R-plasmid transfer frequencies from environmental isolates of Escherichia coli to laboratory and fecal strains

A MODEL ON BACTERIAL RESISTANCE CONSIDERING A GENERALIZED LAW OF MASS ACTION FOR PLASMID REPLICATION

Bacterial plasmids play a fundamental role in antibiotic resistance. However, a lack of knowledge about their biology is an obstacle in fully understanding the mechanisms and properties of plasmid-mediated resistance. This has motivated investigations of real systems in vitro to analyze the transfer and replication of plasmids. In this work, we address this issue with mathematical modeling. We formulate and perform a qualitative analysis of a nonlinear system of ordinary differential equations describing the competition dynamics between plasmids and sensitive and resistant bacteria. In addition, we estimated parameter values from empirical data. Our model predicts scenarios consistent with biological phenomena. The elimination or spread of infection depends on factors associated with bacterial reproduction and the transfer and replication of plasmids. From the estimated parameters, three bacterial growth experiments were analyzed in vitro. We determined the experiment with the highest bacterial growth rate and the highest rate of plasmid transfer. Moreover, numerical simulations were performed to predict bacterial growth.

Evaluation of a Male-Specific DNA Coliphage Persistence Within Eastern Oysters (Crassostrea virginica)

Male-specific coliphages (MSCs) are currently used to assess the virologic quality of shellfish-growing waters and to assess the impact of sewage release or adverse weather events on bivalve shellfish. Since MSC can have either DNA or RNA genomes, and most research has been performed exclusively on RNA MSCs, persistence of M13, a DNA MSC, was evaluated for its persistence as a function of time and temperature within Eastern oysters (Crassostrea virginica). Oysters were individually exposed to seawater containing a total of 10¹⁰ to 10¹² pfu of M13 for 24 h at 15 °C followed by maintenance in tanks with as many as 21 oysters in continuously UV-sterilized water for up to 6 weeks at either 7, 15, or 22 °C. Two trials for each temperature were performed combining three shucked oysters per time point which were assayed by tenfold serial dilution in triplicate. Initial contamination levels averaged 10^6.9 and ranged from 10^6.0 to 10^7.0 of M13. For oysters held for 3 weeks, log₁₀ reductions were 1.7, 3.8, and 4.2 log₁₀ at 7, 15, and 22 °C, respectively. Oysters held at 7 and 15 °C for 6 weeks showed average reductions of 3.6 and 5.1 log₁₀, respectively, but still retained infectious M13. In total, this work shows that DNA MSC may decline within shellfish in a manner analogous to RNA MSCs.

Assessment of species and antimicrobial resistance among Enterobacteriaceae isolated from mallard duck faeces

Mallard ducks have demonstrated to be a likely reservoir for zoonotic E. coli strains; thus, it is possible that these ducks could also act as a reservoir for other Enterobacteriaceae members. The present study was initiated to evaluate the species distribution of Enterobacteriaceae other than E. coli in 175 fresh faecal samples collected from a population of mallard ducks. Sixty-four samples displayed detectable colonies of Enterobacteriaceae (excluding E. coli), which resulted in 75 pulsed-field gel electrophoresis (PFGE) types. Seventy-five single representatives of each PFGE type were subjected to identification with API 32NE and MALDI TOF MS systems due to the practical difficulties in species differentiation of Enterobacteriaceae. Those isolated were found to be from nine genera: Buttiauxella (15 %), Citrobacter (5 %), Enterobacter (32 %), Hafnia (1 %), Leclercia (1 %), Pantoea (7 %), Raoultella (21 %), Rahnella (7 %) and Serratia (11 %). Evaluation of antimicrobial resistance phenotypes using the disc method and detection of resistance genes using the microarray method revealed that these microbes possess resistance to β-lactams, aminoglycosides, macrolides, quinolones, rifamycine, sulphonamides, streptogramins and diaminopyrimidines. In conclusion, mallard ducks harbour a variety of non-pathogenic and pathogenic Enterobacteriaceae species like Enterobacter cloacae and Enterobacter amnigenus in their intestine and could act as a reservoir of resistant Enterobacteriaceae.

