Oxytetracycline Resistance Research Articles

Quantitative Proteomics Reveals Antibiotics Resistance Function of Outer Membrane Proteins in Aeromonas hydrophila.

Outer membrane proteins (OMPs) play essential roles in antibiotic resistance, particularly in Gram-negative bacteria; however, they still have many unidentified functions regarding their behavior in response to antibiotic stress. In the current work, quantitative tandem mass tag labeling-based mass spectrometry was used to compare the outer membrane related proteins between an oxytetracycline-resistant (OXY-R) and its original control stain (OXY-O) in Aeromonas hydrophila. Consequently, a total of 261 commonly altered proteins in two biological repeats were identified including 29 proteins that increased and 28 that decreased. Gene ontology analysis showed that the expression of transport proteins was significantly reduced, and translation-related proteins were downregulated in the OXY-R strain. After using western blotting to validate selected altered proteins, eight OMP-related genes were knocked out and their roles in antibiotic resistance were further evaluated. The survival assays showed that some mutants such as ΔAHA_4281, ΔAHA_2766, ΔAHA_2282, ΔAHA_1181, and ΔAHA_1280 affected the susceptibility of A. hydrophila to antimicrobials. Moreover, the minimum inhibitory concentration assay showed that these candidate mutants also respond differently to other types of antibiotics. Our results reveal several novel outer membrane related proteins of A. hydrophila that play important roles in antibiotic resistance, and as such, may be helpful for screening studies to identify novel drug targets.

Integrative Conjugative Element ICEHs1 Encodes for Antimicrobial Resistance and Metal Tolerance in Histophilus somni.

The objectives of this study were to determine antimicrobial resistance and metal tolerance, and identify associated genes and mobile genetic elements in clinical strains of Histophilus somni isolated from feedlot cattle in Alberta during years 2012–2016 (contemporary isolates, n = 63) and years 1980–1990 (historical isolates, n = 31). Comparison of antimicrobial resistance (AMR) showed a significant increase in resistance among contemporary isolates compared to historical isolates (P < 0.001). Tolerance to copper (Cu) and zinc (Zn) concentrations above 1 mM was observed in 68 and 52% of the contemporary isolates, respectively. The tet(H) gene associated with oxytetracycline resistance and multicopper oxidase (mco) and cation efflux (czcD) genes associated with Cu and Zn tolerance were identified. An integrative conjugative element; ICEHs1, was identified in whole genome sequences of strains resistant to oxytetracycline, which had Cu and Zn minimum inhibitory concentrations (MIC) >1 mM. The length of ICEHs1 was 64,932 bp and it contained 83 genes, including tetracycline resistance gene tetH, a multidrug efflux pump gene ebrB, and metal tolerance genes mco, czcD, and acr3. Comparative genomics of ICEs revealed that ICEHs1 shares high homology with previously described ICEs of Histophilus somni, Pasteurella multocida, and Mannheimia haemolytica. The ICEHs1 is an active element capable of intra- and inter-genus transfer as demonstrated by successful transfer to H. somni and P. multocida recipients. All isolates carrying ICEHs1 were resistant to tetracycline, a commonly used antibiotic in feedlots, and had Cu and Zn MIC higher than 1 mM. Since Cu and Zn are routinely used in feedlots, there is the possibility of co-selection of AMR in H. somni due to selection pressure created by Cu and Zn. Based on results of in-vitro conjugation experiments, ICEHs1 mediated transmission of antimicrobial and metal resistance genes is possible between BRD pathogens in the respiratory tract, potentially undermining treatment options available for histophilosis and BRD.

