Conjugative Plasmid Transfer between Bacteria under Simulated Marine Oligotrophic Conditions

Indigenous food sources as vectors of Escherichia coli and antibiotic resistance

The contamination of surface waters by fecal bacteria, measured by the number of Escherichia coli, is a significant public health issue. When these bacteria are also resistant to antimicrobials, infections are more complicated to treat. While water is regularly tested at recreational sites, wild-harvested foods, known as mahinga kai by the indigenous Māori people of Aotearoa New Zealand, are commonly overlooked as a source of exposure to potential pathogens and antimicrobial resistance (AMR). We investigate two likely sources of risk from harvesting aquatic wild foods. The first is water contact, and the second is contact with/ingestion of the harvest. We used E. coli as a proxy for microbial water quality at harvesting sites. Two popular mahinga kai species were also harvested and assessed. We found antibiotic-resistant bacteria on watercress (Nasturtium officinale) and cockles (Austrovenus stutchburyi). One-third of E. coli isolates were conjugative donors of at least one resistance phenotype. Tank experiments were used to track the internalization of E. coli by Greenshell/lip mussels (Perna canaliculus). Greenshell mussels kept at environmentally relevant concentrations of E. coli were colonized to levels considered unsafe for human consumption in 24 h. Finally, we measured horizontal gene transfer between bacteria within the shellfish, what we termed 'intra-shellular' conjugation. The transmission frequency of plasmid RP4 was significantly higher in mussels than in water alone. Our results indicate that shellfish could promote the dissemination of antibiotic resistance. They highlight the need to limit or reduce human pathogenic bacteria where food is gathered.

d. Maas P, Heederik D, Mevius D, Sørensen SJ, Schmitt H. Horizontal Gene Transfer of an IncP1 Plasmid to Soil Bacterial Community Introduced by Escherichia coli through Manure Amendment in Soil Microcosms

The quantification and identification of new plasmid-acquiring bacteria in representative mating conditions is critical to characterize the risk of horizontal gene transfer in the environment. This study aimed to quantify conjugation events resulting from manure application to soils and identify the transconjugants resulting from these events. Conjugation was quantified at multiple time points by plating and flow cytometry, and the transconjugants were recovered by fluorescence-activated cell sorting and identified by 16S rRNA sequencing. Overall, transconjugants were only observed within the first 4 days after manure application and at values close to the detection limits of this experimental system (1.00-2.49 log CFU/g of manured soil, ranging between 10^-5 and 10^-4 transconjugants-to-donor ratios). In the pool of recovered transconjugants, we found amplicon sequence variants (ASVs) of genera whose origin was traced to soils (Bacillus and Nocardioides) and manure (Comamonas and Rahnella). This work showed that gene transfer from fecal to soil bacteria occurred despite the less-than-optimal conditions faced by manure bacteria when transferred to soils, but these events were rare, mainly happened shortly after manure application, and the plasmid did not colonize the soil community. This study provides important information to determine the risks of AMR spread via manure application.

Temperature and Nutrient Limitations Decrease Transfer of Conjugative IncP-1 Plasmid pKJK5 to Wild Escherichia coli Strains

Plasmid-mediated dissemination of antibiotic resistance among fecal Enterobacteriaceae in natural ecosystems may contribute to the persistence of antibiotic resistance genes in anthropogenically impacted environments. Plasmid transfer frequencies measured under laboratory conditions might lead to overestimation of plasmid transfer potential in natural ecosystems. This study assessed differences in the conjugative transfer of an IncP-1 (pKJK5) plasmid to three natural Escherichia coli strains carrying extended-spectrum beta-lactamases, by filter mating. Matings were performed under optimal laboratory conditions (rich LB medium and 37°C) and environmentally relevant temperatures (25, 15 and 9°C) or nutrient regimes mimicking environmental conditions and limitations (synthetic wastewater and soil extract). Under optimal nutrient conditions and temperature, two recipients yielded high transfer frequencies (5 × 10-1) while the conjugation frequency of the third strain was 1000-fold lower. Decreasing mating temperatures to psychrophilic ranges led to lower transfer frequencies, albeit all three strains conjugated under all the tested temperatures. Low nutritive media caused significant decreases in transconjugants (-3 logs for synthetic wastewater; -6 logs for soil extract), where only one of the strains was able to produce detectable transconjugants. Collectively, this study highlights that despite less-than-optimal conditions, fecal organisms may transfer plasmids in the environment, but the transfer of pKJK5 between microorganisms is limited mainly by low nutrient conditions.

