Bacterial Population Growth Flashcards
What is the bacterial cell cycle?
1: cell elongates, enlarging its volume and DNA is replicated
2:cell wall + plasma membrane begin to constrict
3: cross-wall forms, separating the 2 DNA copies
4: cells separate
What is critical to control and prevention in bacterial growth?
Understanding bacterial growth
This includes environmental microbial contamination, infectious disease caused by bacteria, and pharmaceutical spoilage.
What are the four sequential phases of bacterial population growth?
- Lag phase
- Log (or exponential) phase
- Stationary phase
- Death phase
Each phase represents a different stage in the growth cycle of bacterial populations.
What happens in the lag phase?
- Little or no cell division occurs
- Intense metabolic activity. Individual cells increase in size
What happens in the log/ exponential phase?
- Rapid and constant population growth (exponential manner)
- Number of cells produced > Number of cells dying
What happens in the stationary phase?
- Population size begins to stabilise
- Number of cells produced = Number of cells dying
What is the death phase?
- Population size begins to decrease
- Number of cells produced < Number of cells dying
What is the generation time in bacterial growth?
Time required for a bacteria to complete the cell cycle
*number of bacteria doubles each generation
For example, E. coli divides every 20 minutes, while some bacteria like M. tuberculosis require over 24 hours.
What are biofilms?
Microbial communities embedded within extracellular polymeric substances
Biofilms can shield bacteria from external factors and facilitate nutrient exchange.
What do biofilms do?
▪ Shields bacteria from external factors (e.g.
antibiotics cannot diffuse through it)
▪ Facilitates nutrient exchange
▪ Enables microbes to attach various surfaces
What are the physical requirements for bacterial growth?
*temperature
*pH
*Water activity
What are the chemical requirements for bacterial growth?
*carbon source
*organic growth factors
*oxygen
*nitrogen,sulphur and phosphate
*ions + trace elements
What type of bacteria are psychrophiles?
Cold-loving bacteria that thrive below 15°C
These bacteria have specific temperature requirements for growth.
What type of bacteria are psychrotrophs?
Grow at 20-30℃ but can tolerate colder temperatures
What type of bacteria are mesophiles?
“Middle loving” bacteria with an optimal range of 25-40℃ (most human pathogens)
What type of bacteria are thermophiles?
“Heat loving” bacteria with an optimal range of 50-60℃
What pH range do acidophiles thrive in?
pH < 5.5
Acidophiles are adapted to acidic environments.
What pH range do neutrophils thrive in?
(most pathogens)
pH 6.5-7.5
What pH range do alkaliphiles thrive in?
(rare)
pH >8.0
What is the osmotic pressure’s effect on bacterial cells in a hypertonic solution?
Causes plasmolysis (cell shrinkage)
(exception = halophiles)
This occurs when the concentration of solutes is higher in the surrounding medium than in the cell.
What is the effect of low water activity (A_w) on microbial growth?
Reduces microbial metabolism and growth - causing plasmolysis
This can lead to plasmolysis in bacteria.
What is the effect of high water activity (Aw) on microbial growth?
Supports microbial growth, increasing risk of spoilage
What are the Aw requirements in bacteria?
Gram +ve = 0.8-0.98 Aw
Gram -ve = 0.97 Aw
Halophiles thrive in low Aw
What are each of the chemical requirements for growth for?
carbon = backbone in all organic compounds
N = forms amino acids, DNA/RNA
S= form proteins +some vitamins
P = form DNA,RNA,ATP + phospholipids
Trace elements = used as enzyme cofactors
What are obligate aerobes?
Bacteria that require oxygen to live
e.gPseudomonas aeruginosa
An example is Pseudomonas aeruginosa.
What are Facultative anaerobes?
Can grow with or without oxygen but prefers aerobic conditions
E.g E.coli
What are obligate anaerobes?
Do not tolerate oxygen and are harmed by it
E.gClostridium spp.
What is the purpose of selective media in microbiology?
Suppress unwanted microbes and encourage desired microbes
An example is Saboraud’s Agar, which discourages bacterial growth.
What is differential media?
Allow distinguishing of colonies of different microbes on the same plate
What is an enrichment culture?
Encourages the growth of a desired microbe by increasing very small numbers of an organisms to detectable levels (without suppressing other microbes)
What is a pure culture?
Isolating microorganisms to obtain a population of cells arising from a single cell
This is critical for identifying pathogens.
Fill in the blank: Most bacteria divide by _______.
binary fission
This process allows for rapid population growth.
What is the significance of measuring microbial growth?
To diagnose bacterial infections, assess food safety, and evaluate contamination
This is crucial in clinical and environmental microbiology.
What does the term ‘turbidity’ refer to in microbiology?
Measurement of cloudiness/optical density of liquid media
It is linked to the cell mass and can indicate the population size.
What is the role of agar in microbiology?
Used as a solidifying agent for culture media
Agar liquefies at 100°C and solidifies at ~40°C.
What are the two types of isolating pure culture techniques?
- streaking technique
*colony formation
What are some types of direct measurements of count microbial cells?
*plate count
*filtration
*direct microscopic count
What are some types of indirect measurements to count microbial cells?
*turbidity
*metabolic activity
*cell mass - dry weight
What is the equation for direct microscopic count technique?
number of bacteria/ml = number of cells counted/ volume of area counted
What are the disadvantages to the direct microscopic count?
*difficult to distinguish live/dead bacteria
*often laborious
*often only suitable with high counts
