Polyphasic classification of 0.2 microm filterable bacteria from the western Mediterranean Sea

Viable but nonculturable bacteria and their resuscitation: implications for cultivating uncultured marine microorganisms

Culturing has been the cornerstone of microbiology since Robert Koch first successfully cultured bacteria in the late nineteenth century. However, even today, the majority of microorganisms in the marine environment remain uncultivated. There are various explanations for the inability to culture bacteria in the laboratory, including lack of essential nutrients, osmotic support or incubation conditions, low growth rate, development of micro-colonies, and the presence of senescent or viable but nonculturable (VBNC) cells. In the marine environment, many bacteria have been associated with dormancy, as typified by the VBNC state. VBNC refers to a state where bacteria are metabolically active, but are no longer culturable on routine growth media. It is apparently a unique survival strategy that has been adopted by many microorganisms in response to harsh environmental conditions and the bacterial cells in the VBNC state may regain culturability under favorable conditions. The resuscitation of VBNC cells may well be an important way to cultivate the otherwise uncultured microorganisms in marine environments. Many resuscitation stimuli that promote the restoration of culturability have so far been identified; these include sodium pyruvate, quorum sensing autoinducers, resuscitation-promoting factors Rpfs and YeaZ, and catalase. In this review, we focus on the issues associated with bacterial culturability, the diversity of bacteria entering the VBNC state, mechanisms of induction into the VBNC state, resuscitation factors of VBNC cells and implications of VBNC resuscitation stimuli for cultivating these otherwise uncultured microorganisms. Bringing important microorganisms into culture is still important in the era of high-throughput sequencing as their ecological functions in the marine environment can often only be known through isolation and cultivation.

Structural and Functional Changes of Groundwater Bacterial Community During Temperature and pH Disturbances

In this study, we report the characteristics of a microbial community in sampled groundwater and elucidate the effects of temperature and pH disturbances on bacterial structure and nitrogen-cycling functions. The predominant phyla of candidate OD1, candidate OP3, and Proteobacteria represented more than half of the total bacteria, which clearly manifested as a "low nucleic acid content (LNA) bacteria majority" type via flow cytometric fingerprint. The results showed that LNA bacteria were more tolerant to rapid changes in temperature and pH, compared to high nucleic acid content (HNA) bacteria. A continuous temperature increase test demonstrated that the LNA bacterial group was less competitive than the HNA bacterial group in terms of maintaining their cell intactness and growth potential. In contrast, the percentage of intact LNA bacteria was maintained at nearly 70% with pH decrease, despite a 50% decrease in total intact cells. Next-generation sequencing results revealed strong resistance and growth potential of phylum Proteobacteria when the temperature increased or the pH decreased in groundwater, especially for subclasses α-, β-, and γ-Proteobacteria. In addition, relative abundance of nitrogen-related functional genes by qPCR showed no difference in nitrifiers or denitrifiers within 0.45 μm-captured and 0.45 μm-filterable bacteria due to phylogenetic diversity. One exception was the monophyletic anammox bacteria that belong to the phylum Planctomycetes, which were mostly captured on a 0.45-μm filter. Furthermore, we showed that both temperature increase and pH decrease could enhance the denitrification potential, whereas the nitrification and anammox potentials were weakened.

Predominance of Cand. Patescibacteria in Groundwater Is Caused by Their Preferential Mobilization From Soils and Flourishing Under Oligotrophic Conditions

