Bacterial Physiology
Open and closed systems
| System | Where | Behaviour |
|---|---|---|
| Open system | Organisms growing in nature | Nutrients replenished and wastes removed |
| Closed system | In the laboratory — agar plates, broth tubes | Nutrients run out and wastes are not removed |
The laboratory closed system exists for three reasons, and the lecture lists them: to identify the cause of infection from clinical samples; to study the characteristics and properties of microorganisms; and to prepare biological products such as vaccines, antigens and toxoids.
The growth curve in §4 — the one with four phases that every exam draws — is a property of the closed system, and only of it.
In nature, nutrients keep arriving and waste keeps leaving, so bacteria do not obligingly enter a stationary phase and die off. The curve you memorise is the behaviour of a population trapped in a finite tube. That is worth holding on to, because the stationary phase — where spores, exotoxins and antibiotics appear — is a starvation response. Understand it that way and you never have to memorise which products appear when.
- Open vs closed system? → Open: in nature, nutrients replenished and wastes removed. Closed: in the lab, nutrients run out and wastes accumulate
- Name the three purposes of laboratory culture. → Identify the cause of infection from clinical samples · study microbial characteristics · prepare biological products (vaccines, antigens, toxoids)
Requirements for bacterial growth ★★★
The lecture's summary slide names “the requirements of bacterial growth” as an examinable point. There are three groups: nutrients, pH and temperature, and aeration.
Nutrients — five
- Water
- Carbon source
- Nitrogen source
- Inorganic salts
- Growth factors — organic compounds that a cell must have for growth but cannot synthesise itself
pH — hydrogen ion concentration
| Class | Optimum pH |
|---|---|
| Neutrophile | pH 6.0–8.0 — most medically important bacteria |
| Acidophile | less than pH 5.5 |
| Alkaliphile | more than pH 10.5 |
Temperature
| Class | Optimum temperature |
|---|---|
| Psychrophile | 20–30 °C — low temperature |
| Mesophile | 30–37 °C — the human pathogens |
| Thermophile | 50–60 °C |
Aeration is the third requirement, and it is the basis of the aerobic / anaerobic / facultative anaerobic / microaerophilic classification that the anaerobe unit depends on.
Neutrophile, mesophile. pH 6–8, 30–37 °C.
That is not a coincidence — it is a description of us. A pathogen is, almost by definition, an organism whose optimum growth conditions are the conditions inside a human body. Which is also why the acidophiles and thermophiles of the table are largely environmental curiosities rather than clinical problems, and why the stomach's acidity and a fever are both, in their way, antimicrobial.
- Name the five nutrient requirements. → Water, carbon source, nitrogen source, inorganic salts, growth factors
- Define a growth factor. → An organic compound a cell must have for growth but cannot synthesise itself
- Neutrophile, acidophile, alkaliphile pH? → 6.0–8.0 · below 5.5 · above 10.5
- Psychrophile, mesophile, thermophile temperature? → 20–30 °C · 30–37 °C · 50–60 °C
- Where do human pathogens sit? → Neutrophile and mesophile
Binary fission and generation time ★★
Asexual reproduction in which a cell or organism separates into two cells. This is the mode of bacterial reproduction — another named examinable point on the lecture's summary slide.
The time it takes one cell to undergo binary fission — to divide into two cells.
| Organism | Generation time |
|---|---|
| Escherichia coli — the majority pattern | 20–30 minutes |
| Mycobacterium tuberculosis — the exception | 18–20 hours |
The growth rate constant converts this into arithmetic: it determines the amount of growth occurring in a specified time, or the time required for a specified amount of growth. The lecture's worked figure is td = 10 min (0.17 h), corresponding to a growth rate constant of 4.1 h⁻¹.
Eighteen to twenty hours against twenty minutes is a difference of roughly fiftyfold, and almost everything strange about tuberculosis follows from it.
Culture takes weeks, not overnight — a TB culture is reported at 2–8 weeks, which is why smear microscopy and molecular tests matter so much clinically.
The disease is chronic. An organism doubling once a day cannot produce the explosive 24-hour illness of a pneumococcus; it produces months of cough and weight loss.
Treatment lasts six months. Most antibacterials act on growing cells — as penicillin does on the wall being built. An organism that divides once a day offers very few opportunities to be caught in the act.
One number on a slide; three exam answers.
- Define binary fission. → Asexual reproduction in which a cell separates into two cells
- Define generation time. → The time for one cell to undergo binary fission
- E. coli vs M. tuberculosis generation time? → 20–30 minutes vs 18–20 hours
- Why does TB need six months of therapy? → Most drugs act on dividing cells, and it divides only about once a day
The growth curve ★★★
When a fixed volume of liquid medium is inoculated with microbial cells, and the number of viable cells per millilitre is determined periodically and plotted, showing the growth of cells over time. This is a batch culture — a closed system with finite resources.
| Phase | Growth rate | What happens |
|---|---|---|
| Lag | Zero | Cell division does not occur immediately. Cells must increase in length and synthesise the macromolecules needed for protein synthesis, and the enzymes for cell division |
| Exponential (logarithmic) | Constant | Cells divide at a constant rate and show typical biological properties — staining, shapes, chemical reactions, sensitivity to antimicrobial agents |
| Maximum stationary | Zero | Cell division decreases; total cell number remains constant. Atypical morphology, and spores, exotoxins and antibiotics are produced |
| Decline (death) | Negative | Viable cells decrease exponentially; collapsed or atypical cells |
The exponential phase is the one with typical properties — typical staining, typical shape, typical drug sensitivity. So every characterisation in the laboratory, and every antibiotic sensitivity test, is done on log-phase cells. Test a stationary-phase culture and the Gram stain may mislead you and the sensitivities may be wrong.
