Question 1

Why are less than 1% of bacterial species considered culturable?

Accepted Answer

Most bacterial species are unculturable because scientists do not yet understand their specific metabolic and environmental requirements needed for laboratory growth. Bacteria possess vast metabolic diversity and inhabit extreme environments with conditions difficult to replicate in the lab. Understanding an organism's precise needs for energy generation, temperature, pH, oxygen levels, and other factors is essential for successful cultivation.

Question 2

What is the difference between selective and differential media?

Accepted Answer

Selective media prevent certain bacterial species from growing while promoting others through inhibitory agents like high salt concentration. Differential media contain indicators that reveal specific biochemical features of organisms, such as fermentation ability. Together, these media types allow researchers to identify and isolate particular bacterial species from mixed samples by combining growth inhibition with metabolic identification.

Question 3

How does Mannitol Salt Agar select for and identify Staphylococcus aureus?

Accepted Answer

Mannitol Salt Agar selects for gram-positive organisms tolerant to 6.5% sodium chloride, inhibiting gram-negative bacteria. The differential component, mannitol, is fermented only by S. aureus among clinically significant Staphylococcus species. Fermentation produces acid byproducts that turn the methyl red indicator bright yellow, distinguishing S. aureus from other Staphylococcus species like S. epidermidis.

Question 4

Why do gram-negative bacteria grow on Eosin Methylene Blue medium?

Accepted Answer

Eosin Methylene Blue contains dyes toxic to gram-positive organisms, but the outer membrane of gram-negative bacteria prevents these dyes from entering cells, allowing growth. Lactose-fermenting gram-negative bacteria like E. coli produce acids that lower pH, promoting dye absorption and creating dark, metallic colonies. Non-fermenting species like Proteus vulgaris appear light pinkish-purple.

Question 5

What are fastidious organisms and what special conditions do they require?

Accepted Answer

Fastidious organisms are culturable bacteria requiring very specific growth conditions to proliferate. Examples include Neisseria and Haemophilus species, which need partially broken-down red blood cells, specific growth factors, and high carbon dioxide environments. Without all required components, these organisms will not grow or grow poorly, making them challenging to culture compared to generalist bacteria.

Question 6

How does anaerobic culturing differ from aerobic culturing for bacteria?

Accepted Answer

Anaerobic culturing removes oxygen using sealed gas chamber systems or water-generated hydrogen and carbon dioxide with palladium catalysts. Unlike eukaryotic cells requiring oxygen, prokaryotes can grow anaerobically using fermentation pathways for energy generation. Test organisms on Tryptic Soy Agar may grow poorly under anaerobic conditions if they are not obligate anaerobes.

Question 7

What is enrichment and how does it help isolate specific bacterial species?

Accepted Answer

Enrichment is the process of changing laboratory conditions to select for specific species from mixed cultures. By manipulating media composition and environmental factors like temperature, oxygen levels, or salt concentration, researchers can favor organisms tolerant to those conditions while inhibiting others. This technique combined with pure cultures and streak plating isolation of single bacterial colonies enables identification of target species from complex samples.

Enrichment Cultures: Culturing Aerobic and Anaerobic Microbes on Selective and Differential Medias - Assessment Quiz