Mutational Consequences of Ciprofloxacin in Escherichia coli.

We examined the mutagenic specificity of the widely used antibiotic ciprofloxacin (CPR), which displays weak to moderate mutagenic activity in several bacteria and generates short in-frame deletions in rpoB in Staphylococcus aureus To determine the spectrum of mutations in a system where any gene knockout would result in a recovered mutant, including frameshifts and both short and long deletions, we examined CPR-induced mutations in the thymidylate synthase-encoding thyA gene. Here, any mutation resulting in loss of thymidylate synthase activity generates trimethoprim (Trm) resistance. We found that deletions and insertions in all three reading frames predominated in the spectrum.

Mutagen Synergy: Hypermutability Generated by Specific Pairs of Base Analogs.

Unlabelled: We tested pairwise combinations of classical base analog mutagens in Escherichia coli to study possible mutagen synergies. We examined the cytidine analogs zebularine (ZEB) and 5-azacytidine (5AZ), the adenine analog 2-aminopurine (2AP), and the uridine/thymidine analog 5-bromodeoxyuridine (5BrdU). We detected a striking synergy with the 2AP plus ZEB combination, resulting in hypermutability, a 35-fold increase in mutation frequency (to 53,000 × 10(-8)) in the rpoB gene over that with either mutagen alone.

Exploring Synergy between Classic Mutagens and Antibiotics To Examine Mechanisms of Synergy and Antibiotic Action.

We used classical mutagens in Gram-negative Escherichia coli to study synergies with different classes of antibiotics, test models of antibiotic mechanisms of action, and examine the basis of synergy. We used 4-nitroquinoline 1-oxide (4NQO), zebularine (ZEB), 5-azacytidine (5AZ), 2-aminopurine (2AP), and 5-bromodeoxyuridine (5BrdU) as mutagens (with bactericidal potency of 4NQO > ZEB > 5AZ > 2AP > 5BrdU) and vancomycin (VAN), ciprofloxacin (CPR), trimethoprim (TMP), gentamicin (GEN), tetracycline (TET), erythromycin (ERY), and chloramphenicol (CHL) as antibiotics. We detected the strongest synergies with 4NQO, an agent that oxidizes guanines and ultimately results in double-strand breaks when paired with the bactericidal antibiotics VAN, TMP, CPR, and GEN, but no synergies with the bacteriostatic antibiotics TET, ERY, and CHL.