Mycobacterium avium serovars 2 and 8 infections elicit unique activation of the host macrophage immune responses.

Mycobacterium avium is an opportunistic pathogen whose pathogenesis is attributed to its serovar-specific glycopeptidolipid (ssGPL), which varies among its 31 serovars. To determine if the presence and type of ssGPLs contribute to M. avium pathogenesis, we infected murine macrophages (mφs) with two M. avium wild type (wt) serovars (2 and 8) and their serovar-null strains. We examined the influence of ssGPL (presence and type) on cytokine production in non-activated (-IFN-γ) and activated (+IFN-γ) mφs, and the bacterial intra-mφ survival over a 6-day infection process. Serovar-2 infections activated TNF-α production that increased over the 6 day period and was capable of controlling the intra-mφ serovar-2 null strain. In contrast, the serovar-8 infection stimulated a strong pro-inflammatory response, but was incapable of removing the invading pathogen, maybe through IL-10 production. It was clear that the intracellular growth of serovar-null in contrast to the wt M. avium strains was easily controlled. Based on our findings and the undisputed fact that M. avium ssGPL is key to its pathogenesis, we conclude that it is not appropriate to dissect the pathogenesis of one M. avium serovar and apply those findings to other serovars.

Mycobacterium avium and modulation of the host macrophage immune mechanisms.

Pathogenesis by mycobacteria requires the exploitation of host-cell signaling pathways to enhance intracellular survival and persistence of the pathogen. Among patients with end-stage acquired immune-deficiency syndrome, disseminated infection with Mycobacterium avium, a member of the M. avium complex (MAC), is the most common bacterial infection. The virulence and intrinsic multidrug resistance of this pathogen has been attributed in part to its unique cell wall, which is a complex array of hydrocarbon chains containing the arabinogalactan-peptidoglycan mycolic acid core found in all mycobacteria, surrounded by a second electron-dense layer made up, in part, of serovar-specific glycopeptidolipids (GPLs) found only in MAC. Via cell-surface receptors, M. avium, an intra-macrophage (mφ) pathogen, can modulate various host signaling pathways such as the mitogen-activated protein kinase and nuclear factor κB pathways. The modulation of specific mφ signaling cascades can result in the regulation of pro- and anti-inflammatory cytokine production, and the process of phagolysosome fusion. The outcome of this M. avium-host mφ interaction could result in host disease or death of the invading pathogen. This review will focus on the immunomodulation aspects of M. avium pathogenesis as well as the role of GPLs as virulence factors.

snr-1 gene is required for nitrate reduction in Pseudomonas aeruginosa PAO1.

Pseudomonas aeruginosa is able to use nitrate for both assimilation and anaerobic respiration. One set of genes, designated snr (for "shared nitrate reduction"), have been recently cloned and partially characterized. In this study, we demonstrate that the snr-1 gene encodes a predicted 52.5-kDa protein that is 82% similar to a unique cytochrome c of Desulfomonile tiedjei DCB-1. Importantly, the Snr-1 protein sequence of P. aeruginosa differed from that of the cytochrome c of D. tiedjei primarily in the first 25 amino acids, which are required for membrane attachment in D. tiedjei. In P. aeruginosa, the Snr-1 protein hydropathy profile indicates that it is a soluble protein. An isogenic snr-1::Gm insertional mutant was unable to grow aerobically with nitrate as a sole nitrogen source or anaerobically with nitrate as an electron acceptor. Complementation of the snr-1::Gm mutant with the snr-1 gene restored the wild-type phenotypes. Interestingly, anaerobic growth rates were significantly higher in the snr-1 mutant harboring a multicopy plasmid containing snr-1. In contrast, aerobic growth rates of the restored mutant using nitrate as the sole nitrogen source were similar to those of the wild type. Transcriptional lacZ fusions demonstrated that snr-1 was not regulated by molybdate, oxygen, or nitrate.

Snr, new genetic loci common to the nitrate reduction systems of Pseudomonas aeruginosa PAO1.

Pseudomonas aeruginosa is able to both assimilate and dissimilate nitrate. On the basis of the characteristics of mutants unable to dissimilate or assimilate nitrate to nitrite, it was revealed that two different sets of genes (represented by Class I and Class II mutants) were shared between the nitrate-to-nitrite reduction steps of both pathways. The genes represented by Class I and II mutants have been separated into distinct genetic loci using two cosmids, pAD1695/96. The two different genetic loci have been designated snr (shared nitrate reduction) and mol (MoCo processing genes) based on the phenotypic characteristics of the mutants complemented. Restriction analyses of pAD1695/96 followed by subcloning confirmed the complementation results. The snr loci, which represent a unique and hitherto uninvestigated set of genes for nitrate reduction, were mapped on the P. aeruginosa chromosome by linkage analysis with sex factor FP2.