Question 10 of 10: Bacterial Cell Wall Structure, Virus Comparison, and Pure Culture

Question 10: Bacterial Cell Wall Structure, Virus Comparison, and Pure Culture (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) Gram-positive vs. Gram-negative cell wall structure. Gram-positive cells have a single plasma membrane surrounded by a thick peptidoglycan layer (20–80 nm, often 20–40 sheets), cross-linked by peptide bridges and interwoven with negatively charged teichoic and lipoteichoic acids that thread through the wall and anchor to the membrane. Gram-negative cells have a much thinner peptidoglycan layer (2–7 nm, essentially one or a few sheets) confined to a periplasmic space between the plasma membrane and a second, outer membrane; the outer membrane's outer leaflet is built from lipopolysaccharide (LPS), whose lipid A component is the endotoxin responsible for much of Gram-negative pathogenicity, and it is perforated by porin proteins that control permeability. The Gram stain itself works because the crystal violet–iodine dye complex is physically trapped inside the thick, multilayered Gram-positive wall during the alcohol/acetone decolorization step (retaining purple), whereas it washes straight out through the thin Gram-negative wall, leaving only the pink safranin counterstain.

(b) Animal, plant and bacterial viruses — similarities and differences. Similarities: all viruses are obligate intracellular parasites (no independent metabolism or ribosomes of their own), consist minimally of a nucleic acid genome (DNA or RNA, never both) packaged in a protein capsid, and reproduce by hijacking the host's biosynthetic machinery rather than by binary fission. Differences: bacteriophages (bacterial viruses) commonly have complex tailed morphology (icosahedral head + contractile tail + tail fibres, e.g. T4) adapted to inject DNA through a rigid bacterial cell wall while the capsid stays outside; many follow a lytic cycle ending in host lysis, or a lysogenic cycle where the phage genome integrates as a prophage. Animal viruses are frequently enveloped (host-membrane-derived lipid envelope studded with viral glycoproteins, e.g. influenza, HIV) and enter by membrane fusion or endocytosis since animal cells lack a rigid wall; they may be DNA or RNA, and RNA animal viruses in particular show wide genome architectures (positive-sense, negative-sense, or segmented, e.g. influenza's 8 segments enabling reassortment). Plant viruses (e.g. tobacco mosaic virus) are typically simple rod-shaped or icosahedral, non-enveloped, and cannot cross the rigid, cellulose plant cell wall unaided — they instead rely on a vector (insects, nematodes, fungi) or mechanical wounding to breach the wall, and then spread cell-to-cell through plasmodesmata using virus-encoded movement proteins rather than by extracellular release and re-infection the way many animal viruses do.

(c) Pure culture and its importance. A pure culture is a population of cells descended from a single parental cell (a clone), containing only one microbial species/strain with no other organisms present — typically obtained by streak-plating (or serial dilution/pour-plating) to isolate single, well-separated colonies, each of which is assumed to have arisen from one original cell. Pure culture is foundational because: (i) it satisfies Koch's postulates, which require isolating a suspected pathogen in pure culture before it can be causally linked to a disease; (ii) it is the only basis for reliable identification and characterization — biochemical tests, antibiotic-susceptibility profiles, and genome sequencing are meaningless if the sample is a mixture of organisms; (iii) it is essential for reproducible kinetics and yield-coefficient measurements (exactly the $\mu$, $Y_{XS}$, $Y_{PS}$ values used throughout Q1–Q4, Q7 above) — a contaminated culture invalidates any stoichiometric or kinetic parameter derived from it; and (iv) it protects industrial fermentations from contamination-driven productivity loss or off-spec product, since a competing or product-degrading contaminant organism can consume substrate, produce inhibitory byproducts, or (worst case) force a batch to be discarded entirely.