Mechanism of lethal action of 2,450-MHz radiation on microorganisms.

Various bacteria, actinomycetes, fungi, and bacteriophages were exposed to microwaves of 2,450 +/- 20 MHz in the presence and in the absence of water. It was found that microorganisms were inactivated only when in the presence of water and that dry or lyophilized organisms were not affected even by extended exposures. The data presented here prove that microorganisms are killed by "thermal effect" only and that, most likely, there is no "nonthermal effect"; cell constituents other than water do not absorb sufficient energy to kill microbial cells.     

Microbiological profile in water-supply conduits from a clogged well

This article presents the results of an investigation involving bacterioflora in a water well clogged for the presence of biomass. The water well, placed in a zone near Rome, showed some problems about the water quality and about the extraction of water. The examination of the interior of the pipes showed the presence of biomass. The biomass was examined microscopically and bacteriological analyses were carried out on it. Heterotrophic bacteria were enumerated with three different media by direct count, Pseudomonas sp., yeasts and fungi also by spread plate method. The anaerobic Sulphate Reducing Bacteria were investigated by "Most Probable Number" technique. The results of the analyses showed the presence of protozoa and algae. Moreover high quantity of bacterial flora as heterotrophic bacteria and Pseudomonas sp. were revealed. Sulphate Reducing Bacteria were enumerated in low quantities. Sphaerotilus natans, Actinomyces and Rhodotorula were identified. The clogging problems arose from the presence of filamentous microorganisms as Sphaeroilus natans and Actinomyces sp. When microorganisms of this kind are present in aquifers they can multiply massively if the conditions are favorable.     

Detection of Legionella species in reclaimed water and air with the EnviroAmp Legionella PCR kit and direct fluorescent antibody staining.

Reclaimed water is an important resource for areas with inadequate water supplies. However, there have been few studies on the variety of microorganisms found in this type of water, since typically reclaimed water is examined only for the presence of coliform bacteria. Many microorganisms, including the legionellae, are known to be more resistant to chlorine than are coliform bacteria. Previously, we detected > 10(3) Legionella cells per ml in primary and secondary sewage effluents and observed no significant reduction in population numbers throughout the treatment process. In this study, we detected Legionella spp. in chlorinated effluent by using an EnviroAmp Legionella PCR kit and direct fluorescent antibody (DFA) staining. However, we were not able to isolate Legionella spp. from either natural or seeded reclaimed water samples. This suggests that the Legionella spp. detected by the PCR and DFA methods may be injured or viable but nonculturable after exposure to the high residual chlorine levels typically found in this type of water source. The numbers of coliform bacteria were low (< 2 cells per 100 ml) in most reclaimed water samples and were not correlated with the presence or absence of Legionella spp. We also collected air samples from above a secondary aeration basin and analyzed them by using the PCR, DFA, and plate culture methods. Legionella spp. were detected in the air obtained from above the secondary basin with all three methods. We concluded that the PCR was superior to the culture and DFA methods for detecting Legionella spp. in environmental water samples.     

Bioavailability of Chlorocatechols in Naturally Contaminated Sediment Samples and of Chloroguaiacols Covalently Bound to C2-Guaiacyl Residues

Bacteria in anaerobic enrichment cultures that dechlorinated a range of chlorocatechols were used to examine the stability of endogenous chlorocatechols in a contaminated sediment sample and in interstitial water prepared from it. During incubation of the sediment sample for 450 days with or without added cells, there was a decrease in the concentration of solvent-extractable chlorocatechols but not in that of the total chlorocatechols, including sediment-associated components. In the presence of azide, the decrease in the concentrations of the former was eliminated or substantially decreased. Control experiments in which 3,4,5-trichlorocatechol was added to the sediment suspensions after 130 days showed that its dechlorination was accomplished not only by the added cells but also by the endemic microbial flora. It was concluded that the endogenous chlorocatechols in the sediment were not accessible to microorganisms with dechlorinating activity. On the other hand, microorganisms were apparently responsible for decreasing the solvent extractability of the chlorocatechols, and this effect decreased with increasing length of exposure time. Similar experiments carried out for 70 days with the sediment interstitial water showed that the chlorocatechols that were known to be associated with organic matter were also inaccessible to microbial dechlorination. Experiments with model compounds in which 4,5,6-trichloroguaiacol and tetrachloroguaiacol were covalently linked to C₂-guaiacyl residues showed that these compounds were resistant to O demethylation or dechlorination during incubation with a culture having these activities. The only effect of microbial action was the quantitative reduction in 12 days of the C′1 keto group to an alcohol which was stable against further transformation for up to 65 days. The results of these experiments are consistent with the existence of chlorocatechols and chloroguaiacols in contaminated sediments and illustrate the cardinal significance of bioavailability in determining their recalcitrance to dechlorination and O demethylation, respectively. It is suggested that bioavailability is an important factor in determining the persistence of xenobiotics in natural ecosystems and that its omission represents a serious limitation in the interpretation of many laboratory experiments directed towards determining the persistence of xenobiotics in aquatic ecosystems.     

