For contemporary pH1N1 viruses, the cost or advantage of drug resistance is so subtle that different groups have come to different conclusions about its relevance. In a hospital setting there have been reports that suggest that patient-to-patient transmission of drug-resistant virus has occurred amongst immunocompromised individuals. 16 Hamelin et al. 17 showed that oseltamivir-resistant pH1N1 virus was equally virulent as its wild-type counterpart in mice and ferrets and did transmit to co-housed animals, though they did not assess droplet transmission. Seibert et al. 18 used guinea pigs and ferrets in both contact and droplet transmission studies and concluded that the drug-resistant mutant could potentially circulate in the community. However, a detailed analysis of their data reveals that for one of the viruses they tested, there was a 2 day delay in transmission to half the contact-exposed guinea pigs and for the other strain of pH1N1 there was a reduction in droplet transmission to 88% rather than 100% of exposed sentinels. The ferret experiments were conducted with n¼ 1 so it is difficult to be sure of their significance. Similarly, in the manuscript from Kiso et al., 19 the conclusion is again that the H275Y mutant transmits through the air between ferrets, but there is a 2 day delay in transmission of one of the mutant strains of virus studied. Conversely, Duan et al. 20 found that the drug-resistant virus did not transmit between ferrets by the respiratory droplet route and that in co-infected animals, the wild-type virus outgrew the resistant mutant and was uniquely transmitted to contact animals. Thus, the current picture from animal experiments is confused and discrepant. This might be partly due to the use of different strains of pH1N1 virus as well as different experimental protocols used by the various investigators. Anecdotal evidence from the clinic shows that, in most instances, contemporary drug-resistant variants of pH1N1 were replaced by drug-susceptible variants when the selective pressure of oseltamivir was removed, suggesting that wild-type isolates are fitter in vivo in humans. 21 Whether the subtle fitness deficit reported here for one particular strain of drug-resistant mutant pH1N1 virus explains the epidemiological observation that mutant virus has not circulated through the community is not clear, since many other factors including heterogeneous mixing of populations and stochastic effects may influence whether a particular virus mutant predominates. Nonetheless the virus competition assay conducted in HAE cultured cells described here offers an alternative biologically relevant model as a useful adjunct to animal studies and this system may more accurately reflect the environment in which virus replicates in otherwise healthy humans. Information from a variety of model systems should be combined to guide the appropriate use of oseltamivir. Such knowledge clearly needs to be revised specifically for each novel influenza virus that emerges either as a seasonal strain by drift or as a pandemic virus by antigenic shift.  Ketamine inhibits tumor necrosis factor secretion by RAW264.7 murine macrophages stimulated with antibiotic-exposed strains of community-associated, methicillin-resistant Staphylococcus aureus BACKGROUND: Infections caused by community-associated strains of methicillin-resistant Staphylococcus aureus (CA-MRSA) are associated with a marked and prolonged host inflammatory response. In a sepsis simulation model, we tested whether the anesthetic ketamine inhibits the macrophage TNF response to antibiotic-exposed CA-MRSA bacteria via its antagonism of N-methyl-D-aspartate (NMDA) receptors. RAW264.7 cells were stimulated for 18 hrs with 10(5 )to 10(7 )CFU/mL inocula of either of two prototypical CA-MRSA isolates, USA300 strain LAC and USA400 strain MW2, in the presence of either vancomycin or daptomycin. One hour before bacterial stimulation, ketamine was added with or without MK-801 (dizocilpine, a chemically unrelated non-competitive NMDA receptor antagonist), APV (D-2-amino-5-phosphono-valerate, a competitive NMDA receptor antagonist), NMDA, or combinations of these agents. Supernatants were collected and assayed for TNF concentration by ELISA. RESULTS: RAW264.7 cells exposed to either LAC or MW2 in the presence of daptomycin secreted less TNF than in the presence of vancomycin. The addition of ketamine inhibited macrophage TNF secretion after stimulation with either of the CA-MRSA isolates (LAC, MW2) in the presence of either antibiotic. The NMDA inhibitors, MK-801 and APV, also suppressed macrophage TNF secretion after stimulation with either of the antibiotic-exposed CA-MRSA isolates, and the effect was not additive or synergistic with ketamine. The addition of NMDA substrate augmented TNF secretion in response to the CA-MRSA bacteria, and the addition of APV suppressed the effect of NMDA in a dose-dependent fashion. CONCLUSIONS: Ketamine inhibits TNF secretion by MRSA-stimulated RAW264.7 macrophages and the mechanism likely involves NMDA receptor antagonism. These findings may have therapeutic significance in MRSA sepsis. Infections caused by community-associated strains of methicillin-resistant Staphylococcus aureus (CA-MRSA) present a major public health problem because of recent increases in the incidence of these infections [1, 2] . In a 2007 report, the Centers for Disease Control concluded that Staphylococcus aureus is now the most important cause of serious and fatal infection in the United States [3] . The prototypical USA400 strain, MW2, (CDC nomenclature for this strain of MRSA) was first isolated in 1999 from a Midwest child with fatal CA-MRSA pneumonia [4] . In 2003, the prototypical USA300 CA-MRSA strain, LAC, was isolated from Los Angeles County patients with skin and soft tissue infections, severe pneumonia and sepsis. Recently, concerns about CA-MRSA infections were heightened after reports of severe invasive staphylococcal infections in some patients infected with the novel 2009 H1N1 influenza A virus [5, 6] .