Despite the widely observed predominance of Cand. Patescibacteria in subsurface communities, their input source and ecophysiology are poorly understood. Here we study mechanisms of the formation of a groundwater microbiome and the subsequent differentiation of Cand. Patescibacteria. In the Hainich Critical Zone Exploratory, Germany, we trace the input of microorganisms from forested soils of preferential recharge areas through fractured aquifers along a 5.4 km hillslope well transect. Cand. Patescibacteria were preferentially mobilized from soils and constituted 66% of species-level OTUs shared between seepage and shallow groundwater. These OTUs, mostly related to Cand. Kaiserbacteraceae, Cand. Nomurabacteraceae, and unclassified UBA9983 at the family level, represented a relative abundance of 71.4% of the Cand. Patescibacteria community at the shallowest groundwater well, and still 44.4% at the end of the transect. Several Cand. Patescibacteria subclass-level groups exhibited preferences for different conditions in the two aquifer assemblages investigated: Cand. Kaiserbacteraceae surprisingly showed positive correlations with oxygen concentrations, while Cand. Nomurabacteraceae were negatively correlated. Co-occurrence network analysis revealed a central role of Cand. Patescibacteria in the groundwater microbial communities and pointed to potential associations with specific organisms, including abundant autotrophic taxa involved in nitrogen, sulfur and iron cycling. Strong associations among Cand. Patescibacteria themselves further suggested that for many groups within this phylum, distribution was mainly driven by conditions commonly supporting a fermentative life style without direct dependence on specific hosts. We propose that import from soil, and community differentiation driven by hydrochemical conditions, including the availability of organic resources and potential hosts, determine the success of Cand. Patescibacteria in groundwater environments.

Proteobacteria and Bacteroidetes are major phyla of filterable bacteria passing through 0.22 μm pore size membrane filter, in Lake Sanaru, Hamamatsu, Japan

141 filterable bacteria that passed through a 0.22 μm pore size filter were isolated from Lake Sanaru in Hamamatsu, Japan. These belonged to Proteobacteria, Bacteroidetes, Firmicutes, or Actinobacteria among which the first two phyla comprised the majority of the isolates. 48 isolates (12 taxa) are candidates assignable to new bacterial species or genera of Proteobacteria or Bacteroidetes.

Metagenomic analysis of 0.2-μm-passable microorganisms in deep-sea hydrothermal fluid

We pyrosequenced the bulk DNA extracted from microorganisms that passed through 0.2-μm-pore-size filters and trapped by 0.1-μm-pore-size filters in the hydrothermal fluid of the Mariana Trough. Using the 454-FLX sequencer, we generated 202,648 sequences with an average length of 173.8 bases. Functional profiles were assigned by the SEED Annotation Engine. In the metagenome of the 0.2-μm-passable microorganisms, genes related to membrane function, including potassium homeostasis classified as membrane transport, and multidrug-resistance efflux pumps classified as virulence, were dominant. There was a higher proportion of genes pertinent to the subsystem of membrane transport in our metagenomic library than in other oceanic and hydrothermal vent metagenomes. Genes associated with a RND-type efflux transporter for exogenous substances were specifically identified in the present study. After a comparative analysis with the genome of the known ultramicrobacterium Sphingopyxis alaskensis RB2256, we discovered 1,542 cases of significant hits (E < 1 × 10(-2)) in our metagenome, and 1,172 of those were related to the DNA repair protein RadA. In this way, the microbial functional profile of 0.2-μm-passable fraction in the present study differs from oceanic metagenomes in the 0.2-μm-trapped fractions and hydrothermal vent metagenomes reported in previous research.

Enzymatic, outer membrane proteins and plasmid alterations of starved Vibrio parahaemolyticus and Vibrio alginolyticus cells in seawater

The marine bacteria Vibrio parahaemolyticus and V. alginolyticus were incubated in seawater for 8 months to evaluate their adaptative responses to starvation. The starved cells showed an altered biochemical and enzymatic profiles, respectively, on Api 20E and Api ZYM systems and an evolution to the filterable minicells state capable to pass membrane pore size 0.45 microm. Outer membrane proteins patterns of stressed bacteria were also altered. Indeed, these modifications were manifested by the appearance and/or disappearance of bands as well as in the level of expression of certain proteins. Plasmids profiles analysis showed that V. alginolyticus ATCC 33787 lost three plasmids, whereas other tested strains conserved their initial profiles.

