Carbapenem-resistant Enterobacteriaceae (CRE) have emerged as major causes of health care-associated infections worldwide. This diverse collection of organisms with various resistance mechanisms is associated with increased lengths of hospitalization, costs of care, morbidity, and mortality. The global spread of CRE has largely been attributed to dissemination of a dominant strain of Klebsiella pneumoniae producing a serine β-lactamase, termed K. pneumoniae carbapenemase (KPC). Here we report an outbreak of KPC-producing CRE infections in which the degree of horizontal transmission between strains and species of a promiscuous plasmid is unprecedented. Sixteen isolates, comprising 11 unique strains, 6 species, and 4 genera of bacteria, were obtained from 14 patients over the first 8 months of the outbreak. Of the 11 unique strains, 9 harbored the same highly promiscuous plasmid carrying the KPC gene blaKPC. The remaining strains harbored distinct blaKPC plasmids, one of which was carried in a strain of Klebsiella oxytoca coisolated from the index patient and the other generated from transposition of the blaKPC element Tn4401. All isolates could be genetically traced to the index patient. Molecular epidemiological investigation of the outbreak was aided by the adaptation of nested arbitrary PCR (ARB-PCR) for rapid plasmid identification. This detailed molecular genetic analysis, combined with traditional epidemiological investigation, provides insights into the highly fluid dynamics of drug resistance transmission during the outbreak.

Tumor necrosis factor (TNF)-alpha-dependent apoptosis of alveolar macrophages (AM) after infection with avirulent Mycobacterium tuberculosis (Mtb) results in bacillary death and the destruction of a growth niche for the pathogen. This response is minimized after infection with virulent strains of Mtb. To study the genetic control of Mtb-induced apoptosis, we used microarrays to interrogate the expression profile of infected human AM. Although we found variation in gene expression between different donors of AM, a set of genes were constant for each condition. A group of proapoptotic genes were downregulated after infection by virulent Mtb strain H37Rv, whereas infection with avirulent Mtb H37Ra led to a gene expression profile that would favor macrophage apoptosis. Neutralizing TNF in macrophage cultures infected with H37Ra changed the gene expression profile to one that resembled the profile of macrophages infected with H37Rv. These data reveal that apoptosis-related genes are regulated differently by virulent or attenuated Mtb strains, and are consistent with the hypothesis that virulent Mtb interfere with TNF death signaling. Given the importance of TNF in host defense against tuberculosis, the ability to repress the expression of genes activated by TNF may constitute a bacillary virulence mechanism.