Expanding ecological assessment by integrating microorganisms into routine freshwater biomonitoring.

Bioindication has become an indispensable part of water quality monitoring in most countries of the world, with the presence and abundance of bioindicator taxa, mostly multicellular eukaryotes, used for biotic indices. In contrast, microbes (bacteria, archaea and protists) are seldom used as bioindicators in routine assessments, although they have been recognized for their importance in environmental processes. Recently, the use of molecular methods has revealed unexpected diversity within known functional groups and novel metabolic pathways that are particularly important in energy and nutrient cycling. In various habitats, microbial communities respond to eutrophication, metals, and natural or anthropogenic organic pollutants through changes in diversity and function. In this review, we evaluated the common trends in these changes, documenting that they have value as bioindicators and can be used not only for monitoring but also for improving our understanding of the major processes in lotic and lentic environments. Current knowledge provides a solid foundation for exploiting microbial taxa, community structures and diversity, as well as functional genes, in novel monitoring programs. These microbial community measures can also be combined into biotic indices, improving the resolution of individual bioindicators. Here, we assess particular molecular approaches complemented by advanced bioinformatic analysis, as these are the most promising with respect to detailed bioindication value. We conclude that microbial community dynamics are a missing link important for our understanding of rapid changes in the structure and function of aquatic ecosystems, and should be addressed in the future environmental monitoring of freshwater ecosystems.

Prevalence of undiagnosed atrial fibrillation in elderly individuals and potential cost-effectiveness of non-invasive ambulatory electrocardiographic screening: The ASSERT-III study.

BACKGROUND: In approximately 10% of patients with implanted pacemakers or defibrillators, previously unrecognized atrial fibrillation (AF) is detected within 3 months. It is unknown whether elderly patients without implanted devices have a similar prevalence of undiagnosed AF using non-invasive ECG monitoring, and if this approach to screening in this population is cost-effective. METHODS: Individuals ≥80 years old attending outpatient clinics without a history of AF and with hypertension and one additional risk factor underwent 30 days of continuous ECG monitoring with an option for an additional 30 days of monitoring if no AF was detected. The primary outcome was AF ≥ 6 min. Cost-effectiveness to prevent stroke was estimated using a Markov model based on observed AF detection rates and data from the published literature. RESULTS: Among 129 patients enrolled, 100 initiated monitoring for an average duration of 36 ±â€¯21 days. The proportion of patients that completed at least 30 days of monitoring was 59%. Average age was 84 ±â€¯3 years and mean CHA2DS2-VASc score was 4.5 ±â€¯1.2. AF ≥ 6 min was documented in 14%, ≥6 h in 8%, and ≥24 h in 3%. One week of monitoring costed $50,000 per quality-adjusted life-year-gained, 30 days and 60 days of monitoring costed $70,000 and $84,000, respectively. CONCLUSIONS: Continuous non-invasive ECG monitoring is feasible in elderly patients. Undiagnosed AF is present in many elderly individuals, with 1 in 7 having episodes lasting ≥6 min. One week of monitoring may be cost-effective for stroke prevention in this population.

Strong, weak, and missing links in a microbial community of the N.W. Mediterranean Sea.

Planktonic microbial communities often appear stable over periods of days and thus tight links are assumed to exist between different functional groups (i.e. producers and consumers). We examined these links by characterizing short-term temporal correspondences in the concentrations and activities of microbial groups sampled from 1 m depth, at a coastal site of the N.W. Mediterranean Sea, in September 2001 every 3 h for 3 days. We estimated the abundance and activity rates of the autotrophic prokaryote Synechococcus, heterotrophic bacteria, viruses, heterotrophic nanoflagellates, as well as dissolved organic carbon concentrations. We found that Synechococcus, heterotrophic bacteria, and viruses displayed distinct patterns. Synechococcus abundance was greatest at midnight and lowest at 21:00 and showed the common pattern of an early evening maximum in dividing cells. In contrast, viral concentrations were minimal at midnight and maximal at 18:00. Viral infection of heterotrophic bacteria was rare (0.5-2.5%) and appeared to peak at 03:00. Heterotrophic bacteria, as % eubacteria-positive cells, peaked at midday, appearing loosely related to relative changes in dissolved organic carbon concentration. Bacterial production as assessed by leucine incorporation showed no consistent temporal pattern but could be related to shifts in the grazing rates of heterotrophic nanoflagellates and viral infection rates. Estimates of virus-induced mortality of heterotrophic bacteria, based on infection frequencies, were only about 10% of cell production. Overall, the dynamics of viruses appeared more closely related to Synechococcus than to heterotrophic bacteria. Thus, we found weak links between dissolved organic carbon concentration, or grazing, and bacterial activity, a possibly strong link between Synechococcus and viruses, and a missing link between light and viruses.

