Genomic backgrounds of major methicillin-resistant Staphylococcus aureus epidemic clonal lineages present in fully susceptible isolates collected in the 1960s in Denmark

Abstract number: 1134_02_194

Gomes A.R., Westh H., Chung M., Lencastre H.

Objectives:

Most methicillin-resistant Staphylococcus aureus (MRSA) isolates identified among blood isolates collected in Denmark between 1957 and 1970 belonged to either phage group III or the closely related 83A complex and had a PSTM antibiotype. Recently, some of these strains were shown to belong to the same genetic backgrounds as contemporary epidemic MRSA by MLST and spa typing and Danish MSSA strains from the 60s with a PST antibiotype were proposed to have been the recipients of the mecA gene in those lineages. Among these were strains that belonged to ST247, ST5 and ST30, a single locus variant of ST36. In this study we sought to investigate the genetic backgrounds of isolates that were fully susceptible or resistant to penicillin only from phage group 83A in order to try to trace the evolutionary trajectory of those MRSA lineages. We also studied MSSA and MRSA from other phage groups in order to investigate whether these were associated with genetic backgrounds that reached an evolutionary ‘dead-end’.

Methods:

Eighty-two MSSA and MRSA strains isolated from the blood between 1957 and 1973 in Denmark were characterized by MLST, spa and SCCmec typing.

Results:

Most susceptible or P-resistant isolates from phage groups 83A or III belonged to ST8 or ST5, while four isolates were ST254, the first MSSA strains identified with this genetic background. STs 30, 45 and 25 were represented by MSSA strains from other phage groups, which also included several singletons. ST250 was the most common genotype among nontypeable MRSA isolates.

Conclusions:

Representatives of some of the current major epidemic MRSA lineages were identified among fully susceptible isolates collected in the 60s suggesting that these were lineages which carried genetic traits important for a superior epidemicity before acquisition of antibiotic resistance while singletons may represent less fit genotypes. In particular, ST254 MSSA provided the first evidence to the existence of this derivative of ST8 before the introduction of the mecA gene.