Session 4: Bacterial Culture

what is a selective medium?

a medium that encourages the growth of some microorganisms and simultaneously suppresses the growth of other microorganisms

(nonselective media are free of inhibitors and will usually support the growth of most microorganisms)

what is a differential medium?

growth medium with some constituents that causes an observable change (ex. change in color or pH) in the medium when a particular chemical reaction occurs

(allows the possibility of distinguishing between different bacterial organisms or groups of organisms)

what is an enriched medium?

a growth medium that contains special nutrients or ingredients that allow for the growth of as many organisms as possible

what kind of medium is Sheep Blood Agar (SBA)?

(most common) nonselective, enriched medium

used as a primary isolation medium for clinical specimens

hemolysis can be seen in Streptococcus species

how can Sheep Blood Agar (SBA) be made selective media?

through the addition of:

one or more antibiotics

one or more specific chemicals

what is alpha hemolysis in Sheep Blood Agar (SBA)?

biochemical change in heme of RBCs

what is beta hemolysis in Sheep Blood Agar (SBA)?

partial to complete lysis of RBCs

what is gamma hemolysis in Sheep Blood Agar (SBA)?

no lysis of RBCs

what are other bacteria called for hemolytic reactions in Sheep Blood Agar (SBA)?

(other than Streptococcus species)

bacteria from all other genera are referred to as either “hemolytic” or “non-hemolytic”

(beta-type or alpha-type can be used)

what kind of medium is MacConkey agar?

selective medium that is most commonly used to inhibit gram (+) microorganisms

differential medium that distinguishes between lactose fermenters and non-fermenters

what are the selective ingredients for MacConkey agar?

bile salts and crystal violet

what are the differential ingredients for MacConkey agar?

lactose → sole carbohydrate

what does MacConkey agar test for?

lactose fermentation producing mixed acid end-products

what generates the MacConkey agar test results?

pH indicator = neutral red indicator (precipitates as shades of red when the pH is lower than 6.8) (more acidic conditions)

what happens when there is strong fermentation in MacConkey agar?

red colonies surrounded by zone of precipitate

what happens when there is no fermentation in MacConkey agar?

slightly pink colonies after 24-48 hours

what happens when there is slow/weak fermentation in MacConkey agar?

colorless colonies

what kind of medium is Mueller-Hinton agar (MHA)?

medium that allows growth of many different organisms

general nutrient medium for non-fastidious microorganisms

standard medium for disk-diffusion antibiotic susceptibility testing

what kind of organisms cannot grow in Mueller-Hinton agar (MHA)?

organisms that are considered fastidious

what was Mueller-Hinton agar (MHA) originally designed for?

for the isolation of pathogenic Neisseria sp.

(starch in medium is able to absorb toxic components that inhibit growth of the organism)

what kind of medium is Bile Esculin agar?

selective medium → inhibits microorganisms that are unable to grow in 40% bile

differential medium → distinguishes microorganisms that are able to hydrolyze esculin in the presence of 40% bile

what are the exceptions for the selective part of Bile esculin agar?

Enterococci

Group D streptococci

Listeria

what is the reaction occuring inside Bile Esculin agar?

esculin is hydrolyzed by esculinase

esculin reacts with ferrous nitrate for color change

what is the positive test result for esculin hydrolysis in Bile Esculin agar?

dark brown to black color surrounding the growth of the microorganism

what is the negative test result for esculin hydrolysis in Bile Esculin agar?

no change in color/light brown surrounding the growth of the microorganism

what is the goal of streak plating?

to produce isolated colonies of bacteria on an agar medium

(designed to dilute the sample; isolated colonies consist of many bacterial cells)