Survival of the blaNDM-harbouring Escherichia coli in tropical seawater and conjugative transfer of resistance markers

Anthropogenic contamination of coastal-marine water is responsible for introducing multidrug-resistant bacteria such as the pNDM-harbouring Escherichia coli into the seafood chain. This study was conducted to understand the survivability of a multidrug-resistant, the New Delhi Metallo-β-lactamase-producing E. coli (AS-EC121) in tropical seawater at room temperature (28-32 °C) compared to E. coli K12 strain. The experimental and control strains were inoculated at 6 log CFU/ml level into seawater. After an initial sharp decline in counts, AS-EC121 and K12 strains showed a gradual loss of viability after week-1 of inoculation. AS-EC121 was undetectable after day-56, while K12 colonies disappeared a week later, from day-63. The conjugation experiment revealed that pNDM was transferable to a recipient E. coli strain in seawater. This study suggests that the multidrug-resistant, pNDM-harbouring E. coli is able to survive in seawater for over 2 months stably maintaining the resistance plasmid. The resistance genotypes do not seem to compromise the survivability of MDR E. coli and the stability of plasmid provides ample opportunities for dissemination of plasmids among co-inhabiting bacteria in the coastal-marine environments.

Biofilms: hot spots of horizontal gene transfer (HGT) in aquatic environments, with a focus on a new HGT mechanism

Biofilms in water environments are thought to be hot spots for horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs). ARGs can be spread via HGT, though mechanisms are known and have been shown to depend on the environment, bacterial communities and mobile genetic elements. Classically, HGT mechanisms include conjugation, transformation and transduction; more recently, membrane vesicles (MVs) have been reported as DNA reservoirs implicated in interspecies HGT. Here, we review the current knowledge on the HGT mechanisms with a focus on the role of MVs and the methodological innovations in the HGT research.

The Conjugation Window in an Escherichia coli K-12 Strain with an IncFII Plasmid

Many studies have examined the role that conjugation plays in disseminating antibiotic resistance genes in bacteria. However, relatively little research has quantitively examined and modeled the dynamics of conjugation under growing and nongrowing conditions beyond a couple of hours. We therefore examined growing and nongrowing cultures of Escherichia coli over a 24-h period to understand the dynamics of bacterial conjugation in the presence and absence of antibiotics with pUUH239.2, an IncFII plasmid containing multiantibiotic- and metal-resistant genes. Our data indicate that conjugation occurs after E. coli cells divide and before they have transitioned to a nongrowing phase. The result is that there is only a small window of opportunity for E. coli to conjugate with pUUH239.2 under both growing and nongrowing conditions. Only a very small percentage of the donor cells likely are capable of even undergoing conjugation, and not all transconjugants can become donor cells due to molecular regulatory controls and not being in the correct growth phase. Once a growing culture enters stationary phase, the number of capable donor cells decreases rapidly and conjugation slows to produce a plateau. Published models did not provide accurate descriptions of conjugation under nongrowing conditions. We present here a modified modeling approach that accurately describes observed conjugation behavior under growing and nongrowing conditions.IMPORTANCE There has been growing interest in horizontal gene transfer of antibiotic resistance plasmids as the antibiotic resistance crisis has worsened over the years. Most studies examining conjugation of bacterial plasmids focus on growing cultures of bacteria for short periods, but in the environment, most bacteria grow episodically and at much lower rates than in the laboratory. We examined conjugation of an IncFII antibiotic resistance plasmid in E. coli under growing and nongrowing conditions to understand the dynamics of conjugation under which the plasmid is transferred. We found that conjugation occurs in a narrow time frame when E. coli is transitioning from a growing to nongrowing phase and that the conjugation plateau develops because of a lack of capable donor cells in growing cultures. From an environmental aspect, our results suggest that episodic growth in nutrient-depleted environments could result in more conjugation than sustained growth in a nutrient rich environment.

