Lecture 2: Bacterial nutrition Flashcards
defined culture media
- know exact composition
- know every component and the amount of that component
- make with specific chemicals
–> specific sugars, amino acids, vitamins
complex culture media
- do not know exact composition
- use complex sources of nutrients
–> glucose, beef extract, yeast extract, peptones
what does transport media look like
usually a liquid broth
transport media
- used for temporary storage of a sample when its going from a patient to the lab
- want it to arrive at the lab in the exact same way it left the patient
- contain only buffers and salts
- lacks carbon, nitrogen and organic growth factors
what does enriched media look like
liquid broth or solid agar
enriched media
- contains general nutrient supplements (eg: serum or yeast extract) to provide precursor molecules
- molecules rely on media to supply essential amino acids which they cant make themselves
what does enrichment media look like
usually a liquid broth
enrichment media
- encourage the growth of one particular organism (especially when its clinically important)
- gives a competitive edge to the desired microbe, which then becomes the dominant species
- commonly used in the clinical lab
enrichment media example
–> Faecal specimen suspected of containing salmonella
- conc of pathogens usually low
- conc of intestinal normal microflora high
- tetrathionate broth holds the normal flora in the lag phase of growth while promoting log growth of the pathogen, as salmonella has tetrathionate reductase enzymes
what does selective media look like
usually solid agar
selective media
- encourages the growth of some organisms while specifically inhibiting the growth of others
selective media examples
- Mannitol salt agar
7.5% sodium chloride - inhibits all except staphylococci
–> selective for staphylococci - Hektoen agar
bile salts - inhibits gram positive and some (non enteric) gram negative bacteria
–> selective for gram negative enteric (gut) bacteria
what does differential media look like
usually a solid agar
differential media
- contains indicators that visually distinguish between organisms
differential media: mannitol salt agar example
–> Has two differential ingredients
1. mannitol
2. phenol red (pH indicator)
mannitol fermented –> acid
mannitol fermenters (S.aureus) change the agar pink –> yellow = pathogens
non mannitol fermenters (S. epidermidis) agar colour not changed
mannitol salt agar flow diagram
mannitol –(mannitol dehydrogenase)–> D-Fructose –(fructokinase)–> D-fructose-6-phosphate –(glycolysis)–> pyruvic acid –(fermentation) –> organic acid
differential media : hektoen agar example
–> Has 7 different differential elements
1. lactose
2. sucrose
3. salicin
4. acid fuchsin
5. bromothymol blue
carbs fermeneted = acid (yellow/pink)
fermenters colony colour = yellow/pink
non-fermenters (shigella and salmonella) = blue/green
- sodium thiosulphate
- ferric ammonium citrate (source of ion)
–> H2S production gives black colonies or black centres
–> In this way salmonella (blue/green with black centres) and shigella (blue/green) can be differentiated from other members of the enterobacteriaceae family (yellow/pink)
what specialized media does blood agar fall under
enriched and differential
how is blood agar differential media
- bacterial enzymes (haemolysins) act on animal cytoplasmic membrane
- observe by streaking organism on blood agar
- during bacterial growth, haemolysin released and lyses the surrounding red blood cells, creating a zone haemolysis
different haemolysis
a- haemolysis = partial lysis of RBC –> green discolouration
B- haemolysis = complete lysis of RBC –> completely clear zone (indicates a pathogen)
Y-haemolysis = non haemolytic (not very common)
batch culture
the bacteria grows and then dies due to lack nutrients and buildup of toxins
continuous culture
keep microbes growing instead of them growing and dying like in batch culture
chemostat
a bioreactor that continuously supplies nutrients to microorganisms while removing waste
primary metabolites
formed during exponential phase, essential for growth of the microbe
secondary metabolites
formed during the secondary phase, not essential for growth
