Bacterial Population Growth (w19) Flashcards
How do most prokaryotes reproduce ?
By binary fission
Describe the steps to bacterial cell division …
1) cell elongates, enlarging it volume and DNA is replicated.
2) cell wall and plasma membrane begins to constrict.
3) cross-wall forms, completely separating the two DNA copies.
4) the cells separate.
- this is also known as binary fission in bacteria
What are the phases of bacterial population growth ?
1) LAG phase (no increase in population but intense activity preparing for population growth)
2) LOG phase - or exponential (exponential increase on population)
3) stationary phase (period of equilibrium, microbial deaths balance production of new cells)
4) DEATH phase (population is decreasing at a logarithmic rate)
What is LAG phase of bacterial population growth ?
- little or no cell division occurs.
- intense metabolic activity, individual cells increase in size.
What is the LOG or exponential phase of bacterial population growth ?
- rapid and constant population growth (exponential manner)
- number of cells produced > number of cells dying
What occurs during the stationary phase of bacterial population growth ?
- population size begins to stabilize
- number of cells produced = number of cells dying
What occurs during the death phase of bacterial population growth ?
- population size begins to decrease
- number of cells produced < number of cells dying
What happens to the number of cells in each generation during binary fission ?
It doubles
What are the physical requirements for bacterial growth ?
- temperature
- pH
- osmotic pressure
What are the chemical requirements for bacterial growth ?
- carbon source
- ions, trace elements
- oxygen
- nitrogen, sulphur, and phosphate
- organic growth factors
What type of bacteria grow between pH 6.5 and 7.5 ?
Neutrophiles
What bacteria grow in acidic environments (pH 0-5) ?
Acidophiles
What bacteria prefer the pH range of 8.0-11.5 ?
Alkalphiles
What does hypertonic environments (higher in solutes than inside the cell) cause the bacterial cell to do ?
Causes plasmolysis, due to high osmotic pressure, the water will move from the inside of the cell to outside of cell potentially causing it to burst.
What is inoculum ?
The introduction of microbes into a medium
What is agar ?
Agar is a complex polysaccharide, used as a solidifying agent for culture media in Petri plates (liquefies at 100C and Solidifies at ~40C)
- Generally not metabolized by microbes
What does selective culture media do ?
Suppress unwanted microbes and encourage desired microbes
E.g. Saboraud’s Agar: 5.6pH discourages bacterial growth. Used to isolate fungi.
What does differential culture media do ?
Allow distinguishing of colonies of different microbes on the same plate
E.g. Blood Agar: to distinguish bacteria that destroy red blood cells (hemolysis).
What does enrichment culture do ?
Encourages the growth of a desired microbe by increasing very small numbers of a desired organisms to detectable levels (without suppressing other microbes).
What does obligate aerobes require and what is it ?
Obligate Aerobes - require oxygen to live. E.g. Pseudomonas, causing infections in humans, mostly in hospital patients
What are the three classifications of microorganisms based on their oxygen requirements ?
- obligate aerobes
- facultative anaerobes
- obligate anaerobes
What are facultative anaerobes ?
Facultative anaerobes – can grow via fermentation or anaerobic respiration when oxygen is not available. Grow best in aerobic conditions. E.g. E.coli
What are obligate anaerobes ?
Obligate anaerobes – do not tolerate oxygen and are harmed by it. E.g. Clostridium bacteria that cause tetanus and botulism
What is a culture medium ?
Culture medium: Nutrients prepared for microbial growth in a laboratory (chemical requirements)
What is a culture ?
Culture: Microbes growing in/on culture medium at appropriate conditions (physical requirements)
How is a pure culture obtained ?
To obtain a pure culture, individual organisms must be isolated
Streak-plate method is commonly used
What are aseptic techniques ?
Procedures under suitably controlled conditions to maintain the sterility, free from external sources of contamination
How are microorganisms isolated in pure culture (list the two steps) ?
(a) streaking techinque, in which a sterile loop is inserted into a sample and streaked onto a plate in a pattern (e.g. 3 sectors), to obtain individual colonies.
Next
(b) Colony formation: A population of cells arising from a single cell (also referred to as CFU, colony forming unit).
Why is measuring microbial growth crucial for a variety of applications ?
Measuring the microbial growth is crucial for a variety of applications:
- Diagnose bacterial infections from patient specimens (blood, urine, etc)
- Food safety and process hygiene assessment against relevant criteria
- Microbiologic sampling of environmental sources
- Assessing microbial contamination of sterile pharmaceutical products - test for sterility for Quality Assurance (YEAR 3)
- Microbiological assessment of non-sterile pharmaceutical products (e.g. topical use preparation, herbal remedies) comply with the specifications / acceptance criteria outlined in the British Pharmacopoeia (YEAR 3)
What is direct measurements for measuring microbial growth ?
List the three methods …
Direct measurements–count microbial cells
1) Plate count
2) Filtration
3) Direct microscopic count
What is indirect measurements for measuring microbial growth ?
List the three methods …
Indirect measurements–count microbial cells
1) Turbidity (mass)
2) Metabolic activity
3) Cell mass - Dry weight
What is serial dilution ?
Sequential dilutions in a stepwise manner
What is membrane filtration ?
A method used for low counts, where a filter is applied to a Petri dish and bacteria can grow as colonies on the surface. Before this a solution is passed through a cellulose filter (0.45 μM) that collects and retains bacteria (bacteria size > pore size)
What does direct microscopic count rely on ?
Direct microscopic count = Rely on light microscopy and a cell counter and involves placing a small amount of samples on a microscope slide with a special grid
How do you calculate the number of bacteria per ml ?
= number of cells counted / volume of area counted
What are some barriers to direct microscopic count ?
- Difficult to distinguish live/dead bacteria
- Often laborious
- Only suitable with high counts
Define metabolic activity …
Metabolic activity - amount of metabolic product is proportional to the population size
What is turbidity/ cell mass ?
Turbidity/Cell mass - measurement of cloudiness/optical density (linked to the cell mass) of liquid media by a spectrophotometer
