--- title: "Microbial growth: generation time, growth curves and continuous culture" description: "During exponential growth, generation time can be calculated from population counts and elapsed time. Logarithmic counts plotted against linear time give a straight line. A typical closed batch cultur" canonical: https://lightmysky.com/learn/science/microbial-growth-generation-time-growth-curves-and-continuous-culture-mt_ltzgjIBytk source: https://lightmysky.com/learn/science/microbial-growth-generation-time-growth-curves-and-continuous-culture-mt_ltzgjIBytk.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Microbial growth: generation time, growth curves and continuous culture During exponential growth, generation time can be calculated from population counts and elapsed time. Logarithmic counts plotted against linear time give a straight line. A typical closed batch culture passes through lag, exponential, stationary and death phases. A chemostat can maintain steady conditions through nutrient supply and culture outflow. Subject: Science · Area: Microbiology · Ages 18 to 20 Page: https://lightmysky.com/learn/science/microbial-growth-generation-time-growth-curves-and-continuous-culture-mt_ltzgjIBytk ## Ready when they can - Calculates generation time from two counts and the time between them. - Explains why constant exponential growth gives a straight line when logarithmic population counts are plotted against linear time. - Says what a chemostat controls that a closed flask cannot. ## Lesson: Counting cells that double Many bacteria and archaea reproduce by binary fission: one parent divides into two daughter cells. Mutations can occur, so genetic identity is not guaranteed. During constant exponential growth without cell loss, generation time g is the time for the population to double. Nutrients, temperature and oxygen conditions affect growth. **Example.** A population grows from 40 to 320 cells in six hours at a constant doubling rate, without cell loss. Trace 40, 80, 160, 320: three doublings. Divide elapsed time by doublings: g = 6/3 = 2 hours. To predict a later count, divide elapsed time by g, then double the starting count once per complete generation. *(drawing: Three doubling intervals connect four counts. Equal time intervals multiply the count by the same factor.)* A typical closed batch culture has four phases. During lag, cells adjust and population size changes little. During exponential, or log, growth, generation time is roughly constant. Nutrient shortage or waste buildup can lead to stationarity, when new cells roughly balance deaths. During death, viable counts decline as deaths exceed divisions. *(drawing: This sequence shows phase order. It is not a graph of population size or phase duration.)* A logarithm gives an exponent: log2(8) = 3 because 2 × 2 × 2 = 8. Counts 1, 2, 4, 8 have base-2 logs 0, 1, 2, 3. Equal ratios have equal log spacing, so constant exponential growth plots as a straight line with linear time and logarithmic counts. For initial count N0 and later count N during exponential growth, doublings n = log(N/N0)/log(2). Use a calculator's log key with the same base throughout. Then g = elapsed time/n. A chemostat supplies fresh medium and removes mixed culture at equal volume rates, keeping volume constant. At a suitable flow rate, nutrient limitation can maintain steady population density while cells divide. New cells replace cells removed in outflow. Temperature and oxygen must also suit the organism. Excessively fast flow can wash the population out. Doublings per unit time equal 1/g. A strain doubling every 20 minutes has three doublings per hour; one doubling every 60 minutes has one. Natural logarithms use base e, approximately 2.718. A calculator's ln key gives ln(2), approximately 0.693. Each doubling adds ln(2) to the natural log of the count, so specific growth rate is ln(2)/g, about 0.693/g, not 1/g. **Recap.** Use exponential-phase counts to find generation time. Define graph axes and rate units. A chemostat can keep density steady while cells divide and leave in outflow. *Lesson, questions, worked solutions and flashcards written for this dataset. OpenStax Microbiology was consulted for factual study.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Archaea: different membranes, extreme habitats, separate domain](https://lightmysky.com/learn/science/archaea-different-membranes-extreme-habitats-separate-domain-mt_isKmuoGgAn) ## Optional connections - [Population Growth Curves and Carrying Capacity](https://lightmysky.com/learn/science/population-growth-curves-and-carrying-capacity-mt_KX6rhHF5kj) ## Opens up - [Applied Microbiology: Fermentation, Bioremediation and Host Microbiomes](https://lightmysky.com/learn/science/applied-microbiology-fermentation-bioremediation-and-host-microbiomes-mt_WcfGvV_dD9)