Conjugative Gene Transfer between Nourished and Starved Cells of Photobacterium damselae ssp. damselae and Escherichia coli

Horizontal gene transfer (HGT) between bacteria with different habitats and nutritional requirements is important for the spread of antibiotic resistance genes (ARG). The objective of the present study was to clarify the effects of organic matter on HGT between nourished and starved bacteria. We demonstrated that conjugation ability is affected by the nutritional conditions of the cell and environment. A filter mating HGT experiment was performed using Photobacterium damselae ssp. damselae, strain 04Ya311, a marine-origin bacterium possessing the multidrug-resistance plasmid pAQU1, as the donor, and Escherichia coli as the recipient. The donor and recipient were both prepared as nutrient-rich cultured and starved cells. Filter mating was performed on agar plates with and without organic nutrients. The transcription of the plasmid-borne genes tet(M) and traI was quantitated under eutrophic and oligotrophic conditions. The donor P. damselae transferred the plasmid to E. coli at a transfer rate of 10⁻⁴ under oligotrophic and eutrophic conditions. However, when the donor was starved, HGT was not detected under oligotrophic conditions. The addition of organic matter to starved cells restored conjugative HGT even after 6 d of starvation. The transcription of traI was not detected in starved cells, but was restored upon the addition of organic matter. The HGT rate appears to be affected by the transcription of plasmid-associated genes. The present results suggest that the HGT rate is low in starved donors under oligotrophic conditions, but is restored by the addition of organic matter.

Physicochemical Factors That Favor Conjugation of an Antibiotic Resistant Plasmid in Non-growing Bacterial Cultures in the Absence and Presence of Antibiotics

Horizontal gene transfer (HGT) of antibiotic resistance genes has received increased scrutiny from the scientific community in recent years owing to the public health threat associated with antibiotic resistant bacteria. Most studies have examined HGT in growing cultures. We examined conjugation in growing and non-growing cultures of E. coli using a conjugative multi antibiotic and metal resistant plasmid to determine physiochemical parameters that favor horizontal gene transfer. The conjugation frequency in growing and non-growing cultures was generally greater under shaken than non-shaken conditions, presumably due to increased frequency of cell collisions. Non-growing cultures in 9.1 mM NaCl had a similar conjugation frequency to that of growing cultures in Luria-Bertaini broth, whereas those in 1 mM or 90.1 mM NaCl were much lower. This salinity effect on conjugation was attributed to differences in cell-cell interactions and conformational changes in cell surface macromolecules. In the presence of antibiotics, the conjugation frequencies of growing cultures did not increase, but in non-growing cultures of 9.1 mM NaCl supplemented with Cefotaxime the conjugation frequency was as much as nine times greater than that of growing cultures. The mechanism responsible for the increased conjugation in non-growing bacteria was attributed to the likely lack of penicillin-binding protein 3 (the target of Cefotaxime), in non-growing cells that enabled Cefotaxime to interact with the plasmid and induce conjugation. Our results suggests that more attention may be owed to HGT in non-growing bacteria as most bacteria in the environment are likely not growing and the proposed mechanism for increased conjugation may not be unique to the bacteria/plasmid system we studied.

Conjugative Plasmid Transfer in Xylella fastidiosa Is Dependent on tra and trb Operon Functions

The insect-transmitted plant pathogen Xylella fastidiosa is capable of efficient horizontal gene transfer (HGT) and recombination. Natural transformation occurs at high rates in X. fastidiosa, but there also is evidence that certain strains of X. fastidiosa carry native plasmids equipped with transfer and mobilization genes, suggesting conjugation as an additional mechanism of HGT in some instances. Two operons, tra and trb, putatively encoding a conjugative type IV secretion system, are found in some but not all X. fastidiosa isolates, often on native plasmids. X. fastidiosa strains that carry the conjugative transfer genes can belong to different subspecies and frequently differ in host ranges. Using X. fastidiosa strain M23 (X. fastidiosa subsp. fastidiosa) or Dixon (X. fastidiosa subsp. multiplex) as the donor strain and Temecula (X. fastidiosa subsp. fastidiosa) as the recipient strain, plasmid transfer was characterized using the mobilizable broad-host-range vector pBBR5pemIK. Transfer of plasmid pBBR5pemIK was observed under in vitro conditions with both donor strains and was dependent on both tra and trb operon functions. A conjugative mechanism likely contributes to gene transfer between diverse strains of X. fastidiosa, possibly facilitating adaptation to new environments or different hosts.IMPORTANCEXylella fastidiosa is an important plant pathogen worldwide, infecting a wide range of different plant species. The emergence of new diseases caused by X. fastidiosa, or host switching of existing strains, is thought to be primarily due to the high frequency of HGT and recombination in this pathogen. Transfer of plasmids by a conjugative mechanism enables movement of larger amounts of genetic material at one time, compared with other routes of gene transfer such as natural transformation. Establishing the prevalence and functionality of this mechanism in X. fastidiosa contributes to a better understanding of HGT, adaptation, and disease emergence in this diverse pathogen.

