Microbiology Practice Exam — Free

Each experiment is designed to rule mechanisms in or out, so work through them in turn.

Experiment 1 shows that genetic transfer happens between living cells in a mixed culture, which is consistent with conjugation, transformation or transduction.

Experiment 2 is the decisive one. The filter blocks cells but not DNA or phage. If transfer were by transformation (uptake of naked DNA from the medium) or by transduction (delivery of DNA inside a bacteriophage), the DNA or the phage would cross the filter and recipients would still become resistant. No transfer when the cells cannot touch means the mechanism requires direct cell-to-cell contact, which is the defining feature of conjugation: a sex pilus draws donor and recipient together, a mating bridge forms, and a single strand of the plasmid is passed across and copied in the recipient.

Experiment 3 confirms this from the other direction. DNase destroys free DNA in the medium, so transformation would have been abolished. Conjugation is unaffected because the DNA travels through the mating bridge and is never exposed to the medium. Transduction is also DNase-resistant because the DNA is protected inside a capsid, but transduction was already excluded by experiment 2.

Transposition (option D) moves DNA between sites within one cell's genome; it cannot by itself move a gene from one strain to another.

The transfer is by conjugation (option A).

Option B is the tempting choice if you focus only on the fact that a plasmid is involved, but transformation is DNase-sensitive and does not need cell contact, so experiments 2 and 3 both rule it out.

During exponential growth each cell divides in two every generation, so after $n$ generations the number of cells is

Solve for the number of generations by taking the ratio of the two counts; the units of cells/mL cancel:

Because $256=2^{8}$, $n = 8$ generations. If the ratio were not an exact power of two you would use $n = \dfrac{\log N - \log N_0}{\log 2}$, which gives the same result here: $\log 256 / \log 2 = 2.408 / 0.301 = 8.0$.

The generation time is the elapsed time divided by the number of generations. Convert 4.0 h to minutes first so the answer is in the units asked for:

A quick check: starting from $5.0\times10^{3}$ cells/mL and doubling eight times gives $5.0\times10^{3} \times 256 = 1.28\times10^{6}$ cells/mL, the measured count. A common mistake is to treat the ratio 256 itself as the number of generations, which gives a nonsensical generation time of under one minute.

Generation time: 30 min.

(a) The D-value is the time at a given temperature that reduces the population by one log, that is, by 90 %. Going from $10^{6}$ spores per vial to a survival probability of $10^{-6}$ is a reduction of 12 orders of magnitude, so 12 D-values are needed:

(b) The z-value is the temperature change that alters the D-value by a factor of 10. Lowering the temperature makes the organism harder to kill, so D increases:

The same 12 log reductions are still required:

So a 6 °C shortfall roughly quadruples the required holding time, which is why autoclave validation uses biological indicators and probes inside the load rather than trusting the chamber gauge alone.

(c) The calculations assume first-order (logarithmic) death kinetics: in each equal time interval the same fraction of the surviving population is killed, so a plot of log survivors against time is a straight line with slope $-1/D$. This is also why sterility is expressed as a probability rather than as zero survivors; the line approaches zero but never reaches it.

(a) 18 min at 121 °C; (b) $D_{115} \approx 6.0$ min and about 72 min at 115 °C; (c) first-order (log-linear) death kinetics.