3.4 microbiology Flashcards
three shapes of bacteria
bacillus (rod)
coccus (sphere)
spirillus (sprial)
other way bacteria can be classified
by the structure of its cell wall
how can you distinguish between gram -ve and gram +ve bacteria
gram stain
what colour will gram +ve bacteria turn
purple
what colour will gram -ve bacteria stain
red
why do the bacteria stain different colours
due to the different chemical composition and structure of their cell wall
structure of gram +ve bacteria
thick outer wall made of peptidoglycan (murein) with no outer lipopolysaccharide.
how does staining work on gram +ve bacteria
stain can bind due to the absence of lipopolysaccharide (also why gram +ve are more susceptible to penicillin and lysozyme
how does the structure of gram +ve bacteria allow for purple staining
allows the crystal violet/iodine complex to be retained within the cell-staining the cells purple
structure of gram -ve bacteria
thinner peptidoglycan cell wall and an outer lipopolysaccharide membrane
what does lipopolysaccharide do to gram-ve bacteria
protects the peptidoglycan so they are not affected by lysozymes or penicillin. a different class of antibiotics must be used instead
explain the staining of gram negative cells
gram negative cell walls lose the outer lipopolysaccharide membrane and the thin peptidoglycan walls allow the purple stain complex to be washed away. gram negative cells are not stained by the gram stain (remain colourless) but stain red after counter-staining with safranin
what are the stage of staining
fixation
crystal violet
iodine treatment
decolorisation
counter stain with safranin
explain crystal violet
crystal violet is the basic dye that binds to peptidoglycan
peptidoglycan
=meurin
explain iodine treatment
iodine makes the crystal violet bind more strongly to the peptidoglycan
explain decolorisation
acetone- alcohol decolourises the gram -ve bacteria as it removes unbound crystal violet and the lipopolysaccharide layet
explain counter-stain with safranin
safranin is used as a counter-stain, it stains the gram -ve bacteria red
what do bacteria have different of
oxygen requirements
three different types of bacteria (oxygen requirements)
obligate anaerobe
obligate aerobe
facultative anaerobe
explain growth of an obligate aerobe
growth is inhibited in the absence of oxygen
explain growth of an obligate anaerobe
growth is inhibited in the presence of oxygen
explain growth of a facultative anaerobe
grows best in the presence of oxygen. but can respire anaerobically if they need to
what would a tube where bacteria was clustered at the top of the tube show
they need oxygen for respiration and growth. their growth is inhibited in the absence of oxygen, they must be obligate aerobes
what would a tube where bacteria was clustered at the bottom of the tube show
bacteria found away from the source of oxygen, they must be obligate anaerobe
what would a tube where bacteria was spread throughout the tube show
spread throughout but mainly found towards the top. these bacteria can grow in the absence of oxygen, but grow best with oxygen, they are facultative anaerobes
what conditions are needed for micro-organisms growth (culturing conditions)
nutrient (glucose and nitrogen)
vitamins and mineral salts
temperature
pH
oxygen
why is glucose needed for culturing
where would you use it
needed for respiration
can be in the broth (liquid medium) or agar (solid)
why is nitrogren needed for culturing
where would you use it
nucleic acid, amino acid synthesis
in organic molecules and in organic forms (nitrate ions)
why are vitamins and mineral salts needed for culturing
what would you use
growth factors
vitamins e.g. biotin
mineral salts e,g, Na+
why is optimum temperature needed for culturing
optimum?
25/45 degrees
enzymes regulate bacterial metabolism
optimum is 25/45 with mammalian pathogens at 37
why is optimum pH needed for culturing
most bacteria favour slightly alkaline conditions (pH)
fungi grow better in slightly acidic conditions
why is oxygen temperature needed for culturing
needed for respiration
what are aseptic techniques used for
to prevent;
-contamination of the environment by the microorganisms being handled
-contamination of the bacterial cultures by unwanted microorganisms from the environment
examples of sterilisation
- passing metal transfer tools (inoculating loop) through a roaring/blue bunsen flame until they glow red
-using pre-sterilised petri dishes
-sterilising any glassware used under high pressure and high temperature
how to pour a sterile agar plate using aseptic techniques
-open the culture bottle cap using your little finger and do not place the cap or bottle on the bench
-flame the neck of the bottle in a blue bunsen flame
-work close to the bunsen flame as the updraft helps prevent contamination
-open the sterile petri dish lid at an angle
-pour in the molten agar and close lid immediately. swirl gently to remove air bubbles
-secure lid with tape
how can you count bacteria grown in a liquid culture
can be counted directly (by counting each cell) or indirectly (by measuring tubidity)
what is the turbidity
cloudiness of the culture medium
how do you measure the turbidity
using a colorimeter
what are the 2 types of direct counts
total counts
viable counts
what does the total count refer to
include both living and dead cells
what does the viable count refer to
only count living or actively growing cells and therefore underestimate population cells
viable count method
counts cells which are unable to grow into visible colonies on an agar plate. serial dilution is used if the population density of the sample is too high to count,
what is a colony
cluster of cells arising from a single bacterium or fungal spore
what is a pathogen
a microogranism that causes disease in its host
what is aseptic techniques
laboratory practice thatmaintains sterility of apparatus and prevents contamination
what can we work out from counting the colonies
work out the original number of bacteria in our sample
