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

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_CTX-M-1 in E. coli.

Using an in vitro faecal micro-cosmos model, the selective properties of zinc were investigated using an E. coli strain with bla_CTX-M-1 encoded by a natural IncI1 resistance plasmid (MG1655/pTF2) and another strain where the same gene was located on the chromosome (MG1655::bla_CTX-M-1). 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_CTX-M-1 under the experimental zinc concentrations was determined by quantitative polymerase chain reaction.

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_CTX-M-1 in the two ESBL strains increased at increasing zinc concentrations and varied with the growth phase.

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_CTX-M-1 mRNA expression levels seemed to increase with increasing zinc concentrations, but varied with growth phase, but not gene location.