**10. Conclusions**

**Experiment Observations/treatment Reference**

control groups were 67% and 10% on day 5, respectively.

involved in protection

mice

S13 phage against lung-derived lethal septicemia by *S. aureus* strain SA27 in a mouse model.

192 Frontiers in Frontiers in Staphylococcus Aureus *Staphylococcus aureus*

Endolysin LysGH15 derived from staphylococcal phage GH15 was used against MRSA *in vivo* using mice and *in vitro*

PlySs2 bacteriophage lysine derived from *Streptococcus suis* was used to treat MRSA which cause bacteremia in mice

Nine endolysins within an homology group sharing SH3b domain but diverse classes of peptidoglycan hydrolyses (PGHs) from *S. aureus* were tested to determinate their antimicrobial activity

Endolysins

suggesting that bacterial components, such as bacteriocins, were not

Intranasal application of *S. aureus* strain SA27 induced 93% lethality in 3 days. S13 phage was done administered 6 h postinfection with 0.2 ml of solution of 15 × 1010 PFU/ml. The survival rates of phage administered and [77]

[78]

[79]

[80]

The administration of phage S13 reduced the *S. aureus* cell densities with significant phage replication in different tissues and it rescued the infected

Mice were infected with 2× of the minimum lethal dose of MRSA. The bacterial growth in spleens was determined 1–24 h after the lethal infection. Although the number of bacteria in spleens decreased slightly 6–12 h after infection, it increased until death. In contrast, the number of MRSA cells in spleens declined by 2 log units at 5 h after LysGH15 treatment (50 μg/ mouse) in the lethal MRSA-infected mice and continue decreasing to reach

an undetectable level. Also, LysGH15 treatment could modulate

treatment with PlySs2 only 6% of mice survived

**9. A functional molecular epidemiology approach to isolate bacteriophages**

As stated previously, particular genetic lineages are related to host specificity and pathogenic strategies of *S. aureus*. In a previous work, we isolated and typed *S. aureus* isolates from bovine mastitis in backyard farms in México. Most of these isolates were related to CC5 subgroups ST97 and ST126 and present diverse *spa*-types [56,57]. An isolate of ST8 (CA, human-associated) genetic background was also found. Several isolates from different STs were selected

**Table 4.** Use of phages and endolysins against *S. aureus* infections using animals models.

**against specific genetic lineages of** *S. aureus*

inflammation reducing the levels of IL-6, IL-4 and IFN-γ mRNA in spleens

Mice were infected i.p. with MRSA (MW2). PlySs2 protected mice and result in 89% survival in a bacteremia model, while in the control group without

Proteins were expressed, purified and tested for staphylococcal activity *in vitro*. Cut sites from endolysins were determined. PGHs show different degrees of activity *in vitro*. Some PGHs can eliminate biofilms. Six of the nine PGHs protected from death at 100% of infected mice with MRSA

> Bacteriophages and their endolysins in its natural or recombinant forms have proven to function in animals and animal models to control diverse forms of *S. aureus* infections. More structure-function studies of endolysins will contribute to design recombinant enzybiotics for the control of *S. aureus* infections. Functional molecular epidemiology is the applied use of the knowledge generated by molecular epidemiology to establish strategies for the control of infectious diseases [58] such as bacteriophage therapy. Bacteriophage selection using finely typed strains will help to properly select phages for therapy and to analyze the host range of the isolated bacteriophages. The strains typed by molecular approaches may also be useful to test ranges of activity of phage-derived endolysins. These, along with genetic engineering for

the study and expression of endolysins, will help to design better biotechnological approaches for the control of infectious diseases.