Influence of size, shape, and flexibility on bacterial passage through micropore membrane filters

Sterilization of fluids by means of microfiltration is commonly applied in research laboratories as well as in pharmaceutical and industrial processes. Sterile micropore filters are subject to microbiological validation, where Brevundimonas diminuta is used as a standard test organism. However, several recent reports on the ubiquitous presence of filterable bacteria in aquatic environments have cast doubt on the accuracy and validity of the standard filter-testing method. Six different bacterial species of various sizes and shapes (Hylemonella gracilis, Escherichia coli, Sphingopyxis alaskensis, Vibrio cholerae, Legionella pneumophila, and B. diminuta) were tested for their filterability through sterile micropore filters. In all cases, the slender spirillum-shaped Hylemonella gracilis cells showed a superior ability to pass through sterile membrane filters. Our results provide solid evidence that the overall shape (including flexibility), instead of biovolume, is the determining factor for the filterability of bacteria, whereas cultivation conditions also play a crucial role. Furthermore, the filtration volume has a more important effect on the passage percentage in comparison with other technical variables tested (including flux and filter material). Based on our findings, we recommend a re-evaluation of the grading system for sterile filters, and suggest that the species Hylemonella should be considered as an alternative filter-testing organism for the quality assessment of micropore filters.

Quantification of the filterability of freshwater bacteria through 0.45, 0.22, and 0.1 microm pore size filters and shape-dependent enrichment of filterable bacterial communities

Micro-filtration is a standard process for sterilization in scientific research, medical, and industrial applications, and to remove particles in drinking water or wastewater treatment. It is generally assumed, and confirmed by quantifying filtration efficiency by plating, that filters with a 0.1-0.45 microm pore size can retain bacteria. In contrast to this assumption, we have regularly observed the passage of a significant fraction of natural freshwater bacterial communities through 0.45, 0.22, and 0.1 microm pore size filters. Flow cytometry and a regrowth assay were applied in the present study to quantify and cultivate filterable bacteria. Here we show for the first time a systematic quantification of their filterability, especially their ability to pass through 0.1 microm pore size filters. The filtered bacteria were subsequently able to grow on natural assimilable organic carbon (AOC) with specific growth rates up to 0.47 h(-1). We were able to enrich bacteria communities that pass preferentially through all three pore size filters at significantly increased percentages using successive filtration-regrowth cycles. In all instances, the dominant microbial populations comprised slender spirillum-shaped Hylemonella gracilis strains, suggesting shape-dependent selection during the filtration process. This quantification of the omnipresence of microfilterable bacterial in natural freshwater and their regrowth characteristics demand a change in the sterile filtration practice used in industrial and engineering applications as well as scientific research.

Phylotype diversity of deep-sea hydrothermal vent prokaryotes trapped by 0.2- and 0.1-microm-pore-size filters

Eleven 16S rRNA gene clone libraries including 34 archaeal and 72 bacterial phylotypes were constructed from total 708 clones of hydrothermal vent prokaryotes trapped by 0.2- and 0.1-microm-pore-size filters. Crenarchaeota and Proteobacteria phylotypes dominated the archaeal and bacterial populations, respectively. Novel unaffiliated phylotypes occurred only in the 0.1-microm-trapped populations.

Detection and isolation of ultrasmall microorganisms from a 120,000-year-old Greenland glacier ice core