Tetracycline resistance gene tet(M) of a marine bacterial strain is not accumulated in bivalves from seawater in clam tank experiment and mussel monitoring

Antibiotic resistance genes (ARGs) are found in marine as well as terrestrial bacteria. Bivalves are known to accumulate chemical pollutants and pathogenic microbes, however, the fate of ARGs in bivalves after the intake of ARG-possessing bacteria is not known. Here we show that the copy number of oxytetracycline resistance gene tet(M) increased rapidly in the clam digestive tract by filtering water, then remained constant over 96h in a tank experiment even with the addition of tet(M)-possessing bacteria every 24h. >99.9% of the added tet(M) was decomposed, reaching a balanced state. Environmental sampling of mussel digestive tract and seawater supported the hypothesis that tet(M) was decomposed in bivalves as tet(M) was present in seawater from April to October at a concentration of 10−5 to 10−6 copies/16S, whereas tet(M) in mussels was mostly below the detection limit. Two (April) and three (July and October) individual mussels were positive for tet(M) with a concentration equivalent to that of seawater. We therefore conclude that bivalves do not accumulate tet(M) from seawater.

Risk of prophylactic antibiotics in livestock and poultry farms; a growing problem for human and animal health

Purpose: Veterinary antibiotics are physiologically highly active substances which are being used on a large scale as therapeutic drugs and as feed additives in modern agricultural practice. Major purposes of use of antibiotics are prevention and treatments of bacterial infections and for the improvements of growth rates of farm animals. Therefore, the present study was focused on quantification of some selected groups of antibiotics; tetracycline [Tetracycline (TET), Oxytetracycline (OTC)] and penicillin [Amoxicillin (AMX), Ampicillin (AMP)] in wastewater discharge drains in large-scale livestock and poultry farms and to isolate antibiotic resistant bacteria. Methods: Twenty wastewater discharge drains in livestock and poultry farms were selected to collect samples for the study. Solid-Phase Extraction (SPE) was employed and antibiotic quantification was done by High Performance Liquid Chromatography (HPLC). Isolation of resistance (r) bacteria was done by standard pour plate method where Minimum Inhibitory Concentration (MIC) of bacteria was determined using the range of 60 to 720ppm concentrations of the antibiotics by agar dilution method. Results: Oxytetracycline (55%), and tetracycline (38%) were recorded as most frequent antibiotics in samples and having concentrations of 0.005 ppm and 0.004 ppm respectively. The highest concentration of AMX was recorded as 0.003±0.004 ppm. However, AMP was not detected during the study period. Oxytetracycline and tetracycline resistance bacteria were identified as Enterococcus sp., E. faecium, E.coli and Clostridium sp. while, A. baumanii, E. clocae, A. lwoffii and H.pylori were identified as AMPr and AMXr bacteria by 16S rRNA sequencing. The MIC values of tetracycline (TET, OTC) resistant bacteria ranged from 360 ppm to 720 ppm whereas for penicillin (AMX, AMP) from 360 to 760 ppm. Conclusion: The contamination of antibiotics leads to develop antibiotic resistance in environmental bacteria. Thus, the results of the study indicate that presence of antibiotic resistant bacteria may limit the effectiveness of antibiotics in treating animal illness, thereby causing a potential risk to the productivity of livestock and poultry farms.

Improvement of oxytetracycline production mediated via cooperation of resistance genes in Streptomyces rimosus.

Increasing the self-resistance levels of Streptomyces is an effective strategy to improve the production of antibiotics. To increase the oxytetracycline (OTC) production in Streptomyces rimosus, we investigated the cooperative effect of three co-overexpressing OTC resistance genes: one gene encodes a ribosomal protection protein (otrA) and the other two express efflux proteins (otrB and otrC). Results indicated that combinational overexpression of otrA, otrB, and otrC (MKABC) exerted a synergetic effect. OTC production increased by 179% in the recombinant strain compared with that of the wild-type strain M4018. The resistance level to OTC was increased by approximately two-fold relative to the parental strain, thereby indicating that applying the cooperative effect of self-resistance genes is useful to improve OTC production. Furthermore, the previously identified cluster-situated activator OtcR was overexpressed in MKABC in constructing the recombinant strain MKRABC; such strain can produce OTC of approximately 7.49 g L-1, which represents an increase of 19% in comparison with that of the OtcR-overexpressing strain alone. Our work showed that the cooperative overexpression of self-resistance genes is a promising strategy to enhance the antibiotics production in Streptomyces.

