The spread plate method of isolation is different from the streak plate method of isolation in that
a. only spread plates will produce individual colonies (CFUs)
b. only pathogens may be cultured on the spread plate
c. spread plates can be inoculated from both broth and agar stock cultures, whereas streak plates can only be made from broth stocks
d. a culture with high density must be diluted in liquid medium first before inoculation to the spread plate
The correct answer and explanation is:
Correct Answer: d. a culture with high density must be diluted in liquid medium first before inoculation to the spread plate
The spread plate method and the streak plate method are both microbiological techniques used to isolate pure colonies from a mixture of microorganisms. However, they differ significantly in how the inoculum is applied and in the conditions required for effective isolation.
The spread plate method involves applying a known volume of a diluted microbial sample onto the surface of an agar plate and then evenly spreading it using a sterile spreader. This method is typically used when quantifying microorganisms, especially in samples with a known or expected microbial load. Because the sample is spread across the surface, if the microbial density is too high, colonies will grow too close together, preventing clear isolation. Therefore, dilution of the sample is essential before plating. This allows for the formation of well-separated colonies, each arising from a single microbial cell or colony-forming unit (CFU). Hence, spread plating requires prior dilution to reduce colony crowding.
In contrast, the streak plate method does not require dilution in a separate liquid medium. Instead, it relies on a mechanical dilution by streaking the inoculum across different zones of the agar surface with an inoculating loop. This gradually thins out the number of cells, leading to the isolation of individual colonies in the later streaks. This technique is particularly useful for isolating colonies from a high-density sample without the need for liquid dilution.
Other choices are incorrect. Option a is false because both methods can produce individual colonies. Option b is false since spread plates are not limited to pathogens. Option c is incorrect because both broth and agar cultures can be used in either method, though broth is more commonly used. Therefore, option d best distinguishes the spread plate method.