True morphology of the Azotobacteraceae-filterable bacteria

Growth of Azotobacter vinelandii on Soil Nutrients

Azotobacter vinelandii cells grew well in a medium made from soil and distilled water which contained little or no carbohydrate. They utilized p-hydroxybenzoic acid and other phenolic acids, soil nitrogen, and water-soluble mineral substances. Seventeen soils which supported excellent growth of A. vinelandii contained 11 to 18 different phenolic acids each, including p-hydroxybenzoic, m-hydroxybenzoic, vanillic, p-coumeric, syringic, cis- and trans-ferrulic, and other unidentified aromatic acids. Three white, chalky "caliche" soils which were taken from areas where no plants grew failed to support the growth of A. vinelandii, and these contained no, two, and three phenolic acids, respectively. A. vinelandii did not fix nitrogen when growing in dialysates of soils which contained numerous phenolic acids. Growth was ample and rapid in most of the soils tested, but cell morphology was different from that usually seen in chemically defined, nitrogen-free media which contain glucose.

Influence of size, shape, and flexibility on bacterial passage through micropore membrane filters

Sterilization of fluids by means of microfiltration is commonly applied in research laboratories as well as in pharmaceutical and industrial processes. Sterile micropore filters are subject to microbiological validation, where Brevundimonas diminuta is used as a standard test organism. However, several recent reports on the ubiquitous presence of filterable bacteria in aquatic environments have cast doubt on the accuracy and validity of the standard filter-testing method. Six different bacterial species of various sizes and shapes (Hylemonella gracilis, Escherichia coli, Sphingopyxis alaskensis, Vibrio cholerae, Legionella pneumophila, and B. diminuta) were tested for their filterability through sterile micropore filters. In all cases, the slender spirillum-shaped Hylemonella gracilis cells showed a superior ability to pass through sterile membrane filters. Our results provide solid evidence that the overall shape (including flexibility), instead of biovolume, is the determining factor for the filterability of bacteria, whereas cultivation conditions also play a crucial role. Furthermore, the filtration volume has a more important effect on the passage percentage in comparison with other technical variables tested (including flux and filter material). Based on our findings, we recommend a re-evaluation of the grading system for sterile filters, and suggest that the species Hylemonella should be considered as an alternative filter-testing organism for the quality assessment of micropore filters.

Effect of linear alkylbenzene sulfonates on the growth of aerobic heterotrophic cultivable bacteria isolated from an agricultural soil

An enrichment culture technique was used to isolate soil bacteria capable of growing in the presence of two different concentrations of linear alkylbenzene sulfonates (LAS) (10 and 500 microg ml(-1)). Nine bacterial strains, representatives of the major colony types of aerobic heterotrophic cultivable bacteria in the enriched samples, were isolated and subsequently identified by PCR-amplification and partial sequencing of the 16S rRNA gene. Amongst the isolates, strains LAS05 (Pseudomonas syringae), LAS06 (Staphylococcus epidermidis), LAS07 (Delftia tsuruhatensis), LAS08 (Staphylococcus epidermidis) and LAS09 (Enterobacter aerogenes), were able to grow in pure culture in dialysed soil media amended with LAS (50 microg ml(-1)). The three Gram-negative strains grew to higher cell numbers in the presence of 50 microg ml(-1) of LAS, compared to LAS-unamended dialysed soil medium, and were selected for further testing of their ability to use LAS as carbon source. However, HPLC analysis of culture supernatants showed that the three strains can tolerate but not degrade LAS when grown in pure cultures. A higher concentration of soluble phosphates was recorded in dialysed soil media amended with LAS (50 microg ml(-1)) compared to unamended control media, suggesting an effect of the surfactant that enhanced the bioavailability of P from soil. The presence of LAS at a concentration of 50 microg ml(-1) had an important impact on growth of selected aerobic heterotrophic soil bacteria, a deleterious effect which may be relevant for the normal function and evolution of agricultural soil.

Quantification of the filterability of freshwater bacteria through 0.45, 0.22, and 0.1 microm pore size filters and shape-dependent enrichment of filterable bacterial communities

Micro-filtration is a standard process for sterilization in scientific research, medical, and industrial applications, and to remove particles in drinking water or wastewater treatment. It is generally assumed, and confirmed by quantifying filtration efficiency by plating, that filters with a 0.1-0.45 microm pore size can retain bacteria. In contrast to this assumption, we have regularly observed the passage of a significant fraction of natural freshwater bacterial communities through 0.45, 0.22, and 0.1 microm pore size filters. Flow cytometry and a regrowth assay were applied in the present study to quantify and cultivate filterable bacteria. Here we show for the first time a systematic quantification of their filterability, especially their ability to pass through 0.1 microm pore size filters. The filtered bacteria were subsequently able to grow on natural assimilable organic carbon (AOC) with specific growth rates up to 0.47 h(-1). We were able to enrich bacteria communities that pass preferentially through all three pore size filters at significantly increased percentages using successive filtration-regrowth cycles. In all instances, the dominant microbial populations comprised slender spirillum-shaped Hylemonella gracilis strains, suggesting shape-dependent selection during the filtration process. This quantification of the omnipresence of microfilterable bacterial in natural freshwater and their regrowth characteristics demand a change in the sterile filtration practice used in industrial and engineering applications as well as scientific research.

