Question 10 of 10: Bacterial Classification, Fungal Spores, and Plasmids

Question 10: Bacterial Classification, Fungal Spores, and Plasmids (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) Five major characteristics used to classify bacteria. (1) Morphology — cell shape (coccus, bacillus, spirillum/vibrio), arrangement (chains, clusters, tetrads), and size. (2) Gram-stain reaction / cell-wall structure — Gram-positive (thick peptidoglycan, retains crystal violet) vs. Gram-negative (thin peptidoglycan plus an outer membrane). (3) Metabolic and physiological characteristics — energy source (photo- vs. chemotroph), carbon source (auto- vs. heterotroph), oxygen requirement (obligate/facultative aerobe or anaerobe), and biochemical test profiles (catalase, oxidase, fermentation patterns). (4) Genetic/molecular characteristics — 16S rRNA gene sequence phylogeny, %G+C content, and DNA–DNA hybridization, which underlie modern taxonomic classification. (5) Serological/antigenic characteristics — reaction of cell-surface antigens with specific antisera (serotyping) and bacteriophage susceptibility (phage typing), historically important for strain-level identification (e.g. of pathogens).

(b) Fungal spores and five asexual types. A fungal spore is a small (often single-celled) reproductive and dispersal unit, produced either sexually or asexually, that is typically resistant to desiccation and adapted to be carried by wind, water, or a vector until it lands in a favourable environment and germinates into a new hypha. Five asexual types: (1) Sporangiospores — formed inside an enclosed sac (sporangium) at the tip of a specialized hypha (sporangiophore); large numbers are released when the sac ruptures (e.g. Rhizopus). (2) Conidia — formed exogenously (not enclosed) at the tip or side of a conidiophore, often in dry chains; among the most common fungal spore types (e.g. Aspergillus, Penicillium). (3) Arthrospores (arthroconidia) — formed by fragmentation of an existing hypha at its septa into separate, thick-walled segments (e.g. Coccidioides). (4) Blastospores — formed by budding from a parent cell or hypha, characteristic of many yeasts (e.g. Candida, Saccharomyces asexual reproduction). (5) Chlamydospores — thick-walled resting spores formed by the rounding-up and wall-thickening of an existing hyphal segment; primarily a survival structure for adverse conditions rather than a dispersal structure (e.g. some Candida, Fusarium).

(c) Why plasmid number varies between bacteria. Plasmids are extrachromosomal, self-replicating DNA elements that are not required for normal growth under standard conditions, but that can carry accessory genes conferring a conditional selective advantage — antibiotic or heavy-metal resistance, virulence factors and toxins, catabolic pathways for unusual carbon sources, bacteriocin production, conjugative transfer functions, or (in symbionts such as Rhizobium) nodulation genes. Bacteria living in variable, competitive, or stressful environments (fluctuating antibiotic exposure, heavy-metal contamination, changing available substrates) benefit from carrying one or more such plasmids because they provide a fast, dispensable, horizontally-transferable route to new traits without permanently altering the core chromosome; different plasmids belonging to different incompatibility groups can co-exist in the same cell, allowing several unrelated plasmids to accumulate. Conversely, bacteria in stable, unchallenging niches — or highly specialized organisms under strong selection for a minimal, streamlined genome (e.g. some obligate intracellular symbionts) — may carry no plasmids at all, because replicating extra DNA has a metabolic cost with no offsetting benefit; once a plasmid's carried trait stops being useful, natural selection tends to eliminate it (plasmid "curing" under non-selective conditions is routinely observed experimentally), so plasmid content reflects a continuously-updated cost–benefit balance rather than a fixed species trait.