**2.2 Laboratory detection of MRSA**

Phenotypic tests for laboratory identification of *Staphylococcus* species are relatively simple, with the employment of the catalase and coagulase tests, both positive. However, definitive confirmation requires the employment of additional tests or the Matrix Assisted Laser Desorption Ionization - Time Of Flight Mass Spectrometry (MALDI-TOF) [24], since both *S. aureus* and *S. pseudointermedius* (in addition to other species of staphylococci, such as *S. lugdunensis*) are coagulase positive. The detection of *mec*A and *mec*C genes by polymerase chain reaction (PCR) is also a complementary alternative for the correct identification of methicillin resistant species [25]. Alternatively, phenotypic tests to confirm methicillin resistance are often performed because they have low cost and reliable results. In this context, the behavior of the bacteria is evaluated by disk-diffusion on Mueller Hinton agar with 30 μg Cefoxitin disk for *S. aureus* (MRSA) [24]. The test consists of preparing a bacterial suspension in sterile 0.9% NaCl with density equivalent to 0.5 McFarland standard (1.5 x 108 CFU/mL). Next, a cotton swab is soaked in the freshly prepared solution and is seeded on the Mueller Hinton agar surface. After application of the antimicrobial disks and appropriate incubation (35°C/24 hours), the behavior against the antibiotics is verified according to the measurements of the inhibition halos formed around the tested disks [21], and it is interpreted according to the current reference guidelines used in each health service.

## **2.3 MRSA colonization and MRSA infection**

Historically MRSA was described in humans in 1961 [26], while MRSA colonization and infection in animals was first reported in 1972 in asymptomatic dogs in Nigeria and a case of bovine mastitis in Belgium [23]. Around 25–30% of the human population is asymptomatically colonized by *S. aureus* in their nostrils [22, 27]. Humans and animals with nasal colonization by *S. aureus* and MRSA are considered to be at higher risk for developing infections and transmission of bacteria and, since colonization usually precedes infection [26]. In this sense, there is a great public health concern because domestic animals are potential reservoirs of these pathogens, with subsequent transmission to humans. The

colonization of people in contact with colonized animals has been described. In addition, it has been shown that transmission can occur from animal to human as well as from human to animal [20]. The epidemiological success of *S. aureus*related pathogens depends not only on its ability to produce virulence factors but also on its *fitness*, that is, its ability to grow and persist in its hosts, promoting colonization [28].

It is now well established that MRSA isolates are often non-susceptible to different classes of antibiotics and are considered multidrug-resistant (MDR) when resistance is observed for at least three different classes of antimicrobials [25]. The great adaptability of this pathogen is due to its expressive genetic plasticity, in which approximately 25% of the *S. aureus* chromosome consists of mobile genetic elements, such as chromosomal cassettes, transposons, plasmids, and bacteriophages, which can be acquired through horizontal transfer [29].

When human MRSA infections persist, worsen, or recur despite surgical treatment, additional use of systemic antibiotic therapy is required [27]. Different clinical treatment options are available to combat MRSA infections, including vancomycin. Although this drug is the main therapeutic option, there are several limitations in its use, such as the achievement of optimal serum concentration, long-term treatment, renal toxicity, and restricted route of administration (intravenous) [30]. In the veterinary field, there is no effective therapy to treat MRSA infections, so prevention and control measures are critical to contain the further spread of MRSA [21]. While this challenge remains unresolved, successful treatment of infections may require the development of new antibiotics and the use of bacteriophages and phage-derived lytic proteins [29] as alternative therapeutic resources.

#### **2.4 Bacteriophages as anti-MRSA agents**

With the emergence of MRSA, staphylococcal infections have become difficult to control. MRSA is typically resistant to beta-lactams and can even present resistance to other antimicrobials [20], thus requiring new therapeutic alternatives. In this sense, phage therapy resurfaces as a promising tool for the control of unwanted bacteria, since it consists of the use of viruses, called bacteriophages, capable of infecting and killing prokaryotes without harming human or animal cells.

#### *2.4.1 What are bacteriophages?*

Bacteriophages, also known as phages, are viruses that infect and lyse prokaryotes. They are considered the most numerous infectious entities on the planet, being found in different environmental matrices, such as sewage, water, soil, among others [31]. Phages have been proposed as an alternative resource to the problem of resistant bacteria since they infect bacterial cells and, at the end of their reproduction cycle, promote the lysis of the host bacterium [18, 32]. After their discovery in 1917, phages were successfully used for the treatment of several bacterial infections [31]. However, the advent of antibiotics and their industrial-scale production, coupled with the lack of adequate studies and the poor understanding of phage biology at the time, resulted in the abandonment of studies related to these viruses as therapeutic agents in most institutions. A few places followed up on these studies, such as Eastern Europe, mainly Russia, Georgia, and Poland. Truly, the production and use of phages for prophylaxis and therapy never stopped in the last two countries mentioned [33]. From these countries emerged the main research in the phage therapy field.

#### *Bacteriophages as Anti-Methicillin Resistant* Staphylococcus aureus *Agents DOI: http://dx.doi.org/10.5772/intechopen.98313*

Subsequently, the indiscriminate use of antibiotics enabled progressive bacterial resistance, leading to the resumption of studies with phages. Thus, bacteriophages and their products, such as enzymes released at the end of their replication cycle, were once again considered as therapeutic agents [32]. Phage therapy is the use of bacteriophages to eliminate bacterial pathogens, and fortunately, innovative research techniques have made several advances in the field possible. One of the most important discoveries has been the distinction between the replication cycles carried out by phages. The replication of these viruses occurs mainly through two cycles: the lysogenic and the lytic.
