Agriculturally Sourced Multidrug-Resistant Escherichia coli for Use as Control Strains

Bacteriological control strains with known characteristics ensure consistency and reproducibility of assay performance across different laboratories and are an important cornerstone of quality control in the microbiology laboratory. Ideally, control strains should be representative of the assay target and be widely available from reputable sources. However, for work involving antibiotic resistance most controls come from human and veterinary clinical sources and are not optimized for work in agriculturally impacted environments or not widely available. The objective of this work was to identify and make widely available two E. coli isolates sourced from agricultural production settings that could be used as external controls supporting method development, research and environmental monitoring for extended spectrum β-lactamase producing (ESBL) and tetracycline resistant Escherichia coli. Previously collected E. coli suspects were screened based on antimicrobial susceptibility testing data, then confirmed as E. coli and characterized both phenotypically and genotypically. The positive control strain, ARS-C301 was ESBL positive and contained the CTX-M-55 and tet(A) genes, and the negative control strain, ARS-C101 was negative for both targets. Here we introduce two agriculturally sourced, fully characterized, and genetically sequenced control strains for use as laboratory controls in research involving extended-spectrum β-lactamase producing (ESBL) and tetracycline-resistant Escherichia coli isolated from the environment, available via publicly accessible culture collections, and commercially as a quantitative pellet.