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At least 1,153 records · Page 64Linked to original sources
PubMed2001

Stochastic modelling of the growth of a microbial population under changing temperature regimes.

The application of models of microbial growth to the design of food safety systems requires consideration of the effect of arbitrary changes in external variables on growth of bacteria. In particular, the effect of changes in external variables, such as temperature, on the probability that the microbial population size will not exceed acceptable levels at a given time needs to be predicted. This paper presents a method of calculating the time-dependent probability distribution of the microbial population size under arbitrary changes of temperature through time. To illustrate this method, the effect of a sudden temporary increase in temperature on the evolution of the probability distribution of Lactobacillus plantarum population size is presented. The effect of this change in temperature on the time taken for the population to reach a critical size, with a given probability, is also calculated and the application of this calculation to the design of HACCP protocols is discussed.

Food Microbiology↗
PubMed2003

Novel Proteorhodopsin variants from the Mediterranean and Red Seas.

Proteorhodopsins, ubiquitous retinylidene photoactive proton pumps, were recently found in the widespread uncultured SAR86 bacterial group in oceanic surface waters. To survey proteorhodopsin diversity, new degenerate sets of proteorhodopsin primers were designed based on a genomic proteorhodopsin gene sequence originating from an Antarctic fosmid library. New proteorhodopsin variants were identified in Red Sea samples that were most similar to the original green-light absorbing proteorhodopsins found in Monterey Bay California. Unlike green-absorbing proteorhodopsins however, these new variants contained a glutamine residue at position 105, the same site recently shown to control spectral tuning in naturally occurring proteorhodopsins. Different proteorhodopsin variants were also found in the Mediterranean Sea. These proteorhodopsins formed new and distinctive proteorhodopsin groups. Phylogenetic analyses show that some of the new variants were very different from previously characterized proteorhodopsins, and formed the deepest branching groups found so far among marine proteorhodopsins. The existence of these varied proteorhodopsin sequences suggests that this class of proteins has undergone substantial evolution. These variants could represent functionally divergent paralogous genes, derived from the same or similar species, or orthologous proteorhodopsins that are distributed amongst divergent planktonic microbial taxa.

Amino Acid Sequence↗
PubMed1997

Functional complementation of the yeast divalent cation transporter family SMF by NRAMP2, a member of the mammalian natural resistance-associated macrophage protein family.

The mammalian NRAMP gene family has two members, NRAMP1 and NRAMP2 that encode integral membrane proteins. Nramp1 is expressed exclusively in macrophages where it is found in the phagosomal membrane, and NRAMP1 mutations cause susceptibility to infection by abrogating the capacity of macrophages to control intracellular microbial replication. Nramp2 is highly similar to Nramp1, but is expressed in several tissues and cell types. The Nramp protein family is remarkably conserved throughout evolution, and recent data suggest that the mammalian Nramp2 and the yeast homologues Smf1 and Smf2 transport divalent cations. We tested whether structural similarity between the mammalian Nramp and the yeast Smf proteins results in functional complementation in yeast. Wild-type and mutant variants of the Nramp1 and Nramp2 proteins were expressed in a yeast mutant bearing null alleles at the SMF1 and SMF2 loci, and complementation of the phenotypes of this yeast mutant was investigated. Nramp2, but not Nramp1, was found to complement hypersensitivity to EGTA of the smf1/smf2 mutant under oxidative stress conditions (methyl viologen). We also observed that the smf1/smf2 double mutant is hypersensitive to growth at alkaline pH (pH 7.9) and that Nramp2 could complement this phenotype as well. Complementation by Nramp2 was specific and required a functional protein as independent mutations in residues highly conserved in all members of the Nramp family abrogated Nramp2 complementation. Since Mn2+ was the only divalent cation capable of completely suppressing both the EGTA and pH phenotypes, our results suggest that Nramp2 can transport Mn2+ in yeast.

Amino Acid Sequence↗
PubMed2003

Nitrogenase gene diversity and microbial community structure: a cross-system comparison.

Biological nitrogen fixation is an important source of fixed nitrogen for the biosphere. Microorganisms catalyse biological nitrogen fixation with the enzyme nitrogenase, which has been highly conserved through evolution. Cloning and sequencing of one of the nitrogenase structural genes, nifH, has provided a large, rapidly expanding database of sequences from diverse terrestrial and aquatic environments. Comparison of nifH phylogenies to ribosomal RNA phylogenies from cultivated microorganisms shows little conclusive evidence of lateral gene transfer. Sequence diversity far outstrips representation by cultivated representatives. The phylogeny of nitrogenase includes branches that represent phylotypic groupings based on ribosomal RNA phylogeny, but also includes paralogous clades including the alternative, non-molybdenum, non-vanadium containing nitrogenases. Only a few alternative or archaeal nitrogenase sequences have as yet been obtained from the environment. Extensive analysis of the distribution of nifH phylotypes among habitats indicates that there are characteristic patterns of nitrogen fixing microorganisms in termite guts, sediment and soil environments, estuaries and salt marshes, and oligotrophic oceans. The distribution of nitrogen-fixing microorganisms, although not entirely dictated by the nitrogen availability in the environment, is non-random and can be predicted on the basis of habitat characteristics. The ability to assay for gene expression and investigate genome arrangements provides the promise of new tools for interrogating natural populations of diazotrophs. The broad analysis of nitrogenase genes provides a basis for developing molecular assays and bioinformatics approaches for the study of nitrogen fixation in the environment.

