| Swarming motility is a flagella-dependent surface motility and bacterial cells move out from inoculation point as a group on the surface of solid medium. There are two main characteristics of this motility. Firstly, compared with swimmers in liquid medium, swarmer cells differentiate dramatically in shape, metabolism and expression of some proteins. Secondly, swarming is a group behavior instead of an individual behavior like swimming. It was showed that single bacterial cell was unable to move out on the solid medium surface.More and more new species were found to have swarming motility which is believed to be a universal phenomenon. Enterobacteria are main pathogen of urethral infection. Results of study on Proteus mirabilis indicated that ability of Proteus mirabilis to invade human urethral cells is coupled to motility and swarming differentiation.Citrobacter freundii is a gram-negative bacterium distributing widely in natural environment. C.freundii is also a typical opportunistic pathogen which mainly cause urethral and respiration infection. C.freundii has flagella which enable cells to swim in aqueous environments. But before this study, there were no report and study on swarming in C.freundii. Results of preliminary study showed that there were distinct differences between C.freundii and P. mirabilis. Study on molecular mechanism of swarming motility in C.freundii which can be induced a less vigorous swarming will help understanding the phenomenon completely.C.freundii ATCC8090 was chosed as study object and a swarming test model was established. A method of transposon mutagenesis was used to screen swarming mutants of C.freundii. By analysis of the mutated genes which were inserted and linked to the transposon, a general knowledge of molecular mechanism of swarming motility in C.freundii was gained and the results build up a basis for a following study.The main content and important results of this study are listed in the following:1 It was showed that C.freundii was able to be induced a less vigorous swarming andonly move on the plate with 0.4%-0.7% agar concentration range. Swarming motility was absence when casamino acids or ammonium salt used as the sole source of nitrogen although bacteria can grow in the two medium. All carbon sources prompted swarming, but glucose was optimal. SW medium was selected for following experiments to unify test condition.2 In the model, swarmers exhibited obvious differentiation including elongated shape, increasing expression of flagella, inhibition of aerobic respiration and increasing hydrophilicity of bacterial cell surface.3 A technique of transposon mutagenesis involving the use of Mini-Tn5 on a suicide plasmid was developed for Citrobacter freundii. Analysis of the resulting exconjugants indicated that Mini-Tn5 Km transposed in C.freundii chromosome at a high frequency, ca. 9.2 10-6 .The resulting mutants were stable and retained the transposon-encoded antibiotic resistance when incubated for three passages under nonselective condition. The frequency of auxotrophic mutants in the population confirmed that the insertion of the transposon was random and the C.freundii chromosome did not contain significant insertional hot spots of transposition.4 85 swarming mutants were screened from 4047 exconjugants. 36 mutants were defective in swimming motility. Of these, 16 mutants were fla+ and 20 were fla-. Another 49 mutants were defective in swarming but not swimming. Of these, 14 mutants showed abnormal swimming behaviour, 35 mutants were normal and were further divided into che-(17 mutants) and che+(18 mutants).5 Inverse PCR was chosed to determine DNA sequence flanking transposon from three PCR methods. Based on characteristic of transposon sequence, Taq I was used directly without selecting process to simplify approach.6 Results of sequencing and homology analysis showed that mutations were in genes for flagellar expression and energy transfer, lipopolysaccharide,exopolysaccharide cellular metabolism, regulatory pr...
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