An unwanted Valentine in New York - A new study describes the epidemic expansion of community acquired MRSA (CA-MRSA) within an orthodox Jewish community of Brooklyn, NYC.

The USA300 clone is characterised, in part, by the presence of Panton Valentine Leukocidin – a toxin that damages skin and soft tissues, often causing abscesses. The authors describe the evolutionary epidemiology of a new variant of this nationally dominant clone. Their analysis uncovers a series of unfortunate events that has led to the development of what may become a serious public health threat.

1. Virulence assays identified a clone-specific prophage (viral DNA integrated into bacterial DNA) that was demonstrated to promote large abscess formation; and abscess formation is known to facilitate the transmission of MRSA.
2. Genomic analysis also identified a mutation of the regulator of the pyrimidine nucleotide biosynthetic operon (pyrR) resulting in metabolic changes that improved colonization ability for this variant.
3. This variant acquired resistance genes for mupirocin and chlorhexidine - both agents commonly used for decolonization and prevention of this type of infection; and this acquisition of resistance genes was possibly facilitated by the clone-specific prophage.

The authors state that “the present report demonstrates how the relationship between virulence and transmissibility in pathogens such as CA-MRSA, in which disease and transmission are tightly linked, can lead to a new antimicrobial-resistance threat”. The PNAS study can be found here.