Molecular characterization of Staphylococcus aureus isolated from skin infection

概要

ABSTRACT OF THE THESIS
Molecular characterization of Staphylococcus aureus isolated from skin infection
by KANAKO MASUDA

Thesis Director:
Dr. Motoyuki Sugai

Molecular epidemiological characterization of total 540 isolates of Staphylococcus aureus
from skin infection in outpatient hospitals and private clinics collected in a clinical laboratory center
in West Japan from 2015 to 2018 was conducted. The isolates included 315 from pus discharge of
open wound, 108 from pus discharge of closed abscess, and 117 from pressure ulcers. Major clonal
complexes (CCs) of these isolates were CC8 (20.9%), CC121 (13.5%), CC15 (13.1%), and CC5
(11.7%). Each CC possessed a specific coagulase serotype and SCCmec type. Overall prevalence of
mecA was 40.2% but the prevalence was significantly different among isolates of three origins: the
percentage of mecA of isolates from open wound, closed abscess, and pressure ulcer were 38.1%,
13.9% and 70.1% respectively. Major CCs of isolates from each infection origin were different:
those from open wound were CC121, CC8, CC15, closed abscess were CC15, CC8, CC45, and
pressure ulcer were CC8, CC5, CC1 respectively. Prevalence of mecA in each CC was strikingly

different, and each CC possessed a unique repertoire of virulence and antimicrobial resistance genes.
CC121, which is more common in isolates from open wound, had a high positive rate of exfoliative
toxin A gene, suggesting that exfoliative toxin is a pathogenic factor associated with open wound.
CC5 carried SCCmec type II and showed much more antimicrobial resistance than other CCs. Most
of the isolates from pressure ulcer were MRSA displaying multiple antimicrobial resistance against
gentamicin, macrolide, and new quinolone. CC8 was the common lineage present in the isolates of
three types of skin infections and ST8 CA-MRSA/J and CA-MSSA/J, carrying multiple virulence
genes, represented 17.7 and 15.0% respectively whereas pvl-positive USA300 lineage (ST8 SCCmec
type IVa) was only 1.8% of CC8. Most of these lineages were isolated from open wound or closed
abscess. On the other hand, most of ST8 isolates from pressure ulcer carried SCCmec type I and
multiple resistance genes but few virulence genes. These results illustrated unique characteristics of
major strain lineages causing skin infections and strongly suggested that there is a strong association
between genotype and pathogenic potential of S. aureus causing different types of skin infection.
Among the skin isolates, we found 11 isolates were oxacillin-susceptible but mecA-positive
showing characteristics of OS-MRSA. Those isolates were determined as MSSA by an automated
antimicrobial susceptibility testing but were positive for mecA by PCR. Whole genome sequencing
indicated all 11 isolates belonged to ST121 and carried SCCmec type V. All these isolates possessed
the same one base substitution in the promoter sequence of mecA in common: the 18th base in the

MecI/BlaI-binding palindrome structure of the mecA repressor was substituted from C to T. This C to
T substitution was reported to be involved in attenuating mecA transcription and corresponding
PBP2′ (PBP2A) production, and was found to be a common nucleotide substitution in the S. aureus
carrying SCCmec type V lineages. In addition, all of these strains increased the oxacillin MIC in the
presence of mupirocin (0.03 μg/ml) which has been reported to increase mecA transcription and
PBP2′ production through inducing stringent responses. Most isolates of ST121 are producing
exfoliative toxin A and major causative agent of bullous impetigo in children. This finding calls
attention not to make a medication mistake in case of treating patient with staphylococcal bullous
impetigo since β-lactam antibiotics are the first-line drugs in case the causative agent is diagnosed as
methicillin-susceptible S. aureus.