Microbiology

Question 1 (14 marks)

(a) Number of generations $n$ from $N = N_0 \cdot 2^{n}$: $2^n = \frac{3.2\times10^6}{5\times10^4} = 64 \Rightarrow n = \log_2 64 = 6.$ 6 generations. (Ratio 1 mark; $\log_2$ setup 1; answer 1) (3)

(b) Time elapsed = 3 h = 180 min over 6 generations: $g = \frac{180}{6} = 30 \text{ min.}$ (setup 1, division 1, answer with units 1) (3)

(c) From $t=0$ to $t=5$ h = 300 min ⇒ $n = 300/30 = 10$ generations: $N = 5\times10^4 \times 2^{10} = 5\times10^4 \times 1024 = 5.12\times10^{7} \text{ cells mL}^{-1}.$ (generations 1, computation 1, answer 1) (3)

(d) Stationary phase (1). Any two reasons (1 each): nutrient depletion; accumulation of toxic waste products (e.g. acids); reduced oxygen/space; rate of division ≈ rate of death. (3)

(e) Viable count measures only living, culturable cells (falls in death phase); turbidity/OD measures all cells including dead intact cells and debris, which still scatter light — so OD stays higher/flat while viable count drops. (2)

Question 2 (13 marks)

(a) Gram-positive (1). Features (any two, 1 each): thick peptidoglycan layer retaining crystal violet–iodine; no outer lipopolysaccharide membrane; teichoic acids present. (3)

(b) Morphology: cocci (spherical); arrangement: clusters (staphylo-) (1). Clusters indicate division in multiple/random planes (2), unlike chains which divide in one plane.

(c) Mechanism (2): transpeptidase cross-links peptidoglycan strands; inhibiting it produces a weak wall so that during growth/division the cell cannot withstand osmotic pressure and lyses. Resistance mechanism (any one, 2): enzyme that degrades/modifies the antibiotic (e.g. β-lactamase); altered target enzyme with reduced binding; efflux pump / reduced permeability.

(d) Conjugation (1): plasmid transferred through a pilus/direct contact from donor to recipient cell (1); because it is on a mobile plasmid it can cross between unrelated species (horizontal gene transfer), spreading resistance quickly (1). (3)

Question 3 (12 marks)

(a) Virus A: lytic cycle — immediate replication, progeny assembly and lysis/bursting of host (2). Virus B: lysogenic cycle — genome integrates as prophage, replicated passively with host DNA, dormant (2). (4)

(b) UV damages DNA and acts as an inducing stress; the prophage is induced to excise and switch to the lytic cycle (2), so the cells produce virus and lyse (1). (3)

(c) Retrovirus (1); enzyme reverse transcriptase (1); it synthesises DNA from the RNA template, which integrates as a provirus (1). High mutation rate because reverse transcriptase lacks proofreading (3′→5′ exonuclease) activity, so replication errors are not corrected (2). (5)

Question 4 (11 marks)

(a) A prion is a misfolded protein that induces normal cellular protein (PrP) to refold into the abnormal form, propagating in a chain reaction — no nucleic acid replication needed (3). Resists sterilisation because it is a protein without nucleic acid, so heat/UV/nucleases that target DNA/RNA do not destroy it; it is highly stable (1). (4)

(b) Viroid: small circular single-stranded RNA with no protein coat/capsid; infects plants. Virus: nucleic acid (DNA or RNA) enclosed in a protein capsid (± envelope); infects animals, plants or bacteria. (structure 2, host 1) (3)

(c) Any two (2 each): decomposition — saprotrophic bacteria/fungi break down dead organic matter releasing nutrients; nitrogen cycle — nitrogen-fixing bacteria (e.g. Rhizobium) convert N₂ to ammonia / nitrifying / denitrifying bacteria; symbiosis (mycorrhizae aiding plant nutrient uptake); carbon cycle recycling. Must name a specific process. (4)

Question 5 (10 marks)

(a) Any three (1 each, precaution + reason): flame the inoculating loop before/after use — kills contaminants; work near a Bunsen flame/updraught — prevents airborne contamination; lift petri lid at an angle only slightly — reduces settling of airborne microbes; flame the neck of culture bottles — sterilises rim. (3)

(b) Valid design (4): control = plate/disc with no antiseptic (or sterile water) (1); measured/dependent variable = zone of inhibition diameter or colony count (1); controlled variables (any, 1) = same bacterial species and concentration, same agar/volume, same incubation temperature and time; repeats for reliability (1). (4)

(c) Fluffy hyphae = fungal contamination (1), most likely from airborne spores due to a breach in aseptic technique (1); the plate is invalid/should be discarded and not used to judge the antiseptic (1). (3)

[
  {"claim":"6 generations give ratio 64", "code":"n=log(64,2); result = (n==6)"},
  {"claim":"Generation time is 30 min over 3h/6 gen", "code":"g=Rational(180,6); result = (g==30)"},
  {"claim":"Density at t=5h is 5.12e7", "code":"N=5*10**4*2**10; result = (N==51200000)"},
  {"claim":"2^10 = 1024 generations factor at 5h", "code":"result = (2**(300//30)==1024)"}
]