ABSTRACT
We propose a simple cellular automaton model for the description of the evolution of a colony of Bacillus subtilis. The originality of our model lies in the fact that the bacteria can move in a pool of liquid. We assume that each migrating bacterium is surrounded by an individual pool, and the overlap of the latter gives rise to a collective pool with a higher water level. The bacteria migrate collectively when the level of water is high enough. When the bacteria are far enough from each other, the level of water becomes locally too low to allow migration, and the bacteria switch to a proliferating state. The proliferation-to-migration switch is triggered by high levels of a substance produced by proliferating bacteria. We show that it is possible to reproduce in a fairly satisfactory way the various forms that make up the experimentally observed morphological diagram of B. subtilis. We propose a phenomenological relation between the size of the water pool used in our model and the agar concentration of the substrate on which the bacteria migrate. We also compare experimental results from cutting the central part of the colony with the results of our simulations.
Subject(s)
Bacillus subtilis/cytology , Bacillus subtilis/physiology , Models, Biological , Rheology/methods , Water Microbiology , Water/chemistry , Computer Simulation , MotionABSTRACT
A stable OprF-deficient mutant for Pseudomonas fluorescens strain MF0 was constructed using reverse genetics. This mutant, called MF372, showed a rounded morphology and grew more slowly in minimal medium, but not in rich medium. Contrary to other Pseudomonas strains, the loss of OprF for strain MF0 was accompanied by an altered outer membrane composition. At least three outer membrane proteins were overexpressed, apparently as a consequence of adaptive mutations. The N-terminal sequence of two of them revealed strong similarities with porins of the OprD family from P. aeruginosa. The data presented here shows that OprF may be an essential protein for this P. fluorescens strain.
Subject(s)
Porins/physiology , Pseudomonas fluorescens/physiology , Molecular Weight , Mutation , Plasmids , Porins/chemistry , Porins/deficiency , Pseudomonas fluorescens/chemistry , Pseudomonas fluorescens/geneticsABSTRACT
Random transcription fusion delivery, with bacterial luciferase genes as reporter, was performed in the psychrotrophic bacterium Pseudomonas fluorescens. Direct screening on plates of the insertions allowed the isolation of fusions into thermoregulated genes with good accuracy, either in a library of insertion fusions, or after genetic transfer of a putative regulatory mutation. Using this method, it was shown that in Ps. fluorescens, nearly 40% of the genes are thermoregulated and belong to at least three classes according to the maximal temperature of expression of the fused genes. This is more than had been estimated by a previous method, and demonstrates the importance of thermoregulation in psychrotrophic bacteria. As this reporter is the first to be used for direct screening for genes regulated by temperature, it should be of great value in the study of mechanisms involved in adaptation to this environmental factor.
Subject(s)
Cold Temperature , Pseudomonas fluorescens/genetics , Artificial Gene Fusion , Escherichia coli/genetics , Gene Expression Regulation, Bacterial , Genes, Reporter/physiology , Luciferases/genetics , Luciferases/metabolism , Luminescent Measurements , Pseudomonas fluorescens/physiology , Recombination, GeneticABSTRACT
In the psychrotrophic bacterium Pseudomonas fluorescens, some genes are thermoregulated: they are maximally expressed at a particular temperature within the broad range of temperatures that allow growth of this bacterium. To study this regulation, random transcriptional insertion fusions were obtained by means of mini-Tn5lacZ1 or mini-Tn5luxAB transposition. One fusion was studied in which beta-galactosidase production was maximal at a low-growth temperature. The mutated gene (that we call xsf) was highly homologous to xseA from Escherichia coli (and from other bacteria) which encodes the large subunit of exonuclease VII. Genetic tools were constructed in order to analyse and manipulate this fusion: a plasmid derived from R68.45 was used for chromosome transfer and a replacement vector was constructed to allow in situ marker exchange of the mini-Tn5lacZ1 by an Hg(r) interposon. This vector was used to make double mutants and hence to study the effect of the insertion in xsf on the expression of other fusions. Six genes were thereby identified with a decreased expression in an xsf- background and with different characteristics of thermoregulation.