[Iatrogenic injuries of the colon during colonoscopy]. / Iatrogenní poranení tracníku pri koloskopii.

Endoscopic examinations of hollow organs, similarly as other invasive operations, are associated with a certain percentage of sometimes very serious complications. Iatrogenic perforations of the gut, in particular during the procedure belong to the most dramatic ones. During the period of 1990-2000 the authors had the opportunity to deal in collaboration with the medical department with five cases.

Changes in bacterial community composition and dynamics and viral mortality rates associated with enhanced flagellate grazing in a mesoeutrophic reservoir.

Bacterioplankton from a meso-eutrophic dam reservoir was size fractionated to reduce (<0.8-microm treatment) or enhance (<5-microm treatment) protistan grazing and then incubated in situ for 96 h in dialysis bags. Time course samples were taken from the bags and the reservoir to estimate bacterial abundance, mean cell volume, production, protistan grazing, viral abundance, and frequency of visibly infected cells. Shifts in bacterial community composition (BCC) were examined by denaturing gradient gel electrophoresis (DGGE), cloning and sequencing of 16S rDNA genes from the different treatments, and fluorescence in situ hybridization (FISH) with previously employed and newly designed oligonucleotide probes. Changes in bacterioplankton characteristics were clearly linked to changes in mortality rates. In the reservoir, where bacterial production about equaled protist grazing and viral mortality, community characteristics were nearly invariant. In the "grazer-free" (0.8-microm-filtered) treatment, subject only to a relatively low mortality rate (approximately 17% day(-1)) from viral lysis, bacteria increased markedly in concentration. While the mean bacterial cell volume was invariant, DGGE indicated a shift in BCC and FISH revealed an increase in the proportion of one lineage within the beta proteobacteria. In the grazing-enhanced treatment (5-microm filtrate), grazing mortality was approximately 200% and viral lysis resulted in mortality of 30% of daily production. Cell concentrations declined, and grazing-resistant flocs and filaments eventually dominated the biomass, together accounting for >80% of the total bacteria by the end of the experiment. Once again, BCC changed strongly and a significant fraction of the large filaments was detected using a FISH probe targeted to members of the Flectobacillus lineage. Shifts of BCC were also reflected in DGGE patterns and in the increases in the relative importance of both beta proteobacteria and members of the Cytophaga-Flavobacterium cluster, which consistently formed different parts of the bacterial flocs. Viral concentrations and frequencies of infected cells were highly significantly correlated with grazing rates, suggesting that protistan grazing may stimulate viral activity.

Predator-specific enrichment of actinobacteria from a cosmopolitan freshwater clade in mixed continuous culture.

We investigated whether individual populations of freshwater bacteria in mixed experimental communities may exhibit specific responses to the presence of different bacterivorous protists. In two successive experiments, a two-stage continuous cultivation system was inoculated with nonaxenic batch cultures of the cryptophyte Cryptomonas sp. Algal exudates provided the sole source of organic carbon for growth of the accompanying microflora. The dynamics of several 16S rRNA-defined bacterial populations were followed in the experimental communities. Although the composition and stability of the two microbial communities differed, numerous members of the first assemblage could again be detected during the second experiment. The introduction of a size-selectively feeding mixotrophic nanoflagellate (Ochromonas sp.) always resulted in an immediate bloom of a single phylotype population of members of the class Actinobacteria (Ac1). These bacteria were phylogenetically affiliated with an uncultured lineage of gram-positive bacteria that have been found in freshwater habitats only. The Ac1 cells were close to the average size of freshwater bacterioplankton and significantly smaller than any of the other experimental community members. In contrast, no increase of the Ac1 population was observed in vessels exposed to the bacterivorous ciliate Cyclidium glaucoma. However, when the Ochromonas sp. was added after the establishment of C. glaucoma, the proportion of population Ac1 within the microbial community rapidly increased. Populations of a beta proteobacterial phylotype related to an Aquabacterium sp. decreased relative to the total bacterial communities following the addition of either predator, albeit to different extents. The community structure of pelagic microbial assemblages can therefore be influenced by the taxonomic composition of the predator community.

