Question 6 of 8: Bacterial Nutrition, Media Types, Transport Mechanisms, and Cell-Wall Structure

Question 6: Bacterial Nutrition, Media Types, Transport Mechanisms, and Cell-Wall Structure (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) Carbon source use, other required nutrients, and media types. Bacteria use their carbon source for two simultaneous purposes: as a catabolic (energy) substrate, oxidized/fermented to generate ATP and reducing power, and as an anabolic (biosynthetic) carbon skeleton, assimilated into new cell material (proteins, nucleic acids, lipids, cell wall). A single carbon source (e.g. glucose) is typically split between these two fates, exactly as quantified by the elemental/electron balances used elsewhere on this paper (Q1–Q4): the fraction not respired to CO₂ ends up as biomass or product. Beyond carbon, bacteria require: nitrogen (amino acids, nucleotides, peptidoglycan — from NH₃, nitrate, or organic N); phosphorus (nucleic acids, phospholipids, ATP); sulfur (cysteine/methionine, some cofactors); trace metals (Mg, Fe, Mn, Zn, etc., as enzyme cofactors); and, for many organisms, growth factors (vitamins, amino acids) that the cell cannot synthesize itself and must obtain preformed from the medium — each of these plays a structural or catalytic role that carbon alone cannot substitute for, which is why a "complete" growth medium must supply all of them, not just an energy source. Enrichment media are formulated to favour growth of a specific (often minority) organism from a mixed population, usually by supplying a substrate or condition only that organism can exploit well, boosting its relative abundance before isolation. Selective media actively suppress unwanted organisms (via an inhibitory agent — a dye, antibiotic, bile salt, or high salt/sugar concentration) while permitting the target group to grow largely unimpeded. Differential media do not restrict growth of anything at all; instead they contain an indicator (a pH dye, a fermentable sugar, blood) that produces a visibly different reaction (colour change, precipitate, haemolysis) depending on the organism's biochemistry, allowing organisms to be distinguished from one another on the same plate. The three purposes are not mutually exclusive — MacConkey agar, for instance, is both selective (bile salts suppress Gram-positives) and differential (lactose fermenters turn pink, non-fermenters stay pale) in a single medium.

(b) Membrane transport mechanisms and Gram-positive vs. Gram-negative cell walls. Materials cross the plasma membrane by several mechanisms of increasing energy/complexity. Passive (simple) diffusion moves small, uncharged, lipid-soluble molecules (O₂, CO₂) down their concentration gradient with no carrier and no energy input. Facilitated diffusion uses a membrane carrier/channel protein to move a solute down its gradient faster than diffusion alone would allow, still without direct energy expenditure. Active transport moves solutes against their concentration gradient, requiring energy — either directly from ATP hydrolysis (primary active transport, e.g. ABC transporters) or indirectly by coupling uphill solute movement to the downhill flow of a second ion along its own electrochemical gradient (secondary active transport / symport–antiport, driven by the proton-motive force). Group translocation is a bacteria-specific mechanism (e.g. the phosphotransferase system, PTS) in which the transported solute is chemically modified (phosphorylated) during transport, so that it emerges on the cytoplasmic side already primed for metabolism. Eukaryotic cells additionally move bulk material (too large for any carrier protein) via endocytosis (phagocytosis of particles, pinocytosis of fluid, receptor-mediated uptake) and exocytosis, vesicle-based mechanisms with no prokaryotic equivalent since prokaryotes lack the internal membrane-trafficking machinery (cytoskeletal motor proteins, vesicle-budding organelles) these processes depend on.

Gram-positive vs. Gram-negative cell walls. Gram-positive bacteria have a single plasma membrane surrounded by a thick (20–80 nm, multi-layered) peptidoglycan wall, with teichoic acids threaded through and covalently linked to it — this thick meshwork traps the crystal-violet–iodine complex during Gram staining, giving the characteristic purple result. Gram-negative bacteria have a much thinner (2–7 nm, essentially single-layer) peptidoglycan layer confined to a periplasmic space, sandwiched between the plasma membrane and a second, outer membrane whose outer leaflet is lipopolysaccharide (LPS) — the thin wall cannot retain the crystal-violet complex once decolourized, so the cells take up the pink safranin counterstain instead. The outer membrane is itself a major functional difference beyond staining: it acts as an additional permeability barrier (via porins) and its LPS is a potent endotoxin, neither of which have a Gram-positive equivalent.