**2. Results and Discussion**

### *2.1. Antibiotics and Y. pseudotuberculosis 488 Susceptibility*

*Y. pseudotuberculosis* 488 (= strain 117), serotype O:1b, was isolated from a patient with FESLF in the Russian Far East region. To elucidate *Y. pseudotuberculosis* porin regulation under antibiotic stress, we chose carbenicillin, tetracycline, kanamycin, and chloramphenicol. These antibiotics have been shown to use porin channels to enter into a bacterial cell [13,14]. Several studies have demonstrated that porin regulation, resulting in decreased penetration of antimicrobials into the cell, contributes to bacterial adaptive resistance [24]. We determined the MIC ranges of the selected antibiotics for *Y. pseudotuberculosis* 488 at 27 ◦C (optimal for bacterial cultivation) and 37 ◦C (human body temperature) since it is known that porin expression is tightly regulated by environmental factors [25,26], predominantly by temperature [27,28]. Such regulation is shown for *Y. pseudotuberculosis* strains in the interactive RNA atlas www.pathogenex.org (*ompF* YPK\_2649; *ompC* YPK\_2839).

*Y. pseudotuberculosis* 488 showed high susceptibility to the tested antibiotics (Table S1). However, cells cultivated at 27 ◦C exhibited a 2-fold increased resistance to kanamycin and tetracycline compared with 37 ◦C, whereas the MICs of carbenicillin and chloramphenicol were not affected by incubation temperature. To induce antibiotic stress, we used subinhibitory concentrations (sub-MICs) of antibiotics corresponding to <sup>1</sup> <sup>2</sup> MICs. Such sublethal antibiotic concentrations reflect the conditions that bacteria may encounter in the natural environments and the human body [29]. Moreover, exposure to sublethal concentration has been reported to play a protective role in the bacterial cell against a wide range of antimicrobials and alter the expression of various bacterial genes that can lead to nonspecific resistance [22].