Associations between antimicrobial resistance genes in fecal generic Escherichia coli isolates from cow-calf herds in western Canada

The objective of this study was to examine associations among the genetic determinants of antimicrobial resistance (AMR) in 207 fecal generic Escherichia coli isolates obtained from 77 cow-calf herds in western Canada. Twenty-three resistance genes corresponding to six different antimicrobial families were assessed using DNA hybridization and PCR. The most common resistance genes in the study sample (207 isolates) were sul2 (48.3%), tet(B) (45.4%), and ant(3'')-Ia (aadA1) (19.3%). Several statistically significant associations between the examined resistance genes were detected. The strongest associations observed were those between genes for resistance to chloramphenicol (catI) and trimethoprim (dhfrI) (odds ratio [OR] = 214; P = 0.0001), sulfonamide (sul1) and chloramphenicol (catI) (OR = 96.9; P = 0.0001), streptomycin [ant(3'')-Ia (aadA1)] and trimethoprim (dhfrI) (OR = 96.2; P = 0.0001), sulfonamide (sul1) and streptomycin [ant(3'')-Ia (aadA1)] (OR = 79.3; P = 0.0001), and tetracycline [tet(B)] and sulfonamides (sul2) (OR = 25.7; P = 0.0001). At least one of the resistance genes corresponding to each nonaminoglycoside family of antimicrobials examined in this study was associated with the two aminoglycoside resistance genes ant(3'')-Ia (aadA1) and aph(3')-Ia. The multiple, strong associations between genes and the diverse nature of the associations described in this study demonstrate the complexity of resistance gene selection in cow-calf herds and should be considered in the planning of AMR control practices for cow-calf operations.

Rule-based modelling of conjugative plasmid transfer and incompatibility

COSMIC-rules, an individual-based model for bacterial adaptation and evolution, has been used to study virtual transmission of plasmids within bacterial populations, in an environment varying between supportive and inhibitory. The simulations demonstrate spread of antibiotic resistance (R) plasmids, both compatible and incompatible, by the bacterial gene transfer process of conjugation. This paper describes the behaviour of virtual plasmids, their modes of exchange within bacterial populations and the impact of antibiotics, together with the rules governing plasmid transfer. Three case studies are examined: transfer of an R plasmid within an antibiotic-susceptible population, transfer of two incompatible R plasmids and transfer of two compatible R plasmids. R plasmid transfer confers antibiotic resistance on recipients. For incompatible plasmids, one or other plasmid could be maintained in bacterial cells and only that portion of the population acquiring the appropriate plasmid-encoded resistance survives exposure to the antibiotics. By contrast, the compatible plasmids transfer and mix freely within the bacterial population that survives in its entirety in the presence of the antibiotics. These studies are intended to inform models for examining adaptive evolution in bacteria. They provide proof of principle in simple systems as a platform for predicting the behaviour of bacterial populations in more complex situations, for example in response to changing environments or in multi-species bacterial assemblages.

Transfer of antibiotic resistance between commensal and pathogenic members of the Enterobacteriaceae under ileal conditions

AIM

To determine the rate of antibiotic resistance transmission between commensal and pathogenic representatives of the Enterobacteriaceae.

METHODS AND RESULTS

Through the use of a validated in vitro simulation of the porcine ileum, the transmission of antibiotic resistance was detected between commensal Escherichia coli, E. coli O157 and Salmonella spp. Countable transconjugant populations arose readily and, in one example, proved capable of indefinite persistence.

CONCLUSIONS

Genetic material conferring antibiotic resistance is readily transmissible between members of the Enterobacteriaceae under ileal conditions. Recipient phenotype influences the persistence of multi-resistant transconjugants.

SIGNIFICANCE AND IMPACT OF THE STUDY

The observation that the conjugal transmission of antibiotic resistance is commonplace under ileal conditions impacts primarily on the risk of food contamination by multi-resistant bacteria. The establishment of a multi-resistant transconjugant population as a dominant member of the microflora maintains a genetic reservoir of antimicrobial resistance.

Influence of tetracycline exposure on tetracycline resistance and the carriage of tetracycline resistance genes within commensal Escherichia coli populations

AIMS

To assess the influence of incremental tetracycline exposure on the genetic basis of tetracycline resistance within faecal Escherichia coli.

