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Selective Media
are designed to supress the growth of unwanted bacteria and encourage the growth of the desired mircobes. e.g. EMB, mannitol salt agar. **S. aureus is capeable of growth on nurtient agar containing 10% salt, this is selective media.
Enrichment media:
-encourages growth of desired microbe. Example: assume soil sample contisn a few phenol degrading bacteria and thousands of other bacteria. *inoculate phenol containing culture medium with the soil and incubate. **transfer 1ml to another flask of the phenol medim and incubate ***transfer 1ml to another flak of the phenol medium and incubate.
antiseptic, first proposed by Lister.
Nutrient broth commonly used in micro is an example of a?
complex media
Pure Cultures
Contain only one species or stain. *most pt specimans and environmental samples contain several different kinds of bacteria.
Method for obtaining pure cultures?
streak plate method.
a population of cells arising from a single cell or spore or from a group ofattached cells
colony formation or CFU
Single colonies can be classified as pure becuase...
there is no other bacteria around to distinguish. *only 1% of all bacteria can be sucessfully cultured. **the aseptic technque is critical
2 ways of presevin bacteria cutures...
Deep Freezing and lyophilization (freeze drying)
Deep Freezing
rapid cooling of pure culture in suspension liquid to -50 to -95 degrees C. Good for several years. *Freezing requires flash freeze to avoid water crystals from forming and killing tthe cell. **Can put in the fridge but they dont last as long.
freeze drying, frozen(-54 to -72 degrees C. and dehydrated in a vacuum. Good for many years.
Reproduction in prokaryotes
by binary fission and budding
binary fission
exponential growth (increases the number of cells not the size of cells)
few bacterial species reproduce by budding, they form a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell and then it separates.
generation time
time required for cells to divide (also known as doubling time) this time in bacteria is very short, E.coli is about 20 minutes, so every 20 minutes they double. M. tuberculosis can be up to 24 hours doubling time.
Bacterial Growth curve
illustrtes the dynamics of growth. 4 phases 1) Lag phase 2) exponential or logarythmic (log) phase 3) stationary phase 4) Death Phase
Lag Phase
the number of cells changes very little because the cells do nt immediately reproduce in a neweduim. this phase can last for one hour or several days.
Log phase
eventuallythe cells begn to divide and enter a period of growth, or larythmic inrease. ellular reproduction is most active during this phase and gen. time reaches a comstant minimum. **PCN works best in log phase bc it inhibits cell wall growth **binary fission is not taking place.
Stationary Phase
eventually growth rate slows and the number of microbial deaths balances the number of new cells, and the population stabilizes. **the have cell walls in this phase
Death Phase
the number of deaths eventually exceds the number of new cells formed and the population enters the death phase or logarythmic decline phase. this phase continues until the population is diminished to a tiny fraction of the number of cells in the previous phase or the population dies entirely. **if you leave bacteria in an incubator for long enough they owuld eventually die bc they build up waster.
Only 2 genus' form endospores
1)Bacillus 2) Clostridium No other bacteria form endospores.
Measuring Bacterial population growth
Viable cell counts: Plate Counts -pour plates or spread plates after serial dilutions and count the CFU's
The advantage of the direct counting method over the pour plate method is that direct counting?
counts both living and dead cells. *pour plate counting only counts living cells.
Addidional Direct cell counts...
counting chambers (slides) for microscope also coulter counter and Flow sytometer Filtration method for low counts. *most comonly used indirect method: Spectophotometry to measure turbidity.
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