Media Flashcards
In what way is MacConkey’s Agar selective?
MacConkey’s Agar inhibits the growth of Gram +ve bacteria.
MacConkey’s Agar is selective.
These two chemical agents contribute to its selectivity.
crystal violet and bile salts
inhibit the growth of Gram +ve bacteria.
In what way is MacConkey’s Agar differential.
MacConkey’s Agar contains lactose and a ph color change indicator. Bacteria able to ferment lactose create an acidic byproduct resulting in a local ph change, and change in color of the colony under growth. Lactose fermenters appear pink on Mac’.
CNA agar is selective for
Gram +ve bacteria
CNA agar is differential for what?
Hemolysis (Beta, Alpha, Gamma)
What mechanisms drive the selectivity of CNA agar?
colistin and nalidixic acid
disrupt the cell membrane of gram-negative organisms.
nalidixic acid
blocks DNA replication in susceptible gram-negative bacteria.
Is blood agar selective or differential?
Blood agar is differential for hemolysis.
PEA agar contains
phenylethyl alcohol (agar)
TMP / SMX contains
TriMethoPrim SulfaMethoXazole
Todd-Hewitt broth contains antimicrobials.
What are those antimicrobials.
gentamicin
nalidixic acid
Todd-Hewitt broths contain antimicrobials in order to suppress what?
Normal vaginal microbiota, while allowing the growth GBS (group B Streptococci).
Antimicrobials gentamicin & nalidixic acid are specific for the suppression of normal vaginal microbiota, while allowing the growth GBS (group B Streptococci)
Is bile esculin media selective or differential?
Bile esculin in BOTH selective and differential.
To what end is bile esculin agar (BEA) employed?
isolate and identify Enterococci and group D Streptococci (based on their ability to hydrolyze esculin)
Esculin hydrolyzes to what?
glucose and esculeTIN Esculetin then reacts with an iron salt (ferric citrate) in the medium to form a phenolic iron complex which produces a dark brown or black color. Esculin + Acid → β-D-glucose + Esculetin Esculetin + Fe3+ → Dark Brown Color
A positive bile esculin test reveals what color? Why?
Positive test is dark brown or black. Esculetin reacts with iron salt (Ferric Citrate) creating the dark color.
Todd Hewitt Broth primarily is used for the growth of what?
beta-hemolytic streptococci Streptococcus pyogenes Streptococcus pneumoniae Streptococcus agalactiae
The ability to hydrolyze hippurate is an alternative test for the presumptive identification of _________ .
S. agalactiae
Streptococci other than S. agalactiae may also be hippurate hydrolysis positive, particularly ___________ .
viridans group streptococci.
All of the viridans group streptococci are leucine aminopeptidase positive and __________ .
pyrrolidonyl arylamidase negative.
A positive bile esculin test is generally indicative of what?
Species belonging to the S. bovis group or enterococci
This enzyme, associated with Gram + bacteria, neutralizes the bactericidal effects of hydrogen peroxide.
catalase
Catalase is a common enzyme which catalyzes the decomposition of _____________ to ________ and _________.
hydrogen peroxide
to
water and oxygen gas
2H2O2 → 2H2O + O2
The enzyme catalase mediates the breakdown of hydrogen peroxide into what?
oxygen and water
The morphologically similar Enterococcus or Streptococcus and Staphylococcus can be differentiated using what test?
catalase test
Enterococcus or Streptococcus (catalase -ve)
Staphylococcus (catalase +ve)
Lactose fermenting strains grow as what color on MacConkey Agar?
red or pink
Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent
Urease (as a virulence factor) acts to break down what?
Urease dissociates
Urea into carbon dioxide and amonia
Urea ⇒ Carbon dioxide and amonia
CH4N20 ⇒ CO2 + NH3
