delphini, Staphylococcus intermedius and S. pseudintermedius) and Laurasiatherian hosts after diverging from Chiropter (bats). Based on this observation, the appearance of genus Staphylococcus was estimated to be about 250 million years ago by molecular clock method using genome-wide datasets ( Fig. 3). Then, the staphylococcal species seem to have

started to colonize and co-evolve with mammals that emerged almost simultaneously about 225 million years ago ( Fig. 3). It is probable that the antecedents of staphylococci, e.g. macrococcal species and old staphylococcal species of S. sciuri-group required the benefit of mecA or mecC genes to protect XL184 themselves from β-lactam-producing environmental microorganisms before their descendants successfully adapted to mammalian hosts. The descendant staphylococcal species, after successful adaptation as mammalian microbial flora, lost mecA or mecC gene, because they became protected

from the assault of β-lactam-producing microorganisms thanks to the host’s immune system. The situation changed, however, in the PARP activity 1940s, when humans started to use penicillin G, threatening the colonizing staphylococci. They first acquired penicillinase plasmid. Then, since the introduction of methicillin in 1960, S. aureus had to regain mecA gene from S. fleurettii via the SCCmec. 1) hVISA, and VISA Some important antibiotic resistance phenotypes of MRSA are acquired

by spontaneous mutations. Rifampin resistance and fluoroquinolone resistance are the most well known examples. Moreover, vancomycin resistance, which has cast a dark shadow on anti-MRSA chemotherapy in the last two decades, is also acquired by mutation. Vancomycin has long been regarded as the last resort for MRSA infection. In 1997, however, the first VISA strain Mu50 was isolated from the surgical wound of a Japanese infant whose infection did not respond favorably to long-term vancomycin therapy [31] and [32]. The vancomycin MIC of Mu50 was 8 mg/L [31]. Now VISA is defined as S. aureus strain having vancomycin MIC of 4 or 8 mg/L. Note that MIC ≤ 2 mg/L is defined as susceptible. However, among the susceptible clinical strains, there are precursor strains for VISA. From the precursor strains, much one-step selection with vancomycin generates VISA at a frequency of 10−6 or above [52]. MIC determination cannot detect such precursor strains. Using 1000 times or more number of cells (or colony forming unit; CFU) of a bacterial strain than used for MIC method (about 104–5 CFU for the test) we can discriminate the precursor strains from really vancomycin-susceptible S. aureus (VSSA). This sensitive method is called analysis of resistant subpopulation (population analysis (PA)), and is an essential tool for the study of vancomycin and methicillin resistance [72]. Fig.