By analyzing the chemotactic behavior of a recently described marine bacterial species, we provide experimental evidence that bacteria are not too small for sensing chemical gradients spatially. The bipolar flagellated vibrioid bacteria (typical size 2 ؋ 6 m) exhibit a unique motility pattern as they translate along as well as rotate around their short axis, i.e., the pathways of the cell poles describe a double helix. The natural habitat of the bacteria is characterized by steep oxygen gradients where they accumulate in a band at their preferred oxygen concentration of Ϸ2 M. Single cells leaving the band toward the oxic region typically return to the band within 16 s following a U-shaped track. A detailed analysis of the tracks reveals that the cells must be able to sense the oxygen gradient perpendicular to their swimming direction. Thus, they can detect oxygen gradients along a distance of Ϸ5 m corresponding to the extension of their long axis. The observed behavior can be explained by the presence of two independent sensor regions at either cell pole that modulate the rotation speed of the polar flagellar bundles, i.e., the flagellar bundle at the cell pole exposed to higher oxygen concentration is rotating faster than the other bundle. A mathematical model based on these assumptions reproduces the observed swimming behavior of the bacteria. F lagellar motility is widespread among prokaryotes living in aquatic environments, and it is estimated that 20% of marine planktonic bacteria are motile (1). Generally, bacteria exhibit motility to find places in their environment where they can maximize their substrate or energy uptake. To achieve this, they have to modulate their motility in response to environmental variables (2). When cell motility is modulated via sensing of chemical signals in the environment, the resulting motility behavior is termed chemotaxis. Best understood is the chemotactic behavior of the flagellated enteric bacterium Escherichia coli (reviewed in refs. 3-5). More or less straight swimming paths (caused by synchronous counter-clockwise rotation of all flagella) are interrupted periodically by random direction changes: so-called tumbles caused by short-term direction reversals of one or more flagella. Chemical attractants or repellants bind to specific receptor protein complexes at the cell surface, which subsequently inhibit or promote, respectively, the phosphorylation of an intracellular freely diffusing signaling protein. The flagellar motors respond to increased phosphorylation levels of the signaling protein with more frequent reversals of flagellar rotation causing more frequent tumbling of the bacteria. Additionally, a dynamic methylation of the receptor protein complexes provides an adaptation mechanism, ensuring that the chemotactic behavior is functioning over a wide range of attractant or repellant concentrations. If a bacterium experiences increasing attractant (or decreasing repellant) concentrations along its swimming path, the described mechanism results in less-frequent ran...

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We describe the morphology and behavior of a hitherto unknown bacterial species that forms conspicuous veils (typical dimensions, 30 by 30 mm) on sulfidic marine sediment. The new bacteria were enriched on complex sulfidic medium within a benthic gradient chamber in oxygen-sulfide countergradients, but the bacteria have so far not been isolated in pure culture, and a detailed characterization of their metabolism is still lacking. The bacteria are colorless, gram-negative, and vibrioid-shaped (1.3-to 2.5-by 4-to 10-m) cells that multiply by binary division and contain several spherical inclusions of poly-␤-hydroxybutyric acid. The cells have bipolar polytrichous flagella and exhibit a unique swimming pattern, rotating and translating along their short axis. Free-swimming cells showed aerotaxis and aggregated at ca. 2 M oxygen within opposing oxygen-sulfide gradients, where they were able to attach via a mucous stalk, forming a cohesive whitish veil at the oxic-anoxic interface. Bacteria attached to the veil kept rotating and adapted their stalk lengths dynamically to changing oxygen concentrations. The joint action of rotating bacteria on the veil induced a homogeneous water flow from the oxic water region toward the veil, whereby the oxygen uptake rate could be enhanced up to six times, as shown by model calculations. The veils showed a pronounced succession pattern. New veils were generated de novo within 24 h and had a homogeneous whitish translucent appearance. Bacterial competitors or eukaryotic predators were apparently kept away by the low oxygen concentration prevailing at the veil surface. Frequently, within 2 days the veil developed a honeycomb pattern of regularly spaced holes. After 4 days, most veils were colonized by grazing ciliates, leading to the fast disappearance of the new bacteria. Several-week-old veils finally developed into microbial mats consisting of green, purple, and colorless sulfur bacteria.Marine sediments and biofilms are characterized by opposing gradients of chemical compounds, which can react with each other via exergonic redox reactions (6, 14). The bestinvestigated gradient systems are sulfide-oxygen or sulfide-nitrate countergradients, and a variety of microorganisms which are specialized in efficiently utilizing the energy released by the redox reaction of the two gradient compounds have been described. Detailed physiological studies of species which can be cultured in homogeneous media, e.g., Thiobacillus spp., have been performed (18). However, many conspicuous gradient microorganisms in natural habitats (e.g., the colorless sulfide oxidizer Thiovulum majus) can be kept in the laboratory only as enrichment cultures, and only a few species have been successfully isolated in pure culture based on gradient media (24). While detailed physiological studies are still lacking, the behavior and motility of gradient microorganisms, especially of the relatively large and easily identifiable colorless and photosynthetic sulfur-oxidizing bacteria, have been described in the literature...

Observations of free-swimming Thiovulum majus cells show that these bacteria exhibit a phobic response as well as true chemotaxis in oxygen gradients. Both phenomena of their chemotactic behavior are integrated into a single model of helical klinotaxis, which is demonstrated by computer simulations.

Transparent sensors for microscopic O 2 imaging were developed by spin coating an ultrathin (<1-to 2-m) layer of a luminescent O 2 indicator onto coverslips. The sensors showed (i) an ideal Stern-Volmer quenching behavior of the luminescence lifetime towards O 2 levels, (ii) homogeneous measuring characteristics over the sensor surface, and (iii) a linear decline of luminescence lifetime with increasing temperature. When a batch of such coverslip sensors has been characterized, their use is thus essentially calibration free at a known temperature. The sensors are easy to use in flow chambers and other growth devices used in microbiology. We present the first application for combined imaging of O 2 and bacteria in a biofilm flow chamber mounted on a microscope equipped with a spinning-disk confocal unit and a luminescence lifetime camera system.