Bioavailability of chlorocatechols in naturally contaminated sediment samples and of chloroguaiacols covalently bound to c(2)-guaiacyl residues

Bacteria in anaerobic enrichment cultures that dechlorinated a range of chlorocatechols were used to examine the stability of endogenous chlorocatechols in a contaminated sediment sample and in interstitial water prepared from it. During incubation of the sediment sample for 450 days with or without added cells, there was a decrease in the concentration of solvent-extractable chlorocatechols but not in that of the total chlorocatechols, including sediment-associated components. In the presence of azide, the decrease in the concentrations of the former was eliminated or substantially decreased. Control experiments in which 3,4,5-trichlorocatechol was added to the sediment suspensions after 130 days showed that its dechlorination was accomplished not only by the added cells but also by the endemic microbial flora. It was concluded that the endogenous chlorocatechols in the sediment were not accessible to microorganisms with dechlorinating activity. On the other hand, microorganisms were apparently responsible for decreasing the solvent extractability of the chlorocatechols, and this effect decreased with increasing length of exposure time. Similar experiments carried out for 70 days with the sediment interstitial water showed that the chlorocatechols that were known to be associated with organic matter were also inaccessible to microbial dechlorination. Experiments with model compounds in which 4,5,6-trichloroguaiacol and tetrachloroguaiacol were covalently linked to C(2)-guaiacyl residues showed that these compounds were resistant to O demethylation or dechlorination during incubation with a culture having these activities. The only effect of microbial action was the quantitative reduction in 12 days of the C'1 keto group to an alcohol which was stable against further transformation for up to 65 days. The results of these experiments are consistent with the existence of chlorocatechols and chloroguaiacols in contaminated sediments and illustrate the cardinal significance of bioavailability in determining their recalcitrance to dechlorination and O demethylation, respectively. It is suggested that bioavailability is an important factor in determining the persistence of xenobiotics in natural ecosystems and that its omission represents a serious limitation in the interpretation of many laboratory experiments directed towards determining the persistence of xenobiotics in aquatic ecosystems.     

Microbial degradation of polycyclic aromatic hydrocarbons in soils affected by the organic matrix of compost

This paper describes the degradation of naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene in soil and soil/compost mixtures. Compost addition facilitated the degradation of 500 mg naphthalene/kg soil and 100 mg/kg each of other polycyclic aromatic hydrocarbons (PAH) within 25 days in soil systems with water contents below the water-holding capacity. By means of a humic acid extraction, it was demonstrated that the decrease of PAH concentrations after compost addition was not caused by a sorption to organic matter preventing PAH analysis. The enhanced PAH degradation was examined in a series of batch experiments with contaminated soil to evaluate whether the effect of compost addition is caused by the microorganisms of the compost itself, by the properties of the organic matrix of the compost material, or by water-soluble fertilising substances. The experiments revealed that the release of fertilising substances from the compost and the shift of soil pH brought about by the compost did not cause the stimulatory effect. The microorganisms inherent to the compost were also not necessary for the enhanced degradation. Sterilised compost was recolonised by soil microorganisms after a lagphase yielding a degradation activity similar to that of the non-sterilised compost. The presence of the solid organic matrix of the compost seemed to be essential for the enhanced degradation. The soil/compost microflora, which was separated from the organic matrix in liquid cultures, exhibited a much lower degrading activity than in the presence of the solid organic material.     