Low Temperature and Modified Atmosphere: Hurdles for Antibiotic Resistance Transfer?

Food is an important dissemination route for antibiotic-resistant bacteria. Factors used during food production and preservation may contribute to the transfer of antibiotic resistance genes, but research on this subject is scarce. In this study, the effect of temperature (7 to 37°C) and modified atmosphere packaging (air, 50% CO2-50% N2, and 100% N2) on antibiotic resistance transfer from Lactobacillus sakei subsp. sakei to Listeria monocytogenes was evaluated. Filter mating was performed on nonselective agar plates with high-density inocula. A more realistic setup was created by performing modified atmosphere experiments on cooked ham using high-density and low-density inocula. Plasmid transfer was observed between 10 and 37°C, with plasmid transfer also observed at 7°C during a prolonged incubation period. When high-density inocula were used, transconjugants were detected, both on agar plates and cooked ham, under the three atmospheres (air, 50% CO2-50% N2, and 100% N2) at 7°C. This yielded a median transfer ratio (number of transconjugants/number of recipients) with an order of magnitude of 10(-4) to 10(-6). With low-density inocula, transfer was only detected under the 100% N2 atmosphere after 10-day incubation at 7°C, yielding a transfer ratio of 10(-5). Under this condition, the highest bacterial density was obtained. The results indicate that low temperature and modified atmosphere packaging, two important hurdles in the food industry, do not necessarily prevent plasmid transfer from Lactobacillus sakei subsp. sakei to Listeria monocytogenes.

Antimicrobial use in aquaculture re-examined: its relevance to antimicrobial resistance and to animal and human health

The worldwide growth of aquaculture has been accompanied by a rapid increase in therapeutic and prophylactic usage of antimicrobials including those important in human therapeutics. Approximately 80% of antimicrobials used in aquaculture enter the environment with their activity intact where they select for bacteria whose resistance arises from mutations or more importantly, from mobile genetic elements containing multiple resistance determinants transmissible to other bacteria. Such selection alters biodiversity in aquatic environments and the normal flora of fish and shellfish. The commonality of the mobilome (the total of all mobile genetic elements in a genome) between aquatic and terrestrial bacteria together with the presence of residual antimicrobials, biofilms, and high concentrations of bacteriophages where the aquatic environment may also be contaminated with pathogens of human and animal origin can stimulate exchange of genetic information between aquatic and terrestrial bacteria. Several recently found genetic elements and resistance determinants for quinolones, tetracyclines, and β-lactamases are shared between aquatic bacteria, fish pathogens, and human pathogens, and appear to have originated in aquatic bacteria. Excessive use of antimicrobials in aquaculture can thus potentially negatively impact animal and human health as well as the aquatic environment and should be better assessed and regulated.

Horizontal gene exchange in environmental microbiota

Horizontal gene transfer (HGT) plays an important role in the evolution of life on the Earth. This view is supported by numerous occasions of HGT that are recorded in the genomes of all three domains of living organisms. HGT-mediated rapid evolution is especially noticeable among the Bacteria, which demonstrate formidable adaptability in the face of recent environmental changes imposed by human activities, such as the use of antibiotics, industrial contamination, and intensive agriculture. At the heart of the HGT-driven bacterial evolution and adaptation are highly sophisticated natural genetic engineering tools in the form of a variety of mobile genetic elements (MGEs). The main aim of this review is to give a brief account of the occurrence and diversity of MGEs in natural ecosystems and of the environmental factors that may affect MGE-mediated HGT.

Sewage Disposal and Wildlife Health in Antarctica

Sewage and its microbiology, treatment and disposal are important to the topic of Antarctic wildlife health because disposal of untreated sewage effluent into the Antarctic marine environment is both allowed and commonplace. Human sewage contains enteric bacteria as normal flora, and has the potential to contain parasites, bacteria and viruses which may prove pathogenic to Antarctic wildlife. Treatment can reduce levels of micro-organisms in sewage effluent, but is not a requirement of the Environmental Protocol to the Antarctic Treaty (the Madrid Protocol). In contrast, the deliberate release of non-native organisms for any other reason is prohibited. Hence, disposal of sewage effluent to the marine environment is the only activity routinely undertaken in Antarctica knowing that it will likely result in the release of large numbers of potentially non-native species.

