Protection by D-amino acids against growth inhibition and lysis caused by beta-lactam antibiotics.

D-Isomers of several amino acids completely protected growing cultures of Escherichia coli against all antibacterial effects of beta-lactam antibiotics up to approximately two to three times the MICs of the antibiotics. L-Isomers of amino acids were ineffective. Protection depended on the concentration and time of addition of the D-amino acids. This appears to be the first demonstration of natural products capable of reversing the antibacterial effects of beta-lactam antibiotics.