Monitoring of drug resistance amplification and attenuation with the use of tetracycline-resistant bacteria during wastewater treatment

The objective of this study was to monitor changes (amplification or attenuation) in antibiotic resistance during wastewater treatment based on the ecology of tetracycline-resistant bacteria. The untreated and treated wastewater were collected in four seasons. Number of tetracycline-(TETR ) and oxytetracycline-resistant (OTCR ) bacteria, their qualitative composition, minimum inhibitory concentrations (MICs), sensitivity to other antibiotics, and the presence of tet (A, B, C, D, E) resistance genes were determined. TETR and OTCR counts in untreated wastewater were 100 to 1000 higher than in treated effluent. OTCR bacterial counts were higher than TETR populations in both untreated and treated wastewater. TETR isolates were not dominated by a single bacterial genus or species, whereas Aeromonas hydrophila and Aeromonas sobria were the most common in OTCR isolates. The treatment process attenuated the drug resistance of TETR bacteria and amplified the resistance of OTCR bacteria. In both microbial groups, the frequency of tet (A) gene increased in effluent in comparison with untreated wastewater. Our results also indicate that treated wastewater is a reservoir of multiple drug-resistant bacteria as well as resistance determinants which may pose a health hazard for humans and animals when released to the natural environment.

Multivariable Analysis of the Association Between Antimicrobial Use and Antimicrobial Resistance in Escherichia coli Isolated from Apparently Healthy Pigs in Japan.

The objective of this study was to investigate the association between antimicrobial agent use and antimicrobial resistance in Escherichia coli isolated from healthy pigs using data from 2004 to 2007 in the Japanese Veterinary Antimicrobial Resistance Monitoring System (JVARM). Fecal E. coli isolates from 250 pigs (one isolate each from a pig per farm) were examined for antimicrobial resistance. Information on the use of antimicrobials within preceding 6 months and types of farms recorded in JVARM was collected and statistically analyzed against the resistance patterns. In the univariate analysis, associations between both therapeutic and feed additive use of antimicrobials, and resistance to dihydrostreptomycin, gentamicin, kanamycin, ampicillin, cefazolin, ceftiofur, oxytetracycline, chloramphenicol, trimethoprim, nalidixic acid, enrofloxacin, colistin, and bicozamycin, and husbandry factors were investigated. In multivariable analysis, generalized estimating equations were used to control geographical intraclass correlation. Confounding for structurally unrelated associations was tested using generalized linear models. The results suggested direct and cross selections in the associations between use of aminoglycosides in reproduction farms and resistance to kanamycin, use of tetracyclines in larger farms and resistance to oxytetracycline, use of beta-lactams and resistance to ampicillin, use of phenicols and resistance to chloramphenicol, and use of fluoroquinolones and resistance to nalidixic acid and enrofloxacin. Coselection was suggested in the use of tetracyclines and chloramphenicol resistance. The associations between use of beta-lactams and dihydrostreptomycin resistance, use of macrolides and ampicillin and oxytetracycline resistance, and use of colistin and kanamycin resistance were significant, but were confounded by the simultaneous use of homologous antimicrobials.

Sulfonamide and tetracycline resistance genes in total- and culturable-bacterial assemblages in South African aquatic environments.

Antibiotic resistant bacteria are ubiquitous in the natural environment. The introduction of effluent derived antibiotic resistance genes (ARGs) into aquatic environments is of concern in the spreading of genetic risk. This study showed the prevalence of sulfonamide and tetracycline resistance genes, sul1, sul2, sul3, and tet(M), in the total bacterial assemblage and colony forming bacterial assemblage in river and estuarine water and sewage treatment plants (STP) in South Africa. There was no correlation between antibiotic concentrations and ARGs, suggesting the targeted ARGs are spread in a wide area without connection to selection pressure. Among sul genes, sul1 and sul2 were major genes in the total (over 10-2 copies/16S) and colony forming bacteria assemblages (∼10-1 copies/16S). In urban waters, the sul3 gene was mostly not detectable in total and culturable assemblages, suggesting sul3 is not abundant. tet(M) was found in natural assemblages with 10-3 copies/16S level in STP, but was not detected in colony forming bacteria, suggesting the non-culturable (yet-to-be cultured) bacterial community in urban surface waters and STP effluent possess the tet(M) gene. Sulfamethoxazole (SMX) resistant (SMXr) and oxytetracycline (OTC) resistant (OTCr) bacterial communities in urban waters possessed not only sul1 and sul2 but also sul3 and tet(M) genes. These genes are widely distributed in SMXr and OTCr bacteria. In conclusion, urban river and estuarine water and STP effluent in the Durban area were highly contaminated with ARGs, and the yet-to-be cultured bacterial community may act as a non-visible ARG reservoir in certain situations.