When the Madrid Protocol was negotiated, the decision to allow release of untreated sewage effluent was considered the only pragmatic option, as a prohibition would have been costly, and may not have been achievable by many Antarctic operators. In addition, at that time the potential for transmission of pathogens to wildlife from sewage was not emphasised as a significant potential risk. Since then, the transmission of disease-causing agents between species is more widely recognised and it is now timely to consider the risks of continued discharge of sewage effluent in Antarctica and whether there are practical alternatives.

Antimicrobial activity observed among cultured marine epiphytic bacteria reflects their potential as a source of new drugs

The surfaces of marine eukaryotes provide a unique habitat for colonizing microorganisms where competition between members of these communities and chemically mediated interactions with their host are thought to influence both microbial diversity and function. For example, it is believed that marine eukaryotes may use their surface-associated bacteria to produce bioactive compounds in defence against competition and to protect the host against further colonization. With the increasing need for novel drug discovery, marine epibiotic bacteria may thus represent a largely underexplored source of new antimicrobial compounds. In the current study, 325 bacterial isolates were obtained from the surfaces of marine algae Delisea pulchra and Ulva australis. Thirty-nine showed to have antimicrobial activity and were identified via 16S rRNA gene sequencing. The majority of those isolates belonged to Alpha- and Gammaproteobacteria. Interestingly, the most commonly isolated bacterial strain, Microbulbifer sp., from the surface of D. pulchra has previously been described as an ecologically significant epibiont of different marine eukaryotes. Other antimicrobial isolates obtained in this study belonged to the phyla Actinobacteria, Firmicutes and Bacteroidetes. Phylogenetically, little overlap was observed among the bacteria obtained from surfaces of D. pulchra and U. australis. The high abundance of cultured isolates that produce antimicrobials suggest that culturing remains a powerful resource for exploring novel bioactives of bacterial origin.

Abundant presence of the gamma-like Proteobacterial pufM gene in oxic seawater

Known anoxygenic photosynthetic bacteria (APB) affiliated to Gammaproteobacteria usually use anaerobic metabolism and are restricted to oxygen-free habitats. Here, we report abundant (average of 34.5%) presence of diverse APB related to gamma-like Proteobacteria in oxic oceanic surface water as indicated by the pufM gene, that encodes the M subunit of the light reaction centre complex. Thus, our sequences were most likely derived from aerobic anoxygenic phototrophs (AAnP). Two genetically distinct genotypes were revealed: one was from the oligotrophic North Pacific Ocean Gyre and the other, was from the trophic East China Sea and Bering Sea. The discovery of abundant presence of novel gamma-like Proteobacterial pufM gene in the oxic seawater extends the functional ecotypes of AAnP.

The enhancement of conjugal plasmid pBHR1 transfer between bacteria in the presence of extracellular metabolic products produced by Microcystis aeruginosa

Conjugal plasmid transfer from Escherichia coli S17-1 (pBHR1) to Pseudomonas stutzeri was investigated in the presence of a cyanophyta Microcystis aeruginosa. The plasmid transfer frequency increased with higher densities of M. aeruginosa. The extracellular metabolic products (EMPs) from M. aeruginosa were found to enhance the plasmid transfer between bacteria. Furthermore, the plasmid transfer frequency in medium containing EMPs was significantly higher than that in culture medium with or without glucose. These results suggest that M. aeruginosa enhances conjugal plasmid transfer between bacteria through its EMPs, and that identity of the carbon source is an important factor affecting conjugal plasmid transfer in aquatic environments.

Approaches to investigating the ecology of plasmids in marine bacterial communities

To better understand prokaryotic gene flux in marine ecosystems and to determine whether or not environmental parameters can effect the composition and structure of plasmid populations in marine bacterial communities, information on the distribution, diversity, and ecological traits of marine plasmids is necessary. This mini-review highlights recent insights gained into the molecular diversity and ecology of plasmids occurring in marine microbial communities.