METHODS AND RESULTS

Through the adoption of a novel combination of multiple breakpoint selection, phenotypic characterization and the application of a polymerase chain reaction based gene identification system it proved possible to monitor the influence of antibiotic exposure on resistance gene possession. Using tetracycline as a case study a clear hierarchy was revealed between tet genes, strongly influenced by host antimicrobial exposure history.

CONCLUSIONS

The antimicrobial exposure regime under which an animal is produced affects both the identity and magnitude of resistance gene possession of a selected bacterial population within its enteric microflora. Among the ramifications associated with such resistance gene selection is the degree of resistance conferred and the carriage of linked resistance determinants. This selection is applied by exposure to antibiotic concentrations well below recognized minimum inhibitory tetracycline concentration breakpoints widely adopted to characterize bacterial 'susceptibility'.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study confirms the ability of minimal antibiotic exposure to select for the continued persistence of resistance genes within the enteric microflora. It is clearly demonstrated that different antimicrobial regimes select for different resistance genes, the implications of which are discussed.

Presence of bacteriophages in animal feed as indicators of fecal contamination

The objectives of this study were to determine if indigenous male specific and somatic bacteriophages could be detected in animal feeds and if isolated phages contained RNA or DNA. Seven fresh feeds, 2 fresh feed ingredients, 7 stored feeds, 2 stored feed ingredients, and 8 samples of poultry diets suspected to contain Salmonella spp. were enriched and spot plated for indigenous phages using Escherichia coli Famp and CN-13 as hosts. Bacteriophage numbers were below detection without enrichment, but both male specific and somatic coliphages were detected in all animal feeds, feed ingredients, and poultry diets after 16 h of enrichment, even after the samples had been stored for 14 months of storage at -20 C. Five out of 9 fresh feeds and 7 out of 8 stored feeds contained RNA somatic phages.

Transfer of erythromycin resistance from poultry to human clinical strains of Staphylococcus aureus

The transfer of ermA and ermC genes, the two most common resistance determinants of erythromycin resistance, was studied with Luria-Bertani broth in the absence of additional Ca(2+) or Mg(2+) ions. Fifteen human and five poultry isolates of Staphylococcus aureus, which were resistant to erythromycin but carried different genetic markers for erythromycin resistance, were used for conjugation. Since both the donors (Amp(s)-Tet(r)) and recipients (Amp(r)-Tet(s)) were resistant to erythromycin, the transconjugants were initially picked up as ampicillin- and tetracycline-resistant colonies. The resistance transfer mechanisms of the chromosomally located erythromycin rRNA methylase gene ermA and the plasmid-borne ermC gene were monitored by a multiplex PCR and gene-specific internal probing assay. Four groups of transconjugants, based upon the transfer of the ermA and/or ermC gene, were distinguished from each other by the use of this method. Selective antibiotic screening revealed only one type of transconjugant that was resistant to ampicillin and tetracycline. A high frequency of transfer (4.5 x 10(-3)) was observed in all of the 23 transconjugants obtained, and the direction of tetracycline and erythromycin resistance marker transfer was determined to be from poultry to clinical isolates. The transfers of the ermA and ermC genes were via transposition and transformation, respectively.

Bacteriophages as indicators of enteric viruses and public health risk in groundwaters

Low concentrations of all types of bacteriophages in groundwater limit their power to predict the presence of enteric viruses. There is little concordance in the literature regarding phage detection methods, thus making comparisons extremely difficult. Different authors have used different hosts, phage concentration methods, and end-point determinations. Also, markedly different volumes of sample have been employed, varying from 1 litre to 400 l. Bacteriophage concentration methods are not reproducible. There has been marked variability among groups in the natural substrates used (for example, beef extract), the type of adsorbing filter used, centrifugation instruments and conditions, and the delivery of the concentrate to the host cells. There is no consensus on the best bacterial host strain. Currently, several are employed with each showing differential sensitivities and specificities. In particular, host stability must be considered. Host stability has two components: the ability of the host to continue to be receptive to the bacteriophage after continued sub-culture, and the lack of lysogenic or temperate bacteriophage in the host cell line which may be randomly and unpredictably activated. There is a lack of consistent recovery of bacteriophages from individual faecal specimens. In particular, only approximately 3% of individual humans carry the FRNA phages. While there is some evidence to indicate that the phages multiply in sewage, it is not clear how they do so since the host pili should not be produced at lower temperatures. These ecological factors need to be understood. Of all the phages thus far studied, Bacteroides fragilis HSP40 has the highest recovery rate from individual people. However, Bacteroides, being an anaerobe, is a difficult host for routine laboratory analysis. Methods for the enumeration of F(+)-specific phages and Bacteroides phages are complex, time-consuming, costly and not reproducible. Conversely, somatic coliphage methods are simpler and results can be available in 4-6 h. The occurrence of phages and viruses in groundwater depends on physicochemical characteristics that control their fate and transport in the groundwater/aquifer environment. There are very little actual data taken from the field that allow an understanding of the ecology and life span of phages in their natural environment. Moreover, the ability of phages to serve as a source of food for other microbes needs to be understood. There has been a lack of association of bacteriophage recovery with gastroenteritis outbreaks due to enteric viruses. There is only a small epidemiological database concerning the occurrence of enteric viruses in groundwater.

