Antonie van Leeuwenhoek

Toxin-antitoxin modules are necessary for the mode of action of several antibiotics. One of the most studied toxin-antitoxin modules is the quorum sensing—dependent MazEF system in Escherichia coli. The quorum sensing factor in this system is called the extracellular death factor (EDF), a linear pentapeptide with the sequence NNWNN. In spite of the extensive research on the mazEF system and the involvement of the quorum sensing factor EDF, the effect of EDF itself on bacteria has not yet been studied. In this research, we determined the effect of EDF and variants on cell growth in the Gram-negative bacterium E. coli and the Gram-positive Bacillus globigii. By aligning the zwf gene (from where EDF originates) of different bacterial species, we found 27 new theoretical variants of the peptide. By evaluating growth curves and light microscopy we found that three EDF variants reduced bacterial cell size in B. globigii, but not in E. coli. The D-peptides did not affect cell size, indicating that the effect is stereospecific. Peptides wherein tryptophan was substituted by alanine also did not affect cell size, which indicates that the effect seen is mediated by an intracellular target.

The existence of a membrane system located at one of the poles of the cell is confirmed in the case ofNitrobacter winogradskyi. Additional information on the structure of this system is presented. Elementary particles with diameters of 90 – 100 Å are embedded on a layer of membrane protein. Identification of the membrane system as the location of nitrite oxidation was achieved by a combination of differential spectrophotometry and electron microscopic study of membrane fragments separated by density gradient centrifugation. The cytochrome chain responsible for nitrite oxidation appears to be located in fragments of the membrane system. Prominent morphological features ofNitrobacter winogradskyi are the high contents in poly-β-hydroxybutyrate and polyphosphate granules.

The amplification by PCR of the intergenic spacer region (IGS) of rDNA followed by restriction fragment length polymorphism (RFLP) analysis was evaluated as a potential method for discriminating the 16 species belonging to the genus Debaryomyces. The digestion of this region with some or all the enzymes used in this study (HapII, HhaI and MboI) produced species-specific patterns that permitted differentiation of the species in the genus. With the exception of Debaryomyces vanrijiae, the technique was also efficient for␣distinguishing the varieties in the species Debaryomyces hansenii (var. hansenii, var. fabryi), Debaryomyces occidentalis (var. occidentalis, var. persoonii) and Debaryomyces polymorphus (var. africanus, var. polymorphus), respectively. PCR-RFLP analysis of the IGS region of rDNA is proposed as a clear and reproducible technique for the practical discrimination of species of the yeast genus Debaryomyces.

A diverse group of bacteria colonize the exo- and endo-rhizospheres of sorghum and play a critical role in its tolerance to drought and other abiotic stresses. Two hundred and eighty endophytic bacteria were isolated from the surface-sterilized roots of four sorghum cultivars that were grown on three soil types at three different phenological stages of growth. The isolates were subjected to in vitro screening for their plant growth promoting traits. Out of 280 isolates, 70 could produce Indole 3-Acetic Acid (IAA), 28 showed N-fixation, 28 could solubilize phosphate, 24 had ACC deaminase activity and 13 isolates were able to produce siderophores. Functional diversity grouping of the isolates indicated one isolate having five PGP traits and two isolates having four PGP traits; two and 29 isolates having three and two PGP traits, respectively. Among the thirty-four isolates that possessed multiple PGP traits, 19 and 17 isolates were able to produce significant quantities of IAA in the presence and absence of l-tryptophan, an inducer. Eight isolates possessed high levels of ACC deaminase activity. PCR–RFLP of the 16Sr RNA gene revealed a distinct clustering and considerable genetic diversity among these functionally characterized isolates. The 16S rRNA gene based identification of the isolates of single and multiple PGP traits revealed phylogenetic dominance of Firmicutes; Acinetobacter, Bacillus, Enterobacter, Geobacillus, Lysinibacillus, Microbacterium, Ochrobactrum, Paenibacillus and Pseudomonas were the major genera present in the endo-rhizosphere of sorghum. Results of this study are constructive in selection of effective rhizobacterial endophytes or consortia for drought stress alleviation in sorghum.

The effect of decreasing oxygen tensions on the physiology and composition ofSaccharomyces cerevisiae was studied in galactose-limited continuous cultures. Between oxygen tensions of 75 and 3 mm Hg the metabolism of galactose was oxidative and cell composition constant. Below 3 mm Hg a more fermentative metabolism was operative resulting in low cell yields and accumulation of ethanol in culture filtrates. This correlated with an increased alcohol dehydrogenase and a decreased particulate NADH oxidase activity of cell extracts. While the cellular contents of RNA and protein were not altered at low oxygen tensions there were marked changes in those of carbohydrates and lipids. Below 3 mm Hg there were decreases in most lipid constituents but those of membrane constituents such as sterol esters and phosphatides together with unsaturated fatty acids were most evident. ATP pool size was lowest and amino acid pool size highest at low oxygen tensions. The increased amino acid pool size was due to higher concentrations of arginine and aspartate.

Several collections of natural isolates of the genus Salmonella and of the species Escherichia coli were studied for the release of viable temperate phages. The results indicated that functional prophage genomes may be a common constituent of all bacterial genomes of the investigated strains. About 99% of the Salmonella phages are capable of generalized transduction of chromosomal host markers and plasmids. The ratio of transducing E. coli phages is significantly lower.

A color test for the detection and titration of neutralizing antibodies against Sendai virus is described. Non-specific inhibitors should be removed by treatment with cholera filtrate R.D.E. In contrast to both the haemagglutination inhibition test and the complement fixation test, which do not allow a differentiation between mumps and Sendai virus infection because of cross reactions, the color test is assumed to be sufficiently specific for Sendai virus infection.

Cultures of a soil pseudomonad grown withn-octane as the sole source of carbon and energy have been shown to accumulate suberic, adipic, acetic and butyric acids. Cultures grown at the expense ofn-octoic acid did not yield either suberic or adipic acids. Whenn-heptane was the growth substrate,n-heptoic acid was detected in the medium. A trace of pimelic acid, the expectedn-alkanedioic acid, also appeared to be present. The principal non-volatile acidic products were recognised to be either hydroxy acids or the lactones of these acids. The formation of suberic and adipic acids fromn-octane is discussed in terms of current views of the biological oxidation ofn-alkanes.