Arginine catabolic mobile element (ACME) is a genomic island of staphylococcus and is considered to confer enhanced ability to survive and growth on host bacterial cells. ACME has been typically identified in Panton-Valentine Leukocidin (PVL)-positive ST8 methicillin-resistant Staphylococcus aureus (MRSA) with SCCmec type IVa (USA300 clone), and it is also found in other lineages at low frequency. Prevalence and molecular characteristics of PVL+and/or ACME+ MRSA were investigated for 624 clinical isolates collected from outpatients in northern Japan from 2013 to 2014. Both PVL genes and ACME type I were detected in nine isolates (1.4%), which were ST8-MRSA-SCCmec IVa/spa type t008/agr-I; whereas solely PVL genes were positive in two isolates, ST30-MRSA-SCCmec IV and ST59-MRSA-SCCmec V. ACME type II' (previously referred to as ACME ΔII) was detected in 36 isolates (5.8%) with SCCmec II and V (32 and 4 isolates, respectively), exhibiting an increased rate within SCCmec II-MRSA (7.1%) compared with our previous studies (0.86-4.5%, 2008-2011). ACME II'-positive MRSA strains were classified into ST5-SCCmec IIa/V or ST764-SCCmec IIa belonging to five different spa types, with t002 being dominant. They harbored mostly enterotoxin gene clusters (seg-sei-sem-sen-seo-seu) and some more enterotoxin genes (seb1, seb2, sec3, sel, sep), showing resistance to more antimicrobials than ST8-MRSA-SCCmec IVa. ACME-SCCmec composite island (CI) of the 36 ACME II'-positive MRSA was classified into five types (ii)-(vi), among which type (ii) (orfX-ΨSCCΔJ1 SCCmec I-ACME II'-SCCmec II) was dominant and subdivided into the A3 variant and the less common A2 variant. CI types (v) and (vi) were considered novel genetic organizations having speG (acetyltransferase genes for polyamines) in inserted SCC4610/SCC266-like genetic elements. The present study revealed increased prevalence and genetic diversity of the ST5/ST764-MRSA-SCCmec II with ACME II' in northern Japan.
Keywords: ACME II′; Japan; MRSA; ST5/ST764; speG.