BMS 114 · Common Core (Tronc commun)Section 5.1 · Bacterial growth, nutrition and culture
Section 18 of 33
Binary fission and the growth curve
Explain how a bacterium becomes a population by binary fission, describe and account for the four phases of the growth curve, calculate the number of generations and the generation time from two counts, and state where bacteria obtain their energy.
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Why this matters
The number of bacteria in a wound, a urine sample or a bag of intravenous fluid doubles on a nearly fixed clock, and that clock decides how fast an infection becomes dangerous, how long a culture must be incubated and why an antibiotic dose given late can lose the ground the earlier doses had won. Understanding binary fission and the growth curve lets you predict what a germ is doing between two observations, explain to a patient why a course of treatment must be finished, and see why some organisms take weeks before a laboratory can name them. This section also gives you the small piece of arithmetic that turns a pair of counts into a generation time.
What you need to know first
You need only the school biology of the cell and one idea you will use again and again: a bacterium is a single cell that divides into two. All the arithmetic is ordinary multiplication and division, and every calculation is written out in full. Words such as generation time, septum and carrying capacity are defined in Words to know before they are used.
Words to know (5)
Binary fission
The asexual division of a bacterial cell into two identical daughter cells after its DNA has been copied and a cross-wall has formed in the middle of the cell. One Escherichia coli cell becoming two is binary fission; the two daughters repeat the same cycle.
Generation time (doubling time)
The time needed for a population to double in number under stated conditions of temperature, medium and aeration. Escherichia coli may double every 20 to 30 minutes; Mycobacterium tuberculosis takes about 24 hours.
Growth curve
A graph of the number of living cells in a closed culture against time, showing the lag, exponential, stationary and death phases. A laboratory plots the growth curve of a broth to see when the cells are dividing fastest.
Carrying capacity
The largest population that a given environment can support with the nutrients, space and oxygen available to it. When the broth reaches its carrying capacity, the stationary phase begins.
Septum
The cross-wall that forms in the middle of an elongating bacterial cell and develops into the wall separating the two daughter cells. A chain of streptococci is a series of cells whose septa formed in the same plane.
One idea at a timeIdea 1 / 5
In one sentence Bacteria multiply by binary fission: a cell grows until it almost doubles, copies its DNA and splits into two identical daughter cells.
One cell becomes two: binary fission and geometric growth
Binary fission is the asexual reproduction of bacteria. Under ideal conditions of food, water and warmth, a bacterial cell grows until it has almost doubled in size, copies its single loop of DNA, and then divides in the middle to produce two daughter cells. The two daughters grow at the same rate as the parent and divide again, so one cell becomes two, two become four, four become eight, and the numbers climb in a geometric progression rather than by simple addition. Under ideal conditions this cycle repeats without pause, which is why a population can rise from a handful of cells to millions while the conditions remain favourable.
For example
One Escherichia coli cell that divides every 30 minutes can become about 65 000 cells (2¹⁶) in eight hours if nothing limits it.
Pause and check yourself
A bacterial cell copies its DNA and then forms a cross-wall in the middle. What has happened, and what are the products?
Show the answer
The cell has completed binary fission. DNA replication was followed by elongation and the formation of a septum, which developed into a cell wall dividing the elongated cell into two genetically identical daughter cells.
