A bacterial culture ($200 \: \mu l$ containing $1.8 \times 10^9$ cells) was treated with an antibiotic $Z$ ($50 \: \mu g$ per $ml$) for $4$ h at $37^\circ C$. After this treatment, the culture was divided into two equal aliquots.
Set A: $100 \: \mu l$ was plated on Luria agar.
Set B: $100 \: \mu l$ was centrifuged, the cell pellet washed and plated on Luria agar.
After incubating these two plates for $24$ h at $37^ \circ C$, Set A plate showed no colonies, whereas the Set B plate showed $0.9 \times 10^9$ cells. This experiment showed that the antibiotic $Z$ is