bacterial growth curves

Meaning: In a closed culture, nutrients are not replenished and wastes build up, so growth passes through four phases. Lag: cells adjust to the medium and make enzymes, with little division. Exponential (log): cells divide by binary fission at a constant generation time, and the population doubles each cycle. Stationary: nutrients fall and wastes accumulate, so division roughly balances cell death and the viable count levels off. Death: cells die faster than they divide, so the viable count falls.

Equation: N = N₀ × 2^(t/g), where N is the cell number at time t (a count, unitless), N₀ is the starting number (unitless), t is elapsed time (s), and g is the generation time (s). The exponent t/g is the number of generations.

On the MCAT: A passage gives a growth curve, usually with log cell number on the y-axis, and asks which phase a condition affects. Examples include adding fresh nutrients, moving cells to a new medium, or limiting a nutrient. You may also compute cells after a given number of doublings, or compare slopes, where a steeper exponential slope means a shorter generation time.

Common confusion: A flat stationary phase does not mean cells have stopped living or metabolizing. New cells still form, but net growth is about zero. Also, lag phase is not dormancy, because cells are actively preparing to divide. Exponential growth looks like an upward curve on a linear axis and a straight line on a log axis.