**5. ST398 evolution and genetic diversity**

24 h [19, 41]. These and other studies indicate that CC398 appears to be frequently shared between animals and humans and is capable of causing infections in both species [25, 70, 164]. Transmission of MRSA between animals and humans is not new, but the MRSA isolates, in

Analysis of MRSA and MSSA from animals and humans spanning 19 countries and four continents indicated that the CC398 lineage originated in humans as an MSSA [167]. The wholegenome sequencing analysis by Price et al. [167] demonstrated that livestock-associated MRSA CC398 lost an immune-evasion cluster (IEC) as it evolved from its human-adapted MSSA. All of the HA-MSSA strains carry ØSa3 prophage in association with human innate immunomodulatory genes that play crucial roles in human niche adaptation (**Figure 2**) [167]. The prophageassociated virulence and adaptation genes are not necessary for nonhuman hosts, therefore, ØSa3 is mostly absent in livestock strains. After their introduction to livestock, MSSA CC398 acquired resistance to methicillin and tetracycline. Since tetracycline is heavily used in animal farming, the tetracycline resistance gene *tet*(M) is nearly universal among livestock-associated MRSA CC398 and MSSA isolates. The MSSA and MRSA CC398 isolates found in humans with direct livestock contact exhibited the same molecular patterns (i.e., ØSa3 prophage negative, *tet*(M)-positive) as the livestock-associated strains, indicating human re-adaptation [167], and were also reported in isolates epidemiologically associated with human-to-human transmission in multiple countries and continents [168]. During the host jump from humans to animals, MRSA CC398 strains also acquired resistance to copper and zinc because of their use in animal feed [130]. The vast majority of LA-MRSA CC398 strains carry SCC*mec* type Vc, which

It is quite possible that LA-MRSA CC398 strains would eventually acquire certain genetic traits (additional antibiotic resistance and virulence factors) that would allow *S. aureus* to colonize both hosts and become a more formidable zoonotic agent [167]. Since ST398 strains are deficient in one or more restriction modification systems [169, 170], this adaptation process may have already occurred, as *pvl*-positive ST398 MRSA strains have been isolated from

most cases, represent an initial human-to-animal transmission [24, 49, 165, 166].

56 Frontiers in Frontiers in Staphylococcus Aureus *Staphylococcus aureus*

contains the *czrC* gene that confers resistance to copper and zinc.

**Figure 2.** Gain or loss of genes as *S. aureus* jumps from human to livestock animals.

In spite of similarities between LA- and HA-MRSA isolates, significant amounts of genetic diversity among *spa* and SCC*mec* cassette types have been documented in ST398 [6, 57, 175]. For instance, ST398 appears to have evolved by multiple acquisitions of the SCC*mec* elements, such as SCC*mec* types II, III, IV, IVa, and V [176]. In the Netherlands, two farms were found to have MRSA ST398 with identical *spa* types, but different SCC*mec* types, suggesting that divergent SCC*mec* elements were inserted into the clonal MSSA [57]. Similarly, MSSA ST398 (*spa* type t899), MRSA ST398–IVa (*spa* type t899), and MRSA ST398–V (*spa* type t108) were found in dust samples, nasal swabs, and a blood isolate from workers on the same pig farm [177], suggesting multiple acquisitions of SCC*mec* cassettes by MSSA precursors. Coagulase-negative staphylococci in the farming environment are suspected as sources of SCC*mec* [176], and the progeny of emerging MRSA strains are spreading locally rather than globally [178–180]. While SCC*mec* acquisition seems to be fairly common in MRSA ST398, the transfer of staphylococcal toxin genes, including the Panton-Valentine leukocidin gene (*pvl*) appears to be rarer [20, 43, 44, 57, 165, 181–184]. Only a handful of studies have found *pvl* positive ST398 [20, 165, 185–187]. Additionally, horizontal transfer of the protein A gene has been suggested, due to the finding of the *spa* type t899 in both ST398 strains and ST9 strains [29, 30, 80, 177].
