## 3 Results and Discussion  
Gentamicin has been shown to inhibit bacterial protein synthesis and eliminate _Salmonella_ tissue cell culture invasion [19]. This makes it a useful treatment for differentiating between external bacterial cells and internalized cells. All three strains were observed to be susceptible to gentamicin. Both _Salmonella_ Heidelberg ARI-14 and \(S\). Typhimurium ATCC 14,028 yielded average inhibition zones of 20.7 \(\pm\) 1.1 and 20.7 \(\pm\) 0.1, respectively. _Escherichia coli_ strain 25922 was used as a control and exhibited a zone of inhibition of 19.0 \(\pm\) 0. To further quantify susceptibility, the gentamycin was serially diluted in a growth medium, cells were added, and aliquots were plated for enumeration. Growth was detected on agar plates for all three organisms after the third dilution. The MIC for gentamicin was 125 \(\upmu\)g/mL for all three strains. These results validated using gentamicin during invasion assays as a means to kill cell-adherent extracellular bacteria. Expected MIC ranges have been reported by [PERSON] [20] for determining the susceptibility of several bacteria to a wide selection of antibiotics, as well as a list of appropriate controls for inclusion when determining MIC responses. As a control, their suggested MIC range for _Enterobacteriaceae_ susceptibility against gentamicin is 0.03 to 128 mg/L using _E. coli_ 25,922 [20]. Our results are consistent with this range. When [PERSON] et al. [21] determined the cell invasion responses of several poultry-associated \(S\). Enteritidis isolates, they noted a gentamicin MIC of <0.125 \(\upmu\)g for all the isolates and used 100 \(\upmu\)g per mL for their Caco-2 invasion assays. [PERSON] et al. [22] examined the MIC of \(S\). Typhimurium 14,028 and \(S\). Virchow and reported MIC values of 125 \(\upmu\)g per mL for both servovars. In the current study, we examined the ability of _Salmonella_ strains from different servovars to attach and invade two cell lines, namely, Caco-2 and HD11. We only evaluated two servovars for the Caco-2 study, the well-characterized \(S\). Typhimurium serovar strain and the previously uncharacterized \(S\). Heidelberg serovar. Our rationale for the Caco-2 cell study was to only compare the \(S\). Heidelberg directly with the standard \(S\). Typhimurium vi typically used in these types of tissue culture studies to assess whether the invasion response of the \(S\). Heidelberg isolate was similar to \(S\). Typhimurium. As expected, the percentage attachment for each respective _Salmonella_ strain was greater than the ability of the respective strain to invade the cells. _Salmonella_ Heidelberg and \(S\). Typhimurium yielded percentages of adhesion of 28.8 \(\pm\) 6.37 and 18.1 \(\pm\) 6.25, respectively, to Caco-2 cells (Table 2). However, the Caco-2 invasion ability of these respective strains was lower, 1.37 \(\pm\) 0.25 for \(S\). Heidelberg and 1.52 \(\pm\) 0.02 for \(S\). Typhimurium (Table 3). The underlying detailed mechanisms that intracellular pathogens such as _Salmonella_ use to penetrate the host epithelium continue to be investigated. Cultured mammalian cells as in vitro models have been used to study the interaction and internalization of _Salmonella_[23; 24; 25]. The invasion of cultured epithelial cells is commonly used to measure _Salmonella_ pathogenicity [26; 27; 21]. Previous studies report that several environmental stimuli such as osmolarity [28; 29], carbohydrate availability [30], and oxygen availability [31; 32; 33] influence _Salmonella_ invasion of cultured mammalian cells. For example, [PERSON] et al. [25] reported that _Salmonella_ exposed high concentrations of short-chain fatty acids (SCFAs) at levels similar to the lower regions of the gastrointestinal tract, along with pH changes, could influence the association and invasion of Hep-2 cells. [PERSON] et al. [21] reported that in cultured Caco-2 cells, isolateswith high invasiveness could invade and/or survive more extensively within chicken macrophage cells compared to low-invasive isolates.