What is the goal of performing a three-zone streak plate?

The goal of performing a three-zone streak plate is to isolate single colonies from mixed cultures. This method is a critical technique in microbiology used to obtain pure cultures from samples containing multiple types of bacteria.

By streaking the inoculating loop across the agar surface in three distinct zones, the bacteria are progressively diluted across the medium. As you streak, fewer bacteria are transferred to the later zones, allowing individual cells to grow into separate colonies that are spaced apart. Each colony ideally arises from a single bacterial cell, ensuring that the culture you obtain is pure and consists of only one species. This purity is essential for subsequent identification and characterization of the microorganism present in the original mixed culture.

While promoting aerobic growth, creating bacterial mixtures, and measuring growth rates are important tasks in microbiology, they do not capture the primary objective of the three-zone streak plate technique, which is fundamentally focused on isolation for further study.