Medios de cultivo selectivos y diferenciales

Summary

This video explains various selective and differential microbiological culture media used in laboratories to isolate and identify specific microorganisms. It covers media like the Methylene Blue Agar, MacConkey Agar, and Cromo Cool Agar, which differentiate and inhibit the growth of specific bacteria based on properties like lactose fermentation or hemolysis. The script also describes specialized media for detecting Staphylococcus, Bacillus, Pseudomonas, and other species, emphasizing their distinctive color changes and reactions to specific substrates. Overall, it provides an in-depth look at how these media facilitate microbiological analysis.

Takeaways

• 😀 Selective culture media are used to grow specific microorganisms by inhibiting others, while differential media help distinguish between different species in a mixture.
• 😀 The Methyl Blue Agar (MV) is selective and differential, used for growing Gram-negative bacteria from the Enterobacteriaceae family, inhibiting Gram-positive bacteria and differentiating lactose and/or sucrose fermenters.
• 😀 The colonies of microorganisms that ferment lactose or sucrose on MV agar appear dark in the center with blue or pink edges, some with a metallic sheen.
• 😀 The MacConkey Agar (MAC) is another selective and differential medium used to isolate Gram-negative bacilli, inhibiting the growth of Gram-positive bacteria.
• 😀 In MacConkey Agar, lactose fermenters produce red or pink colonies, while non-fermenters form colorless colonies, with the medium turning yellow due to pH increase.
• 😀 CromoCool agar allows for the simultaneous detection of total coliforms, using two substrates that turn salmon or blue based on the presence of specific enzymes like beta-galactosidase.
• 😀 Blood agar is used to identify the hemolytic capabilities of microorganisms, with clear zones indicating beta-hemolysis, green zones for alpha-hemolysis, and no color change for gamma-hemolysis.
• 😀 Starch agar helps identify microorganisms that produce amylase enzymes, which hydrolyze starch into monosaccharides, leaving clear zones around the inoculation site.
• 😀 Baird-Parker Agar is selective for Staphylococcus species, with potassium tellurite and lithium chloride inhibiting other microorganisms, while coagulase-positive staphylococci produce gray-black colonies.
• 😀 Egg yolk agar detects lecithinase and lipase production, forming a precipitate around colonies as a result of insoluble calcium and magnesium salts from fatty acids.
• 😀 The Mannitol Salt Agar (MSA) is selective for Bacillus species, where mannitol fermentation turns the medium from red to yellow, while non-fermenters keep it pink or red.
• 😀 Cetrimide agar is selective for Pseudomonas aeruginosa, enhancing pigment production and fluorescence under UV light, allowing for the identification of this bacterium.

Q & A

What is the main purpose of selective media in microbiology?

Selective media are used to isolate specific microorganisms from a mixture by containing substances that inhibit the growth of unwanted species while promoting the growth of the species of interest.

How do differential media work in microbiology?

Differential media are used to differentiate between microorganisms in a mixture based on distinct characteristics, such as the ability to ferment certain sugars or produce specific enzymes.

What is the function of methylene blue in the MacConkey Agar (MV)?

Methylene blue in MV agar serves to inhibit the growth of gram-positive bacteria while allowing the growth of gram-negative bacteria from the Enterobacteriaceae family.

How can you differentiate lactose fermenters from non-fermenters on MacConkey Agar?

Lactose fermenters produce colonies that appear dark in the center with a bluish or pinkish edge and sometimes metallic sheen, while non-fermenters produce colorless colonies on the medium.

What is the primary function of bile salts and crystal violet in the MacConkey Agar?

Bile salts and crystal violet inhibit the growth of gram-positive bacteria, thereby selectively promoting the growth of gram-negative bacteria in the agar.

What does the color change in the Mannitol Salt Agar indicate?

The color change in Mannitol Salt Agar indicates whether the microorganism can ferment mannitol, which causes the pH indicator to turn from red to yellow in acidic conditions.

What does the appearance of clear zones around colonies on an agar plate suggest?

Clear zones around colonies indicate the hydrolysis of specific compounds, such as starch or blood cells, often due to the production of enzymes like amylase or hemolysins.

How does the Cetrimide Agar select for Pseudomonas aeruginosa?

Cetrimide Agar is selective for Pseudomonas aeruginosa because it contains cetrimide, which inhibits the growth of other microorganisms, allowing the growth of Pseudomonas species.

What is the significance of the fluorescence observed under UV light in Pseudomonas aeruginosa colonies?

Pseudomonas aeruginosa colonies fluoresce under UV light due to the production of the characteristic pigment pyoverdine, which is a marker for this microorganism.

What is the role of the egg yolk in the Egg Yolk Agar?

The egg yolk in the Egg Yolk Agar helps detect the production of lecithinase and lipase enzymes by bacteria, which break down lipids and result in a clear zone around the colonies.