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Topic 5: Isolation of Bacteria by Dilution Techniques
- Mixed cultures are of little use in studying microorganisms because of the difficulty they present in determining which organism is responsible for any observed activity.
- Pure culture: culture containing a single kind of microbe
- Required to study concepts such as growth characterisitics, pathogenicity, metabolism, and antibiotic susceptibility
- Bacteria is too small to separate directly therefore indirect methods must be used
- Joseph Lister, in 1870s, attempted to obtain pure cultures by performing serial dilutions until each of his containers theoretically contained one bacterium
- Success very limited and contamination was common
- Dilution methods commonly used for isolation of bacteria
- streak plate
- spread plate
- pour plate
- Streak plate is most common isolation method
- a loop is used to streak the mixed sample many times over the surface of a solid culture medium in a petri plate
- theoretically process of streaking the loop repeatedly over the agar surface causes the baceria to fall off the loop one by one and ultimately to be distributed over the agar surface, where each cell develops into a colony
- Spread and pour plate are quantitative techniques that allow determination of the number of bacteria in a sample.
- In spread plate technique, small amount of previously diluted speciment is spread over the surface of a solid medium using a spreading rod.
- In pour plate technique, small amount of diluted sample is mixed with melted agar and poured into empty, sterile petri dishes
- A plate with between 25 to 250 colonies is selected to determine the number of bacteria in the orginial sample.
- Fewer than 25 colonies is inaccurate because a single contaminant causes at least a 4% error.
- Number of bacteria in the original sample is calculated using:
- Colony-forming units per ml = number of colonies / (dilution X amount plated)
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Explore Top NotesNoteStudied by 17 peopleNoteStudied by 12 peopleNoteStudied by 112 peopleNoteStudied by 21 peopleNoteStudied by 16 peopleNoteStudied by 26 people
4.15 Beethoven
NoteStudied by 17 peopleSoftball Study Guide
NoteStudied by 12 peopleChapter 1: The Comparative Method
NoteStudied by 112 peopleWhy is the rent so high? Final review
NoteStudied by 21 peopleClimate Change - Exploring Earth Climate, Prevailing Winds, and How Heat Traps In Our Atmosphere
NoteStudied by 16 peopleCOMPUTERS NOTES
NoteStudied by 26 people