The abundant microbial population in a 3,043-m-deep Greenland glacier ice core was dominated by ultrasmall cells (<0.1 microm3) that may represent intrinsically small organisms or starved, minute forms of normal-sized microbes. In order to examine their diversity and obtain isolates, we enriched for ultrasmall psychrophiles by filtering melted ice through filters with different pore sizes, inoculating anaerobic low-nutrient liquid media, and performing successive rounds of filtrations and recultivations at 5 degrees C. Melted ice filtrates, cultures, and isolates were analyzed by scanning electron microscopy, flow cytometry, cultivation, and molecular methods. The results confirmed that numerous cells passed through 0.4-microm, 0.2-microm, and even 0.1-microm filters. Interestingly, filtration increased cell culturability from the melted ice, yielding many isolates related to high-G+C gram-positive bacteria. Comparisons between parallel filtered and nonfiltered cultures showed that (i) the proportion of 0.2-microm-filterable cells was higher in the filtered cultures after short incubations but this difference diminished after several months, (ii) more isolates were obtained from filtered (1,290 isolates) than from nonfiltered (447 isolates) cultures, and (iii) the filtration and liquid medium cultivation increased isolate diversity (Proteobacteria; Cytophaga-Flavobacteria-Bacteroides; high-G+C gram-positive; and spore-forming, low-G+C gram-positive bacteria). Many isolates maintained their small cell sizes after recultivation and were phylogenetically novel or related to other ultramicrobacteria. Our filtration-cultivation procedure, combined with long incubations, enriched for novel ultrasmall-cell isolates, which is useful for studies of their metabolic properties and mechanisms for long-term survival under extreme conditions.

Erythrobacter luteolus sp. nov., isolated from a tidal flat of the Yellow Sea in Korea

A Gram-negative, non-spore-forming, yellow-pigmented, slightly halophilic bacterial strain, SW-109(T), was isolated from a tidal flat of the Yellow Sea in Korea, and subjected to a polyphasic taxonomic study. This isolate did not produce bacteriochlorophyll a and contained ubiquinone-10 as the predominant respiratory lipoquinone and C(18 : 1)omega7c as the major fatty acid. The DNA G + C content was 60.3 mol%. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain SW-109(T) is phylogenetically affiliated to the genus Erythrobacter of the family Sphingomonadaceae. Strain SW-109(T) exhibited levels of 16S rRNA gene sequence similarity to the type strains of Erythrobacter species of 94.0-96.3 %, making it possible to categorize strain SW-109(T) as a species that is separate from previously recognized Erythrobacter species. On the basis of its phenotypic properties and phylogenetic distinctiveness, SW-109(T) (= KCTC 12311(T) = JCM 12599(T)) was classified as the type strain of a novel Erythrobacter species, for which the name Erythrobacter luteolus sp. nov. is proposed.

Broad diversity of viable bacteria in 'sterile' (0.2 microm) filtered water

Filtration of liquids through 0.2-microm filters is a common and often-used method for the removal of microorganisms from heat-sensitive solutions. Such 0.2 microm filtrations are frequently referred to as 'sterile filtration', reflecting the general belief that all living organisms are excluded from the filtrate. However, it has been demonstrated that a few bacterial species can pass through 0.2-microm filters. Despite these observations, it is generally assumed that most bacteria are unable to pass through these filters. In contrast to this assumption, a wide diversity of 19 bacterial taxa was isolated by the acclimatization method from 0.2 microm-filtrated freshwater samples. All isolates were able to grow on substrate-rich agar plates. The size of the isolated strains ranged from <1 to >10 microm in cell length. The obtained isolates belong to the Bacteroidetes, Alphaproteobacteria, Betaproteobacteria, Actinobacteria and Spirochaetes. Previous studies which did not use the acclimatization method reported that only two out of the 19 taxa passed through 0.2- or 0.45-microm filters. The majority of the obtained isolates were more closely related to uncultured than to previously cultured bacteria. It is assumed that the obtained taxa represent only a small fraction of the bacteria able to pass through 0.2-microm filters, and it is concluded that microbiologists may have dramatically underestimated the diversity of 0.2 microm-filterable bacteria.

