A fresh look at the definition of susceptibility of Streptococcus pneumoniae to beta-lactam antibiotics

Definitions for susceptibility or resistance of Streptococcus pneumoniae to penicillin were not developed until penicillin-resistant pneumococci appea red in South Africa in the late 1970s. The definition that was accepted (wh ich still remains in use) and later definitions of resistance to most other beta -lactam antibiotics were derived from laboratory and clinical data re lating to the treatment of meningitis, not otitis media, sinusitis, or pneu monia. An understanding of the origin of these definitions helps to resolve the apparent paradox that infections of the respiratory tract due to seemi ngly beta -lactam-resistant pneumococci may still respond well to standard doses of these drugs. A recently sanctioned change in the definition of sus ceptibility to amoxicillin is helpful in eliminating the paradox for this d rug, but it may create further confusion by implying that, on a microgram b asis, amoxicillin is substantially more effective than penicillin or third- generation cephalosporins. This article examines definitions of susceptibil ity and resistance of pneumococci, highlighting areas that have led to conf usion and proposing a new way of understanding them.