Laboratory Culture of Microorganisms Flashcards
What are the essential elements for all microorganisms?
C, O, N, H, P, S, Se
What is the culture media?
- nutrient preparations devised to support the growth (reproduction) of microorganisms
- contains macronutrients and micronutrients (trace metals, growth factors)
- can be liquid or solid (agar)
What are the bases of classification for culture media?
- chemical composition: defined and complex
- physical nature: liquid, semisolid, solid
- function: supportive, enriched, selective, differential
What is defined media?
- exact composition is qualitative and known
- very specific
- usually used for one particular organism
What is complex media?
- contain ingredients of unknown composition and/or concentration
- one complex ingredient automatically makes the media complex
- complex ingredient examples: peptones, extracts, agar
What is general purpose media?
- supports the growth of many microorganisms
- typically a complex media
- ex: tryptic soy agar (TSA)
What is enriched media?
- general purpose media supplemented with highly nutritious substances such as blood
- also typically complex media
- ex: chocolate agar (boiled blood agar)
What is minimal media?
- contains the minimal necessities for growth of the wild type
- only contains inorganic salts, a simple carbon source, and water
What is selective media?
- favors the growth of some microorganisms and inhibits the growth of others
- ex: EMB agar selects for gram-negative bacteria
What is differential media?
- allows you to distinguish between groups of microorganisms based on their biological characteristics
- ex: blood agar (hemolytic v. non hemolytic bacteria)
- ex: MacConkey agar (lactose fermenters v. non-fermenters)
What are alpha hemolytic bacteria?
- bacteria that partially break down blood
- leave a yellow/green ring around them in blood agar
What are beta hemolytic bacteria?
- bacteria that fully break down blood
- leave a clear ring around them in blood agar
What are gamma hemolytic bacteria?
- bacteria that do not break down blood
- no ring around them in blood agar
What is a pure culture?
- a population of cells arising from a single cell
- allows for the study of a single type of microorganism
How is a pure culture obtained?
- a mixture of cells is applied to an agar surface so that individual cells are well separated from each other
- streak plate, pour plate, and spread plate techniques
How does streak plating work?
- Loop is sterilized and a loopful of inoculum is removed from the tube
- Initial streak is worked in well in one corner of the agar plate
- Subsequent streaks are at angles to the first streak
- After incubation, you should obtain isolated colonies at the end of your streak
How does spread plating work?
- small volume of diluted culture is transferred to an agar surface
- culture is spread evenly over surface with a sterile bent glass rod (hockey stick)
How does pour plating work?
- diluted samples are mixed with liquid agar
- mixture of cells and agar are poured into sterile culture dishes
- some colonies may be embedded into the agar (still counted in your plate count)
What is a countable plate for either pour or spread plating?
30-300 colonies
How is cell concentration determined from a plate count?
- find the dilution plate that is countable (30-300 colonies)
- divide the number of colonies by the total dilution
- if you added 1 mL to your plate, then you are done, but if you added 0.1 mL, then you need to multiply by 10
How do microbes grow on solid surfaces?
- species form characteristic colonies (color, texture, arrangement)
- differences in growth rate from edges to center us due to oxygen, nutrients, toxic products, and cell death
- oxygen is available for cells on top
- nutrients are available for cells on bottom
- toxic products are produced from fermentation by cells who don’t have oxygen access
What are the different ways to characterize bacterial colonies?
- form
- elevation
- margin
- color
- consistency
