Induction of the mar operon by miscellaneous groceries

Aim: To investigate the potential of non-antibacterial consumer products to act as inducers of the multiple antibiotic resistance (mar) operon of Escherichia coli SPC105. Methods and Results: Wells were cut into chemically defined agar medium (CDM) contained within Petri dishes. Molten agar slurries were prepared by mixing known quantities of 35 consumer products with molten CDM and these were pipetted into each well. Plates were overlaid with molten CDM (5 ml), containing 40 mug ml(-1) X-gal and approx. 1000 CFU ml(-1) of an overnight culture of E. coli SPC105 containing a chromosomal marO(II)::lacZ fusion. After incubation (37degreesC, 24 h), plates were examined for zones of growth inhibition and the presence of a blue coloration, indicative of mar (marO(II)::lacZ) induction. Of the 35 products tested (nine herbs and spices, 19 food and drinks and seven household products), 24 (69%) of the items produced inhibitory zones and 22 (63%) of the items induced mar expression. Apple puree was inhibitory but did not induce marO(II)::lacZ. Mustard, chilli and garlic were shown to be powerful inducers of marO(II)::lacZ. Overall six products were shown to be powerful marO(II)::lacZ inducers. None of these made hygiene claims. Conclusions: In addition to induction by specific biocides and antibiotics, mar is induced by the exposure of bacteria to natural substances, many of which are common to a domiciliary setting. Significance and Impact of the Study: Concern that the overuse of antibacterials within consumer products might select for mar-mediated resistance is shortsighted and fails to recognize the ubiquity of inducers in our environment.