Read detailed explanations
Counting generations: the arithmetic the notes leave out
The notes describe the growth curve in words but never give the arithmetic behind it, and the arithmetic is what turns a curve into a number you can act on. Because each cell divides into two, the count after n doublings is the starting count multiplied by 2 raised to the power n: N = N₀ × 2ⁿ. Written out, the sequence runs 2, 4, 8, 16, 32, 64, 128 — an exponent, not a fixed addition. To recover the number of generations, rearrange the same expression with logarithms. A specimen that begins with 10³ viable cells per millilitre and ends with 10⁹ has grown a millionfold, so n = log₂ 10⁶ = 6 ÷ 0.301, which is approximately 20 generations. The generation, or doubling, time is then the elapsed time divided by the number of generations: g = t ÷ n. If those 20 doublings occupied five hours, that is 300 minutes, so g = 300 ÷ 20 = 15 minutes. The same relationship runs backwards, and that is how a laboratory predicts a result. A urine specimen containing an organism whose generation time is 30 minutes, left standing overnight for eight hours, passes through 16 doublings, so 10⁴ cells become 10⁴ × 2¹⁶, roughly 6.6 × 10⁸ — a concentration that changes the diagnosis. In practice nobody counts every cell. You plate a measured volume, count the colony-forming units that appear, and multiply by the dilution factor to recover the original concentration; a serial dilution of one in ten repeated six times is a 10⁶-fold dilution, and forgetting one step throws the answer out by a factor of ten. Two cautions keep the arithmetic honest. It applies only while nutrients are plentiful and waste is not limiting, so it describes the exponential region and nothing else; and a single count is a snapshot of a moving process, not a fixed property of the species.
Why the exponential phase is the dangerous phase, and what that means for the antibiotic clock
The exponential phase is the clinically dangerous phase for three linked reasons, and each reason has a consequence for the moment a dose is given. First, the number of organisms rises by multiplication rather than addition, so a delay of one generation time can double the burden that the immune system and the drug must handle: in an infection whose generation time is 30 minutes, four hours of unchecked growth is a 256-fold increase, eight doublings.
Second, the products of that growth are what injure the patient. Many pathogens release toxins, enzymes and inflammatory fragments in proportion to their numbers, which is why the tissue destruction of necrotising fasciitis and the collapse of Gram-negative sepsis track the population size.
Third, populations under antibiotic pressure select resistant mutants: the larger the population and the lower the drug concentration, the greater the chance that a pre-existing resistant variant survives and expands. This is why most antibiotics act best against dividing cells. The β-lactam drugs, penicillin among them, bind the transpeptidase enzymes — the enzymes that cross-link peptidoglycan, the mesh that gives a growing wall its strength — so they kill cells that are actively remodelling a septum and are far less effective against cells resting in the lag phase. Bactericidal drugs kill the organism outright; bacteriostatic drugs, by contrast, only stop it growing and depend on the immune system to clear the survivors, and that distinction matters in a patient with a deep abscess or a weakened immune response. Dosing schedules are built on these facts. The aim is to keep the free drug concentration above the minimum inhibitory concentration — the lowest concentration that visibly stops growth — for long enough between doses, which is why the interval is prescribed rather than chosen by convenience, and why a delayed or omitted dose is not merely late but opens a window in which survivors multiply. Giving the antibiotic at the scheduled hour, checking the rate and volume of an intravenous infusion, recording a dose that was vomited, and reporting an omitted dose are all ways of protecting the treatment itself. Swallowing a drug with food that binds it, or crushing a modified-release tablet, can produce exactly the low concentration that favours resistance.
What the death phase does and does not mean for a sterilising procedure
The death phase on a growth curve looks reassuring, and it is why many believe that time alone makes things safe. It does not. The curve describes a closed batch after the food is exhausted and the waste has accumulated: the number of cells still able to reproduce falls because cells are dying faster than they are replaced, but the fall is a net count, not the disappearance of the organism. Dead cells, intact toxins and fragments remain in the medium, so a culture that has passed through its death phase is not automatically harmless. Many organisms also survive dormant: bacterial endospores can persist for years in a dried state and germinate again when conditions improve. A sterilising procedure therefore does not wait for nature to take its course; it drives the population down deliberately and predictably. The relevant arithmetic is the logarithmic, or decimal, reduction: a lethal agent kills a constant fraction of the survivors per unit of time, so each D-value — the time needed to reduce the population tenfold — removes ninety per cent of what remains. A process that achieves a sterility assurance level of 10⁻⁶ leaves a probability of one surviving organism in a million; that is a probability statement, and it is met by validated steam, heat or chemical cycles, not by hoping. An instrument that is not visibly soiled is not therefore sterile. A wrapped pack that has become damp is no longer treated as sterile, because moisture can carry organisms through the wrapper. The load, the packaging and the removal of air all decide whether saturated steam actually penetrates to the surfaces that must be sterilised, which is why an autoclave cycle is controlled by pressure, temperature and time together and checked with chemical or biological indicators. Filtration and boiling are not substitutes: filtration removes bacteria and larger particles but not viruses, mycoplasmas or toxins, and boiling destroys most vegetative cells but cannot be relied on to destroy endospores. The safe rule is that sterility is an absolute property produced by a validated process, while the death phase of a growth curve is only a description of a population in decline.