Membrane damage and microbial inactivation by chlorine in the absence and presence of a chlorine-demanding substrate

The relationship between cell inactivation and membrane damage was studied in two gram-positive organisms, Listeria monocytogenes and Bacillus subtilis, and two gram-negative organisms, Yersinia enterocolitica and Escherichia coli, exposed to chlorine in the absence and presence of 150 ppm of organic matter (Trypticase soy broth). L. monocytogenes and B. subtilis were more resistant to chlorine in distilled water. The addition of small amounts of organic matter to the chlorination medium drastically increased the resistance of both types of microorganisms, but this effect was more marked in Y. enterocolitica and E. coli. In addition, the survival curves for these microorganisms in the presence of organic matter had a prolonged shoulder. Sublethal injury was not detected under most experimental conditions, and only gram-positive cells treated in distilled water showed a relevant degree of injury. The exposure of bacterial cells to chlorine in distilled water caused extensive permeabilization of the cytoplasmic membrane, but the concentrations required were much higher than those needed to inactivate cells. Therefore, there was no relationship between the occurrence of membrane permeabilization and cell death. The addition of organic matter to the treatment medium stabilized the cytoplasmic membrane against permeabilization in both the gram-positive and gram-negative bacteria investigated. Exposure of E. coli cells to the outer membrane-permeabilizing agent EDTA increased their sensitivity to chlorine and caused the shoulders in the survival curves to disappear. Based on these observations, we propose that bacterial envelopes could play a role in cell inactivation by modulating the access of chlorine to the key targets within the cell.     

Clostridium perfringens and somatic coliphages as indicators of the efficiency of drinking water treatment for viruses and protozoan cysts.

To find the most suitable indicator of viral and parasitic contamination of drinking water, large-volume samples were collected and analyzed for the presence of pathogens (cultivable human enteric viruses, Giardia lamblia cysts, and Cryptosporidium oocysts) and potential indicators (somatic and male-specific coliphages, Clostridium perfringens). The samples were obtained from three water treatment plants by using conventional or better treatments (ozonation, biological filtration). All samples of river water contained the microorganisms sought, and only C. perfringens counts were correlated with human enteric viruses, cysts, or oocysts. For settled and filtered water samples, all indicators were statistically correlated with human enteric viruses but not with cysts or oocysts. By using multiple regression, the somatic coliphage counts were the only explanatory variable for the human enteric virus counts in settled water, while in filtered water samples it was C. perfringens counts. Finished water samples of 1,000 liters each were free of all microorganisms, except for a single sample that contained low levels of cysts and oocysts of undetermined viability. Three of nine finished water samples of 20,000 liters each revealed residual levels of somatic coliphages at 0.03, 0.10, and 0.26 per 100 liters. Measured virus removal was more than 4 to 5 log10, and cyst removal was more than 4 log10. Coliphage and C. perfringens counts suggested that the total removal and inactivation was more than 7 log10 viable microorganisms. C. perfringens counts appear to be the most suitable indicator for the inactivation and removal of viruses in drinking water treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inactivation of Salmonella enterica serovar enteritidis by ultrasonic waves under pressure at different water activities

The inactivation of Salmonella enterica serovar Enteritidis by ultrasonic waves (20 kHz; 117- microm wavelength) under pressure (175 kPa) at nonlethal temperatures (manosonication [MS]) and lethal temperatures (manothermosonication [MTS]) in media of different water activities has been investigated. Heat decimal reduction time values increased 30 times when the water activity was decreased from nearly 1 to 0.96, but the MS resistance was increased only twofold. The inactivation of Salmonella serovar Enteritidis by ultrasound under pressure at low water activities was a phenomenon of the "all-or-nothing" type. A synergistic lethal effect was observed between heat and ultrasound in media with reduced water activity; the lower the water activity, the greater the synergistic effect. This work could be useful for improving sanitation and preservation treatments of foods, especially those which are sensitive to temperature and those in which components protect microorganisms to heat. It also contributes to our knowledge of microbial inactivation mechanisms by MS and MTS treatments.     

Growth of phenol-mineralizing microorganisms in fresh water.

