Lab Exam 2: Ex. 4-1; 4-2 Flashcards
What is the most common means of isolating an organism from a mixed culture?
streak for isolation
selective media
media designed to enhance the isolation procedure by inhibiting growth of some organisms while encouraging the growth of others.
differential media
selective media that contains indicators to expose differences between organisms
Give an example of a situation in which selective media would be used.
Clinical microbiologists, who are familiar with human pathogens and the types of infections they cause choose selective media that will screen out normal flora that also are likely to be in a sample.
defined
“chemically defined” - each of its ingredients is known and in exactly what amounts.
undefined
“complex” - it contains one or more ingredient(s) of unknown composition and/or amount such as yeast extract, beef extract, digest of gelatin, etc.
What is the purpose of nutritional components?
Nutritional components are selected to obtain the optimum growth of the organisms being tested for or suspected of being in the sample.
What do inhibitors do?
make the medium selective - they are designed to exploit weaknesses in specific groups of organisms and thus prevent or inhibit their growth, while allowing other organisms to grow.
In what ways do inhibitors function?
- interrupting DNA synthesis or expression of a gene.
- at the enzymatic level, interfering with a critical reaction.
- interfere with membrane permeability, thus upsetting homeostasis and starting a cascade of catastrophic changes inside the cell.
What can be added to inhibit organisms that can’t tolerate high osmotic pressures?
sodium chloride
What component almost always makes the medium differential?
substrate
What does differentiation and identification of organisms frequently rely on?
their differing abilities to perform a specific chemical reaction or set of reactions and to do them in a way that can be observed.
indicators
make a desired or expected reaction visible
What is Phenylethyl Alcohol Agar?
an undefined, selective medium that inhibits the growth of many Gram-negative and some Gram-positive organisms (primarily selects for Gram-positive microbes). It is not a differential medium because no distinction is made between organisms that successfully grow on it.
What does PEA stand for?
Phenylethyl Alchohol Agar
What is Phenylethyl Alcohol Agar (PEA) used for?
it is used to isolate staphylococci and streptococci (including enterococci and lactococci) from specimens containing mixtures of bacterial flora.
What contaminants does PEA typically screen out?
Escherichia coli and Proteus species
What microbes did we use in our PEA lab and what was the result?
Escherichia coli and Staphlyococcus epidermidis
- There was growth of both microbes but considerably more growth of the Se which is Gram-positive on the PEA plate (all growth on the nutrient agar plate)
What is Columbia CNA with 5% sheeps blood agar?
an undefined, differential, and selective medium that allows growth of Gram-positive organisms (especially staphylococci, streptococci, and enterococci) and stops or inhibits growth of most Gram-negative organisms.
What is the selective agent of PEA?
Phenylethyl alcohol - the PEA itself makes it selective
What does Columbia CNA select for?
Gram-positive microbes
What does PEA Select for?
Gram-positive microbes
What are the selective agents for Columbia CNA?
colistin and nalidixic acid
What are the differential agents for Columbia CNA?
sheeps blood
What is Columbia CNA differentiating?
whether or not a microbe is hemolytic or not
What microbes did we use in our CNA lab and what was the results?
Enterococcus faecalis - medium growth on CNA plate, Escherichia coli - no growth on CNA plate, Staphylococcus epidermidis - heavy growth on CNA plate
(all growth on regular nutrient agar plate)
On the CNA plate how can you distinguish a hemolytic microbe from a non-hemolytic microbe?
The non hemolytic microbe will grow normal; the hemolytic microbe will have space around it in the red agar.
