Bacterial mutagenicity screening in the pharmaceutical industry

The methods for detecting carcinogens and mutagens with the Salmonella mutagenicity test were described previously (Ames et al., 1975b). The present paper is a revision of the methods. Two new tester strains, a frameshift strain (TA97) and a strain carrying an ochre mutation on a multicopy plasmid (TA102), are added to the standard tester set. TA97 replaces TA1537. TA1535 and TA1538 are removed from the recommended set but can be retained at the option of the investigator. TA98 and TA100 are retained. We discuss other special purpose strains and present some minor changes in procedure, principally in the growth, storage, and preservation of the tester strains. Two substitutions are made in diagnostic mutagens to eliminate MNNG and 9-aminoacridine. Some test modifications are discussed.

The SOS response aids bacterial propagation by inhibiting cell division during repair of DNA damage. We report that inactivation of the ftsI gene product, penicillin binding protein 3, by either beta-lactam antibiotics or genetic mutation induces SOS in Escherichia coli through the DpiBA two-component signal transduction system. This event, which requires the SOS-promoting recA and lexA genes as well as dpiA, transiently halts bacterial cell division, enabling survival to otherwise lethal antibiotic exposure. Our findings reveal defective cell wall synthesis as an unexpected initiator of the bacterial SOS response, indicate that beta-lactam antibiotics are extracellular stimuli of this response, and demonstrate a novel mechanism for mitigation of antimicrobial lethality.