| Summary: | The antimicrobial susceptibility testing was conducted on 174 single isolates from poultry farms in Serbia and it was determined that seven <i>Salmonella</i> spp. were multidrug resistant. Sixteen serotypes were detected, but only serotype Infantis confirmed reduced susceptibility to colistin. Seven colistin resistant <i>Salmonella</i> Infantis were studied in detail using the WGS approach. Three sequence types were identified corresponding to different epizootiology region. The isolate from the Province of Vojvodina 3842 and isolates from Jagodina (92 and 821) are represented by the sequence type ST413 and ST11, respectively. Four isolates from Kraljevo are ST32, a common <i>S.</i> Infantis sequence type in humans, poultry and food. The fosfomycin resistance gene <i>fosA7</i> in isolate 3842 and the <i>vgaA</i> gene in isolate 8418/2948 encoding resistance to pleuromutilins were reported for the first time in serovar Infantis. The changes in relative expression of the <i>phoP</i>/<i>Q</i>, <i>mgrB</i> and <i>pmrA/B</i> genes were detected. Single nucleotide polymorphisms of the <i>pmrB</i> gene, including transitions Val164Gly or Val164Met, and Arg92Pro are described. Analyses of quinolone resistance determining region revealed substitutions Ser83Tyr in GyrA protein and Thr57Ser and Ser80Arg in ParC protein. Based on WGS data, there are two major clusters among analyzed <i>Salmonella</i> Infantis isolates from central Serbia.
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