Regrowth of Pseudomonas aeruginosa and other bacteria after the bactericidal action of carbenicillin and other beta-lactam antibiotics.

Exposure of Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus to bactericidal concentrations of beta-lactam antibiotics in broth culture resulted in a decrease in viability over the first 6--8 hr, followed by regrowth which was not due to the selection of resistant variants or loss of antibiotic potency. During incubation, bacteria adhered to the surface of the culture vessel and multiplied despite the presence of bactericidal concentrations of antibiotic in the medium. It is concluded that the phenomenon of "regrowth" results from such adhesion and the subsequent dispersal of some of these cells into the culture medium. The significance of these findings is discussed in relation to the treatment of infection, the determination of minimal bactericidal concentrations, and the phenomena of tolerance and persisters.