Understudied Bacteria on Egg Surfaces May Harbor Genes Encoding Resistance to Antibiotics and Disinfectants

The microbiological safety of eggs has focused largely on screening for pathogenic bacteria and their antimicrobial resistance (AMR). However, with growing awareness of the widespread distribution of antibiotic resistance genes (ARGs) in environmental bacteria, we investigated the presence of ARGs among underexplored non-fastidious culturable bacteria isolated from table eggs. A total of 54 ready-to-use table eggs were purchased from different vendors, and the shell surfaces were sampled for non-fastidious culturable aerobic bacteria. All isolates (n = 66) were identified using a MALDI-TOF MS biotyper. All isolates were tested for phenotypic susceptibility to tetracycline (a low-cost antimicrobial agent known to be used in poultry farms), and whole-genome sequencing was performed only on tetracycline non-susceptible (TNS) isolates (n = 16). The most common egg isolate was Staphylococcus equorum (n = 21), followed by Mammaliicoccus sciuri (n = 7), Psychrobacter pulmonis (n = 4) and Staphylococcus xylosus (n = 4). Within the TNS subset, 81.3% (13/16) carried tet(K), while the remaining isolates carried tet(33), tet(M) or tet(L). Also, in this TNS subset, the next most common co-carried ARGs identified were mph(C) and lnu(A), which encode resistance to macrolides and lincosamides, respectively. Furthermore, seven isolates co-carried qacG, which confers resistance to quaternary ammonium compounds (QACs). Though WGS analysis was restricted only to TNS isolates, the co-occurrence of biocide and antibiotic-resistant determinants highlights the necessity of surveillance of non-pathogenic microorganisms associated with animal food products.