resistance has different forms as will be outlined below,
the general definition of resistance is
Resistance is when a previously susceptible organism is no longer inhibited by an antibiotic at levels that can be safely achieved clinically
when all strains of a species are resistant intrinsically (naturally resistant)
Acquired resistance occurs when a previously susceptible strain or species develops an increase in the MIC that takes it beyond the therapeutic range
MIC = Minimum Inhibitory Concentration
the lowest concentration of an antibiotic that COMPLETELY inhibits the growth of a bacterium
MBC = Minimum Bactericidal Concentration
the lowest dose that completely kills a bacterium