## 1 Introduction  
Knowledge about _Aeromonas (A.) salmonicida_ is continuously updated. Complicated and intensively fluctuating taxonomy inside the _Aeromonas_ genus [1; 2; 3] lasted until the beginning of the 21 st century, when five subspecies were officially established: _salmonicida_, _smithia_, _achromogenes_, _masoucida_, and _pectinolytica_[4]. A division of _A. salmonicida_ to \"typical\" and \"atypical\" representatives is frequently applied even if a clear scientific basis to justify this differentiation is lacking. For example, the _A. salmonicida_ subsp. _salmonicida_ has been positioned as the typical representative of the genus, with the reference to its strict pathogenicity for salmonid fish, unlike other _A. salmonicida_ subspecies that are opportunistic pathogens and have a wide range of hosts, including occasionally immunocompromised humans [5; 6]. Therefore, for a long time, _A. salmonicida_ was not considered a human pathogen because it cannot multiply at 37 \({}^{\circ}\)C. Regardless, in the last few years, sporadic cases of human infections with _A. salmonicida_ have been reported, including bacteremia, endocatitis, eye, skin, and surgical wound infection, but without information about which subspecies of _A. salmonicida_ caused the disease, which indicates the necessity of precise molecular subtyping of this causative agent [4; 7; 8; 9; 10]. So far, not a single case of human infection with _A. salmonicida_ subspecies _salmonicida_ has been recorded. In addition, it was recently proven that the pathogenicity of _A. salmonicida_ subsp. _salmonicida_ can be associated not only with salmonid fish but also with a wide range of other fish species like cod, black rockfish, sailfin sandfish, turbot, and goldfish [2; 6]. The appearance of beta-haemolytic, sometimes pigmented colonies on 5% sheep blood agar [6] was also considered to be typical only for subsp. _salmonicida_. Again, it was reported that other A. _salmonicida_ subspecies, as well as _A. hydrophila_, can sporadically have identical colonies as _A._ subsp. _salmonicida_, which depends on the cultivation temperature [11]. Psychrophilic conditions for the optimal multiplication of _A. salmonicida_ subsp. _salmonicida_ is another basis on which it is considered a typical representative of the genus [12; 13]. Yet, in recently published papers on this subject, there is the confusion in the interpretation of what are psychrophilic and what are mesophilic conditions--majority of authors arbitrarily declared temperatures of 22-28 \({}^{\circ}\)C as psychrophilic for _A. salmonicida_ subsp. _salmonicida_[1; 13; 14], thus, ignoring the basic microbiological psychrophilic norm of a maximum 18 \({}^{\circ}\)C; and mesophilic conditions were declared as between 10 \({}^{\circ}\)C and 30 \({}^{\circ}\)C [6; 15]. Indeed, the most researched habitat of _A. salmonicida_ subsp. _salmonicida_ are cold waters (from 2 \({}^{\circ}\)C in winter, to 18 \({}^{\circ}\)C in summer) [6]; however, recent findings clearly show the ability of _A. salmonicida_ subsp. _salmonicida_ to grow and multiply in a wider temperature range, between 4 \({}^{\circ}\)C and up to 30 \({}^{\circ}\)C [7; 11; 13; 16; 17; 11]. This indicates that _A. salmonicida_ subsp. _salmonicida_ is most likely a psychotropic microorganism (0-30 \({}^{\circ}\)C) [2; 12; 15]. Biofilm, or \"multicellular pattern of life organization in bacteria\" is considered a higher evolutionary stage of life organization in bacteria, enabling them to successfully survive in the external environment and in unfavorable conditions [19; 20; 21]. It has been proven that a biofilm is a natural form in which bacteria live in all ecological environments, including open waters, i.e., that bacteria cannot survive in the external environment as unicellular forms but can survive exclusively united in a biofilm [22]. The basic hypothesis is that a biofilm is formed when bacteria are under stress--the lack of nutrients and during suboptimal temperatures [23; 24].