Size Selective Feeding in Cyclidium glaucoma (Ciliophora, Scuticociliatida) and Its Effects on Bacterial Community Structure: A Study from a Continuous Cultivation System.

Three aspects of size selective feeding by the scuticociliate Cyclidium glaucoma were studied in continuous cultivation systems. Firstly, grazing-induced changes in abundance, biomass, and size structure of a bacterial community were investigated. Secondly, we studied possible grazing-protection mechanisms of bacteria as a response to permanent presence of the predator. And finally, we were looking for potential feedback mechanisms within this predator-prey relationship, i.e., how the ciliate population reacted to a changed, more grazing-protected bacterial community. The first stage of the cultivation system consisted of the alga Cryptomonas sp. and the accompanying mixed bacterial community. These organisms were transferred to two second stage vessels, a control stage without ciliates and a second one inoculated with C. glaucoma. After the first week, the abundance of bacteria in the latter decreased by 60% and remained stable until the end of the experiment (65 d), whereas bacterial biomass was less affected (393 mg C L-1 during days 0-7, 281 mg C L-1 afterwards). The mean bacterial cell volume doubled from 0.089 mm3 to 0.167 mm3, which was mainly due to increasing cell widths. During the whole investigation period formation of colonies or filaments was not observed, but we found a clear feedback of ciliates on bacterial size. An increase in bacterial cell volume was always followed by a decline of the predator population, resulting in a yet undescribed type of microbial predator-prey relation. Literature and our own data on the optimal food size range grazed by C. glaucoma showed that bacterial cell width rather than length was responsible for that observed phenomenon. Finally, we suggest that uptake rates of spherical latex beads give only limited information on truly ingestible prey volumes and that prey geometry should be considered in future studies on size selective feeding of protists.

Changes in the Epilimnetic Bacterial Community Composition, Production, and Protist-Induced Mortality along the Longitudinal Axis of a Highly Eutrophic Reservoir.

We studied changes in the epilimnetic bacterial community composition (BCC), bacterial biomass and production, and protistan succession and bacterivory along the longitudinal axis of the canyon-shaped, highly eutrophic Sau Reservoir (NE Spain) during two sampling campaigns, in April and July 1997. Longitudinal changes in BCC from the river inflow to the dam area of the reservoir were detected by using oligonucleotide probes targeted to the kingdom Bacteria, to the alpha, beta, and gamma subclasses (ALFA, BETA, and GAMA) of the class Proteobacteria, and to the Cytophaga/Flavobacterium (CF) cluster. In general, the inflow of the organically loaded Ter river, with highly abundant allochthonous bacterial populations, induced a clearly distinguishable longitudinal succession of the structure of the microbial food web. The most dynamic changes in microbial parameters occurred at the plunge point, the mixing area of river water and the reservoir epilimnion. Changes within members of BETA and CF were the most important in determining changes in BCC, bacterial abundance and biomass. Much less relevant changes occurred within the less abundant ALFA and GAMA bacteria. From the plunge point downstream, we described a significant shift in BCC in the form of decreased proportions of BETA and CF. This shift spatially coincided with the highest values of heterotrophic nanoflagellate bacterivory (roughly doubled the bacterial production). CF numerically dominated throughout the reservoir without any marked longitudinal changes in their mean cell volume. In contrast, very large cells affiliated to BETA clearly dominated in the allochthonous bacterial biomass brought by the river. BETA showed a marked downstream trend of decreasing mean cell volume. We conclude that the observed BCC shift and the longitudinal shift in food web structure (bacteria-heterotrophic nanoflagellates-ciliates) resulted from highly complex interactions brought about by several major factors: varying hydrology, the high localized allochthonous input of organic matter brought by the river, downstream changing substrate availability, and selective protistan bacterivory.

[Prevention and therapy of dehiscent laparotomy wounds]. / Prevence a osetrení dehiscencí laparotomických ran.

Dehiscence of laparotomic wounds is always unpleasant, starting with the resentment of the patient and the lack of confidence leading as far as complete negation of a favourable result of the operation. Due to lack of more suitable materials we elaborated ten years ago our own technique which we use nowadays for treatment but in particular for prevention. We feel that it could be a certain asset because it is simple and financially feasible.

