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Special Article
November 26, 2001

A Fresh Look at the Definition of Susceptibility of Streptococcus pneumoniae to β-Lactam Antibiotics

Author Affiliations

From the Veterans Affairs Medical Center and the Departments of Medicine and Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Tex (Dr Musher); the Department of Medicine, Johns Hopkins Medical School, Baltimore, Md (Dr Bartlett); and the Department of Microbiology, University of Iowa Hospitals and Clinics, Iowa City (Dr Doern).

Arch Intern Med. 2001;161(21):2538-2544. doi:10.1001/archinte.161.21.2538

Definitions for susceptibility or resistance of Streptococcus pneumoniae to penicillin were not developed until penicillin-resistant pneumococci appeared in South Africa in the late 1970s. The definition that was accepted (which still remains in use) and later definitions of resistance to most other β-lactam antibiotics were derived from laboratory and clinical data relating to the treatment of meningitis, not otitis media, sinusitis, or pneumonia. An understanding of the origin of these definitions helps to resolve the apparent paradox that infections of the respiratory tract due to seemingly β-lactam–resistant pneumococci may still respond well to standard doses of these drugs. A recently sanctioned change in the definition of susceptibility to amoxicillin is helpful in eliminating the paradox for this drug, but it may create further confusion by implying that, on a microgram basis, amoxicillin is substantially more effective than penicillin or third-generation cephalosporins. This article examines definitions of susceptibility and resistance of pneumococci, highlighting areas that have led to confusion and proposing a new way of understanding them.

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