Archaea↗
PubMed1978

[Microbial metabolic activity and transmembrane transport phenomena by potentiometric analysis of lipoic acid oxidation-reduction, in a minimal culture medium].

A method of measuring and studying metabolic bacterial activity is proposed, by following the kinetic evolution of the ratio of the oxidized and reduced forms of an electron transporter as a consequence of decreasing oxidizing power--due to oxygen consumption in the culture,--and increasing. Reduction power of bacterial activity. Namely, with minimum composition using salts and glucose the oxido-reduction of lipoic acid is well indicated by a gold electrode without any major bio-or electrochemical interference. A kinetic diffusion reaction theory takes into account the passive or active transmembrane transport of lipoic acid in good agreement with the experimentally observed shapes of the electrical signal. The various types of antibiotic activities are well reflected by the modifications of the signal.

Bacteria↗
PubMed2004

Bugs and the gut: an unstable marriage.

There is a symbiotic relationship between the gastrointestinal microflora and the human host. Commensal bacteria provide essential nutrients to the epithelium and promote healthy immune responses in the gut. Commensal bacteria such as Escherichia coli can, however, transform into pathogens when they acquire genetic material encoding virulence factors such as adhesins, enterotoxins, invasins and cytotoxins. Enterovirulent organisms 'communicate' with the host by a variety of diverse mechanisms; these underpin the pathogenic processes that are essential for the expression of diarrhoeal disease. Many of these mechanisms involve the activation of signal transduction pathways in epithelial cells. Classical pathways include activation of adenylate or guanylate cyclases to produce chloride secretion, and subversion of cytoskeletal functions to effect intimate attachment with or without invasion of epithelial cells. Other systems are also involved, including inflammatory cells and local neuroendocrine networks. Understanding the complex interactions between the human gastrointestinal tract and the commensals and pathogens which it encounters will hopefully help us to exploit further the beneficial effects of the 'marriage' and to find new ways of preventing and treating microbial disease of the intestine which occurs when the symbiotic arrangement breaks down.

Biological Evolution↗
PubMed1973

Transposition of the arsenate resistance locus of Bacillus subtilis strains 23 and 168.

Wild-type Bacillus subtilis strains 23 and 168 are resistant to high concentrations of sodium arsenate. The genetic configurations of the arsenate resistance loci of these two related strains of B. subtilis have been characterized. The transformable 168 strain has a single resistance locus which maps between phe and aroD in the terminal third of the genome. In contrast, strain 23 is shown to have its single arsenate resistance locus between purB and thr in the first third of the bacterial chromosome. Moreover, in strain 23 the chromosomal segment equivalent to the phe-linked asa region of 168 strains is missing. DNA isolated from 23 strains is able to transform 168 arsenate-sensitive strains to resistance and the heterologous 23 DNA is found to preferentially establish a new purB linked asa locus in such transformed cells. Thus, the majority of phenotypically arsenate-resistant cells recovered after exposure of competent 168 sensitive mutants to 23 DNA are "heterozygous" and still retain their phe-linked mutated asa locus. The tolerance of several of these heterologously transformed hybrid strains to arsenate suggests that the 168 and 23 asa gene products are similar, and a transposition model for the evolution of arsenate resistance in B. subtilis is proposed.

Arsenic↗
PubMed2005

Group-beneficial traits, frequency-dependent selection and genotypic diversity: an antibiotic resistance paradigm.

The evolution of group-beneficial traits potentially allows the survival of 'cheaters' that would otherwise be unfit. Here we describe experimental work on group-beneficial traits and the consequences of frequency-dependent selection in the context of bacterial antibiotic resistance. We constructed a 'self-limited antibiotic resistant' (SLAR) strain of Escherichia coli in which a TEM-1 beta-lactamase was anchored to the inner membrane. In pairwise competition experiments between the SLAR strain and ampicillin-sensitive strains, only the SLAR strain survived in the presence of ampicillin. We also constructed a 'shared antibiotic resistant' (SAR) strain in which TEM-1 beta-lactamase protected both the SAR strain and nearby sensitive cells, thus acting as a model for a genetically defined group-beneficial trait. In pairwise competition experiments of the SAR strain against two different sensitive strains of E. coli, we found that the sensitive strains maintained themselves at frequencies of 5-12% in the presence of ampicillin. When the relative cost of the SAR strain was lowered, its equilibrial frequency rose. Sensitive strains also arose from pure cultures of the SAR strain. In these cases, too, the sensitive 'cheaters' were maintained in ampicillin at frequencies comparable to those observed in the previous competitions. These results suggest that traits which benefit other group members can permit survival of genotypes that otherwise would be eliminated by natural selection, and allow the maintenance of greater genetic variation upon which evolution can operate.

