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4
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LAG PHASE:

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Bacteria are adapting to a new environment & not reproducing at
maximum rate

2
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STATIONARY PHASE:
- Total growth = 0
- Number of new cells = number of dying cells
- Due to COMPETITION FOR NUTRIENTS; LACK ESSENTIAL
NUTRIENTS; ACCUMULATION OF TOXIC WASTE; LACK OF
OXYGEN
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1 mm3 volume
- DON’T COUNT CELLS ON GRID LINES
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Number of live cells counted within the 4 corner squares and a mean is
calculated

6A: Counting Cells Using Optical Methods: Turbidity:
Turbidity: method to measure concentration of substance by measuring amount of
light passing through it; a specialised form of COLOURIMETRY
- As number of bacterial cells in culture INCREASES, the sample becomes
MORE TURBID/CLOUDY
- The more turbid a solution, the more light it absorbs and less light
passes through
- Shows how many microorganisms are present

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You can measure TURBIDITY & COUNT CELLS using a
HAEMOCYTOMETER for each sample
- It gives a relationship between TURBIDITY & NUMBER OF
MICROORGANISMS

6A: Counting Cells Using Dilution Plating:
Dilution plating: method to obtain a culture plate with countable number of bacterial
colonies; used to find the TOTAL NUMBER OF VIABLE CELL COUNT
- Based on the idea that colonies on agar plates are GROWN FROM A
SINGLE, VIABLE MICROORGANISM
- Total viable cell count = number of colonies x dilution factor
1
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Use a pipette and collect 1 cm3 of the sample and add it to 9 cm3
of water (0
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Take 1 cm3 of this solution and add it to another 9 cm3 of water (0
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Repeat this till the number of cells in the sample is clear and is at a low
number
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PRIMARY INFECTION:
- Initial stage of TB where TB is inhaled & invades lung cells & multiplies
slowly with no obvious symptoms
- It evades the immune system by becoming encased in TUBULES (contains
dead bacteria and macrophages)
- TUBULES are surrounded with a thick, waxy outer layer that
protects the pathogen from macrophage enzymes
- Can remain dormant as a result and allows bacteria to grow
slowly for many years deep inside lungs
2
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- Mycobacteria targets T-CELLS

6A: How HIV Causes AIDS:

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HIV enters T helper cell, controls the host DNA & replicates it
Host T helper cell is destroyed when virus leaves it
At the same time, T killer cell destroys the heavily infected T helper cells
This causes a large decrease in T helper cells
- Reduces production of PLASMA CELLS which reduces the production of
ANTIBODIES; causes a lack of a defence system

6A: 4 Stages of AIDS:
Stage 1: ACUTE HIV SYNDROME:
- First few weeks after infection:
- Some people feel unwell with FEVER, HEADACHES & TIREDNESS
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Between 3-12 weeks after infection
- HIV antibodies appear in blood
- Individuals = HIV positive

Stage 2: ASYMPTOMATIC STAGE:
- Once the infection is established, all symptoms disappear
- People who HAVE ACCESS TO ANTIRETROVIRAL DRUGS; stage lasts for MANY
YEARS
- People who DON’T

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