Conjugal transfer of natural plasmids between Escherichia coli strains in sterile environmental water

Seven antibiotic-multiresistant Escherichia coli strains, possessing three or four plasmids, capable of transferring their resistance marker at a high frequency, were selected among a total of 300 antibiotic-resistant E. coli strains isolated from natural water--raw and treated wastewater, and brackish water (collected 1 km downstream). These strains were mated with E. coli K-12 C600 nalr, both in sterilized natural water and LB medium at 25 degrees C. Conjugation did occur in all the systems tested, although fewer transconjugants were recovered from raw and treated wastewater experiments. In contrast, in brackish and seawater, the transfer frequency did not significantly decrease in spite of salt contents. In 100% of the cases, transfer of the high-molecular-weight plasmids (> or = 20 kb) was observed, but the small plasmids (2.6-7.5 kb) were only cotransferred in raw or treated wastewater and in brackish water. Moreover, genotypic variation occurred more frequently in natural water than in LB medium.

Transferable antibiotic resistance among thermotolerant coliforms from rural drinking water in India

A total of 231 thermotolerant coliforms was isolated from rural drinking water from four states of India. Of these, 220 isolates were resistant to ampicillin, chloramphemicol, streptomycin and tetracycline. Multiple (MAR), double and single antibiotic resistances were observed in 31.4, 48.6 and 13.7% of the isolates, respectively. Out of 177 antibiotic-resistant isolates examined for transmissibility, only 15.3% were able to transfer their resistances to Escherichia coli K-12 recipient. The resistances were transferred by 32.5% of MAR, 21.9% of double resistant and 7.6% of single resistant isolates. Ampicillin resistance was transferable in 14.69% strains while resistances for the rest of the antibiotics were transferable in less than 4% strains. MAR strains of E. coli and Klebsiella sp. showed highest levels of R-plasmid transfer.

Conjugal transfer of R plasmids to and from Enterobacteriaceae isolated from sewage

The potential of the transfer of natural plasmids between sewage strains has been studied. In vitro transfer was conducted at 37 degrees C in tryptone soya broth and sterile raw sewage as mating media. In situ transfer was carried out in sterile raw sewage within membrane diffusion chambers at 10.6 degrees C. When the recipient was a laboratory strain of Escherichia coli K-12, the in situ frequency values were significantly lower (P less than 0.001) than those obtained in vitro for the same mating pair. When the laboratory recipient was replaced with recipients from the same sewage source, frequency values decreased progressively from the optimum conditions to the most adverse. However, in situ frequency values were higher than those for the same donors mated with a laboratory recipient.

Biotic and abiotic factors affecting plasmid transfer in Escherichia coli strains

The influence of biotic and abiotic factors on plasmid transfer between Escherichia coli strains in terms of the variation in the number of transconjugants formed and the variation in transfer frequency was investigated. The density of parent cells affected the number of transconjugants, reaching a maximum when the cell density was on the order of 10(8) CFU ml-1. As the donor-to-recipient ratios varied from 10(-4) to 10(4), the number of transconjugants varied significantly (P less than 0.001), reaching a maximum with donor-to-recipient ratios between 1 and 10. The concentration of total organic carbon in the mating medium affects both the number of transconjugants and the transfer frequency, being significantly higher (P less than 0.001) when the total organic carbon concentration was higher than 1,139 mg of C liter-1. However, the transconjugants were detected even with less than 1 mg of C liter-1. Linear regression of log10 transconjugants versus mating temperature showed a highly significant regression line (P less than 0.001). Neither the transfer frequency nor the transconjugant number varied significantly in the range of pHs assayed. We can conclude that plasmid transfer by conjugation can take place within a wide range of conditions, even in such adverse conditions as the absence of nutrients and low temperatures.