A method was developed to enumerate the procaryotic and eucaryotic phenol-mineralizing microorganisms present in samples of fresh water. Sixty-five percent or greater mineralization of [U-14C]phenol was considered a positive tube (contained phenol-mineralizing microorganisms) in the most-probable-number technique. Replicate most-probable-number tubes contained no microbial inhibitors, streptomycin and tetracycline, or cyclohexamide and nystatin plus 200 pg to 100 micrograms of phenol per ml. Phenol mineralization rates were obtained by measuring the amount of exogenous phenol that disappeared from solution over time in the presence or absence of the microbial inhibitors. Initially, less than 100 phenol-mineralizing bacteria per ml and 1 phenol-mineralizing fungus per ml were present at both 200 pg and 100 micrograms of phenol per ml. Phenol mineralization rates were 6.3 times greater for the mineralizing bacteria than for the fungi at 200 pg of phenol per ml. Phenol concentrations above 10 micrograms/ml were inhibitory to the microorganisms capable of mineralizing phenol. The phenol mineralizers grew in the water samples in the absence of phenol, indicating that there were sufficient indigenous nutrients in the lake water to support growth. There was no difference in the growth rate of these microorganisms in the presence or absence of 1 ng of phenol per ml, whereas the growth rate was more rapid at 1 microgram of phenol per ml than in its absence. There was a correlation between microbial growth and the amount of phenol mineralized at 1 microgram but not at 1 ng of phenol per ml.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth of phenol-mineralizing microorganisms in fresh water

A method was developed to enumerate the procaryotic and eucaryotic phenol-mineralizing microorganisms present in samples of fresh water. Sixty-five percent or greater mineralization of [U-14C]phenol was considered a positive tube (contained phenol-mineralizing microorganisms) in the most-probable-number technique. Replicate most-probable-number tubes contained no microbial inhibitors, streptomycin and tetracycline, or cyclohexamide and nystatin plus 200 pg to 100 micrograms of phenol per ml. Phenol mineralization rates were obtained by measuring the amount of exogenous phenol that disappeared from solution over time in the presence or absence of the microbial inhibitors. Initially, less than 100 phenol-mineralizing bacteria per ml and 1 phenol-mineralizing fungus per ml were present at both 200 pg and 100 micrograms of phenol per ml. Phenol mineralization rates were 6.3 times greater for the mineralizing bacteria than for the fungi at 200 pg of phenol per ml. Phenol concentrations above 10 micrograms/ml were inhibitory to the microorganisms capable of mineralizing phenol. The phenol mineralizers grew in the water samples in the absence of phenol, indicating that there were sufficient indigenous nutrients in the lake water to support growth. There was no difference in the growth rate of these microorganisms in the presence or absence of 1 ng of phenol per ml, whereas the growth rate was more rapid at 1 microgram of phenol per ml than in its absence. There was a correlation between microbial growth and the amount of phenol mineralized at 1 microgram but not at 1 ng of phenol per ml.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of earthworms on the physiological state of the microbial community at vermicomposting

The effect of earthworms on the microbial community of composts and vermicomposts was assayed by the following parameters: mineralization activity, the levels of physiologically active and growing microbial biomass, the requirement for growth factors, and the spectrum of assimilation of organic substrates by the microbial community. The substrate affinities of microbial enzyme systems in vermicompost were found to be lower than in compost without earthworms, which is evidence of a higher amount of r-strategists in the microbial community of vermicomposts. Physiologically active biomass of microorganisms is higher in peat-based vermicompost than in compost. The microorganisms of vermicomposts and composts experience deficiency in growth factors to a lesser extent than the microorganisms in soil. The presence of earthworms influences the physiological diversity: the Shannon index increases or decreases depending on the type of composted substrate and incubation time. The growth rate of microorganisms increases on various test substrates in the presence of worms.     

Manganese-oxidizing microorganisms inhabiting the phylloplane]

Manganese oxidizing microorganisms belonging to the genus Metallogenium were found in the phylloplane of various trees and grass plants. This is the first evidence that epiphytes participate in the transformation of mineral elements, and that microorganisms belonging to the genus Metallogenium can be found not only in fresh water and soil.     

Membrane Damage and Microbial Inactivation by Chlorine in the Absence and Presence of a Chlorine-Demanding Substrate

The relationship between cell inactivation and membrane damage was studied in two gram-positive organisms, Listeria monocytogenes and Bacillus subtilis, and two gram-negative organisms, Yersinia enterocolitica and Escherichia coli, exposed to chlorine in the absence and presence of 150 ppm of organic matter (Trypticase soy broth). L. monocytogenes and B. subtilis were more resistant to chlorine in distilled water. The addition of small amounts of organic matter to the chlorination medium drastically increased the resistance of both types of microorganisms, but this effect was more marked in Y. enterocolitica and E. coli. In addition, the survival curves for these microorganisms in the presence of organic matter had a prolonged shoulder. Sublethal injury was not detected under most experimental conditions, and only gram-positive cells treated in distilled water showed a relevant degree of injury. The exposure of bacterial cells to chlorine in distilled water caused extensive permeabilization of the cytoplasmic membrane, but the concentrations required were much higher than those needed to inactivate cells. Therefore, there was no relationship between the occurrence of membrane permeabilization and cell death. The addition of organic matter to the treatment medium stabilized the cytoplasmic membrane against permeabilization in both the gram-positive and gram-negative bacteria investigated. Exposure of E. coli cells to the outer membrane-permeabilizing agent EDTA increased their sensitivity to chlorine and caused the shoulders in the survival curves to disappear. Based on these observations, we propose that bacterial envelopes could play a role in cell inactivation by modulating the access of chlorine to the key targets within the cell.     