Evidence for a role of rpoE in stressed and unstressed cells of marine Vibrio angustum strain S14

We report the cloning, sequencing, and characterization of the rpoE homolog in Vibrio angustum S14. The rpoE gene encodes a protein with a predicted molecular mass of 19.4 kDa and has been demonstrated to be present as a single-copy gene by Southern blot analysis. The deduced amino acid sequence of RpoE is most similar to that of the RpoE homolog of Sphingomonas aromaticivorans, sigma(24), displaying sequence similarity and identity of 63 and 43%, respectively. Northern blot analysis demonstrated the induction of rpoE 6, 12, and 40 min after a temperature shift to 40 degrees C. An rpoE mutant was constructed by gene disruption. There was no difference in viability during logarithmic growth, stationary phase, or carbon starvation between the wild type and the rpoE mutant strain. In contrast, survival of the mutant was impaired following heat shock during exponential growth, as well as after oxidative stress at 24 h of carbon starvation. The mutant exhibited microcolony formation during optimal growth temperatures (22 to 30 degrees C), and cell area measurements revealed an increase in cell volume of the mutant during growth at 30 degrees C, compared to the wild-type strain. Moreover, outer membrane and periplasmic space protein analysis demonstrated many alterations in the protein profiles for the mutant during growth and carbon starvation, as well as following oxidative stress, in comparison with the wild-type strain. It is thereby concluded that RpoE has an extracytoplasmic function and mediates a range of specific responses in stressed as well as unstressed cells of V. angustum S14.

Genetic exchange between bacteria in the environment

Nucleotide sequence analysis, and more recently whole genome analysis, shows that bacterial evolution has often proceeded by horizontal gene flow between different species and genera. In bacteria, gene transfer takes place by transformation, transduction, or conjugation and this review examines the roles of these gene transfer processes, between different bacteria, in a wide variety of ecological niches in the natural environment. This knowledge is necessary for our understanding of plasmid evolution and ecology, as well as for risk assessment. The rise and spread of multiple antibiotic resistance plasmids in medically important bacteria are consequences of intergeneric gene transfer coupled to the selective pressures posed by the increasing use and misuse of antibiotics in medicine and animal feedstuffs. Similarly, the evolution of degradative plasmids is a response to the increasing presence of xenobiotic pollutants in soil and water. Finally, our understanding of the role of horizontal gene transfer in the environment is essential for the evaluation of the possible consequences of the deliberate environmental release of natural or recombinant bacteria for agricultural and bioremediation purposes.

In Situ Detection of High Levels of Horizontal Plasmid Transfer in Marine Bacterial Communities

Gene transfer of the conjugative plasmid pBF1 from Pseudomonas putida to indigenous bacteria in seawater was investigated with a detection system for gene transfer based on the green fluorescent protein (GFP) (C. Dahlberg et al., Mol. Biol. Evol. 15:385-390, 1998). pBF1 was tagged with the gfp gene controlled by a lac promoter which is down regulated in the donor cell by a chromosomal repressor (lacIq). The plasmid donor cells (Pseudomonas putida KT2442) subsequently do not express gfp. Transfer to recipient strains lacking the repressor results in expression of gfp. The transconjugant can subsequently be detected by epifluorescence microscopy on a single-cell level. By using this method, transfer of pBF1::gfp and expression of the gfp gene were first shown to occur during nutrient-limiting conditions to several defined recipient bacteria in artificial seawater. Second, we measured transfer of pBF1 from P. putida to the marine bacterial community directly in seawater samples, on a single-cell level, without limiting the detection of gene transfer to the culturable fraction of bacteria. Plasmid transfer was detected on surfaces and in bulk seawater. Seawater bacteria with different morphologies were shown to receive the plasmid. Gene transfer frequencies of 2.3 x 10(-6) to 2.2 x 10(-4) transconjugants per recipient were recorded after 3 days of incubation.

Conjugative plasmids isolated from bacteria in marine environments show various degrees of homology to each other and are not closely related to well-characterized plasmids

Mercury resistance plasmids were exogenously isolated, i.e., recovered after transfer to a model recipient bacterium, from marine air-water interface, bulk water, and biofilm communities during incubation in artificial seawater without added nutrients. Ninety-five plasmids from different environments were classified by restriction endonuclease digestion, and 12 different structural plasmid groups were revealed. The plasmid types isolated from different habitats and from different sampling occasions showed little similarity to each other based on their restriction endonuclease patterns, indicating high variation and possibly a low transfer between microhabitats and/or a different composition of the microbial communities at different sites and times. With another approach in which probes derived from one of the isolated plasmids and a mercury resistance (mer) probe from Tn501 were used, similarities between plasmids from several different groups were found. The plasmids were further tested for their incompatibility by use of the collection of inc/rep probes (B/O, com9, FI, FII, HI1, HI2, I1, L/M, N, P, Q, U, W, Y) described by Couturier et al. (M. F. Couturier, P. Bex, L. Bergquist, and W. K. Maas, Microbiol. Rev. 52:375-395, 1988). Hybridizations did not reveal any identity between the 12 plasmid groups and any of the inc/rep probes tested. The results indicate that plasmids isolated from different marine habitats have replication and/or incompatibility systems that are different from the well-characterized plasmids that are commonly used in plasmid biology. This shows the need for the use of more relevant plasmids in studies of plasmid activity in the environment and development of new inc/rep probes for their characterization.

