A certain antibiotic 'X' is effective only against a few types of harmful microbes and cells. What is the antibiotic 'X'?

Narrow spectrum

Antibiotics are classified based on the range of microbes that are affected by it. Both Gram-positive and Gram-negative bacteria are affected by broad spectrum antibiotics. While narrow spectrum antibiotics only kill or inhibit one specific bacteria, narrow spectrum antibiotics are only effective against either Gram-positive or Gram-negative bacteria.