Bacteria: structure, growth and pathogenesis

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.

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.

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.

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.

Worked example: to a million

Worked example: the halo on the slide

Three common slips

Recap