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The bacterial growth curve: lag, exponential, stationary and death
A closed batch culture followed from inoculation to decline. The curve plots the number of living cells against time: it stays almost flat while the cells build their machinery, climbs steeply while nutrients are plentiful, levels off when the carrying capacity is reached and division is balanced by death, then falls as waste, starvation and oxygen shortage take their toll. Follow the phase markers to see what the population is doing behind the shape of the line.
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01 / 5Lag phase
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Lag phase
The culture has been inoculated but the count hardly moves. The cells are not idle: they are building the enzymes, ribosomes and other machinery needed before the first septum can form, and repairing any damage from the transfer. This is why a lag phase exists at all, and why a culture transferred to fresh medium does not divide immediately.
Schematic of a closed batch culture, not drawn to scale: the vertical axis represents a logarithmic count, so a straight-looking segment is a steady doubling, and the real curve steepens even more than the drawing suggests. Timings are typical ranges and vary with the organism, the medium and the temperature. The death phase is a net decline in a closed vessel and is not a sterilising process.
Key points to remember
✓Bacteria grow in a geometric progression: one cell grows until it almost doubles, copies its DNA, forms a septum and divides by binary fission into two identical daughter cells.
✓A closed culture passes through lag, exponential (log), stationary and death phases; the lag exists because enzymes and ribosomes must be built, and the stationary phase is a balance between division and death, not simple starvation.
✓Daughter cells may separate at once or remain joined, giving pairs, clusters, chains or filaments that help in identification.
✓Generation time ranges from 20 to 30 minutes in Escherichia coli and many Gram-negative bacilli to about 24 hours in the tubercle bacillus.
✓Bacteria obtain energy by respiration at the cytoplasmic membrane — not at the mesosomes, which are regarded as fixation artefacts — by photosynthesis with specialised pigments, or by oxidising inorganic compounds.
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Worked example
From two counts to a generation time. A broth culture is started with a measured number of cells and then sampled twice. At 08:00 it contains 5 × 10⁴ colony-forming units per millilitre; at 12:00 the same broth contains 3.2 × 10⁶ colony-forming units per millilitre. The laboratory asks how many generations occurred, what the generation time is, and whether the result is consistent with a fast-growing organism rather than a slow one.
Find the fold increase by dividing the later count by the earlier one: 3.2 × 10⁶ ÷ 5 × 10⁴ = 64. The population has grown sixty-four-fold, and that single number is all you need before you look for generations.
Convert the fold increase into generations. Every generation doubles the population, so ask how many times 2 must be multiplied by itself to reach 64: 2, 4, 8, 16, 32, 64, which is six generations. The same answer comes from the logarithm to base 2: six doublings.
Divide the elapsed time by the number of generations. From 08:00 to 12:00 is four hours, or 240 minutes, so the generation time is 240 ÷ 6 = 40 minutes.
Interpret the answer instead of only reporting it. A generation time of 40 minutes is in the range of Escherichia coli and many Gram-negative bacilli once the culture is adapted, so a fast-growing organism is plausible. Had the broth been inoculated with Mycobacterium tuberculosis, whose generation time is about 24 hours, six doublings would have taken about six days rather than four hours.