Transfer in Marine Sediments of the Naturally Occurring Plasmid pRAS1 Encoding Multiple Antibiotic Resistance

The results of microcosm experiments performed with the fish-pathogenic bacterium Aeromonas salmonicida acting as a donor showed that promiscuous plasmid pRAS1, which encodes tetracycline resistance, is transferred at a high frequency in marine sediments even in the absence of a selective factor. The presence of oxytetracycline resulted in an increase in the transfer frequency compared with that of a microcosm to which no selective factor was added. Transfer frequencies of 3.4 × 10 -1 transconjugant per recipient and 3.6 transconjugants per donor cell were obtained in a microcosm to which oxytetracycline had been added. Hybridization with a DNA probe specific for plasmid pRAS1 revealed that 45.8% of the oxytetracycline-resistant isolates obtained from a microcosm with no selective pressure carried the plasmid, while 86.8% of the isolates obtained from a microcosm to which oxytetracycline had been added carried the plasmid. Phenotypic characterization of the transconjugants revealed that the plasmid had been transferred to a variety of different biotypes in both microcosms. The diversity among the transconjugants isolated from the microcosm to which oxytetracycline had been added was substantially lower than the diversity among the transconjugants isolated from the microcosm to which no selective agent had been added.

Effect of growth phase and parental cell survival in river water on plasmid transfer between Escherichia coli strains

We evaluated the transfer to and from Escherichia coli of endogenously isolated plasmid material from the River Butrón during the growth of three donor strains and two recipient strains as well as after the survival of these parental cells in river water. Transfer frequency varied greatly during the growth of donor cells, with minimum values in the exponential phase; frequency remained constant, however, during the growth of recipient strains. After survival in river water, donor cells lost their ability for plasmid transfer before any other physiological variations in the cells caused by environmental stress were detected. Under the same conditions and during equal periods, however, no variation in the ability of recipient cells to receive and express plasmid material was observed.

Transfer of multiple drug resistance plasmids between bacteria of diverse origins in natural microenvironments

Plasmids harboring multiple antimicrobial-resistance determinants (R plasmids) were transferred in simulated natural microenvironments from various bacterial pathogens of human, animal, or fish origin to susceptible strains isolated from a different ecological niche. R plasmids in a strain of the human pathogen Vibrio cholerae O1 E1 Tor and a bovine Escherichia coli strain were conjugated to a susceptible strain of the fish pathogenic bacterium Aeromonas salmonicida subsp. salmonicida in marine water. Conjugations of R plasmids between a resistant bovine pathogenic E. coli strain and a susceptible E. coli strain of human origin were performed on a hand towel contaminated with milk from a cow with mastitis. A similar conjugation event between a resistant porcine pathogenic E. coli strain of human origin was studied in minced meat on a cutting board. Conjugation of R plasmids between a resistant strain of the fish pathogenic bacterium A. salmonicida subsp. salmonicida and a susceptible E. coli strain of human origin was performed in raw salmon on a cutting board. R plasmids in a strain of A. salmonicida subsp. salmonicida and a human pathogenic E. coli strain were conjugated to a susceptible porcine E. coli strain in porcine feces. Transfer of the different R plasmids was confirmed by plasmid profile analyses and determination of the resistance pattern of the transconjugants. The different R plasmids were transferred equally well under simulated natural conditions and under controlled laboratory conditions, with median conjugation frequencies ranging from 3 x 10(-6) to 8 x 10(-3). The present study demonstrates that conjugation and transfer of R plasmids is a phenomenon that belongs to the environment and can occur between bacterial strains of human, animal, and fish origins that are unrelated either evolutionarily or ecologically even in the absence of antibiotics. Consequently, the contamination of the environment with bacterial pathogens resistant to antimicrobial agents is a real threat not only as a source of disease but also as a source from which R plasmids can easily spread to other pathogens of diverse origins.