Bacteria can adopt several strategies to become resistant to an antibiotic. Inactivation of the drug molecule can be done by enzymes. Beta-lactamase degrades the beta-lactam ring found in penicillin, preventing it from having an antibiotic effect.
Efflux pumps are special transport pumps found in the cell wall and membrane. These pump out toxins, preventing the antibiotic concentration reaching toxic levels in the cell and thus preventing death.
To overcome these resistance mechanisms, the antibiotic could be administered at a higher concentration. But the concentration of an antibiotic in a patient can only reach a certain level before severe adverse reactions occur. In the case of enzymes, a different class of antibiotic could be used.