Chapter 4 Flashcards
Microbial Growth
The growth of a population (increase in the number of cells not size) in a specific amount of time; lead by cell division
Binary Fission
A primitive form of cell division asexual reproduction) that does not use a spindle fiber apparatus:
- Bacterial cell doubles in size
- Replicates its chromosome (DNA)
- Two chromosomes attach to separates sites (opposite ends) on the plasma membrane
- A cell wall forms between the chromosomes and separates the cells, producing two daughter cells
Budding
A type of asexual reproduction in which a cell forms a bubble-like growth that enlarges and separates from the parent; a few bacteria and some eukaryotes (yeast) replicate this way
Phases of Growth (Closed System)
A microbial lab culture typically passes through 4 distinct, sequential phases of growth:
- Lag Phase
- Exponential Phase
- Stationary Phase
- Death Phase (phase of prolonged decline)
In the lab, cultures usually pass through all phases - not in nature
Lag Phase
Considerable metabolic activiity is occurring as the cells prepare to grow:
- Enzymes and proteins are synthesized to efficiently break down substances in the environment and to construct new macromolecules and ribosomes
- Length varies depending on species
- No perceivable change in numbers
Exponential (Logarithmic) Phase
Cell numbers increase exponentially:
- During each generation time, number of cells in a population increases by a factor of 2
- Slowly at first, but then extremely rapidly
- Lasts 4-10 hours; quickly use up most nutrients and metabolic byproducts accumulate to near toxic levels (oxygen depletion)
- Cells are very virulent, easily cause infection or disease, but are also most susceptible to antibiotics
Stationary Phase
The number of cells doesn’t increase, but changes within cells occur:
- Cells “realize” environmental conditions are turning detrimental to continued growth
- Cells become smaller and produce structures the provide resistance (glycocalyx, endospores, cytoplasmic inclusions)
- Signal to enter this phase may have to do with overcrowding (accumulation of metabolic byproducts, depletion of nutrients, etc.)
- Important phase when treating infections with antibiotics
Death Phase
Cells begin to die out:
- Occurs exponentially but at a low rate
- Cells have depleted intracellular ATP resources
- Not all necessarily die this way (phase of prolonged decline)
- More fit cells survive using nutrients from dying cells
Continuous Culture of Microbes
In nature, nutrients continuously enter the cell’s environment at low concentrations, and populations grow continually at a low but steady rate:
- Growth rate set by the concentration of the scarcest or limiting nutrient
- Other microbes use metabolic byproducts for their own metabolism
Colony Growth of Microbes
Liquid or agar:
- On agar, the position of the cell in the colony determines the environment it is in and what its phase is in the growth curve
- Exponential on the edges, stationary in the middle, death in the center
Generation Time
The length of time it takes one cell to divide into two cells; varies greatly between bacteria:
- Some bacteria can grow every 10 minutes if conditions are excellent
- Some can take 24 hours to finish one division
- The average can reproduce, under optimum conditions, about once every 30 minutes
Rate of Growth Formula
N+ = (No) x 2^n
- N+ = total population of cells in a given amount of time
- No = original number of cells
- n= number of cell divisions in a given amount of time
Reasons Microbes Exist in Many Environments
- Small size
- Easily dispersed
- Need only small quantities of nutrients
- Diverse in their nutritional requirements
Moisture’s Influence on Bacterial Growth
Since they are free-living, independent cells get all their nutrients by diffusion from the surrounding environment, and therefore, need water in order to grow; very resistant to desiccation; dehydration preserves food (may not kill)
Minimum Temperature
Lowest temperature that permits growth and metabolism, but usually at a very slow pace; very cold temperatures do not usually denature proteins or destroy microorganisms
Maximum Temperature
Highest temperature that permits growth and metabolism, usually at depressed rates; going above, enzymes will become denatured and metabolism will stop, destroying the cell
Optimum Temperature
Typically covers a small range in which organism’s metabolic processes and growth are fastest; why fevers (even slight) can be helpful in fighting off bacterial infections
Psychophiles (Cold-Loving)
Bacteria that will grow in temperatures from 5’ C to 20’ C:
- Will not cause infections in humans
- Responsible for the spoiling of refrigerated, and some frozen, food (e.g., blood)
- Perishable items must be frozen far below freezing to retard growth
Mesophiles (Middle-Loving)
Bacteria that will grow in temperatures from 20’ C to 50’ C:
- Optimum temperatures from human pathogens are around 37’ C
- Pathogenic bacteria
- Different organisms will grow best in different regions of the body (e.g., Mycobacterium leprae grows best on the extremities - leprosy)
Thermophiles (Heat-Loving)
Bacteria that will grow in temperatures from 50’ C to 80’ C:
- Found in natural hot springs, volcanic vents, etc.
- The autoclave is used to isolate them in a culture
Hyperthermophiles
Bacteria that will grow in temperatures about 80’ C; found in compost heaps and in boiling hot springs
Acidophiles
Bacteria that live in very acidic environments; optimal pH is below 5.5, but some can live in environments close to 0 (e.g., Helicobacter pylori, which cause ulcers in the stomach; Lactobacillus, which ferments millk)
Neutrophiles
Bacteria that grow best in pHs of 6-8; most human pathogenic bacteria have an optimum pH near 7.3
Alkalinophiles
Bacteria that live in very alkaline environments; optimal pH is above 8.5, but some can live in environments of pHs close to 10 (bleach)