Interactions of Tetrahymena pyriformis, Escherichia coli, Azotobacter vinelandii, and glucose in a minimal medium

Successful invasion of a food web in a chemostat

A food web in a chemostat is considered in which an arbitrary number of competitor populations compete for a single, essential, nonreproducing, growth-limiting substrate, and an arbitrary number of predator populations prey on some or all of the competitor populations. Although any number of predator populations may prey on the same competitor population, each predator population preys on only one competitor population. The dynamics of substrate uptake is modeled by Lotka-Volterra or Michaelis-Menten (Holling type I or II), but the dynamics of competitor uptake is restricted to Lotka-Volterra. Based on certain parameters, the model predicts the asymptotic survival or extinction of each of the different populations and suggests how competitor and/or predator populations could successfully invade the chemostat with or without causing a diverse ecosystem to crash. Similarly, it suggests how the elimination of certain populations could result in a more diverse or less diverse system.

Peristance of bacteria in the presence of viable, nonencysting, bacterivorous ciliates

Laboratory studies of the interactions between a bacterial population and a population of bacterivorous ciliates consistently show that the bacteria are able to persist in the presence of viable ciliates. Reproduction of the bacteria, presumably at the expense of substrates produced by death and lysis of the ciliates and/or by their metabolic activity, has been suggested to be a factor involved in the observed bacterial persistence. Rates and extents of growth ofEscherichia coli in broths of mixed cultures of this bacterium and the ciliateTetrahymena pyriformis were determined in order to provide some data necessary to assess the importance of the suggested factor. In addition, an attempt was made to suppress bacterial growth on produced substrates so that feeding of the ciliates could be studied free of this complication. However, the procedure tested-addition of the antibiotic chloramphenicol (CM) at a concentration of 150μg/ml-led to other complications that made it impossible to obtain the desired information about feeding.

Simultaneous consumption of bacteria and dissolved organic matter byTetrahymena pyriformis

The addition of ciliated protozoa to aquatic microcosms and bench-scale sewage treatment plants increases decomposition rates. This is surprising, inasmuch as protozoa consume bacteria, which are the primary decomposers. One possible mechanism of the increase in decomposition rate is the direct consumption of dissolved organic matter by protozoa that are feeding primarily on bacteria. This possibility was explored experimentally in two-stage continuous cultures, with glucose limitingEscherichia coli in the first stage andE. coli limitingTetrahymena pyriformis in the second. Glycine and histidine were the test nutrients. The results of adding them to the second stages suggested that direct uptake by ciliates does not affect the dynamics of dissolved amino acids in pelagic environments or activated sludge plants. Ciliates might, however, affect the dynamics of amino acid pools in environments high in nutrients and ciliates, perhaps including some microenvironments near decomposing material or in benthic sediments. Direct uptake of dissolved amino acids by ciliates probably does not affect ciliate or bacterial populations substantially.

Experimental and modeling studies of a four-trophic level predator-prey system

Experimental studies of a microbial food chain involving organic carbon substrates,Enterobacter aerogenes, and ciliate protozoansParamecium primaurelia andDldinium nasutum were conducted in stirred, aerated batch cultures. Quantitative measurements were made of organic carbon levels and of cell numbers, mean cell volumes, and total biovolumes for all three microbial populations. A mathematical model based on Monod kinetics was developed to describe this four-trophic level predator-prey system. The model was formulated in terms of biovolume, which is the product of cell numbers and mean cell size, and includes terms for bio-volume decay. Batch culture data were used to derive parameter values, and model simulations were compared to experimental results. Despite the significance ofParamecium-Didinium studies in ecological literature, the entire food chain has not been previously studied or modeled.

Some physiological aspects of the autecology of the suspension-feeding protozoanTetrahymena pyriformis

Feeding, growth, and reproductive responses of the suspension-feeding protozoanTetrahymena pyriformis to shifts up or down of the density of its bacterial food were observed. The rates of feeding, growth, and reproduction were determined by measuring the rates of uptake of viable bacterial cells, of change of mean volume of the protozoan cells, and of change of number of protozoan cells, respectively. The effects of the nutritional status of the protozoans at the time of shifting were observed also. Results are interpreted in terms of the limited polymorphism exhibited in the life cycle of this organism. Responses in all cases seem to reflect a strategy for exploiting a patchy, transient environment, a conclusion already reached by several earlier investigators.