Evaluation of an Escherichia coli host strain for enumeration of F male-specific bacteriophages

A method was developed for the selective enumeration of F male-specific bacteriophages in samples of environmental waters. The host strain for the phages, Escherichia coli HS(pFamp)R, has three antibiotic resistance markers, ampicillin on the Famp plasmid, which codes for pilus production, and streptomycin and nalidixic acid on the chromosome. The strain is resistant to coliphages T2 to T7 and phi X174. More than 95% of the phages from environmental samples which plaqued on the host strain were F male specific. The host bacterium had a higher plaquing efficiency than E. coli K-12 Hfr for F-specific phages in stock suspensions and sewage effluents. The F male-specific phage levels in prechlorinated, secondary-treated sewage effluents generally were about 10(3) to 10(4) PFU/100 ml. The levels in the influents to the sewage treatment plants and in septic tank contents were about 10(5) PFU/100 ml. RNA-containing phages composed about 90% of the total F-specific phage population in sewage effluents.

Antibiotic resistance of Salmonella strains isolated from natural polluted waters

Resistance to 14 antibiotics was tested in 270 Salmonella strains isolated from different aquatic environments. All the strains were sensitive to nalidixic acid (30 micrograms) and cephalothin (30 micrograms) but more than 90% were resistant to tetracycline (30 micrograms). The percentage of strains resistant to other antimicrobial substances depended on the antibiotic and on the isolation source. Twenty-four resistance patterns were recorded in strains isolated from three environmental sources. The only multi-resistance detected in the three ecosystems was that of sulphadiazine and tetracycline (about 20%). The serotypes most frequently detected with multi-resistance to different antibiotics were Salmonella typhimurium and Salm. blockley.

In situ survival of plasmid-bearing and plasmidless Pseudomonas aeruginosa in pristine tropical waters

Two rare wild-type strains of Pseudomonas aeruginosa were mixed in membrane diffusion chambers and then introduced into a natural freshwater environment for 72 h. The plasmid-containing strain (R serotype 15) and the plasmidless strain (H serotype 5) had initial bacterial densities of 2 x 10(5) cells per ml. Samples collected from the chambers were analyzed for viable and direct counts and for acquired-resistance frequencies. Suspected transconjugant-to-donor ratios ranged from 0.5 to 1.3; transfer percentages ranged from 13 to 70%. [3H]thymidine uptake indicated DNA synthesis in both strains as well as in transconjugants. These studies indicate that rare wild-type bacterial strains with large plasmid loads can survive as well as can bacteria with low plasmid loads when exposed to the in situ conditions of a tropical freshwater habitat. These results also suggest that genetic modification of indigenous microbiota through conjugation or transformation is feasible when rare wild-type strains or genetically engineered microorganisms are released in large numbers in tropical aquatic ecosystems.

Frequency and Characteristics of Plasmids in Bacteria Isolated from Deep-Sea Amphipods

Bacterial strains isolated from deep-sea amphipods were identified, classified, and screened for plasmid content. Plasmids were common, with 11 of 16 isolates carrying one or more plasmids; these ranged in size from 2.9 to 63 megadaltons. Several of the strains demonstrated distinctly different phenotypic traits yet contained plasmids of the same molecular weight. Results of agarose gel electrophoresis, DNA hybridization, and restriction analysis indicate that the plasmids detected in these deep-sea isolates are identical, suggesting that transmission may occur in the deep-sea environment and that plasmids are common in some deep-sea habitats.

