Emerging ST121/agr4 community-associated methicillin-resistant Staphylococcus aureus (MRSA) with strong adhesin and cytolytic activities: trigger for MRSA pneumonia and fatal aspiration pneumonia in an influenza-infected elderly

Abstract

The pathogenesis of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) pneumonia in influenza-infected elderly individuals has not yet been elucidated in detail. In the present study, a 92-year-old man infected with influenza developed CA-MRSA pneumonia. His CA-MRSA was an emerging type, originated in ST121/agr4 S. aureus, with diversities of Panton-Valentine leucocidin (PVL)(-)/spat5110/SCCmecV(+) versus PVL(+)/spat159((etc.))/SCCmec (-), but with common virulence potentials of strong adhesin and cytolytic activities. Resistance to erythromycin/clindamycin (inducible-type) and gentamicin was detected. Pneumonia improved with the administration of levofloxacin, but with the subsequent development of fatal aspiration pneumonia. Hence, characteristic CA-MRSA with strong adhesin and cytolytic activities triggered influenza-related sequential complications.

Keywords: Community-associated methicillin-resistant Staphylococcus aureus; ST121/agr4 lineage; elderly community-acquired pneumonia; fatal aspiration pneumonia; influenza.

Fig. 1

Chest X-ray of the patient on admission day 1. A chest radiograph shows bilateral pulmonary infiltrates. Arrow 1 indicates an infiltrative shadow overlapping the second arch in the middle to lower field of the right lung, while arrow 2 indicates consolidation with air bronchograms on the lateral side of the left middle lung field.

Fig. 2

Molecular characteristics (a) and mRNA expression levels of the cytolytic peptide gene (psmα) (b) of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strain KT1. In (a), superscript letters indicate the following: ^athe split hlb gene due to the insertion of phage Sa3; ^beight adhesin genes are common among S. aureus; ^cthe 2,473-bp plasmid pWKT1 (GenBank Accession number, LC086373) carried ermC with the leader peptide sequence, responsible for inducible (ind) resistance to clindamycin; pWKT1 was 100% homologous to the pKH19 of MRSA spat1081 (GenBank Accession number, EU350089) and 99% homologous to the pOC160-1 of ST8 CA-MRSA from Russia (GenBank Accession number, AB982226); and ^daacA-aphD encoded for resistance (r) to gentamicin. (b) Strains KT1 and USA300-0114 are CA-MRSA, and strains N315 and Mu50 are healthcare-associated (HA-) MRSA. The mRNA expression levels of the phenol-soluble modulin α (PSMα) gene (psmα), normalized to 16S rRNA expression levels, were 0.64 ± 0.13, 0.05 ± 0.02, 0.07 ± 0.02 and 0.63 ± 0.13 for strains KT1, N315, Mu50 and USA300-0114, respectively; the psmα expression level of KT1 (no. 1) was similar to that of USA300-0114 (no. 2), and was significantly higher than that of HA-MRSA (p <0.01).