Microbial Physiology最新文献

As an opportunistic pathogen of humans and animals, Staphylococcus aureus asymptomatically colonizes the nasal cavity but is also a leading cause of life-threatening acute and chronic infections. The evolution of S. aureus resulting from short- and long-term adaptation to diverse hosts is tightly associated with mobile genetic elements. S. aureus strains can carry up to four temperate phages, many of which possess accessory genes encoding staphylococcal virulence factors. More than 90% of human nasal isolates of S. aureus have been shown to carry Sa3int phages, whereas invasive S. aureus isolates tend to lose these phages. Sa3int phages integrate as prophages into the bacterial hlb gene, disrupting the expression of the sphingomyelinase Hlb, an important virulence factor under specific infection conditions. Virulence factors encoded by genes carried by Sa3int phages include staphylokinase, enterotoxins, chemotaxis-inhibitory protein, and staphylococcal complement inhibitor, all of which are highly human specific and probably essential for bacterial survival in the human host. The transmission of S. aureus from humans to animals is strongly correlated with the loss of Sa3int phages, whereas phages are regained once a strain is transmitted from animals to humans. Thus, both the insertion and excision of prophages may confer a fitness advantage to this bacterium. There is also growing evidence that Sa3int phages may perform "active lysogeny," a process during which prophages are temporally excised from the chromosome without forming intact phage particles. The molecular mechanisms controlling the peculiar life cycle of Sa3int phages remain largely unclear. Nevertheless, their regulation is likely fine-tuned to ensure bacterial survival within different hosts.

Escherichia coli is unable to grow on polymeric and oligomeric chitin, but grows on chitin disaccharide (GlcNAc-GlcNAc; N,N'-diacetylchitobiose) and chitin trisaccharide (GlcNAc-GlcNAc-GlcNAc; N,N',N''-triacetylchitotriose) via expression of the chb operon (chbBCARFG). The phosphotransferase system (PTS) transporter ChbBCA facilitates transport of both saccharides across the inner membrane and their concomitant phosphorylation at the non-reducing end, intracellularly yielding GlcNAc 6-phosphate-GlcNAc (GlcNAc6P-GlcNAc) and GlcNAc6P-GlcNAc-GlcNAc, respectively. We revisited the intracellular catabolism of the PTS products, thereby correcting the reported functions of the 6-phospho-glycosidase ChbF, the monodeacetylase ChbG, and the transcriptional regulator ChbR. Intracellular accumulation of glucosamine 6P-GlcNAc (GlcN6P-GlcNAc) and GlcN6P-GlcNAc-GlcNAc in a chbF mutant unraveled a role for ChbG as a monodeacetylase that removes the N-acetyl group at the non-reducing end. Consequently, GlcN6P- but not GlcNAc6P-containing saccharides likely function as coactivators of ChbR. Furthermore, ChbF removed the GlcN6P from the non-reducing terminus of the former saccharides, thereby degrading the inducers of the chb operon and facilitating growth on the saccharides. Consequently, ChbF was unable to hydrolyze GlcNAc6P-residues from the non-reducing end, contrary to previous assumptions but in agreement with structural modeling data and with the unusual catalytic mechanism of the family 4 of glycosidases, to which ChbF belongs. We also refuted the assumption that ChiA is a bifunctional endochitinase/lysozyme ChiA, and show that it is unable to degrade peptidoglycans but acts as a bona fide chitinase in vitro and in vivo, enabling growth of E. coli on chitin oligosaccharides when ectopically expressed. Overall, this study revises our understanding of the chitin, chitin oligosaccharide, and chitin disaccharide metabolism of E. coli.

Bacterial predation is a ubiquitous and fundamental biological process, which influences the community composition of microbial ecosystems. Among the best characterised bacterial predators are the myxobacteria, which include the model organism Myxococcus xanthus. Predation by M. xanthus involves the secretion of antibiotic metabolites and hydrolytic enzymes, which results in the lysis of prey organisms and release of prey nutrients into the extracellular milieu. Due to the generalist nature of this predatory mechanism, M. xanthus has a broad prey range, being able to kill and consume Gram-negative/positive bacteria and fungi. Potential prey organisms have evolved a range of behaviours which protect themselves from attack by predators. In recent years, several investigations have studied the molecular responses of a broad variety of prey organisms to M. xanthus predation. It seems that the diverse mechanisms employed by prey belong to a much smaller number of general "predation resistance" strategies. In this mini-review, we present the current state of knowledge regarding M. xanthus predation, and how prey organisms resist predation. As previous molecular studies of prey susceptibility have focussed on individual genes/metabolites, we have also undertaken a genome-wide screen for genes of Pseudomonas aeruginosa which contribute to its ability to resist predation. P. aeruginosa is a World Health Organisation priority 1 antibiotic-resistant pathogen. It is metabolically versatile and has an array of pathogenic mechanisms, leading to its prevalence as an opportunistic pathogen. Using a library of nearly 5,500 defined transposon insertion mutants, we screened for "prey genes", which when mutated allowed increased predation by a fluorescent strain of M. xanthus. A set of candidate "prey proteins" were identified, which shared common functional roles and whose nature suggested that predation resistance by P. aeruginosa requires an effective metal/oxidative stress system, an intact motility system, and mechanisms for de-toxifying antimicrobial peptides.

