Definition: the capacity of disease-causing microorganisms to withstand exposure to drugs previously toxic to them; acquired either through spontaneous mutation or by gradual selection of relatively resistant strains after drug exposure. Pathogenic microorganisms resist antibiotics by various mechanisms, including the production of enzymes (β-lactamases) that chemically inactivate antibiotic molecules. In mixed infections of the respiratory tract, a β-lactamase (penicillinase) produced by one organism ( Haemophilus influenzae) can inactivate penicillin and so block its effectiveness against other organisms in the mixture that possess no resistance of their own ( group A β-hemolytic streptococci). Usually an organism that has acquired resistance to a given antibiotic is resistant to others in the same chemical class. Some bacteria transmit antibiotic resistance to their offspring not chromosomally but via plasmids, which lie outside the bacterial nucleus but perform certain genetic functions. Bacteria of one species can develop resistance to certain antibiotics by acquiring plasmids from bacteria of another species.