Microorganisms in summer snow patches in selected high mountain ranges of Slovakia

Snow is not only abiotic component of cryosphere but also a true functional ecosystem. In the past were the Vysoké Tatry Mts, the highest mountains of the Carpathians, the most studied mountain area in the Europe in terms of cryoflora. The aim of this study was to investigate the occurrence of snow microorganisms and its environmental conditions also in other high mountains of Slovakia. Due to climatic conditions of high altitudes, snow patches persist in deep concave relief shapes until late of June. Therefore these conditions are suitable for snow microorganisms. Sampling of snow microorganisms was carried out from May to September of 2013 and 2014. In selected mountains were mostly found snow alga cf. Chloromonas nivalis and we found also the presence of individuals of cyanobacteria, microscopic fungi, ciliates, rotifer and tardigrade. Recorded occurrence of thaw–froze cycles during august and high intensity of photosynthetically active radiation (PAR) (max. 2133 μmol.m^?2.s^?1) could be considered as stress factors this harsh environment. The pH values of taken samples were slightly acidified, in some cases neutral. The conductivity of water from melted snow had high coefficient of variation (values characteristic for distilled water until values 160 μS.cm^?1). The chemical analyses determined the higher content of N-NO₃ in analysed samples of melted snow in compare with content of N-NH₄ and P-PO₄. Despite these facts paper confirms potential of Slovak high mountains as suitable place for life of snow microorganisms.     

Effect of nitrates on biotransformation of phosphogypsum and phenol uptake in cultures of autochthonous sludge microflora from petroleum refining wastewaters

The effect of nitrates on the biotransformation of phosphogypsum at 30 degrees C in stationary cultures of anaerobic, heterogeneous microflora growing in medium with phenol (250-1,000 mg/L) as sole carbon source was studied. The microorganisms used in this study were isolated from sludge in biological petroleum-refining wastewater treatment plant. Phosphogypsum (a waste product in the chemical industry that contains approximately 95% CaSO4) was added in amount of 5 g/L, the source of nitrates was KNO3 in concentration equivalent to that of phenol (250-1,000 mg N-NO3/L). The presence of nitrates in heterogeneous cultures has an inhibitory effect on the process of phosphogypsum biotransformation and stimulates the uptake of phenol. We have found that in cultures in medium containing phenol, phosphogypsum and nitrates at least three physiological groups of microorganisms were present. These were phenol-biodegrading microorganisms not requiring an external electron acceptor, sulfate-reducing bacteria biodegrading phenol or intermediate products of its breakdown and denitrifying bacteria not utilising phenol as a carbon source. On solid medium these bacteria together formed heterogeneous single colonies. In spite of repeated attempts we were unable to isolate pure strains and the only result of these measures was loss of denitrification ability in medium with phenol.     

Characterization of Microbial Arsenate Reduction in the Anoxic Bottom Waters of Mono Lake,California

Dissimilatory reduction of arsenate (DAsR) occurs in the arsenic-rich, anoxic water column of Mono Lake, California, yet the microorganisms responsible for this observed in situ activity have not been identified. To gain insight as to which microorganisms mediate this phenomenon, as well as to some of the biogeochemical constraints on this activity, we conducted incubations of arsenate-enriched bottom water coupled with inhibition/amendment studies and Denaturing Gradient Gel Electrophoresis (DGGE) characterization techniques. DAsR was totally inhibited by filter-sterilization and by nitrate, partially inhibited (~50%) by selenate, but only slightly (~25%) inhibited by oxyanions that block sulfate-reduction (molybdate and tungstate). The apparent inhibition by nitrate, however, was not due to action as a preferred electron acceptor to arsenate. Rather, nitrate addition caused a rapid, microbial re-oxidation of arsenite to arsenate, which gave the overall appearance of no arsenate loss. A similar microbial oxidation of As(III) was also found with Fe(III), a fact that has implications for the recycling of As(V) in Mono Lake's anoxic bottom waters. DAsR could be slightly (10%) stimulated by substrate amendments of lactate, succinate, malate, or glucose, but not by acetate, suggesting that the DAsR microflora is not electron donor limited. DGGE analysis of amplified 16S rDNA gene fragments from incubated arsenate-enriched bottom waters revealed the presence of two bands that were not present in controls without added arsenate. The resolved sequences of these excised bands indicated the presence of members of the epsilon ( Sulfurospirillum ) and delta ( Desulfovibrio ) subgroups of the Proteobacteria , both of which have representative species that are capable of anaerobic growth using arsenate as their electron acceptor.     

