967 research outputs found
Antibiotic residues and R-plasmid selection: are in vitro methods good models?
Three clones of E. coli, one of which was harbouring a tetracycline resistance plasmid were inoculated together into the stomach of axenic mice. Without antibiotic selective pressure, the R-Plasmid bearing strain became dominant in the faeces of mice, while the R-plasmid free strain was eliminated. When the R-plasmid bearing strain was given to mice 4 days after the inoculation with the R-plasmid free strain, it was repressed and remained at the stable level of 10(4.5) organisms per g of faeces. But a rapid spread of the R-plasmid was observed, tetracycline resistant bacteria become dominant within one day, and replace the tetracycline sensitive E. coli. The tetracycline resistance plasmid did not disadvantage the mediating strain in the gut, even in the absence of antibiotic pressure. In contrast Lebek and Egger (1983), studying the same strains in vitro, found that in a chemostat the plasmid bearing strain was overgrown by the plasmid free strain. These results strongly suggest that in vitro interactions between E. coli strains cannot be directly extrapolated to in vivo conditions. For the determination of the no-effect level of antibiotic residue on the selection of R-factor in the gut, studies should be made in vivo
Extensions of the chemostat model with flocculation
In this work, we study a model of the chemostat where the species are present
in two forms, isolated bacteria and under an aggregated form like attached
bacteria or bacteria in flocks. We show that our general model contains a lot
of models which were previously considered in the literature. Assuming that
flocculation and deflocculation dynamics are fast with respect to the growth of
the species, we construct a reduced chemostat-like model in which both the
growth functions and the apparent dilution rate depend on the density of the
species. We also show that such a model involving monotonic growth rates may
exhibit bistability, while it may only occur in the classical chemostat model
when the growth rate in non monotonic
Growth of an Adherent Mixed Microbial Culture in a Substrate Limited Single State Chemostat
This study was supported by the Office of Water Resources Research U.S. Department of the Interior under Project Number A-021-OHIOA steady-state was established between a C. lividum and a Pseudomonas sp at a dilution rate of 0.27 hr^-1 when the growth limiting substrate was citrate. During both pure and mixed continuous culture studies, the C. lividum adhered to the wall of the chemostat and the Pseudomonas showed no such tendency visually. This system offers a convenient model for studying the importance of bacterial adherence to certain aquatic ecosystems such as river sediments and sewage treatment.Summary -- Introduction -- Methods -- Results -- Discussion -- Reference
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Adaptation of bacteriophage to variable environments
Because a virus is an obligate cellular parasite, the host is a key part of its environment. Viruses may expand their ecological niche by switching host. Successful host switching can be influenced by ecological and evolutionary factors, genetic constraints and fitness within new hosts. An outcome of host switching is reduced fitness exhibited by viruses, a phenomenon observed in the evolution of viral disease emergence and resistance. To understand the genetic basis of this cost, investigations are required at the genotypic and phenotypic level.
A host switching paradigm was developed using the model bacteriophage φX174 which was propagated with its laboratory bacterial host Escherichia coli C and with the novel host Salmonella enterica serovar Typhimurium, LT2 strain IJ750 or Escherichia coli K-12 mutant strain JWO196-2 designated as E. coli K-12gmhB-mut. A chemostat was used to achieve steady-state conditions for propagation of φX174 and bacterial cells. Two experiments were performed using this approach. In the first, φX174 was cultured on E. coli K-12gmhB-mut for 3 days (~206 generations). In the second experiment, φX174 was cultured on E. coli C and S. Typhimurium for four consecutive periods of 10 days (~720 generations), alternating between the two hosts.
For the second chemostat experiment, the fitness and attachment rates of each viral population were measured using qPCR in liquid culture in order to identify and characterise fitness costs associated with host-switching. Deep sequencing of chemostat samples was also carried out to identify allelic changes occurring before and after host switches. Viral samples were chosen to capture substitutions associated with each host across the experiment (which might explain observed changes in fitness) and time series were picked to identify the dynamics of adaptation on a new host. Bacterial host strains were not sampled in this study.
