Purpose: The purpose of this study was to determine how subjecting Escherichia coli O157:H7 to common slaughter/fabrication stressors prior to antimicrobial exposure impacts the minimum inhibitory concentration (MIC).
Methods: E. coli O157:H7 (ATCC 33150) was grown in tryptic soy broth for 18 h at 37°C and then exposed to one of seven stresses: no stress (control), salt, acid, alkaline, heat, cold, and freeze-thaw. Cultures were centrifuged and pelletized cells were adjusted to a 0.5 McFarland standard prior to inoculating Mueller-Hinton broth and dosing into a 96-well microplate containing various concentrations of antimicrobials (lauric arginate, citric acid plus hydrochloric acid, peroxyacetic acid plus acetic acid and hydrochloric acid, lactic acid plus citric acid, and lactic acid). MICs were determined from absorbance value (600 nm) interpretations following 18 h of growth at 37°C.
Results: Statistical analysis of MIC absorbance values indicated a significant (P ≤ 0.05) difference in absorbance for concentration, as expected. However, no difference (P > 0.05) in absorbance values was detected for stress or stress*concentration interaction compared to controls for all antimicrobials.
Significance: Continued prudent use of antimicrobials in concert with other interventions (stressors) is unlikely to reduce antimicrobial efficacy against E. coli O157:H7. However, investigation into multiple hurdles (e.g., heat shock plus acid shock), and additional cocktails of various STEC serogroups, is warranted. Furthermore, transformation of absorbance values to quantifiable microbial populations may elucidate differences in antimicrobial efficacy and should be explored.