LAUSR

OBJECTIVES Zinc oxide is used to prevent post-weaning diarrhea 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 E. coli and affects the expression of blaCTX-M-1 in E. coli. METHODS Using an in vitro fecal micro-cosmos model, the selective properties of zinc was investigated using an E. coli strain with blaCTX-M-1 encoded by a natural IncI1 resistance plasmid (MG1655/pTF2) and another strain where the same gene was located on the chromosome (MG1655::blaCTX-M-1). The micro-cosmos was seeded with fecal material containing an increasing concentration of zinc (0mM to 8mM). Outcome measurements consisted of CFU of the inoculated ESBL E. coli and natural occurring coliforms as determined by plate counting on MacConkey with and without 5mg/L cefotaxime as well as total viable bacteria determined on LA without cefotaxime. Expression of blaCTX-M-1 under the experimental zinc concentrations was determined by qPCR. RESULTS The proportion of MG1655/pTF2 of the total viable bacteria was significantly higher at high zinc concentrations (6 and 8mM) compared to low concentrations (0-4mM). The mRNA levels of blaCTX-M-1 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 natural occurring coliforms was impacted differently when exposed to zinc oxide. The blaCTX-M-1 mRNA expression levels seemed to increase with increasing zinc concentrations, but varied with growth phase, but not gene location.