Erythrobacter seohaensis sp. nov. and Erythrobacter gaetbuli sp. nov., isolated from a tidal flat of the Yellow Sea in Korea

Two Gram-negative, non-spore-forming, slightly halophilic strains, SW-135T and SW-161T, which were isolated from a tidal flat of the Yellow Sea in Korea, were subjected to a polyphasic taxonomic study. The two isolates lacked bacteriochlorophyll a and contained ubiquinone-10 (Q-10) as the predominant respiratory lipoquinone and C18 : 1 ω7c and C17 : 1 ω6c as the major fatty acids. The DNA G+C contents of strains SW-135T and SW-161T were 62·2 and 64·5 mol%, respectively. Phylogenetic analyses based on 16S rRNA gene sequences showed that the two strains fall within the radiation of the cluster comprising Erythrobacter species. Strains SW-135T and SW-161T exhibited a 16S rRNA gene sequence similarity value of 96·9 % and a mean DNA–DNA relatedness level of 12·3 %. Sequence similarities between strains SW-135T and SW-161T and the type strains of recognized Erythrobacter species ranged from 96·7 to 98·5 %. Levels of DNA–DNA relatedness were low enough to indicate that strains SW-135T and SW-161T represent members of two species separate from all recognized Erythrobacter species. On the basis of polyphasic taxonomic data, strains SW-135T (=KCTC 12228T=DSM 16221T) and SW-161T (=KCTC 12227T=DSM 16225T) were classified as two novel Erythrobacter species, for which the names Erythrobacter seohaensis sp. nov. and Erythrobacter gaetbuli sp. nov. are proposed, respectively.

Erythrobacter aquimaris sp. nov., isolated from sea water of a tidal flat of the Yellow Sea in Korea

Three Gram-negative, non-motile, non-spore-forming, slightly halophilic rods (strains SW-110T, SW-116 and SW-140) were isolated from sea water of a tidal flat of the Yellow Sea in Korea and subjected to a polyphasic taxonomic study. The three isolates did not produce bacteriochlorophyll a and were characterized chemotaxonomically by having ubiquinone-10 as the predominant respiratory lipoquinone and C18 : 1 ω7c and C17 : 1 ω6c as the major fatty acids. The DNA G+C content of the three isolates was between 62·2 and 62·9 mol%. Strains SW-110T, SW-116 and SW-140 showed no difference in their 16S rRNA gene sequences, and their mean level of DNA–DNA relatedness was 94·8 %. Phylogenetic analyses based on 16S rRNA gene sequences showed that the three strains form a distinct phylogenetic lineage within the cluster comprising Erythrobacter species. Similarities between the 16S rRNA gene sequences of strains SW-110T, SW-116 and SW-140 and the type strains of Erythrobacter species ranged from 98·4 % (with Erythrobacter longus) to 97·7 % (with Erythrobacter flavus). Levels of DNA–DNA relatedness between strains SW-110T, SW-116 and SW-140 and the type strains of all recognized Erythrobacter species were in the range 5·3–12·7 %. On the basis of polyphasic taxonomic data, strains SW-110T, SW-116 and SW-140 were classified as a novel Erythrobacter species, for which the name Erythrobacter aquimaris sp. nov. is proposed. The type strain is SW-110T (=KCCM 41818T=JCM 12189T).

Erythrobacter flavus sp. nov., a slight halophile from the East Sea in Korea

Two gram-negative, motile, non-spore-forming, yellow-pigmented and slightly halophilic strains (SW-46T and SW-52) were isolated from sea water of the East Sea, Korea, and subjected to a polyphasic taxonomic study. Strains SW-46T and SW-52 were characterized chemotaxonomically by having ubiquinone-10 (Q-10) as the predominant respiratory lipoquinone and C18:1 omega7c as the major fatty acid. Their DNA G + C content was 64.0-64.1 mol%. Strains SW-46T and SW-52 showed 1 bp difference in their 16S rDNA sequences and a mean DNA-DNA relatedness level of 94.4%. Phylogenetic analysis based on 16S rDNA sequences showed that strains SW-46T and SW-52 fall within the alpha-subclass of the Proteobacteria and form a coherent cluster with Erythrobacter longus, Erythrobacter litoralis and Erythrobacter citreus. Levels of 16S rDNA similarity between strains SW-46T and SW-52 and the type strains of these three Erythrobacter species were 96.5-97.9%. Levels of DNA-DNA relatedness between strains SW-46T and SW-52 and the type strains of E. longus, E. litoralis and E. citreus were 3.6-14.7%. Therefore, on the basis of phenotypic properties, phylogeny and genomic data, strains SW-46T and SW-52 should be placed in the genus Erythrobacter as a novel species, for which the name Erythrobacter flavus sp. nov. is proposed. The type strain is SW-46T (= KCCM 41642T = JCM 11808T).