[Treatment of ingrown toenails]. / Osetrení zarustajících nehtu palcu na dolních koncetinách.

Surgery of ingrowing toe nails are one of the most frequent operations in ambulatory practice. It is well known that a minor operation which relieves the patient of his complaints is appreciated more than the most complicated operation. Based on the article of Dr. Krticka we wish to present our own modification of an operation called Klapp's operation. We found that the results when using a modified vertical U stitch are better than when the classical technique is applied.

Morphological and compositional shifts in an experimental bacterial community influenced by protists with contrasting feeding modes.

In a two-stage continuous-flow system, we studied the impacts of different protozoan feeding modes on the morphology and taxonomic structure of mixed bacterial consortia, which were utilizing organic carbon released by a pure culture of a Rhodomonas sp. grown on inorganic medium in the first stage of the system. Two of three second stages operated in parallel were inoculated by a bacterivorous flagellate, Bodo saltans, and an algivorous ciliate, Urotricha furcata, respectively. The third vessel served as a control. In two experiments, where algal and bacterial populations grew at rates and densities typical for eutrophic waters, we compared community changes of bacteria, algae, and protozoa under quasi-steady-state conditions and during the transient stage after the protozoan inoculation. In situ hybridization with fluorescent oligonucleotide probes and cultivation-based approaches were used to tentatively analyze the bacterial community composition. Initially the cell size distribution and community structure of all cultivation vessels showed similar patterns, with a dominance of 1- to 2.5-(mu)m-long rods from the beta subdivision of the phylum Proteobacteria ((beta)-Proteobacteria). Inoculation with the ciliate increased bacterial growth in this substrate-controlled variant, seemingly via a recycling of nutrients and substrate released by grazing on algae, but without any detectable effect on the composition of bacterial assemblage. In contrast, an inoculation with the bacterivore, B. saltans, resulted in a decreased proportion of the (beta)-Proteobacteria. One part of the assemblage (<4% of total bacterial numbers), moreover, produced large grazing-resistant threadlike cells. As B. saltans ingested only cells of <3 (mu)m, this strategy yielded a refuge for (symbl)70% of total bacterial biomass from being grazed. Another consequence of the heavy predation in this variant was a shift to the numerical dominance of the (alpha)-Proteobacteria. The enhanced physiological status of the heavily grazed-upon segment of bacterial community resulted in a much higher proportion of CFU (mean, 88% of total bacterial counts) than with other variants, where CFU accounted for (symbl)30%. However, significant cultivation-dependent shifts of the bacterial community were observed toward (gamma)-Proteobacteria and members of the Cytophaga/Flavobacterium group, which demonstrated the rather poor agreement between cultivation-based approaches and oligonucleotide probing.

Contrasting bacterial strategies to coexist with a flagellate predator in an experimental microbial assemblage.

We studied predator-induced changes within a slowly growing mixed microbial assemblage that was sustained by algal exudates in a continuous cultivation system. In situ hybridization with fluorescent monolabeled oligonucleotide probes was used for a tentative community analysis. This method also allowed us to quantify the proportions of predators with ingested bacteria of different taxonomic groups. In addition, we determined grazing rates on bacteria with fluorescently labelled prey. Bacteria belonging to the alpha and beta subdivisions of the phylum Proteobacteria ((alpha)- and (beta)-Proteobacteria, respectively) showed very different responses to the addition of a bacterivorous flagellate, Bodo saltans. Within one day, filamentous protist-inedible bacteria developed; these belonged to the (beta)-Proteobacteria and constituted between 8.7 and 34% of bacteria from this subgroup. Total abundance of (beta)-Proteobacteria decreased from 3.05 x 10(sup6) to 0.23 x 10(sup6) cells ml(sup-1), and estimated cell division rates were low. Other morphologically inconspicuous protist-edible bacteria belonging to the (alpha)-Proteobacteria were found to respond to predation by an increase in growth rate. Although these bacteria were heavily grazed upon, as on average >85% of flagellate cells had ingested (alpha)-Proteobacteria, they numerically dominated after the addition of B. saltans (mean, 1.35 x 10(sup6) cells ml(sup-1)). It was thus mainly those fast-dividing strains of (alpha)-Proteobacteria that supported the growth of the flagellate population. We conclude that bacteria in mixed assemblages can adopt at least two distinct strategies as a reaction to intense flagellate predation: to outgrow predation pressure or to develop inedible, inactive filaments. Since these strategies occurred within 24 h after the addition of the flagellate, we hypothesize that chemical stimuli released by the predator may have triggered bacterial responses.

