Yeast extract tsa ye

Information about the strains of L. monocytogenes used in the present study is summarized in Table 1. Bacterial stock cultures were kept at -80°C in Brain heart infusion (BHI) broth (Oxoid Limited, UK) with 20% glycerol added as cryogenic agent. Fresh cultures in their early stationary growth phase were prepared for each experiment by inoculating a loopful of the frozen culture in BHI and incubating at 37°C for 18 h. BHI was chosen as medium with optimal growth rates under controllable conditions based on the EURL technical guidance document (European Union Reference Laboratory for Listeria monocytogenes [EURL Lm], 2014 ). The resulting cell suspension of L. monocytogenes contained about 10⁹ colony forming units (CFU) mL^-1. Microbial counts were performed by spreading 0.1 mL of the appropriate serial dilution in peptone water (PW; Merck, DE) on the non-selective medium Tryptone Soya Agar supplemented with 0.6% Yeast Extract (TSA + YE; Oxoid Limited, UK), incubating at 37°C and colony counting after 24 h.

To examine the effect of GSE against the tested pathogenic bacteria, cells (obtained as described in Section 2.1) were appropriately diluted in TSBYE medium and transferred via pipetting to 1% and 4% TSBYE-GSE solutions. Additionally, all bacteria were inoculated in TSBYE without GSE (controls). For L. monocytogenes, different initial inoculum concentrations (10⁵ and 10⁸ CFU/mL) and growth phases (stationary and mid-exponential) were studied. Testing different initial cell concentrations reveals if the same GSE concentration would be effective for heavier contaminations. The initial population of E. coli and S. Typhimurium was approximately 10⁵ CFU/mL and only stationary phase cells were studied, as the inhibitory effect of GSE was much weaker than that for L. monocytogenes even for (stress-adapted) stationary phase cells where maximum resistance is expected (see Section 3). All tested parameters can be seen in Table 1.
After inoculation, the samples were kept at 37 °C. The bacterial survival was systematically monitored for up to 24 h post-treatment (at 0, 2, 4, 8, 12, 18, and 24 h) using the plate count method in non-selective agar, i.e., Tryptone Soy Agar supplemented with 0.6% of Yeast Extract (TSAYE, Oxoid Ltd., Basingstoke, UK).