lab 6 Flashcards
skin is rather inhospitable for microorganisms to live
bc it is dry and has a low ph between 4 and 6
certain microorganisms flourish in the moister regions
armpits , genitals, face and scalp
underarm odour is cause by bacteria living in your armpits, breaking down substances release in your sweat and releasing foul smelling waste products
normal biota of your skin is composed of two groups
transient
resident
transient microbes
you skin picks up from the environment , they are unable to establish a permanent residence , they may remain on your skin for a few hours , day or months
resident microbes
they thrive on your skin and can be considered your normal microbiota
most are gram positive which are better adapted to the dry conditions
these organisms also convert the oils on your skin to low moleculer weight unsaturated fatty acids, which have antimicrobial activity and prevent growth of gram negative bacteria and fungi
when you knock out the pop of gram positive bacteria, such as when you take certain antibiotics , gram negatives and yeasts can flourish
yeast infections occur when the
normal microbiota is inhibited raising the pH to nearly neutral and allowing the yeasts to take over
staphylococcus is the main genus of gram positive bacteria found on the skin
staphylococcus epidermis and staphylococcus aureus
staphylococcus epidermis
non-pigmented,( white colonies), non pathogenic bacterium that accounts for a large proportion of the bacteria on your skin
staphylococcus aureus
less common
usually forms yellow colonies and is often found associated with pimples and boils as well as being the cause of ton of the most common types of food poisoning
responsible for causing many nosocomial infections
s. au
an infection caused while in the hospital that was not present when the patient was admitted
strains of s. au that are resistant to every known antibiotic are booming more common and are known as superbugs
macConkey agar
used to differentiate lactose fermenters which turn bright red to pinkish purple, from those unable to use lactose
strong lactose fermenter is gram negative
to support bacterial growth, a medium must contain
-carbon source (unless the bacterium is and autotroph)
-a nitrogen source (unless its a nitrogen-fixer)
-and inorganic salts
medium may or may not require the addition of various growth factors
if the medium is to be used for plates then
agar must be added
if the medium will be used to select for a particular group of organisms then
a selective agent should be added
for differentiating between groups of organisms a
pH indicator is normally used
defined or synthetic medium
all of the elements are in a known quantity
complex medium
the exact components of the medium are not known
this occurs when te complex nutrient sources is added to the medium
yeast extract, peptone and trypton are all examples of complex nutrient sources ie you dont know the exact proportions of chemical elements in these ingredients
enriched medium
these are complex media to which an extra nutrient source such as blood or serum is added to encourage the growth of fastidious heterotrophs
selective medium
supports the growth of the wanted organisms and inhibits the growth of all others
a medium can be made selective by adding an inhibitory agent such as crystal violet (which inhibits gram positive organisms) or by removing a component necessary for growth
Nitrogen-free media selects for those bacteria that can fix atmospheric nitrogen. antibiotics are often used to make a medium selective
differential medium
allows many organisms to grow, but lets you discriminate between different bacteria base on the appearance of the colony or the presence of a zone around the colony
agar
a complex sulphur containing polysaccharide extracted from a marine alga that is commonly used to solidify petri plates. although agar is organic, it is relatively difficult to break down and cannot used as a nutrient sources by most terrestrial bacteria when added to media agar will melt around 95 and solidify at about 43
amino acids
growth factors required by all bacteria for synthesis of proteins. if a bacterium cannot synthesize a particular amino acid itself, then it must be provided in the growth medium for that organism to grow
beef extract
an extract of beef concentrated into a paste that contains a large amount of amino acids and peptides
it is also a source of carbohydrates, vitamins and minerals and trace metals
bile salts
these are used to simulate gut conditions and are effective fore selection of enteric organisms
gram positive organisms are inhibited by bile salts in macConkey agar, colonies produces by strong lactose fermenting microbes will exhibit dark red ring of precipitated bile salts around the colony
blood agar
an agar medium used to determine if an organism produces hemolysis which will break down the red blood cells in the medium. different patterns of hemolysis can be observed.
- B-hemolysis is characterized by a clear zone around the colonies and is a characteristic of staphylococcus aureus which causes numerous skin and wound infections and streptococcus pyogenes, which causes strep throat (complete lysis of blood cells-complete zone of clearing on the plate )
- alpha-hemolysis greenish coloured zones from the partial destruction of the red blood cells and are characteristic of streptococcus pneumoniae (incomplete lysis of red blood cells and results in a green zone of clearing)
- lots of bacteria are also non-hemolytic and will not produce a zone of clearing on the place .gamma-hemolysis
is blood agar selective or differential or both?
