Animal & Veterinary
ESCHERICHIA COLI O157:H7 MARR: GENETIC ANALYSIS OF ITS ROLE IN GROWTH IN THE PRESENCE AND ABSENCE OF ANTIBIOTICS
FDA Veterinarian Newsletter September/October 2001 Volume XVI, No V
The marRAB operon is a regulatory locus that controls multiple drug resistance in gram negative bacteria such as Escherichia coli and Salmonella by altering the expression of many chromosomal genes. MarR, a member of phenolic-binding regulatory proteins is the transcriptional repressor of the operon. A mutation within the marR gene in E. coli K-12 leads to constitutive transcriptional activation of marRAB, resulting in decreased influx and increased efflux of toxic agents. marR from the foodborne pathogen E. coli O157:H7 has been amplified by PCR. Its sequence is 98% identical to MarR from E. coli K-12. Upon exposure of E. coli O157:H7 to chloramphenicol (Cm) we isolated a mutant that does not express MarR. This mutant grew on higher concentrations of tetracycline (2.5 mg/ml), nalidixic acid (4 mg/ml) and ciprofloxacin (0.025 mg/ml) compared to the parent strain.
The role of MarR in growth was analyzed. We compared the growth of MarR mutant to the parent strain, in the presence and absence of Cm (7 mg/ml), under various conditions. No significant differences were observed in growth curves of the parent and the mutant in the conditions examined (rich and minimal media, acidic conditions and at a temperature range of 24-42°C). Those results demonstrate that under standard growth conditions the multiple antibiotic resistant mutant is highly competitive with the susceptible parent.
Conditions required for induction of antibiotic resistance of MarR mutant were examined. The mutant grew at 30°C and below on rich media supplemented with Cm, but growth was slow at 37°C, and no growth occurred above 37°C. No growth occurred in minimal broth supplemented with Cm. MarR mutant was pre-conditioned by culturing for 17 h in Luria broth containing Cm. Significantly greater resistance was observed with the pre-conditioned mutant. In general, regardless of antibiotic, growth rates were greater and lag periods shorter for the pre-conditioned mutant. Results support the hypothesis of amplifiable multiple antibiotic resistance.