Microbial competition

Populations of microorganisms inhabiting a common environment complete for nutrients and other resources of the environment. In some cases, the populations even excrete into the environment chemicals that are toxic or inhibitory to their competitors. Competition between two populations tends to eliminate one of the populations from their common habitat, especially when competition is focused on a single resource and when the populations do not otherwise interact. However, a number of factors mitigate the severity of competition and thus competitors often coexist.

Grazing by protozoa as selection factor for activated sludge bacteria

In continuous culture enrichments that were inoculated with activated sludge and were fed with polymeric substrates, freely dispersed single-celled bacteria belonging to theCytophaga group dominated among the initial populations, irrespective of the activated sludge source. These populations were grazed by flagellated protozoa which after several days reached high cell densities. Other morphologic bacterial groups such as spiral-shaped or filamentous bacteria then became dominant. In defined mixed culture experiments with bacterial isolates from the enrichment cultures, it was shown that a "grazing-resistant"Microcyclus strain outgrew aCytophaga strain in the presence of grazing protozoa. In contrast, theCytophaga strain competed successfully with theMicrocyclus strain and with other "grazing-resistant" strains under protozoa-free conditions. Furthermore, it was demonstrated that assumed grazing resistance factors such as floccing or filamentous growth were lost by some of the strains when they were grown for several generations in continuous culture under the same conditions, but in the absence of protozoa.

Growth responses of ciliate protozoa to the abundance of their bacterial prey

The growth rate or numerical response of five species of bactivorous ciliates to the abundance ofEnterobacter aerogenes was examined in monoxenic culture. The ciliatesColpidium campylum, C. colpoda, Glaucoma scintillons, G. frontata, andCyclidium glaucoma were isolated from a small pond. Four were grown in shaken cultures, while three were grown in cultures in which the bacteria were allowed to settle on the bottom of the culture vessel. Of the seven response curves generated, four had distinct thresholds, so that the Michaelis-Menten model usually fitted to ciliate numerical response curves was not appropriate. In shaken cultures, half-saturation prey densities ranged from 5.5 × 10(6) to 42.9 × 10(6) bacteria/ml. In unshaken cultures, half-saturation densities ranged from 0.057 × 10(6) to 14.6 × 10(6) bacteria/cm(2). Two species grown on both suspended and settled bacteria attained higher growth rates and had lower half-saturation prey densities feeding on settled bacteria.

Competition for mixed substrates by microbial populations

A model for the growth of an organism on multiple substrates was developed, assuming that each substrate has a competitive inhibition effect on the uptake of other substrates. The model was extended to examine mixed substrates, showing that the coexistence of several species at steady state in continuous cultures is possible, even when all the organisms all strongly prefer the one substrate. The diversity of nutrient sources in a real system may be a key factor in supporting a heterogeneous microbial population.

Protozoan feeding and bacterial wall growth

Monod's model is often assumed to describe the kinetics of feeding of a protozoan population on a bacterial population in a chemostat. An earlier study (J. L. Jost et al., J. Bacteriol., 113, 84 (1973)) of the feeding of Tetrahymena pyriformis on either Escherichia coli or Azotobacter vinelandii found that this model correctly predicted the occurrence of sustained oscillations of population densities but made predictions of minimum bacterial population densities that were much smaller than those observed. The earlier study removed the discrepancy between the model and data by replacing Monod's model with a different model. It is shown in the present study that the discrepancy can be explained equally as well if Monod's model for the feed relation is retained and if, in addition, growth of bacteria on the chemostat walls is allowed for in the model equations.

Differential counting in mixed cultures with coulter counters

A critical comparison of Coulter, viable, and microscope counts for several mixed cultures of microorganisms has been made. This investigation shows that Coulter counting can provide reliable estimates of microbial numbers in mixed cultures. Precautions and limitations of Coulter counting in mixed cultures are discussed.