The phenotype measures indicated the pleiotropic costs of host switching, that is a reduction in phage fitness was observed when this was tested on the host used prior to switching, and this may be explained by changes in the attachment rate. The genotype data revealed sets of changes that could be identified as signatures of adaptation to each host, although control data indicate that these may arise during DNA preparation, implicating synthesis of replicative form DNA in the host as a source of selective constraint. Some host-specific alleles and some shared alleles were identified and their fitness effects were examined in isolation after reconstruction of these alleles in the ancestor via targeted mutagenesis. The fitness effects observed for reconstructed mutants were in the direction expected although they do not fully account for the observed costs of host switching.
By analysing different phenotypes and genotypes produced during evolution, a detailed view of φX174’s adaptation to different hosts was obtained. The results support the idea that costs associated with pathogen-host adaptation may be host-specific, associated with specific mutations, acquired early and persist. Examining these is relevant for understanding emerging infectious diseases
Long term adaptation of a microbial population to a permanent metabolic constraint: overcoming thymineless death by experimental evolution of Escherichia coli
BACKGROUND: To maintain populations of microbial cells under controlled conditions of growth and environment for an indefinite duration is a prerequisite for experimentally evolving natural isolates of wild-type species or recombinant strains. This goal is beyond the scope of current continuous culture apparatus because these devices positively select mutants that evade dilution, primarily through attachment to vessel surfaces, resulting in persistent sub-populations of uncontrollable size and growth rate. RESULTS: To overcome this drawback, a device with two growth chambers periodically undergoing transient phases of sterilization was designed. The robustness of this device was assessed by propagating an E. coli strain under permanent thymine starvation for over 880 days, i.e. metabolic conditions notoriously known to lead to cell death and clogging of cultivation vessels. Ten thousand generations were required to obtain a descendant lineage that could resist thymine starvation and had recovered wild-type growth rate. CONCLUSIONS: This approach provides a technological framework for the diversification and improvement of microbial strains by long-term adaptation to inescapable metabolic constraints. An E. coli strain that is totally resistant to thymineless death was selected
Nonautonomous chemostats with variable delays
The appearance of delay terms in a chemostat model can be fully justified since the future behavior of a dynamical system does not in general depend only on the present but also on its history. Sometimes only a short piece of history provides the relevant influence (bounded or finite delay), while in other cases it is the whole history that has to be taken into account (unbounded or infinite delay). In this paper a chemostat model with time variable delays and wall growth, hence a nonautonomous problem, is investigated. The analysis provides sufficient conditions for the asymptotic stability of nontrivial equilibria of the chemostat with variable delays, as well as for the existence of nonautonomous pullback attractors
Chemostats with random inputs and wall growth
Chemostat refers to a laboratory device used for growing microorganisms in a cultured environment, and has been regarded as an idealization of nature to study competition modeling of mathematical biology. The simple form of chemostat model assumes that the availability of nutrient and its supply rate are both fixed. In addition the tendency of microorganism to adhere to surfaces is neglected by assuming the flow rate is fast enough. However, these assumptions largely limit the applicability of chemostat models to realistic competition systems. In this paper, we relax these assumptions and study the chemostat models with random nutrient supplying rate or random input nutrient concentration, with or without wall growth. This leads the models to random dynamical systems and requires the concept of random attractors developed in the theory of random dynamical systems. Our results include existence of uniformly bounded non-negative solutions,
existence of random attractors and geometric details of random attractors for different value of parameters.Ministerio de Economía y Competitividad (España)Consejería de Innovación, Ciencia y Empresa (Junta de Andalucía
Laboratory Growth Systems in Biofilm Research
The huge variety of ecosystems that we collectively refer to as biofilm is reflected by the numerous different systems available to grow them in the laboratory. The relationship between in situ systems, microcosms and laboratory models is defined and discussed. The first two represent holistic approaches designed to assess the structure and function of particular biofilms: the last is analytical and reductionist, aiming to isolate specific functions of biofilms in order to understand properties that can apply to biofilm in general. Properties of a model can be completely understood whilst this is unlikely with natural ecosystems because of the possibility of unculturable species which could play an unrecognised but important part in its structure and function. A range of systems is reviewed. These include simple surfaces exposed to nutrient in different ways, flow systems such as the Robbins device and constant shear devices such as the Rototorque and the Fowler cell adhesion measurement module. The constant depth film fermenter (CDFF) is described as are membrane based models including the membrane biofilm and the perfused biofilm reactors. Some examples of microcosms are described. The concept and value of steady state biofilm is introduced in terms of the CDFF and of fluidised bed reactors. A number of commercially available film fermenters are listed in the appendix
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