Growth and Nutrition of Bacteria Questions and Answers

Answer: D

The number of bacteria is \(1 \times 2^{100}\).

A division every 30 minutes means two generations per hour, so in 50 hours there are \(50 \times 2 = 100\) generations. Starting from one bacterium, the count doubles each generation:

\[ \begin{aligned} N &= N_{0} \times 2^{n} = 1 \times 2^{100} \\ &\approx 1.27 \times 10^{30} \end{aligned} \]

\(2 \times 10^{10}\) and \(2 \times 10^{20}\) are far too small, since \(2^{100}\) is about \(10^{30}\), and \(1 \times 10^{50}\) is far too large. (In practice the culture would run out of nutrients long before, but the question asks for unrestricted growth.)

Answer: B

The answer is 10, 60 minutes.

Exponential growth gives \(N = N_{0} \times 2^{n}\):

\[ \begin{aligned} 10^{4} &= 10 \times 2^{n} \\ 2^{n} &= 1000 \\ n &= \frac{\log 1000}{\log 2} = 9.97 \approx 10 \end{aligned} \]

Ten generations in ten hours means a generation time of 600/10 = 60 minutes. 120 minutes would mean only five generations; 20 and 40 generations would give \(10 \times 2^{20} \approx 10^{7}\) or more cells, far above \(10^{4}\).

Answer: C

The correct match is (P)-(iv), (Q)-(ii), (R)-(i), (S)-(iii).

• Streptomyces grows as branched hyphae; its aerial hyphae lay down septa at the tip and break into chains of spores, the picture in (iv).
• Rhodopseudomonas acidophila, a purple non-sulfur bacterium, divides by budding: a small bud forms at one end and grows into a daughter cell, as in (ii).
• Bacillus subtilis divides by ordinary binary fission, a septum splitting the cell into two equal halves, as in (i).
• Nocardia forms a mycelium that fragments into rod-shaped and coccoid cells, as in (iii).

Options A, B and D all deny Streptomyces its spore chains (iv): A and B give it budding and D gives it binary fission. B also gives binary fission to Rhodopseudomonas, which buds.

Answer: D

The statement that is NOT correct is the rate of generation of new cells during exponential growth phase is constant.

In exponential growth the specific growth rate and the generation time are constant, but the population doubles every generation, so the number of new cells produced per unit time keeps increasing. A constant rate of producing new cells would describe linear, not exponential, growth.

The other statements are accepted. Stationary-phase cells need longer to adapt to fresh medium, so their lag phase is longer. Death often follows a logarithmic (exponential) decline. In stationary phase some cells grow on nutrients released by others that die, which is called cryptic growth.

Answer: C

Such organisms are lithotrophs.

The prefix litho- (rock) refers to the source of electrons or energy: lithotrophs obtain energy by oxidising inorganic compounds such as ammonia, nitrite, hydrogen sulfide, ferrous iron or hydrogen, as nitrifying and sulfur-oxidising bacteria do.

Organotrophs obtain energy from organic compounds. Phototrophs obtain it from light. Autotrophs are defined by their carbon source, \(CO_{2}\), not by their energy source; many lithotrophs are also autotrophs, but the term for using inorganic energy sources is lithotroph.

Answer: A

The correct match is P - iii; Q - i; R - ii; S - v.

• A psychrophile grows best at low temperature, below about 15 °C.
• A barophile (piezophile) thrives under high hydrostatic pressure, such as the deep sea, here more than 380 atm.
• A mesophile grows at moderate temperatures, roughly 15 to 45 °C, the range of most human pathogens.
• A halophile requires high salt, over about 2 M NaCl for extreme halophiles.

Item iv, pH below 3.0, describes an acidophile and matches none of the four. The other options give the barophile low temperature or acidity, or the halophile high pressure, which mismatches the names.

Answer: A

The survivors decline exponentially.

Density-independent mortality means each cell has the same chance of dying per unit time, regardless of how many cells remain. The death rate is then proportional to the number of survivors:

\[ \begin{aligned} \frac{dN}{dT} &= -kN \\ N &= N_{0}\,e^{-kT} \end{aligned} \]

The same fraction is killed in each interval, which is why antibiotic killing curves are straight lines on a log scale. A linear fall would need a constant number killed per unit time; a sigmoidal curve describes density-dependent growth with a limit; and a parabola does not arise from a constant per-cell death rate.