Factors affecting the measurement of antibiotic resistance in bacteria isolated from lake water

It is more difficult to obtain a reliable assessment of antibiotic resistance in populations of aquatic bacteria than in those populations which are well characterized (e.g. bacteria of medical and veterinary significance). Factors which influence the results include the bacterial taxa involved, their site of origin and the methods and media used to isolate and subculture the bacteria, and to perform the sensitivity tests. Examples of these effects are provided. The resistance profiles obtained with populations of aquatic pseudomonads depend on the species composition of the population. Resistance patterns in aquatic bacteria varied with the site from which they were isolated; a higher incidence of resistance was recorded along shorelines and in sheltered bays than in the open water. The inclusion of antibiotics in the media employed for primary isolation increased the number of individual and multiple resistances recorded. A similar effect was observed with increased inoculum size in the sensitivity disc method but this could be reversed by raising the incubation temperature. The medium used to conduct the test also affected the results and many aquatic bacteria failed to grow on media such as Iso-Sensitest Agar. It is recommended that the sensitivity disc method is adopted for aquatic bacteria because it permits interpretation of a wider range of response. Comparison of the incidence of antibiotic resistance in different habitats will remain meaningless, however, until comprehensive methods for the identification of bacteria are developed and the techniques used for sensitivity testing are standardized.

A note on antibiotic-resistant Escherichia coli in adult man, raw sewage and sewage-polluted River Tigris in Mosul, Nineva

A total of 600 isolates of Escherichia coli were isolated, over a 9 month period during 1984, from healthy human adults, raw sewage and the sewage-polluted River Tigris in Nineva. Over 90% of these organisms were E. coli type 1, but only 8.3% could be serogrouped as enteropathogenic E. coli. Resistance of these organisms to 11 antimicrobial drugs was assessed. Over 40% were antibiotic-resistant and of these 77.1% were resistant to more than one antibiotic. The minimal inhibitory concentration of ampicillin for 193 selected strains from the various sources was determined and ranged from less than 0.625-greater than 160 micrograms/ml. The high incidence of antibiotic-resistant E. coli in this locality and the possible implications to human health are discussed.

A note on antibiotic resistance in the bacterial flora of raw sewage and sewage-polluted River Tigris in Mosul, Iraq

Polluted water samples collected from the River Tigris in the vicinity of a raw sewage outfall were examined for the incidence of antibiotic resistance among coliform bacteria on three occasions during 1983. Eighty percent or more of the coliform bacteria were resistant to one or more antibiotics. At the same time, raw sewage samples were examined for the incidence of antibiotic-resistant bacteria, and Escherichia coli, Pseudomonas spp. and Staphylococcus spp. were selected for sensitivity testing. Collectively, more than 90% of the 480 strains of the three organisms were resistant to one or more antibiotics. The minimal inhibitory concentration (MIC) of ampicillin for twenty-nine strains including coliforms, E. coli, Klebsiella sp., Serratia sp., Ps. aeruginosa, Pseudomonas sp., Micrococcus sp., Staph. aureus, Streptococcus faecalis and Bacillus sp. from raw sewage and polluted River Tigris water was determined and that for Ps. aeruginosa was 250 micrograms/ml. The high incidence of antibiotic-resistant bacteria in natural waters could be related to the widespread use of antibiotics in this locality.

Plasmid coding for transferable drug resistance in bacteria isolated from cultured rainbow trout

The occurrence of drug resistance and plasmid-mediated transferability was investigated in 170 strains belonging to eight bacterial groups isolated from cultured rainbow trout. It was found that 87.6% of the strains were resistant to at least one drug, with the highest percentages of resistance being detected for ampicillin (54.7%), sulfadiazine (46.5%), nitrofurantoin (38.2%), and chloramphenicol (37.0%). Six enterobacteria, two Vibrio, and one Aeromonas isolate transferred resistance factors to Escherichia coli K-12. The most common transmissible R factor determined resistance to chloramphenicol and sulfadiazine, demonstrating an association between a specific plasmid and the resistance pattern transferred. The presence of chloramphenicol in fish food was detected by bioassay. In general, transfer frequencies were similar in primary and secondary matings, which indicate the potential water-borne dissemination of these R plasmids.

R-plasmid transfer in Salmonella spp. isolated from wastewater and sewage-contaminated surface waters

A total of 865 Salmonella isolates from wastewaters and sewage-contaminated natural waters were tested for antimicrobial resistance by using NR10 medium and incubation at 43 degrees C. Of the strains, 12.7% were resistant to one or more of the compounds tested, and 30% transferred resistance to an Escherichia coli recipient. The highest minimal inhibitory concentrations were ca. 1,000 micrograms/ml. Transfer frequencies ranged from 10(-3) to 10(-7).

