microbiology Flashcards
what are bacteria cell walls made up of
bacteria walls are made of a polymer of proteins and sugars known as peptidoglycan or murein
what is gram staining
a technique used to differentiate between the classes of gram-negative or gram-positive bacteria
what is a 1) spherical bacteria 2) rod shaped bacteria 3) spiral shaped bacteria called?
1) coccus
2) bacillus
3) spirillum
outline the process of gram staining
1) sterilize the inoculating loop using a roaring flame of a nearby bunsen burner
2) pick up a loop of bacteria and place it on a slide
3) heat fixate the bacteria to the slide by passing over a flame
4) flood with crystal violet
3) wash with alcohol
4) flood with iodine
5) wash with alcohol
6) flood with counterstain pink safranin
what is the difference between gram positive and gram negative bacteria
positive
- stains purple
- thick layer of peptidoglycan
- no lipopolysaccharide layer
negative
- stains red/pink
- thin layer of peptidoglycan
- protective layer of lipopolysaccharide
why do gram negative bacteria stain pink
the cell wall doesn’t retain the crystal violet due to the presence of a lipopolysaccharide layer
this layer is lost when washed with alcohol
the peptidoglycan layer retains the pink safranin counterstain
why doe gram positive bacteria stain purple
the peptidoglycan layer retains the crystal violet stain
what is an obligate aerobe
a form of bacteria which can only metabolise in the presence of oxygen, to can not survive in anaerobic conditions
what is an obligate anaerobe
a form of bacteria which can only survive in the absence of oxgen
what is a facultative anaerobe
a form of bacteria that usually respires aerobically but can switch to anaerobic respiration if there is a lack of oxygen
what are aseptic techniques
techniques used in the culturing of microorganisms under sterile conditios to prevent contamination
list some basic aseptic techniques
1) sterilize bench with antibacterial cleaner
2) always flame the bottleneck or inoculating loop under a roaring flame
3) always open lids at an angle
4) sterilize equipment before use in an autoclave
5) always have a bunsen burner nearby to create convection currents
outline how to culture microorganisms
1) sterilize inoculating loop under a roaring flame and let cool
2) open Petri dish lid at an angle and take a sample
3) transfer bacteria to an agar plate using a sterile inoculating loop
4) placed two pieces of tape securing the lid
5) invert the agar plate and incubate
6) ensure incubation doesn’t exceed 25 degrees and makes sure the lid is not airtight to avoid culturing pathogenic bacteria
what are the conditions necessary for culturing bacteria
1) a carbon source
2) a nitrogen source
3) growth factors ie vitamins or salts
why is temperature important in culturing bacteria
bacterial metabolism is regulated by enzymes, the range of 25-45v degrees is optimum for most bacteria
why is PH important to consider when culturing bacteria
most bacterial metabolism is regulated by enzymes, optimum PH is usually slightly alkaline
what is the difference between a spread and streak plate
spread- microorganisms are spread evenly using a sterile spreader
streak- obtain single colonies by rotating the plate to build layers of culture on at least three separate streaks
what is the difference between total count and viable count of bacterial colonies
total count= includes the total of all living and all dead bacteria present in a given volume
viable count = the total number of living bacteria in a given volume
how can bacteria be measured indirectly
by measuring the turbidity of the culture
how is a viable count conducted
add a known volume of microorganisms to the plate
incubate
count number of colonies
what is assumed when using a viable count
that only one cell has given rise toa single colony
what is an issue with the one cell one colony assumption
- doesn’t account for the clumping of cells in the original inoculum resulting in a lower estimate of numbers
what is a serial dilution
a sequence of dilutions where the dilution factor is constant
used to dilute a stock solution
what is a haemocytometer
a more accurate way of calculating colonies however is not able to distinguish between living and dead cells so produce a total count
