Bacteria: structure, growth and pathogenesis
Introduced| English | Meaning |
|---|---|
| lipopolysaccharide | The endotoxin in the outer membrane of Gram-negative bacteria that can trigger septic shock. |
| capsule/ˈkæpsjuːl/ | A slimy polysaccharide coat around some bacteria that resists phagocytosis. |
| exotoxins | Proteins secreted by living bacteria that damage the host, often very potently, such as tetanus and diphtheria toxins. |
| endotoxin | The LPS of the Gram-negative wall, released when bacteria die. |
| growth curve | The pattern of a bacterial population over time in culture: lag, exponential growth, stationary and death phases. |
One cell at breakfast, a billion by evening
- A single E. coli that reaches the bladder at breakfast can become a billion cells by the evening, because it divides every 20 minutes.
- A bacterium's wall, its weapons and its growth rate decide what disease it causes, how fast, and which drugs can reach it.
Two kinds of wall
- Gram-positive bacteria have a thick peptidoglycan wall with teichoic acids and no outer membrane.
- Gram-negative bacteria have a thin peptidoglycan layer inside an outer membrane whose outer face is lipopolysaccharide 脂多糖 (LPS), with porins as gates.
Tools for attack
- A capsule 荚膜 is a slippery coat that resists phagocytosis; pili stick bacteria to surfaces; flagella let them swim.
- People without a spleen are at risk from capsulated bacteria such as Streptococcus pneumoniae, because the spleen clears them from the blood.
Match each structure to what it does or marks.
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Structure explains both the stain and the disease.
Two kinds of toxin
- Exotoxins 外毒素 are proteins that bacteria release, each with a precise target: diphtheria toxin stops protein synthesis; haemolysins burst cells.
- Endotoxin 内毒素 is the LPS of the Gram-negative wall, released when bacteria die. It triggers fever and, in large amounts, septic shock.
What is endotoxin?
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Endotoxin is part of the wall and is released when Gram-negative bacteria die.
Growth is doubling
- With generation time g, cell number grows as $N = N_0 \times 2^{t/g}$. E. coli doubles in about 20 minutes: 10 hours is 30 generations, $2^{30} \approx 10^9$ cells.
- In a closed culture the growth curve 生长曲线 has four phases: lag, exponential (log), stationary and death.
A bacterium doubles every 20 minutes. Starting from one cell, how many cells are there after 4 hours?
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4 hours = 240 minutes = 12 generations, and 2¹² = 4096.
Put the phases of a closed bacterial culture in order.
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Cells adjust, then double steadily, then run out of food, then die.
Slow growers
- The tuberculosis bacterium doubles only every 15–20 hours, so in 10 hours it completes about half a generation.
- Slow growth means disease that builds over weeks and cultures that take weeks: the arithmetic sets the clinical pace.
In 10 hours a tuberculosis bacterium completes only about half a generation.
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It doubles every 15–20 hours, so 10 hours is about half a doubling.
Worked example: to a million
- From one cell to about a million needs 20 doublings ($2^{20} \approx 10^6$).
- E. coli (20 minutes): 20 × 20 = 400 minutes, under 7 hours. Tuberculosis (20 hours): 20 × 20 = 400 hours, about 17 days.
A bacterium doubles every 20 minutes. How many hours does one cell take to reach 2²⁰ (about a million) cells?
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20 doublings × 20 minutes = 400 minutes ≈ 6.67 hours.
Worked example: the halo on the slide
- (Fictional) Sputum Gram stain: Gram-positive diplococci shaped like lances, each with a clear halo.
- The halo is the capsule of S. pneumoniae. The stain shows the very structure that lets it escape phagocytes.
What is the clear halo around pneumococci on a stained slide?
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The capsule does not take up stain, so it shows as a halo; it is what protects the bacterium from phagocytes.
Three common slips
- Calling LPS an exotoxin. It is part of the wall, released at death: endotoxin.
- "Fast growth means a pathogen." Harmless gut E. coli grows just as fast.
- "No growth means no organism." Slow or fussy organisms need the right medium and time.
Recap
- Gram-positive: thick peptidoglycan. Gram-negative: thin wall plus an outer membrane with LPS.
- Capsules resist phagocytes; exotoxins are released proteins; endotoxin is wall LPS.
- $N = N_0 \times 2^{t/g}$: a 20-minute doubler makes 10⁹ in 10 hours; a 20-hour doubler half a generation.