as Bacillus and Clostridium.

What is the function of endospores?

Survival under harsh conditions (heat, drought, lack of nutrients).

What is sporulation?

Process by which vegetative cells form endospores under stress.

What is germination?

Process by which spores return to vegetative form under favorable conditions.

What is the typical unit of bacterial size measurement?

Micrometer (μm), one-millionth of a meter.

Name the smallest and largest known bacteria.

Smallest: Mycoplasma (0.2 μm); Largest: Thiomargarita magnifica (~1 cm).

List the main bacterial shapes.

Coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped), spirillum (rigid spiral), spirochete (flexible spiral), branched filamentous, pleomorphic.

What determines bacterial arrangement?

The plane of division and whether daughter cells remain attached.

What does Gram staining differentiate?

Cell wall structure: Gram-positive (purple) vs. Gram-negative (pink).

Why do Gram-positive cells retain crystal violet stain?

Thick peptidoglycan becomes dehydrated and less permeable during alcohol wash.

Why do Gram-negative cells lose crystal violet stain?

Alcohol dissolves outer membrane and damages thin peptidoglycan, allowing dye to wash out.

What are acid-fast bacteria and why do they resist decolorization?

Mycobacterium species; waxy mycolic acid layer prevents acid-alcohol from removing stain.

What is the Ziehl-Neelsen stain used for?

Differentiating acid-fast bacteria (pink) from non-acid-fast (blue).

List the taxonomic hierarchy used for bacteria.

Kingdom → Phylum → Class → Order → Family → Genus → Species.

What is the binomial naming system?

Two-word naming: Genus (capitalized) + species (lowercase), both italicized.

What is the ratio of microbial to human cells in the body?

Microbial cells outnumber human cells.

What is the normal microbiota?

Community of microorganisms living symbiotically with the host, beneficial for health.

List functions of the normal microbiota.

Help digestion, synthesize vitamins, protect against pathogens, and regulate immunity.

What are the three types of symbiotic relationships?

Parasitism (harmful), mutualism (beneficial), commensalism (neutral).

What are opportunistic pathogens?

Normally harmless microbes that cause disease when the host’s immunity is compromised.

What is dysbiosis?

Disruption of normal microbiota balance, leading to disease.

Give an example of disease caused by dysbiosis.

Antibiotic use suppresses normal flora, allowing Clostridium difficile overgrowth → colitis.