R-plasmid transfer in a wastewater treatment plant

Enteric bacteria have been examined for their ability to transfer antibiotic resistance in a wastewater treatment plant. Resistant Salmonella enteritidis, Proteus mirabilis, and Escherichia coli were isolated from clinical specimens and primary sewage effluent. Resistance to ampicillin, chloramphenicol, streptomycin, sulfadiazine, and tetracycline was demonstrated by spread plate and tube dilution techniques. Plasmid mediation of resistance was shown by ethidium bromide curing, agarose gel electrophoresis, and direct cell transfer. Each donor was mated with susceptible E. coli and Shigella sonnei. Mating pairs (and recipient controls) were suspended in unchlorinated primary effluent that had been filtered and autoclaved. Suspensions were added to membrane diffusion chambers which were then placed in the primary and secondary setting tanks of the wastewater treatment plant. Resistant recombinants were detected by replica plating nutrient agar master plates onto xylose lysine desoxycholate agar plates that contained per milliliter of medium 10 micrograms of ampicillin, 30 micrograms of chloramphenicol, 10 micrograms of streptomycin, 100 micrograms of sulfadiazine, or 30 micrograms of tetracycline. Mean transfer frequencies for laboratory matings were 2.1 X 10(-3). In situ matings for primary and secondary settling resulted in frequencies of 4.9 X 10(-5) and 7.5 X 10(-5), respectively. These values suggest that a significant level of resistance transfer occurs in wastewater treatment plants in the absence of antibiotics as selective agents.

In situ studies with membrane diffusion chambers of antibiotic resistance transfer in Escherichia coli

Coliform bacteria were isolated from raw sewage and sewage effluent-receiving waters and tested for their antibiotic susceptibility patterns and their ability to transfer antibiotic resistance to Escherichia coli K-12 C600. An environmental isolate of E. coli (MA527) capable of transferring antibiotic resistance to C600 was mated, both in vitro and in situ, with an antibiotic-sensitive E. coli environmental isolate (MA728). In situ matings were conducted in modified membrane diffusion chambers, in the degritter tank at the Grant Street (Melbourne, Fla.) sewage treatment facility, and in the sewage effluent-receiving waters in Melbourne, Fla. The transfer frequencies in situ were 3.2 x 10(-5) to 1.0 x 10(-6), compared with 1.6 x 10(-4) to 4.4 x 10(-5) observed in vitro. Transfer was shown to occur in raw sewage but was not detected in the effluent-receiving waters. The presence of a 60-megadalton plasmid species in both donor and transconjugants, but not in the recipients, provided physical evidence for the transfer of antibiotic resistance in situ.

Conjugal transfer of R-plasmid R1drd-19 in Escherichia coli below 22 degrees C

The conjugal transfer of R-plasmids is known to occur at temperatures above 22 degrees C. We found that R1drd-19 is transferable below 22 degrees C, and we discuss this finding in the context of plasmid transfer in environmental waters.

Antibiotic-resistant bacteria in drinking water

We analyzed drinking water from seven communities for multiply antibiotic-resistant (MAR) bacteria (bacteria resistant to two or more antibiotics) and screened the MAR bacterial isolates obtained against five antibiotics by replica plating. Overall, 33.9% of 2,653 standard plate count bacteria from treated drinking waters were MAR. Two different raw water supplies for two communities carried MAR standard plate count bacteria at frequencies of 20.4 and 18.6%, whereas 36.7 and 67.8% of the standard plate count populations from sites within the respective distribution systems were MAR. Isolate identification revealed that MAR gram-positive cocci (Staphylococcus) and MAR gram-negative, nonfermentative rods (Pseudomonas, Alcaligenes, Moraxella-like group M, and Acinetobacter) were more common in drinking waters than in untreated source waters. Site-to-site variations in generic types and differences in the incidences of MAR organisms indicated that shedding of MAR bacteria living in pipelines may have contributed to the MAR populations in tap water. We conclude that the treatment of raw water and its subsequent distribution select for standard plate count bacteria exhibiting the MAR phenotype.