Isolation of novel ultramicrobacteria classified as actinobacteria from five freshwater habitats in Europe and Asia

We describe the first freshwater members of the class Actinobacteria that have been isolated. Nine ultramicro-size (<0.1 microm(3)) strains were isolated from five freshwater habitats in Europe and Asia. These habitats represent a broad spectrum of ecosystems, ranging from deep oligotrophic lakes to shallow hypertrophic lakes. Even when the isolated strains were grown in very rich media, the cell size was <0.1 microm(3) and was indistinguishable from the cell sizes of bacteria belonging to the smaller size classes of natural lake bacterioplankton. Hybridization of the isolates with oligonucleotide probes and phylogenetic analysis of the 16S rRNA gene sequences of the isolated strains revealed that they are affiliated with the class Actinobacteria and the family Microbacteriaceae. The previously described species with the highest levels of sequence similarity are Clavibacter michiganensis and Rathayibacter tritici, two phytopathogens of terrestrial plants. The 16S rRNA gene sequences of the nine isolates examined are more closely related to cloned sequences from uncultured freshwater bacteria than to the sequences of any previously isolated bacteria. The nine ultramicrobacteria isolated form, together with several uncultured bacteria, a diverse phylogenetic cluster (Luna cluster) consisting exclusively of freshwater bacteria. Isolates obtained from lakes that are ecologically different and geographically separated by great distances possess identical 16S rRNA gene sequences but have clearly different ecophysiological and phenotypic traits. Predator-prey experiments demonstrated that at least one of the ultramicro-size isolates is protected against predation by the bacterivorous nanoflagellate Ochromonas sp. strain DS.

Viable cytophaga-like bacterium in the 0.2 microm-filtrate seawater

A strain of the Cytophaga-like bacterium (CLB), Nano-1, was isolated from the 0.2 microm-filtrate of natural seawater. Both cellular fatty acid and 16S rDNA sequence analyses indicated that Nano-1 is closely affiliated to the marine gliding CLB genus, Microscilla. Nano-1 was observed to undergo cyclic morphological change typical of the genus Microscilla, and sub-0.2-microm cells were formed in the late stationary phase. The sub-0.2-microm cells were repeatedly revived and subcultured. Formation of the sub-0.2-microm cells seems to be adaptive for oligotrophic growth and starvation survival.

Sphingomonas alaskensis strain AFO1, an abundant oligotrophic ultramicrobacterium from the North Pacific

Numerous studies have established the importance of picoplankton (microorganisms of < or =2 microm in length) in energy flow and nutrient cycling in marine oligotrophic environments, and significant effort has been directed at identifying and isolating heterotrophic picoplankton from the world's oceans. Using a method of diluting natural seawater to extinction followed by monthly subculturing for 12 months, a bacterium was isolated that was able to form colonies on solid medium. The strain was isolated from a 10(5) dilution of seawater where the standing bacterial count was 3.1 x 10(5) cells ml(-1). This indicated that the isolate was representative of the most abundant bacteria at the sampling site, 1.5 km from Cape Muroto, Japan. The bacterium was characterized and found to be ultramicrosized (less than 0.1 microm(3)), and the size varied to only a small degree when the cells were starved or grown in rich media. A detailed molecular (16S rRNA sequence, DNA-DNA hybridization, G+C mol%, genome size), chemotaxonomic (lipid analysis, morphology), and physiological (resistance to hydrogen peroxide, heat, and ethanol) characterization of the bacterium revealed that it was a strain of Sphingomonas alaskensis. The type strain, RB2256, was previously isolated from Resurrection Bay, Alaska, and similar isolates have been obtained from the North Sea. The isolation of this species over an extended period, its high abundance at the time of sampling, and its geographical distribution indicate that it has the capacity to proliferate in ocean waters and is therefore likely to be an important contributor in terms of biomass and nutrient cycling in marine environments.

