, Proc. Natl. Acad. Sci. USA 86:6533-6537, 1989; also, this work). In this study, we demonstrated that E. halophila is also negatively phototactic. Video analysis of free-swimming bacteria and the formation of cell distribution patterns as a result of light-color boundaries in an anaerobic suspension of cells revealed the existence of a repellent response toward intense (but nondamaging) blue light. In the presence of saturating background photosynthetic light, an increase in the intensity of blue light induced directional switches, whereas a decrease in intense blue light gave rise to suppression of these reversals. To our knowledge, this is the first report of a true repellent response to light in a free-swimming eubacterium, since the blue light response in Escherichia coli and Salmonella typhimurium (B. L. Taylor and D. E. Koshland, Jr., J. Bacteriol. 123:557-569, 1975), which requires an extremely high light intensity, is unlikely to be a sensory process. The wavelength dependence of this negative photoresponse was determined with narrow band pass interference filters. It showed similarity to the absorption spectrum of the photoactive yellow protein from E. halophila.In free-swimming prokaryotes, behavior at the molecular level involves the measurement of the value of certain chemical or physical parameters in the course of time, as the cell swims in spatial gradients of stimulants and/or repellents (for reviews, see references 2 and 7). Protein molecules with a sensory function, either chemo-or photoreceptors or specific indicators of cellular metabolism, such as the chemiosmotic proton gradient or the pool of certain metabolic intermediates, inform the cell about its present physiological situation. This information is compared with a previously sensed situation, and the change is evaluated as either favorable or unfavorable. Whereas the direction of the swimming of the cell with respect to the spatial gradient is random and remains so, uninfluenced by tactic processes, the time during which the cell continues swimming in a given direction is dependent on whether the integrated sensory signals are positive or negative. In the former case, the cell's tendency to switch its direction is suppressed, whereas in the latter case (i.e., when the sum is negative), it is enhanced.The process of chemotaxis in Escherichia coli has been extensively studied (2,7

A small irregular coccoid methanogenic bacterium (PA T ) was isolated from the hindgut of the cockroach Periplaneta americana. Fluorescence microscopy and transmission electron microscopy of the hindgut of P. americana suggest that the organism occurs abundantly in the microbiota attached to the hindgut wall. The strain produces methane by the reduction of methanol and methylated amines with molecular hydrogen. Acetate, coenzyme M, yeast extract, tryptic soy broth and vitamins are required for growth. The cells lack a rigid cell wall and lyse immediately in buffers of low ionic strength. Maximum rate of growth (specific growth rate, 022 h V1 ) occurs in a rich medium at 39 SC, at a pH range of 72-77 and at a salt concentration below 100 mM NaCl. Sequence analysis of the small-subunit rDNA indicates that strain PA T is related to the family Methanosarcinaceae but does not belong to any previously described genus. Therefore, it is proposed that strain PA T be classified in a new genus, related to the Methanosarcinaceae, as Methanomicrococcus blatticola (type strain PA T l DSM 13328 T ).

Methanomicrococcus blatticola is an obligately anaerobic methanogen that derives the energy for growth exclusively from the reduction of methylated compounds to methane with molecular hydrogen as energy source. Competition for methanol (concentration below 10 microM) and H(2) (concentration below 500 Pa), as well as oxidative stress due to the presence of oxygen are likely to occur in the peripheral region of the cockroach hindgut, the species' normal habitat. We investigated the ecophysiological properties of M. blatticola to explain how it can successfully compete for its methanogenic substrates. The organism showed affinities for methanol (K(m)=5 microM; threshold<1 microM) and hydrogen (K(m)=200 Pa; threshold <0.7 Pa) that are superior to other methylotrophic methanogens (Methanosphaera stadtmanae, Methanosarcina barkeri) investigated here. Thermodynamic considerations indicated that 'methanol respiration', i.e. the use of methanol as the terminal electron acceptor, represents an attractive mode of energy generation, especially at low hydrogen concentrations. Methanomicrococcus blatticola exploits the opportunities by specific growth rates (>0.2 h(-1)) and specific growth yields (up to 7 g of dry cells per mole of methane formed) that are particularly high within the realm of mesophilic methanogens. Upon oxygen exposure, part of the metabolic activity may be diverted into oxygen removal, thus establishing appropriate anaerobic conditions for survival and growth.

Methanomicrococcus blatticola, a methanogenic archaeon isolated from the cockroach Periplaneta americana, is specialised in methane formation by the hydrogen-dependent reduction of methanol, monomethyl-, dimethyl- or trimethylamine. Experiments with resting cells demonstrated that the capability to utilise the methylated one-carbon compounds was growth substrate dependent. Methanol-grown cells were incapable of methylamine conversion, while cells cultured on one of the methylated amines did not metabolise methanol. Unlike trimethylamine, monomethyl- and dimethylamine metabolism appeared to be co-regulated. The central reaction in the energy metabolism of all methanogens studied so far, the reduction of CoM-S-S-CoB, was catalysed with high specific activity by a cell-free system. Activity was associated with the membrane fraction. Phenazine was an efficient artificial substrate in partial reactions, suggesting that the recently discovered methanophenazine might act in the organism as the physiological intermediary electron carrier. Our experiments also showed that M. blatticola apparently lacks the pathway for methyl-coenzyme oxidation to CO2, explaining the strict requirement for hydrogen in methanogenesis and the obligately heterotrophic character of the organism.