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1

Know how To isolate the individual pathogenic organism from a mixed culture and identify the individual bacteria by applying basic microbiological techniques such as Gram stain, biochemical properties and serological testing.

Gram stain: is it gram + or -?Serological test: study of body fluids. Check for antibodies or organisms causing infectionUse differential growth mediums to select out species based on biochemical characteristics.

2

What is a selective medium?

A selective medium is one that permits the growth of the organisms one seeks to recover but suppresses the growth of other undesired organisms. MacConkey agar is a selective medium. It contains bile salts and certain dyes (e.g., crystal violet). The Gram-negative rods are not inhibited by these ingredients and will flourish. Other organisms, particularly the Gram-positive ones, are inhibited.

3

What is a differential medium?

A differential medium is one that enlists a particular bacterial biochemical property to allow for the visible differentiation of one organism or species from another. To differentiate between the Gram-negative species in your mixed culture we make use of the property of lactose fermentation. In a medium containing lactose and a pH indicator, fermenters can be distinguished from the non-fermenters by the color of the colonies. Both MacConkey and Hektoen Enteric Agars are differential media. The lactose fermenting enteric Gram-negative rods appear as orange colonies on Hektoen Enteric Agar and brick red on MacConkey Agar while the lactose-non-fermenting colonies appear as green on Hektoen Enteric Agar and colorless on MacConkey Agar.

4

Tell me about selective and differential media.

Bacteriologists have simplified the process by devising media that combine the properties of both selective and differential media. MacConkey and Hektoen Enteric Agar are both selective and differential media. In the clinical laboratory these media are particularly useful in the isolation of Gram-negative enteric pathogens from stool and throat specimens where Gram-positive organisms would mask their presence.

5

What is an enrichment medium?

An enrichment medium is one that provides a selective advantage for the growth of one organism over other organisms in a polymicrobic specimen. This can be accomplished by growing the mixture in the presence of a chemical that is relatively less toxic for the species to be enriched.

6

Know how To identify a pathogenic organism from a simulated urine sample, to illustrate how many bacteria must be present in samples to detect them by microscopy.

enough to gram staina cloudy culture is an infected culture10^5 bacteria needed to see it in a microscope

7

Understand differential and selective media including identifying which media is differential or selective and how these media are used in microbiology. What are the different types?

Blood agar: Sheep blood cells + nutrient agar, good for determining hemolytic properties (a lot= clearing of cells, a little= green, none=no change around colony)Hektoen Enteric Agar: Isolates salmonella and shigella, inhibits gram + and other organisms of the gut and colon. Lactose fermenting = orange, non lactose fermenting= greenMac Conkey agar: selective and differential for gram - rods. lactose ferment= red, non fermenting= colorlessNutrient media: obtain maximum cuture sensitivity when low # organisms in specimenPIA media: selects for pseudomonas (blue green = pseudomonas aeruginosa)

8

Understand that antimicrobial resistance can be transferred between bacteria and that selective conditions can enrich for these antimicrobial resistant bacteria in a mixed population.

Antibacterial resistance genes can be transferred via plasmids. Certain drugs can induce antibacterial resistance in certain organisms (macrolide-inducable clindomycin resistance). You can plate suspected resistant bacteria on plates containing antibiotic to test if resistance has been acquired via plasmid.

9

3

Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.