**9. A functional molecular epidemiology approach to isolate bacteriophages against specific genetic lineages of** *S. aureus*

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 according to their *spa-*type or their antimicrobial resistance profile. **Table 5** shows examples of phages isolated using the selected molecular-typed *S. aureus* strains. All of the strains used for isolation belong to CC5 subgroup 97, but differed in their *spa*-type and their resistance profile, or belong to the ST8. Twenty-eight bacteriophages were isolated from 10 different *S. aureus* genetic lineages. Host ranges of isolated phages included strains from the same genetic lineage (CC5 subgroup 97). NST-1 corresponds to a new ST that is a single locus variant of ST126. None of these bacteriophages were active against the isolate with ST8 genotype. Restriction fragment length polymorphism with *XbaI* enzyme revealed only four different phage genotypes (data not shown). Phages MICHSAF5 and MICHSAF9 were clustered in the same RFLP group, whereas MICHSAF1 and MICHSAF15 were from different groups.


NB, novobiocin; GM, gentamicin; FOX, cefoxitin; C, chloramphenicol; CC, clindamycin; L, lincomycin; E, erythromycin; LZD, linezolid; ND, not determined.

**Table 5.** Preliminary analysis of phage isolation using a molecular typing background.

It is interesting to note that phages MICHSAF5 and MICHSAF9 were isolated using the same strain as host, and both presented the same host range and RLFP pattern. Phages MICHSAF1 and MICHSAF15 were associated with strains with different STs and resistance patterns, and the genotypes of the susceptible *S. aureus* strains were similar. All strains used for the isolation of bacteriophages and the susceptible strains belonged to CC5 subgroups ST97 and ST126. These results suggest that genetic background of the strain used for isolation of the bacteriophage will determine the host range of the bacteriophage.