Predatory bacteria gained interest in the last 20 years. Nevertheless, only a few species are well characterized. The endobiotic predator Bdellovibrio bacteriovorus invades its prey to consume it from the inside, whereas Myxococcus xanthus hunts as a whole group to overcome its prey. Both species were described to prey on cyanobacteria as well. This minireview summarizes the findings of the last 20 years of predatory bacteria of cyanobacteria and is supplemented by new findings from a screening experiment for bacterial predators of the model organism Anabaena variabilis PCC 7937. Known predatory bacteria of cyanobacteria belong to the phyla Proteobacteria, Bacteroidetes, and Firmicutes and follow different hunting strategies. The underlying mechanisms are in most cases not known in much detail. Isolates from the screening experiment were clustered after predation behaviour and analyzed with respect to their size. The effect of predation in high nitrate levels and the occurrence of nitrogen-fixing cells, called heterocysts, are addressed.

The betaproteobacterial genus Aromatoleum comprises facultative denitrifiers specialized in the anaerobic degradation of recalcitrant organic compounds (aromatic and terpenoid). This study reports on the complete and manually annotated genomes of Ar. petrolei ToN1T (5.41 Mbp) and Ar. bremense PbN1T (4.38 Mbp), which cover the phylogenetic breadth of the genus Aromatoleum together with previously genome sequenced Ar. aromaticum EbN1T [Rabus et al., Arch Microbiol. 2005 Jan;183(1):27-36]. The gene clusters for the anaerobic degradation of aromatic and terpenoid (strain ToN1T only) compounds are scattered across the genomes of strains ToN1T and PbN1T. The richness in mobile genetic elements is shared with other Aromatoleum spp., substantiating that horizontal gene transfer should have been a major driver in shaping the genomes of this genus. The composite catabolic network of strains ToN1T and PbN1T comprises 88 proteins, the coding genes of which occupy 86.1 and 76.4 kbp (1.59 and 1.75%) of the respective genome. The strain-specific gene clusters for anaerobic degradation of ethyl-/propylbenzene (strain PbN1T) and toluene/monoterpenes (strain ToN1T) share high similarity with their counterparts in Ar. aromaticum strains EbN1T and pCyN1, respectively. Glucose is degraded via the ED-pathway in strain ToN1T, while gluconeogenesis proceeds via the reverse EMP-pathway in strains ToN1T, PbN1T, and EbN1T. The diazotrophic, endophytic lifestyle of closest related genus Azoarcus is known to be associated with nitrogenase and type-6 secretion system (T6SS). By contrast, strains ToN1T, PbN1T, and EbN1T lack nif genes for nitrogenase (including cofactor synthesis and enzyme maturation). Moreover, strains PbN1T and EbN1T do not possess tss genes for T6SS, while strain ToN1T does and facultative endophytic "Aromatoleum" sp. CIB is known to even have both. These findings underpin the functional heterogeneity among Aromatoleum members, correlating with the high plasticity of their genomes.

Newly synthesized secretory proteins are released into the lumen of the endoplasmic reticulum (ER). The secretory proteins are surrounded by coat protein complex II (COPII) vesicles, and transported from the ER and reach their destinations through the Golgi apparatus. Sec12p is a guanine nucleotide exchange factor for Sar1p, which initiates COPII vesicle budding from the ER. The activation of Sar1p by Sec12p and the subsequent COPII coat assembly have been well characterized, but the events that take place upstream of Sec12p remain unclear. In this study, we isolated the novel extragenic suppressor of sec12-4, PIN4/MDT1, a cell cycle checkpoint target. A yeast two-hybrid screening was used to identify Pin4/Mdt1p as a binding partner of the casein kinase I isoform Hrr25p, which we have previously identified as a modulator of Sec12p function. Deletion of PIN4 suppressed both defects of temperature-sensitive growth and the partial protein transport observed in sec12-4 mutants. The results of this study suggest that Pin4p provides novel aspects of Sec12p modulations.