Panton-Valentine Leukocidin-Positive Methicillin-Resistant Staphylococcus Aureus with Reduced Vancomycin Susceptibility: An Emerging Trend?

Methicillin-resistant Staphylococcus aureus (MRSA) is a major multidrug-resistant nosocomial pathogen. This class of MRSA, first reported in the early 1960s and now termed healthcare-associated MRSA (HA-MRSA), was followed by a newer class of MRSA, community-associated MRSA (CA-MRSA). The unique feature of the initial CAMRSA included Panton-Valentine leukocidin (PVL), an abscess-associated toxin and also S. aureus spread factor. CA-MRSA usually causes skin and soft-tissue infections, but occasionally causes invasive infections, including (necrotizing) pneumonia, sometimes preceded by respiratory virus infections. The most successful CA-MRSA USA300 (ST8/SCC mec IVa) caused an epidemic in the United States. In Russia, we first detected PVL-positive CAMRSA (ST30/SCC mec IVc) in Vladivostok in 2006, but with no more PVL-positive MRSA isolation. However, we recently isolated four lineages of PVL-positive MRSA in Krasnoyarsk. Regarding chemotherapy against invasive MRSA infections, vancomycin still remains a gold standard, in addition to some other anti-MRSA agents such as teicoplanin, linezolid, and daptomycin. For resistance, vancomycin-resistant MRSA (VRSA) with MICs of ≥16 μg/mL appeared in patients, but cases are still limited. However, clinically, infections from strains with MICs of ≥1.5 μg/mL, even albeit with susceptible MICs (≤2 μg/mL), respond poorly to vancomycin. Some of those bacteria have been bacteriologically characterized as vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA), generally with HA-MRSA genetic backgrounds. The features of the above PVL-positive Krasnoyarsk MRSA include reduced susceptibility to vancomycin, which meets the criteria of hVISA. In this review, we discuss a possible new trend of PVL-positive hVISA, which may spread and threaten human health in community settings.