Changes in Bacteria Recoverable from Subsurface Volcanic Rock Samples during Storage at 4°C

The abundance of viable microorganisms recovered from deep subsurface volcanic rock samples increased after rock perturbation and storage for 1 week at 4°C, while the diversity and evenness of recoverable heterotrophic bacterial communities generally decreased. One sample of each morphologically distinct colony type, recovered both before and after storage of U12n rock samples, was purified and characterized by fatty acid methyl ester (MIDI) and API rapid NFT strips. As determined by MIDI cluster analysis, the composition of the recoverable microbial communities changed with storage of rock samples; some groups of organisms were recovered only before, only after, or at both sample times. In general, the isolates recovered only after storage of rock samples had a greater ability to utilize the carbohydrates included in API test strips and had faster generation times than isolates recovered only on initial plating. The nutritional versatility and faster growth rates of organisms recovered in higher proportions after sample storage provide evidence that some microbial community changes may be due to the proliferation of a few bacterial types. However, because some new genera are recovered only after storage, the possibility also exists that dormant bacterial types are resuscitated during sample perturbation and storage.

Changes of ploidy during the Azotobacter vinelandii growth cycle

The size of the Azotobacter vinelandii chromosome is approximately 4,700 kb, as calculated by pulsed-field electrophoretic separation of fragments digested with the rarely cutting endonucleases SpeI and SwaI. Surveys of DNA content per cell by flow cytometry indicated the existence of ploidy changes during the A. vinelandii growth cycle in rich medium. Early-exponential-phase cells have a ploidy level similar to that of Escherichia coli or Salmonella typhimurium (probably ca. four chromosomes per cell), but a continuous increase of DNA content per cell is observed during growth. Late-exponential-phase cells may contain > 40 chromosomes per cell, while cells in the early stationary stage may contain > 80 chromosomes per cell. In late-stationary-phase cultures, the DNA content per cell is even higher, probably over 100 chromosome equivalents per cell. A dramatic change is observed in old stationary-phase cultures, when the population of highly polyploid bacteria segregates cells with low ploidy. The DNA content of the latter cells resembles that of cysts, suggesting that the process may reflect the onset of cyst differentiation. Cells with low ploidy are also formed when old stationary-phase cultures are diluted into fresh medium. Addition of rifampin to exponential-phase cultures causes a rapid increase in DNA content, indicating that A. vinelandii initiates multiple rounds of chromosome replication per cell division. Growth in minimal medium does not result in the spectacular changes of ploidy observed during rapid growth; this observation suggests that the polyploidy of A. vinelandii may not exist outside the laboratory.

Floc Formation by Azospirillum lipoferum Grown on Poly-β-Hydroxybutyrate

Azospirillum lipoferum RG6xx was grown under conditions similar to those resulting in encystment of Azotobacter spp. A. lipoferum produced cells of uniform shape when grown on nitrogen-free β-hydroxybutyrate agar. Cells accumulated poly-β-hydroxybutyrate and often grew as chains or filaments that eventually lost motility and formed capsules. Within 1 week, vegetative A. lipoferum inocula were converted into microflocs arising from filaments or chains. Cells within microflocs were pleomorphic, contained much poly-β-hydroxybutyrate, and were encapsulated. Some cells had a cystlike morphology. Up to 57% of the dry weight of encapsulated flocs was poly-β-hydroxybutyrate, whereas vegetative cells grown in broth with combined nitrogen had only 3% of their dry weight as poly-β-hydroxybutyrate. Neither encapsulated cells in flocs nor nonencapsulated vegetative cells were significantly desiccation resistant. Under starvation conditions (9 days) only 25% of encapsulated cells remained viable, whereas vegetative cells multiplied severalfold. In short-term germination experiments with encapsulated flocs, nitrate, ammonium, and soil extract promoted formation of motile vegetative cells. Most cells in treatments lacking combined nitrogen eventually depleted their visible poly-β-hydroxybutyrate reserves without germinating. The remaining cells retained the reserve polymer and underwent size reduction.

Growth of Azotobacter vinelandii in dialysed soil medium: studies upon the life cycle

Azotobacter vinelandii ATCC 12837 cultured in dialysed soil medium with addition of 0.5% glucose showed four distinct morphological cell types: large cells, precyst forms, mature cysts and filterable corpuscles (0.3 micron in diameter). These results indicate that Azotobacter is a bacterium with a complex life cycle under certain culture conditions. Intracellular levels of RNA and poly-beta-hydroxybutyric acid were significantly affected when cells grown in dialysed soil were compared with those obtained after growth on defined medium (N-free). Further studies showed that the chemical composition of filterable corpuscles obtained from dialysed soil medium were different from the composition of normal Azotobacter cells produced in both culture media (dialysed soil and defined media). We suggest that filterable corpuscles represent a stage in the life cycle of Azotobacter in their natural environment.