Two counts and the time between them are enough to describe the speed of a culture: fold increase, then generations, then generation time. Notice that the calculation only holds while the population is in its exponential phase, because it assumes that every cell is dividing and none is held back by exhausted nutrients or accumulating waste.
Common misconception
A common beginner error is to read the death phase as a kind of self-sterilisation: the count is falling, so the material must be becoming safe. It is not. The death phase is only a net decline in a closed batch; dead cells, intact toxins and fragments remain, and endospores can survive in a dormant state and germinate later. A second error comes straight from the notes: believing that respiration happens at the mesosomes. Mesosomes are now widely regarded as artefacts created when cells are chemically fixed for electron microscopy, and bacterial respiration belongs to the cytoplasmic membrane. The two errors share a lesson — neither the passage of time nor a shrinking count sterilises anything; that requires a validated method.
Check your reasoning
Think of a specimen you have sent or seen sent for culture. If you knew the generation time of the likely organism, how would that change when you send it, what you tell the patient about the wait, and how you expect the result to look?
Reveal an answer
A short generation time, as in Escherichia coli, means the organism multiplies rapidly in the specimen and in the patient, so the sample must be sent promptly and handled without delay, the patient can be told that a preliminary answer may come within a day, and a heavy growth is expected if the infection is real. A long generation time, as in Mycobacterium tuberculosis, means the specimen is stable enough to be transported but the culture itself is slow, so the patient must be prepared for weeks of waiting, treatment is often started on clinical grounds, and a negative early culture proves nothing.
Where this comes from
BMS 114 General Microbiology I lecture notes (supplied) · “Bacterial Reproduction”, lines 873–912: growth by geometric progression, binary fission with DNA replication, elongation and septum formation, the four growth-curve phases, the arrangements of daughter cells, the generation times of Escherichia coli and the tubercle bacillus, and the three routes by which bacteria obtain energy.
Correction to the supplied notes — mesosomes and the site of respiration · Correction to the supplied notes: lines 909–910 state that respiration occurs at the mesosomes. Mesosomes are now generally regarded as artefacts of chemical fixation for electron microscopy; oxidative respiration in bacteria takes place at the cytoplasmic membrane (standard bacteriology, e.g. Prescott’s Microbiology).
Correction to the supplied notes — the name of the tubercle bacillus · Correction to the supplied notes: lines 906–908 use the older common name “tubercle bacilli”; the organism is Mycobacterium tuberculosis. Its generation time of about 24 hours is as the notes state.
Beyond the supplied notes — the mathematics of population growth · the equation N = N₀ × 2ⁿ, the estimate of generations as n = log₂(N ÷ N₀), and the generation time g = t ÷ n are standard population-growth arithmetic. The notes at lines 874–877 say the population grows by geometric progression but give no formula, and the worked example that converts two counts into a generation time is added here.
Beyond the supplied notes — why the exponential phase matters clinically · the relationship between the growth rate and the speed at which an infection becomes dangerous, the action of β-lactam drugs on dividing cells, the difference between bactericidal and bacteriostatic agents, and the pharmacodynamic reason for keeping the drug concentration above the minimum inhibitory concentration between scheduled doses (antimicrobial dosing and stewardship guidance).
Beyond the supplied notes — the death phase and sterilisation · the logarithmic (decimal) reduction of a microbial population, the D-value, the sterility assurance level, and the limits of filtration and boiling follow standard sterilisation science; the notes at lines 901–902 describe the death phase without saying what it does or does not mean for a sterilising procedure.
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About this course and its sources
Bilingual lessons organised from the supplied BMS 114 General Microbiology I lecture notes (Mr Ngotelo F. Ambene, Mr Lyonga B. Lyonga & Mr Fumbui Louis Ngong). Diagrams are original educational schematics; sections that correct the notes or go beyond them say so in their sources. Nursing and French-language review are pending.