Ampicillin↗
PubMed1990

Antimicrobial resistance in the intensive care unit.

Antimicrobial resistance remains a major clinical problem in the intensive care unit despite the introduction of potent new antibiotics and the application of infection control measures. Impaired antibiotic permeability, antibiotic inactivation and alteration of antibiotic target sites, typically in concert, are mechanisms by which this resistance occurs. Resistance traits may be chromosomally mediated, in which case they may be expressed constitutively or inducibly, or may be mediated by plasmids and/or transposons that may confer resistance to multiple drugs and facilitate spread to other organisms. Bacteria in which clinically significant resistance has now become a potential problem include methicillin-resistant staphylococci, enterococci, a wide variety of Enterobacteriaceae, Pseudomonas aeruginosa, Pseudomonas cepacia, and Xanthomonas (Pseudomonas) maltophilia. Judicious limitation in the use of antimicrobials, especially in prophylactic regimens; use of antibiotics in appropriate doses; and, where possible, avoidance of drugs to which resistance has been shown to emerge rapidly in a specific clinical setting will help to minimize the evolution and spread of resistant bacterial isolates.

Cross Infection↗
PubMed1985

[Evolution of Acinetobacter calcoaceticus in the hospital milieu, from 1971 to 1984].

During the last 10 years the authors have evaluated the increasing part played by Acinetobacter calcoaceticus in nosocomial infections and the increasing resistance of this species to antibiotics. The study involved 850 clinical strains isolated from 1971 to 1984, and 24 antibiotics were tested. A progressive increase in resistance to beta-lactam antibiotics, aminoglycosides and tetracycline was observed, and to date up to 80-90% of the strains are resistant to all but major drugs such as imipenem, ceftazidime, tobramycin and amikacin. A significant difference in susceptibility to the major antibiotics was noticed between Acinetobacter clinical strains isolated before and after 1980. A correlation study between the development of resistant strains and the hospital consumption of antibiotics showed that this was a factor to be taken into account. After the new Bichat Hospital was opened, in 1980, the rate at which var. anitratum strains were isolated rose from 77.5% to 94.5%. The opening of new departments (intensive care units) with a higher risk of infection and a totally different environment probably constitutes the most important factor in this evolution.

Acinetobacter↗
PubMedDate not supplied

Proinflammatory cytokines production and PMN-elastase release from activated PMN cells in the periodontal disease.

The aim of this study was to evaluate the local changes in the crevicular gingival fluid (CGF) determined by the inflammatory and immune response in periodontitis and gingivitis. The selected patients presented gingivitis (n = 9) and periodontitis: aggressive periodontitis (n = 21) and adult periodontitis (n = 8). The crevicular fluid was provided from the gingival and periodontal pocket. The measurement of PMN-elastase in the CGF, using the ELISA method, showed a significant (p < 0.01) increase of the enzyme concentration in the aggressive periodontitis group (62.1 +/- 3.91 ng/ml) comparing to the gingivitis group (33.04 +/- 4.14 ng/ml) but also the increase (p < 0.05) of this enzyme in the adult periodontitis (43.6 +/- 2.16 ng/ml) comparing to the gingivitis, which indicated the evolutive aspects of the inflammatory reaction in these diseases. The increased production of PMN-E is the result of the activation of polymorphonuclear cells (PMN) as a reaction of the microbial attack. Degranulation and release of proteolytic enzymes including elastase, which present cytotoxic capacities, follow the activation of neutrophil granulocytes (PMN). The activated granulocytes release proinflammatory cytokines IL-1, TNF-alpha which augment the inflammatory immune response. The aggressive periodontitis group showed an increased CGF level of IL-1 (780.4 +/- 104 pg/ml) comparing to the gingivitis group (275.5 +/- 78 pg/ml) (p < 0.01). TNF-alpha also presented an increased level (p < 0.01) in the aggressive periodontitis group (16.3 +/- 2.3 pg/ml) comparing to the gingivitis group (4.1 +/- 1.2 pg/ml) as a consequence of the periodontium destruction and of the tissular necrosis in the former group. In conclusion, our study shows a significant increase of the PMN-elastase and proinflammatory cytokines level in CGF of patients with gingivitis and periodontitis. The intensity of the inflammatory response in these diseases is strongly correlated to the activation of the neutrophil granulocytes which release these biological active molecules that could be used as evolution markers of the disease.

Adolescent↗