**1. Introduction**

#### **1.1 The role of** *S. aureus* **in human and animals**

Among the different relevant bacterial genus in Veterinary and Human Medicine, *Staphylococcus* is one of the most frequent opportunist pathogens. The species belonging to this genus present themselves as Gram positive cocci and are related to different communitarian and nosocomial infections, in both humans and animals. The members of *Staphylococcus* spp., especially *Staphylococcus aureus*, are constituents of the normal microbiota of the skin, mucous membranes, and upper respiratory tract of humans [1]. Although *S. aureus* is not considered part of the microbiota of dogs, indexes of 5% [2], 10% [3, 4], and even 20% [5] of nasal colonization by the bacterium were described in canines. Similarly, the cats also are included among the pet target-species potentially colonized by *S. aureus* due to their close proximity to humans, as pets [6]. In the context of proximity, the coexistence between man and dogs is still closer in order of canine aptitudes additional to the condition of the pet, as guide dogs, hunting dogs, guard dogs, among others. Thus,

the pets share daily routines with their owners, establishing affective bonds that emphasizes the importance of the control of transmissible diseases inter-species.

Historically, the first publications related to the human carriage of *S. aureus*, emerged in mid-1940s [7] and showed the relevance of the bacteria in the human infections. On the other side, the approach to this theme in the vet sphere was only evidenced from the year 2000. Regardless, *S. aureus* has zoonotic potential [8], being even more relevant when the bacteria is methicillin-resistant (Methicillin Resistant *S. aureus* or MRSA). The transmission of this emerging zoonotic pathogen among pets and humans [9], including veterinary staff, has been demonstrated [10, 11], implying problems in the public health sphere [12]. In addition, the risk of zoonotic transmission of *S. aureus* may impact directly in the relation between humans and animals, harming the strength of the affective bond. Additionally, the expressive occupational health risk to veterinary professionals must also be considered [13].

#### **1.2 Infections related to** *S. aureus* **and Methicilli Resistant** *S. aureus* **(MRSA)**

*S. aureus* is one of the most structured species in order of the high frequency as etiological agent of infections, as well as the growing prevalence of its resistance to antimicrobials [14]. The health complications arising of the infection by *S. aureus* in humans and animals are diversified and depend on intrinsic factors to the bacteria (virulence factors as extracellular enzymes, capsular polysaccharides, surfaceassociated proteins), as well as the conditions inherent to the host. Clinically, they can limit themselves to localized skin infections, but can cause severe illnesses as septicemia, respiratory tract infections, osteomyelitis, endocarditis, besides food poisoning [9]. Along with the severity of the bacterial infections, the other factor that compromises the recovery of the infected individuals is the bacteria antimicrobial resistance profile. The higher the degree of resistance, the higher will be the restriction to therapeutic alternatives to the treatment of the infection, there may not even be an effective drug. In this regard, the World Health Organization (WHO) suggested in 2017, a list of resistant bacteria considered more relevant in order of antibiotics shortage to treat the diseases. The specialists grouped the pathogens accordingly with the bacterial species and the resistance type shown, resulting in three priority tiers: critical, high, and medium, being Methicillin-Resistant *S. aureus* considered high priority [15].

#### **1.3 Perspectives to MRSA infections treatment**

Alternatively, with the development of the new antibiotics to supplant the resistance, there is the possibility of using viral agents to control unwanted bacteria. Viruses termed "bacteriophages" or "phages" are the most abundant agents in the environment and are host-specific, i.e., they infect only prokaryotes that have their own specific receptors for their adsorption. The absence of such receptors makes phage binding to the target cell as well as subsequent infection impossible, characterizing the specificity of these viruses [16, 17]. Phages are easily recovered from soil, sewage, and feces and their numbers are about 3 to 10 times higher than bacterial counts even though variations exist between ecosystems [18, 19]. Like other viruses, bacteriophages are obligate intracellular, and are characterized according to the replication cycle exhibited after infection of the bacterial host. The cycle can be lytic or lysogenic, but only phages that exclusively perform the lytic cycle are of interest for use as therapeutic agents, since they will promote cell lysis at the end of the cycle [18]**.**