not selective - wide variety of bacteria can grow on it
blood agar is considered an enriched medium- so packed with nutrients that even bacteria that cant grow on t-soy may be able to grow on it
we call these picky eaters fastidious because the require many growth factors
blood agar is differential because you can easily see whether or not bacteria are hemolytic by their appearance on the plate
why do bacteria produce hemolysins
- bc blood serum supplies nutrients to animal cells, we would expect it to be nutrient rich and an excellent growth medium for bacteria. however there is one essential nutrient that is not biologically available in the liquid fraction of blood: iron. animals use iron binding proteins called transferrins to sequester an iron in the blood thus limiting its availability. the activity of transferrins is considered to be an important component of the immune system and a demesnes against bacteria that would otherwise cause blood borne infections
- unfortunately for animals, some enterprising bacteria have discovered that blood has another important pool of iron: the hemoglobin of red blood cells . hemolytic enzymes that destroy red blood cells release the heme, making the iron biologically available
bromcresol purple
pH indicator that is yellow below pH 5.2 and purple in the alkaline range
crystal violet
a basic stain that can also be used in a medium to inhibit the growth of gram positive organisms
glucose
a readily utilized carbon and energy source for heterotrophic bacteria
growth factors
small organic compounds that are essential for the growth of all bacteria if a bacterium cannot synthesize a particular growth factor, then is must be provided in the medium
-amino acids, purines ,pyrimidines ,and vitamins
streptococcus mutans
- responsible for producing the acid that results in the destruction of your tooth enamel
- the bacteria produces the acids as an end result of carbohydrate fermentation and also converts sucrose to dextran, a sticky polysaccharide that helps the bacteria adhere to youre teeth
hemolysins
these are enzymes used to break down red blood cells. this ability is associated with virulence like the level of pathogenicity of the organism and so pathogenic strains of staphylococcus, streptococcus and clostridium exhibit beta hemolysis complete clearing
lactose
a disaccharide that can be used as a carbon and energy source by some bacteria often added to media along with a pH indicator to differentiate between lactose fermenters and lactose non-fermenters
macConkey agar
medium usually used to distinguish bw members of the enterobacteriaceae (gram negative enteric bacteria). one of the distinguishing features of an enteric organisms is the ability to ferment lactose to acid end-products. Acid from lactose fermentation will accumulate in the medium and cause the colony growth to turn dark pink.
strong lactose fermenters produce so much acids that the bile salts in the medium precipitate around the colonies.
weak lactose fermenters will turn a light shade of pink
non-lactose fermenters will appear white or colours
gram positive does not grow on plate at all
macconkey agar is both
selective and differential
- useful in identifying potential human pathogens and used to detect fecal contamination
- lactose fermenters produce acid
s. au gram positive
e. co- strong
s. flexneri-nonlactose
e. aerogens -weak
mannitol
a carbohydrate that is a carbon source for some bacteria
mannitol salt agar
this medium favours those organisms that can tolerate high salt concentrations like staphylococcus and distinguishes those that can ferment mannitol
s. au can - yellow colonies
s. ep cannot white colonies
selective, differential, and complex
white colonies are surrounded by red (original plate colour) whereas yellow colonies are surround by yellow
staphylococcus are halotolerant- can grow in high salt concentrations
complex- peptone
differential bc it van be used to differential bw mannitol fermenters and non fermenters
bacteria that can ferment mannitol will produce acid as an end product of fermentation, acid reacts with the phenol red and changes the colour to yellow
s. ep can still grow by metabolizing other components of the medium like peptones and beef extract
methyl green
a dye that forms a complex with large fragments of DNA. if an organism can produce enzymes to degrade the DNA the dye will become unbound from the DNA and a clear zone will result around the colony
molybdenum
a micronutrient that serves as an important co-factor for enzymes involved in N fixation. often added to N free medium for the culture of N2 fixing bacteria
neutral red
pH indicator that is red below pH 6.8 and clear in the alkaline range
phenol red
a pH indicator that is yellow below pH 6.8 and pink/red in the alkaline range
peptone
hydrolyzed proteins that are formed by the enzymatic or acidic digestion of a variety of materials like soybean meal, casein (milk protein) animal tissues, gelatin, provides a source of carbon and energy for heterotrophic bacteria and supplies some growth factors such as amino acids (soytone, tryptone)
purines and pyrimidines
growth factors required by all bacteria fro synthesis of nucleotides and nucleic acids. if an organisms cannot synthesize a particular purine or pyrimidine, it must be provided to it in the medium
sheep’s bloods
added to blood agar to provide essential growth factors and nutrients for many bacteria but is particularly useful for culturing fastidious organisms.
fastidious organisms are those microbes that are difficult to culture bc they have a very specific nutritional requirements. also helps to differentiate bacteria base on their patterns of hemolysis
sodium chloride
provides a saline environment for osmoregulation at low concentrations (7.5%) of salt that will kill many bacteria
soytone
an enzymatic digest of soybean meal that provides a source of amino acids peptides, vitamins and other growth factors
sucrose
a disaccharide that serves as a source of carbon for some heterotrophs
tomato juice
a source of addition vitamins to support the growth of lactobacillus species and other fastidious lactic acid bacteria. the natural acidity of tomato juice gives the medium a low pH that can encourage the growth of mildly acidophilic bacteria, but may discourage the growth of some neutrophiles
t-soy
tryptic soy agar used for cultivating a wide range of bacteria including some fastidious organisms 15g/L tryptone 5g/L soytone 5g/L NaCl 15g/L agar
tryptone
an enzymatic digest of casein (milk protein) that provides a source of amino acids, peptides, vitamins and other growth factors
vitamin
a growth factor that serves as a precursor for the synthesis of coenzymes, required by all bacteria. if a bacterium cannot synthesize a particular vitamin then is must be provided in the growth medium
yeast extract
an extract of lysed yeast cells which supply a complex mixture of amino acids, vitamins, and other growth factors
