Question 9 of 10: Bacterial Morphology, Prokaryote vs. Eukaryote, and Culture Media

Question 9: Bacterial Morphology, Prokaryote vs. Eukaryote, and Culture Media (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Bacterial shapes. The three basic bacterial cell shapes are coccus (spherical, often seen in clusters, pairs or chains — staphylo-, diplo-, strepto-cocci), bacillus (rod-shaped), and spirillum/spirochete (spiral or helically curved). Similarities between prokaryotic and eukaryotic cells: both are bounded by a phospholipid cell membrane, contain cytoplasm and ribosomes, carry genetic material as DNA, and carry out the same fundamental processes of metabolism, growth and reproduction. Differences: prokaryotes lack a membrane-bound nucleus (DNA resides in an unbound nucleoid region, typically a single circular chromosome) and lack membrane-bound organelles (no mitochondria, endoplasmic reticulum, or Golgi apparatus); they are smaller (roughly 1–10 µm vs. 10–100 µm), carry smaller 70S ribosomes (vs. 80S), divide by simple binary fission rather than mitosis/meiosis, and typically possess a rigid peptidoglycan cell wall (absent, or chemically different — cellulose in plants, chitin in fungi — in eukaryotes).

(b) Carbon source and other nutrients. Heterotrophic bacteria use an organic carbon source (e.g. glucose) for two simultaneous purposes: as an energy source, oxidized via glycolysis, the TCA cycle and an electron-transport chain (or fermented anaerobically) to generate ATP, and as the carbon skeleton for biosynthesis of amino acids, nucleotides, lipids and cell-wall precursors (autotrophic bacteria instead fix inorganic CO₂). Beyond carbon, bacteria require: nitrogen (for amino acids, nucleotides and peptidoglycan, obtained from NH₃, NO₃⁻, N₂-fixation, or organic nitrogen); phosphorus (nucleic acids, ATP, phospholipids); sulfur (the amino acids cysteine and methionine, some cofactors); trace metals such as Mg, K, Ca, Fe, Mn and Zn (enzyme cofactors and electron-transport components); and, for fastidious organisms unable to synthesize them, pre-formed vitamins/growth factors.

Culture media. Selective media contain an inhibitory agent (antibiotic, elevated salt, extreme pH, bile salts, a specific dye) that suppresses growth of unwanted organisms while permitting the target organism to grow — the "selection" happens by exclusion. Differential media contain an indicator system (commonly a fermentable substrate paired with a pH or redox indicator) that allows colonies of different organisms to be visually distinguished by colour or appearance without inhibiting any of them — the "differentiation" happens by contrast, not exclusion (many media, such as MacConkey agar, are simultaneously selective and differential). Enrichment culture instead supplies a specific nutrient, carbon source, temperature, pH or atmosphere that favours the disproportionate growth of a target organism from within a mixed population over an incubation period, increasing its relative abundance before isolation — a kinetic (growth-rate) strategy rather than an inhibitory or indicator-based one.