Decreased necrotizing fasciitis capacity caused by a single nucleotide mutation that alters a multiple gene virulence axis.

Single-nucleotide changes are the most common cause of natural genetic variation among members of the same species, but there is remarkably little information bearing on how they alter bacterial virulence. We recently discovered a single-nucleotide mutation in the group A Streptococcus genome that is epidemiologically associated with decreased human necrotizing fasciitis ("flesh-eating disease"). Working from this clinical observation, we find that wild-type mtsR function is required for group A Streptococcus to cause necrotizing fasciitis in mice and nonhuman primates. Expression microarray analysis revealed that mtsR inactivation results in overexpression of PrsA, a chaperonin involved in posttranslational maturation of SpeB, an extracellular cysteine protease. Isogenic mutant strains that overexpress prsA or lack speB had decreased secreted protease activity in vivo and recapitulated the necrotizing fasciitis-negative phenotype of the DeltamtsR mutant strain in mice and monkeys. mtsR inactivation results in increased PrsA expression, which in turn causes decreased SpeB secreted protease activity and reduced necrotizing fasciitis capacity. Thus, a naturally occurring single-nucleotide mutation dramatically alters virulence by dysregulating a multiple gene virulence axis. Our discovery has broad implications for the confluence of population genomics and molecular pathogenesis research.

Lack of a major role of Staphylococcus aureus Panton-Valentine leukocidin in lower respiratory tract infection in nonhuman primates.

Panton-Valentine leukocidin (PVL) is a two-component cytolytic toxin epidemiologically linked to community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections, including serious invasive infections caused by the epidemic clone referred to as strain USA300. Although PVL has long been known to be a S. aureus virulence molecule in vitro, the relative contribution of this leukotoxin to invasive CA-MRSA infections such as pneumonia remains controversial. We developed a nonhuman primate model of CA-MRSA pneumonia and used it to test the hypothesis that PVL contributes to lower respiratory tract infections caused by S. aureus strain USA300. The lower respiratory tract disease observed in this monkey model mimicked the clinical and pathological features of early mild to moderate S. aureus pneumonia in humans, including fine-structure histopathology. In this experiment using a large sample of monkeys and multiple time points of examination, no involvement of PVL in virulence could be detected. Compared with the wild-type parental USA300 strain, the isogenic PVL deletion-mutant strain caused equivalent lower respiratory tract pathology. We conclude that PVL does not contribute to lower respiratory tract infection in this nonhuman primate model of human CA-MRSA pneumonia.

Lower respiratory tract infection in cynomolgus macaques (Macaca fascicularis) infected with group A Streptococcus.

Group A Streptococcus (GAS), a human-specific pathogen, is best known for causing pharyngitis ("strep-throat") and necrotizing fasciitis ("flesh-eating disease"). However, the organism is also an uncommon but important cause of community-acquired bronchopneumonia, an infection with an exceptionally high mortality rate. Inasmuch as little is known about the molecular pathogenesis of GAS lower respiratory tract infection, we sought to develop a relevant human infection model. Nine cynomolgus macaques were infected by intra-bronchial instillation of either sterile saline or GAS (10(5) or 10(7) CFU). Animals were continuously monitored and sacrificed at five days post-inoculation. Serial bronchial alveolar lavage specimens and tissues collected at necropsy were used for histologic and immunohistochemical examination, quantitative microbial culture, lung and blood biomarker analysis, and in vivo GAS gene expression studies. The lower respiratory tract disease observed in cynomolgus macaques mimicked the clinical and pathological features of severe GAS bronchopneumonia in humans. This new monkey model will be useful for testing hypotheses bearing on the molecular pathogenesis of GAS in the lower respiratory tract.