Influence of zinc on CTX-M-1 β-lactamase expression in Escherichia coli.

OBJECTIVE

Zinc oxide is used to prevent post-weaning diarrhoea in pigs as an alternative to antimicrobial growth promoters. This study aims to determine if the use of zinc oxide selects for extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and affects the expression of bla in E. coli.

METHODS

Using an in vitro faecal micro-cosmos model, the selective properties of zinc were investigated using an E. coli strain with bla encoded by a natural IncI1 resistance plasmid (MG1655/pTF2) and another strain where the same gene was located on the chromosome (MG1655::bla). The micro-cosmos was seeded with faecal material containing an increasing concentration of zinc (0-8 mM). Outcome measurements consisted of colony-forming units (CFU) of the inoculated ESBL E. coli and naturally occurring coliforms as determined by plate counting on MacConkey with and without 5 mg/L cefotaxime as well as total viable bacteria determined on Luria agar without cefotaxime. Expression of bla under the experimental zinc concentrations was determined by quantitative polymerase chain reaction.

RESULTS

The proportion of MG1655/pTF2 of the total viable bacteria was significantly higher at high zinc concentrations (6 and 8 mM) compared with low concentrations (0-4 mM). The messenger RNA (mRNA) levels of bla in the two ESBL strains increased at increasing zinc concentrations and varied with the growth phase.

CONCLUSION

The growth of the inoculated CTX-M-1-encoding E. coli MG1655 strains and naturally occurring coliforms was impacted differently when exposed to zinc oxide. The bla mRNA expression levels seemed to increase with increasing zinc concentrations, but varied with growth phase, but not gene location.