Influence of Food Microorganisms on Staphylococcal Growth and Enterotoxin Production in Meat

Forty-four microorganisms were studied for their influence on staphylococcal growth and enterotoxin production. Inhibition was found to be more common than stimulation. Two types of inhibition were observed: inhibition of staphylococcal growth, and inhibition of enterotoxin formation with no apparent effect on growth. By use of a plate test, 12 of the 44 food microorganisms were found to inhibit staphylococcal growth at 35 C. Of the 12, 3 also inhibited growth at 25 C. No significant differences in inhibition were observed with the 15 strains of enterotoxigenic staphylococci. In meat slurries, inhibition of staphylococcal growth was found to be greater at 25 C than at 35 C. Results on inhibition obtained from the plate test could not be correlated with the effect of the organisms in slurries. Environmental conditions were found to affect markedly the influence of food microorganisms on staphylococci. Of the 44 food microorganisms studied, only Bacillus cereus was observed to stimulate significantly staphylococcal growth and enterotoxin formation. Stimulation was more pronounced with Staphylococcus aureus 196E than with other strains of enterotoxigenic staphylococci. Bacillus megaterium and Brevibacterium linens were inhibited by staphylococci. These organisms were completely inhibited when inoculated in mixed cultures with staphylococci. In pure cultures, good staphylococcal growth was found to be accompanied by enterotoxin production; however, in the presence of food microorganisms, good staphylococcal growth occurred without the formation of detectable levels of enterotoxin A.     

Identification of crude-oil components and microorganisms that cause souring under anaerobic conditions

Oil souring has important implications with respect to energy resources. Understanding the physiology of the microorganisms that play a role and the biological mechanisms are both important for the maintenance of infrastructure and mitigation of corrosion processes. The objective of this study was to identify crude-oil components and microorganisms in oil-field water that contribute to crude-oil souring. To identify the crude-oil components and microorganisms that are responsible for anaerobic souring in oil reservoirs, biological conversion of crude-oil components under anaerobic conditions was investigated. Microorganisms in oil field water in Akita, Japan degraded alkanes and aromatics to volatile fatty acids (VFAs) under anaerobic conditions, and fermenting bacteria such as Fusibacter sp. were involved in VFA production. Aromatics such as toluene and ethylbenzene were degraded by sulfate-reducing bacteria (Desulfotignum sp.) via the fumarate-addition pathway and not only degradation of VFA but also degradation of aromatics by sulfate-reducing bacteria was the cause of souring. Naphthenic acid and 2,4-xylenol were not converted.     

Potential usefulness of bacteriophages that infect Bacteroides fragilis as model organisms for monitoring virus removal in drinking water treatment plants.

The presence of bacteriophages at different stages in three drinking water treatment plants was evaluated to study the usefulness of phages as model organisms for assessing the efficiency of the processes. The bacteriophages tested were somatic coliphages, F-specific coliphages, and phages infecting Bacteroides fragilis. The presence of enteroviruses and currently used bacterial indicators was also determined. Most bacteriophages were removed during the prechlorination-flocculation-sedimentation step. In these particular treatment plants, which include prechlorination, phages were, in general, more resistant to the treatment processes than present bacterial indicators, with the exception, in some cases, of clostridia. Bacteriophages infecting B. fragilis were found to be more resistant to water treatment than either somatic or F-specific coliphages or even clostridia. Enteric viruses were found only in untreated water in low numbers, and consequently, the efficiency of the plants in the removal of viruses could not be evaluated with precision. The numbers and frequencies of detection of the various microorganisms in water samples taken in the distribution network served by the three plants confirm the results found in the finished water at the plants.