The stationary phase is the one that produces spores, exotoxins and antibiotics. Read that as a survival response to starvation and it makes sense: the organism armours itself (spore), poisons the environment that is starving it (exotoxin), and kills the competition (antibiotic).
It is also, incidentally, why Clostridium botulinum in a sealed tin becomes dangerous over time rather than immediately.
- Name the four phases and their growth rates. → Lag (zero) · exponential (constant) · maximum stationary (zero) · decline (negative)
- Why is there a lag phase? → Cells must lengthen and synthesise macromolecules for protein synthesis and the enzymes for division
- Which phase shows typical biological properties? → Exponential — staining, shape, chemical reactions, drug sensitivity
- What is produced in the stationary phase? → Spores, exotoxins and antibiotics
- What kind of culture is a growth curve? → A batch culture — a closed system with finite resources
⭐ The seven medically important anabolic products ★★★
“Seven medically important anabolic products of bacteria” is on the lecture's summary slide, and three of the seven are named there as definitions to learn — pyrogen, antibiotic and bacteriocin. Bacteriocin was set in 2019.
| # | Product | Definition / note |
|---|---|---|
| 1 | Pyrogen | A polysaccharide produced by bacteria that causes a rise in body temperature |
| 2 | Toxin | Exotoxin and endotoxin — compared in full in the Bacterial Infections unit, where the 2020 paper sets them |
| 3 | Invasive enzyme | Enzymes that let the organism spread through tissue |
| 4 | Antibiotic | A microbial product that inhibits or kills other microorganisms |
| 5 | Bacteriocin ⭐ | An agent produced by a bacterium that inhibits or kills closely related species, or even different strains of the same species |
| 6 | Vitamin | e.g. the vitamin K supplied by gut flora |
| 7 | Pigment | Used in identification — e.g. the golden pigment of Staphylococcus aureus |
For four marks, add the discriminating point: a bacteriocin has a narrow spectrum, acting on organisms closely related to the producer, whereas an antibiotic is a microbial product that inhibits or kills other microorganisms generally. Bacteriocins are also used in typing — colicin typing of E. coli, pyocin typing of Pseudomonas — because the pattern of which strains a given isolate can kill is an epidemiological fingerprint.
Both are microbial products that kill microbes. The difference is aim.
An antibiotic is broad — a weapon against the microbial world at large, which is exactly why we can borrow it as a drug.
A bacteriocin is narrow — a weapon against your own relatives, the organisms competing for the identical niche. Nobody competes with you for a niche more fiercely than a strain of your own species.
That narrowness makes bacteriocins poor drugs and excellent typing reagents.
- Name the seven anabolic products. → Pyrogen · toxin · invasive enzyme · antibiotic · bacteriocin · vitamin · pigment
- Define pyrogen. → A polysaccharide produced by bacteria that causes a rise in body temperature
- Define antibiotic. → A microbial product that inhibits or kills other microorganisms
- Define bacteriocin. → An agent produced by a bacterium that inhibits or kills closely related species, or different strains of the same species
- Antibiotic vs bacteriocin? → The antibiotic is broad-spectrum; the bacteriocin is narrow, aimed at close relatives — hence its use in typing
Revision
The lecture's own summary slide — these are the examinable points
- Definitions: pyrogen, antibiotic and bacteriocin — §5
- The requirements of bacterial growth — §2
- The mode of bacterial reproduction — §3
- The characteristics of the growth curve — §4
- Seven medically important anabolic products of bacteria — §5
The whole unit on one screen
| Question | Answer |
|---|---|
| Five nutrients? | Water · carbon · nitrogen · inorganic salts · growth factors |
| Growth factor? | An organic compound needed for growth but not synthesisable by the cell |
| Human pathogen pH and temperature? | Neutrophile pH 6–8 · mesophile 30–37 °C |
| Mode of reproduction? | Binary fission |
| Generation times? | E. coli 20–30 min · M. tuberculosis 18–20 h |
| Four phases? | Lag · exponential · maximum stationary · decline |
| Typical properties in which phase? | Exponential |
| Spores, exotoxins, antibiotics in which phase? | Stationary |
| Pyrogen? | A bacterial polysaccharide causing a rise in body temperature |
| Antibiotic? | A microbial product inhibiting or killing other microorganisms |
| Bacteriocin? ⭐ | A bacterial agent killing closely related species or strains |
- Define bacteriocin for 4 marks. → A bacterial product killing closely related species or strains; narrow-spectrum, unlike an antibiotic; used for typing
- List the seven anabolic products. → Pyrogen, toxin, invasive enzyme, antibiotic, bacteriocin, vitamin, pigment
- Describe the growth curve. → Batch culture in a closed system: lag (zero rate, synthesis) → exponential (constant, typical properties) → stationary (zero, spores/exotoxins/antibiotics) → decline (negative)
- Why is TB different? → Generation time 18–20 h against 20–30 min — slow culture, chronic disease, months of therapy