Evaluation of Extracellular, High-affinity beta-N-acetylglucosaminidase Measurements from Freshwater Lakes: An Enzyme Assay to Estimate Protistan Grazing on Bacteria and Picocyanobacteria

Protistan community grazing rates upon both bacterioplankton and autotrophic picoplankton were estimated using fluorescently-labeled prey and by measurement of extracellular hydrolysis of 4-methylumbelliferyl (MUF) beta-N-acetylglucosaminide in a eutrophic reservoir and an oligo-mesotrophic lake during phytoplankton blooms. In addition, enzyme methods were optimized in bacterivorous flagellate cultures by two enzyme assays, based on fluorometric detection of protistan digestive activity, which were compared and calibrated independently against flagellate bacterivory. Enzymatic hydrolyses of MUF beta-N,N',N''-triacetylchitotriose and MUF beta-N-acetylglucosaminide were measured in cell-free (sonicated) and whole-cell (unsonicated) samples. The hydrolysis of both substrates, using the whole-cell enzyme assay at in situ pH, was correlated significantly with total grazing rate of Bodo saltans. Thus the whole-cell enzyme assay with MUF beta-N-acetylglucosaminide was used for freshwater samples. High-affinity (Km < 1 µmol l-1) and low-affinity (Km > 100µmol l-1) enzymes were distinguished kinetically in most samples from both systems studied. Activities (Vmax) of the high-affinity enzyme varied from 0.24 to 1.43 nmol l-1 h-1. Protistan community grazing on bacterioplankton was in the range of 0.15-1.36 µg C l-1 h-1 both for lake and reservoir, the differences being observed in grazing on picocyanobacteria (lake, 0.03-0.22 µg C l-1 h-1; reservoir, 0.35-1.56 µg C l-1 h-1). The enzyme activities were correlated significantly with the protistan grazing both on bacterioplakton (rs=0.62, P<0.001) and total procaryotic picoplankton (the sum of organic carbon grazed from bacteria and picocyanobacteria, rs=0.73, P<0.001) in the eutrophic reservoir. Weaker relationships (rs=0.42) with a lower slope were found for the oligo-mesotrophic lake. Ingestion rate studies are time-consuming and the digestive enzyme assay with MUF beta-N-acetylglucosaminide presents a rapid alternative for estimating total protistan prokaryotic picoplanktivory in freshwaters.

[Surgical wound closure with continuous sutures]. / Uzáver operacní rány pokracujícím stehem.

The author describes the technique of closure of surgical wounds by continuous suture. It is a vertical mattress suture, as described by Donatti-Mc Millen, which is made continuously along the whole wound. It may involve either one layer, depending on the depth of the wound, or it can be used after the usual treatment of the subcutaneous layer. For suture monophil fibre with atraumatic threading is optimal but any other material can be used for suture. The advantage is the simple and rapid character of the procedure, adequate haemostasis and perfect adaptation of the margins of the wound. A balanced tension of the fibre in the whole wound reduces the risk of ischaemisation of a single stitch. The stitch can be readily removed, and if properly done, the cosmetic effect is satisfactory. The economy of material is not negligible either. The authors have used it as a matter of routine since 1984.

[Traumatic rupture of the greater pectoral muscle tendon]. / Traumatická ruptura slachy velkého prsního svalu.

The authors describe the case-history of a rare covered injury of the musculus pectoralis major, abruption of the attachment of its tendon on the humerus. In the world literature only three major groups were described, the majority of publications pertain only to individual observations. In the development of the injury always a very similar mechanism is involved, a considerable proportion are sports injuries. The injury is adequately defined from the diagnostic aspect, it may cause embarrassment because it is uncommon. As the loss of function is marked, surgical repair is considered the method of choice. The importance of early operation is emphasized. The described technique of reinsertion produced results satisfactory from the patient's point of view.

[High ileus with a prolonged course]. / Vysoký ileus s protrahovaným prubehem.

The authors describe the atypical development of a so-called high ileus caused by a bulky phytobezoar made up of tropical fruit. it is of interest that the organism managed to cope for two months relatively well with a condition described in surgical practice as acute abdomen where urgent or at least acute intervention is essential. The authors draw attention to pitfalls of out-patient care which may involve risk of delay due to incomplete evaluation by the specialists involved. This risk is particularly great during the present period of restructuring of the health services "hunting for a"high point score".