Enzymatic manganese(II) oxidation by a marine alpha-proteobacterium

A yellow-pigmented marine bacterium, designated strain SD-21, was isolated from surface sediments of San Diego Bay, San Diego, Calif., based on its ability to oxidize soluble Mn(II) to insoluble Mn(III, IV) oxides. 16S rRNA analysis revealed that this organism was most closely related to members of the genus Erythrobacter, aerobic anoxygenic phototrophic bacteria within the alpha-4 subgroup of the Proteobacteria (alpha-4 Proteobacteria). SD-21, however, has a number of distinguishing phenotypic features relative to Erythrobacter species, including the ability to oxidize Mn(II). During the logarithmic phase of growth, this organism produces Mn(II)-oxidizing factors of approximately 250 and 150 kDa that are heat labile and inhibited by both azide and o-phenanthroline, suggesting the involvement of a metalloenzyme. Although the expression of the Mn(II) oxidase was not dependent on the presence of Mn(II), higher overall growth yields were reached in cultures incubated with Mn(II) in the culture medium. In addition, the rate of Mn(II) oxidation appeared to be slower in cultures grown in the light. This is the first report of Mn(II) oxidation within the alpha-4 Proteobacteria as well as the first Mn(II)-oxidizing proteins identified in a marine gram-negative bacterium.

Detection and characterization of filterable heterotrophic bacteria from rural groundwater supplies

AIMS

The chemical/physical environment of groundwater may contribute to the existence of a subpopulation of small-sized bacteria (filterable bacteria) that fails to be trapped on conventional 0.45 microm-pore-size membrane filters during routine bacteriological water quality analyses. Efforts were directed to determining an efficient recovery method for detection of such cells.

METHODS AND RESULTS

Individual groundwater supplies in a rural setting were examined by a double membrane filtration procedure to determine the presence of heterotrophic plate count (HPC) bacteria capable of escaping detection on conventional pore size (0.45 microm) membrane filters but retained on 0.22 microm-pore-size filters. Since optimum cultural conditions for recovery of filterable bacteria are not well defined, initial efforts focused on evaluation of various media (R2A, m-HPC and NWRI) and incubation temperatures (15, 20, 28 and 35 degrees C) for specific recovery of filterable bacteria. Maximum recovery of small-sized HPC bacteria occurred on low-nutrient concentration R2A agar incubated for 7 d at 28 degrees C. Similarly, identical cultural conditions gave enhanced detection of the general HPC population on 0.45 microm-pore-size filters. A 17-month survey of 10 well water supplies conducted with the cultural conditions described above resulted in detection of filterable bacteria (ranging in density from 9 to 175 cfu ml-1) in six of the groundwater sources. The proportion of filterable bacteria in any single sample never exceeded 10% of the total HPC population. A majority of the colonies appearing on the 0.22 microm membrane filters was pigmented (50-90%), whereas the proportion of colonies demonstrating pigmentation on the larger porosity filters failed to exceed 50% for any of the samples (19-49%).

CONCLUSION

A reliable recovery method was developed for the detection of filterable bacteria from groundwater. During a subsequent survey study using this procedure, filterable bacteria were detected in a majority of the groundwater supplies examined; however, the density of filterable bacteria in any single sample never exceeded 10% of the total HPC population. Identification of randomly selected isolates obtained on the 0.22 microm filters indicated that some of these filterable bacteria have been implicated as opportunistic pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

We have determined the presence of small-sized HPC bacteria in ground water that may go undetected when using standard porosity membrane filters for water quality analyses. Further study is needed to assess the significance and possible health risk associated with presence of filterable bacteria in drinking water supplies from groundwater sources.