Question

Sabouraud dextrose agar CC is selective for: a. all fungi b. non-saprophytic fungi c. bacteria d. viruses

Short answer

Sabouraud dextrose agar (SDA) CC is selective for all fungi.

Step-by-step solution

Understanding the Medium

Sabouraud dextrose agar (SDA) is a type of growth medium that contains peptones. It is often used for the cultivation of fungi and has a low pH that inhibits bacterial growth. It is not a selective medium for bacteria, viruses, or non-saprophytic fungi.

Identifying the Selective Property

The agar's low pH is selective, mostly promoting the growth of fungi due to their greater acid tolerance compared to many bacteria.

Choosing the Correct Option

Since SDA is designed to encourage fungal growth while inhibiting bacterial growth, and it is not specially formulated to select for non-saprophytic fungi or viruses, the best answer is that it is selective for 'all fungi.'

Key concepts

Microbial Growth Media

Understanding the right environment for microbial growth is essential for proper identification and study of microorganisms. Microbial growth media are specially formulated preparations that support the cultivation of microbes in a controlled setting.

These media can be found in various forms, such as liquid broths or solid gels, like agar. Sabouraud dextrose agar (SDA), for instance, is one such specialized solid medium. It's enriched with dextrose as a carbohydrate source and peptones to provide protein. Its low pH, usually around 5.6, not only favors the growth of fungi but actively discourages the survival of many bacteria, which generally prefer a neutral to slightly alkaline pH.

In laboratories, different growth media are critical for isolating and identifying microorganisms, conducting sensitivity tests, and for various educational purposes.

Cultivation of Fungi

Fungi are an incredibly diverse group of organisms, and cultivating them requires an understanding of their unique needs. SDA, with its acidic environment, is tailored to support the cultivation of fungi.

Fungi generally have a higher tolerance for acidic conditions than bacteria, which is why SDA's low pH helps select for them. This quality makes SDA ideal for mycological studies. It supports the growth of yeast and molds, which are frequently of medical relevance, by providing an environment where they can thrive largely free from bacterial contamination.

To cultivate fungi effectively on SDA, it is streaked with the sample and then incubated at a suitable temperature, often around 25 to 30 degrees Celsius. Over time, the growth pattern, color, and morphology of the colonies provide clues to the fungi's identity, aiding in diagnosis and research.

Selective Media Properties

Understanding the properties of selective media like SDA is key in microbiology because they allow researchers to isolate specific types of microorganisms. Selective media contain components that favor the growth of certain microbes while inhibiting others. This is achieved through the inclusion of certain chemicals, antibiotics, salts, or by adjusting pH levels.

For SDA, the primary selective agent is the low pH. Fungi's acid tolerance allows them to grow, while many bacteria cannot survive under such conditions. Thus, SDA selectively permits the growth of 'all fungi,' consistent with the solution to the textbook exercise.

In practice, selective media are invaluable for simplifying the identification of organisms from mixed cultures and are extensively utilized in medical, food, and environmental microbiology to ensure accurate results are obtained when focusing on particular microbial groups.