[An inveterate incarceration of an umbilical hernia treated by radical excision]. / Zastaralá inkarcerace umbilikální hernie resená radikální excizí.

The authors describe the case-history of an elderly female patient with an inveterate incarcerated relapsing umbilical hernia. The patient's age, and obviously also associated diseases, were the cause of subjective underrating of the severity of the condition, late admission to hospital and surgical intervention. The syndrome of advanced ileus had already developed as well as local changes complicating or rather making classical surgical treatment impossible. The applied technique which can be described as "no touch" with regard to the hernia and its contents is considered as one of the dominating features of successful treatment. The second dominating feature is a satisfactory interdisciplinary collaboration in treating the condition which at first appeared infaust.

4-Methylumbelliferyl-beta-N-Acetylglucosaminide Hydrolysis by a High-Affinity Enzyme, a Putative Marker of Protozoan Bacterivory.

Hydrolysis of an artificial fluorogenic substrate, 4-methylumbelliferyl-beta-N-acetylglucosaminide, has been studied in a monoculture predator-prey system with either a flagellate (Bodo saltans) or a ciliate (Cyclidium sp.) fed upon pure bacterial culture (Aeromonas hydrophila or Alcaligenes xylosoxidans). Aeromonas hydrophila produced a low-affinity beta-N-acetylglucosaminidase-like enzyme (K(m), >>100 mumol liter) but Alcaligenes xylosoxidans did not. Inoculation of both bacterial strains with bacterivorous protozoa induced the occurrence of another, high-affinity, beta-N-acetylglucosaminidase-like enzyme (K(m), <0.5 mumol liter). The latter enzyme showed significant, close correlations with total grazing rates of both B. saltans (r = 0.96) and Cyclidium sp. (r = 0.89) estimated by using uptake of fluorescently labelled bacteria. Further significant correlations between several protozoan parameters and kinetic parameters of this enzyme suggest its likely protozoan origin. If both types of enzyme occurred together, they could be satisfactorily distinguished by using kinetic data analysis. Hence, measurements of beta-N-acetylglucosaminidase-like activities might be promising to use to improve estimations of protozoan bacterivory.

Estimates of bacterial growth rate constants from thymidine incorporation and variable conversion factors.

Thymidine incorporation into DNA is widely used to estimate rates of bacterial growth and secondary production in aquatic systems. The procedure requires the use of several conversion factors and assumptions to convert rates of thymidine uptake to rates of carbon production. Perhaps the most controversial of the conversion factors is that which converts rates of incorporation to the rate of cell production. During a year-long study in Lake Arlington, Texas, we empirically determined conversion factors from bacterial growth in filtered (1.0 µm porosity) and diluted (1:9) lake water. Bacterial growth rate constants determined from changes in cell abundance were compared to growth rate constants estimated from empirically derived (both instantaneous and annually averaged), theoretical, and modeled conversion factors. Single value conversion factors (i.e., theoretical, or average of 19 empirically determined conversion factors) did not yield estimates of bacterial growth rate constants that compared favorably to growth estimates from changes in bacterial abundance. Conversion factors, determined from a regression model based on empirically determined conversion factors, gave annual growth estimates that were similar to those obtained from changes in cell abundance.

Direct and indirect evidence of size-selective grazing on pelagic bacteria by freshwater nanoflagellates.

Size-selective grazing of three heterotrophic nanoflagellates (with cell sizes of 21, 44, and 66 mum) isolated from Lake Arlington, Texas was examined by using a natural mixture of fluorescence labelled lake bacteria. Sizes of ingested bacteria in food vacuoles were directly measured. Larger bacterial cells were ingested at a frequency much higher than that at which they occurred in the assemblage, indicating preferential flagellate grazing on the larger size classes within the lake bacterioplankton. Water samples were collected biweekly from June through September, 1989, fractionated by filtration, and incubated for 40 h at in situ temperatures. The average bacterial size was always larger in water which was passed through 1-mum-pore-size filters (1-mum-filtered water) (which was predator free) than in 5-mum-filtered water (which contained flagellates only) or in unfiltered water (in which all bacterivores were present). The increase of bacterial-cell size in 1-mum-filtered water was caused by a shift in the size structure of the bacterioplankton population. Larger cells became more abundant in the absence of flagellate grazing.