Azospirillum brasilense Sp 7 was grown in a nitrogen free medium in a chemostat with malate as the sole carbon source. Several steady states were established. Malate limitation occurred at dilution rates less than 0.05 (hr -1). Maximum growth rate and KS were calculated from Monod's model. Poly-~3-hydroxybutyrate content was found to decrease under malate limitation from 18.5% to about 2%. Protein exudation increases under these conditions. The acetylene reduction assay was shown to be not suitable for continuous culture systems. A model for the physiological properties of A. brasilense in a malate limited chemostat is developed.Azospirillum brasilense Sp 7 is the major organism responsible for nitrogenase activity in the rhizosphere of several tropical grasses (1). The rhizosphere microflora is mainly supported by carbonaceous compounds liberated by intact roots (2). In these experiments, the continuous system is used in order to understand and describe some processes in the rhizosphere (3). A chemostat has certain properties which reflect ecologically important characteristics. The microbial growth is limited by the availability of a selectable substrate (4).Due to its economic importance, work had been done previously on the physiology of A. brasilense in batch culture systems (S-7) and continuous culture systems (8-10). Carbon-limited continuous cultures of A, brasilense however have not yet been studied on N-free medium (8). For this reason, studies on the physiological properties of this organism in a carbon limited chemostat were undertaken. The effect of oxygen on the nitrogen fixing ability and the efficiency of the nitrogenase was the main objective of the studies (8-10). MATERIAL AND METHODSBacterial strain and inoculum. Azospirillum brasilense Sp 7 (DSM 1690, 447 ATCC

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SummaryA chemostat was used as a model system to study competitive interactions of diazotrophic microorganisms. Enrichment experiments were carried out under microaerobic conditions (8.7 tzmol O2/1) with malate as the sole carbon source. The starting material was a Korean rice soil including intact root pieces. The enrichment process was governed by the dilution rate. High dilution rates resulted in the enrichment ofAzospirillum lipoferum, whereas low dilution rates led to the predominance of an unidentified organism, named Isolate R. Dilution rates were set in the range from D = 0.005 to D = 0.1 h -1 . The growth kinetics of both organisms followed Monod's model in the enrichment eultnre. From the experiments, the maximum specific growth rate ofA. lipoferum and Isolate R were 0.069 h -1 and 0.025 h -1 , respectively. The corresponding Ks-values were 8.4 and 0.9 (mg .1-1 ). The point of theoretical coexistence of both organisms was calculated to occur at a substrate concentration of s = 3.0 (mg.1-1 ) with a growth of rate ts = 0.018 h -1 . Hence the preset nutritional niches occupied by at least two organisms. Azospirillum lipoferum seems to represent the copiotroph microflora and Isolate R is of the oligotroph type. In addition to its high substrate affinity Isolate R liberated ca. 75% of the fixed nitrogen into the medium, which indicates its potential role for mutualistic interactions in the rhizosphere.

In recent decades, biofilm-associated infections have become a major problem in many medical fields, leading to a high burden on patients and enormous costs for the healthcare system. Microbial infestations are caused by opportunistic pathogens which often enter the incision already during implantation. In the subsequently formed biofilm bacteria are protected from the hosts immune system and antibiotic action. Therefore, the development of modified, anti-microbial implant materials displays an indispensable task. Thermoplastic polyurethane (TPU) represents the state-of-the-art material in implant manufacturing. Due to the constantly growing areas of application and the associated necessary adjustments, the optimization of these materials is essential. In the present study, modified liquid silicone rubber (LSR) surfaces were compared with two of the most commonly used TPUs in terms of bacterial colonization and biofilm formation. The tests were conducted with the clinically relevant bacterial strains Staphylococcus aureus and Staphylococcus epidermidis. Crystal violet staining and scanning electron microscopy showed reduced adhesion of bacteria and thus biofilm formation on these new materials, suggesting that the investigated materials are promising candidates for implant manufacturing.

Medical device-related infections are becoming a steadily increasing challenge for the health care system regarding the difficulties in the clinical treatment. In particular, cardiovascular implant infections, catheter-related infections, as well as infective endocarditis are associated with high morbidity and mortality risks for the patients. Antimicrobial materials may help to prevent medical device-associated infections and supplement the currently available therapies. In this study, we present an easy-to-handle and simplified in vivo model to test antimicrobial materials in the bloodstream of mice. The model system is composed of the implantation of a bacteria-laden micro-stent scaffold into the murine tail vein. Our model enables the simulation of catheter-related infections as well as the development of infective endocarditis specific pathologies in combination with material testing. Furthermore, this in vivo model can cover two phases of the biofilm formation, including both the local tissue response to the bacterial biofilm and the systemic inflammatory response against circulating bacteria in the bloodstream that detached from a mature biofilm.