The laboratory tool invented by Julius Petri and used to culture bacteria is the Petri dish. Its shallow, lidded design holds a nutrient medium—usually agar—on which microorganisms can grow into visible colonies.
Julius Richard Petri developed the modern dish in 1887 while working as an assistant to Robert Koch. Earlier culture methods used glass plates placed inside moist chambers, but Petri’s design made the culture container itself a covered dish. The lid helped reduce contamination while still allowing scientists to observe the sample.
In practice, a laboratory worker inoculates the agar by streaking a sample across its surface. Individual bacterial cells can multiply into separate colonies, allowing researchers to isolate, identify, and study them. Plates are often incubated upside down so condensation does not drip onto the culture.
A common mix-up is calling the dish a test tube or microscope: those tools may be used in microbiology, but they do not serve the same culturing role. Robert Koch developed an important precursor, and Walther Hesse helped introduce agar as a useful solid growth medium, so the modern method reflects several scientists’ contributions.