Antimicrobial susceptibility and genetic characterisation of oxytetracycline-resistant Edwardsiella tarda isolated from diseased eels

Edwardsiellosis is one of the most important bacterial diseases in eels. Edwardsiella tarda (E. tarda) isolates (n=94) from diseased eels were screened against the eight most commonly used antimicrobial agents...

Estudio de susceptibilidad antimicrobiana de Salmonella enterica en muestras de origen animal y alimentario

Bacterial resistance to one or more antimicrobiak is worrisome. To determine the susceptibility to antimicrobials of Salmonella entérica isolates from animáis and food, from the Laboratory of Veterinary Microbiology at the University of Concepción. The samples were isolated according to traditional microbiological methods standardized protocols. Resistance was determined by the Kirby-Bauer method and minimal inhibitory concentration (MIC), following Clinical and Laboratory Standards Institute recommendations (2008). Nine serotypes were identified among the 68 isolates. Strains were resistant to one or more antibiotics and 11 patterns of resistance were identified. Multidrug resistance (MDR) was observed in 20.5% of the strains tested. The most common was Oxytetracycline resistance (69.1%). Infood, the predominant serotype was S. Derby (2.9%) and S. Senftenberg (2.9%), which is commonly found infood intended for animal consumption. In samples of animal origin, the predominant serotypes were S. infantis (33.8%) and S. Group E (3.9;-;-) (23.5%). The frequeney of resistance found and the impending risk that these strains could reach humans through the food chain, should prompt a follow-up study of this pathogen.

Vanadium accelerates horizontal transfer oftet(M) gene from marinePhotobacteriumtoEscherichia coli

Vanadium is a contaminant from steel additive and ship fuel in coastal and port areas, and its effect on marine microbes remains largely unknown. We showed that vanadium accelerates transfer of the tetracycline resistance gene tet(M) from Photobacterium to Escherichia coli, and found a positive correlation between the concentration of vanadium in natural marine sediment and the rate of oxytetracycline resistance. These results suggest the possibility that vanadium may play a role in the preservation and horizontal transfer of antibiotic resistance genes in the marine environment.

Molecular cloning and functional characterization of an ATP-binding cassette transporter OtrC from Streptomyces rimosus

BackgroundThe otrC gene of Streptomyces rimosus was previously annotated as an oxytetracycline (OTC) resistance protein. However, the amino acid sequence analysis of OtrC shows that it is a putative ATP-binding cassette (ABC) transporter with multidrug resistance function. To our knowledge, none of the ABC transporters in S. rimosus have yet been characterized. In this study, we aimed to characterize the multidrug exporter function of OtrC and evaluate its relevancy to OTC production.ResultsIn order to investigate OtrC’s function, otrC is cloned and expressed in E. coli The exporter function of OtrC was identified by ATPase activity determination and ethidium bromide efflux assays. Also, the susceptibilities of OtrC-overexpressing cells to several structurally unrelated drugs were compared with those of OtrC-non-expressing cells by minimal inhibitory concentration (MIC) assays, indicating that OtrC functions as a drug exporter with a broad range of drug specificities. The OTC production was enhanced by 1.6-fold in M4018 (P = 0.000877) and 1.4-fold in SR16 (P = 0.00973) duplication mutants, while it decreased to 80% in disruption mutants (P = 0.0182 and 0.0124 in M4018 and SR16, respectively).ConclusionsThe results suggest that OtrC is an ABC transporter with multidrug resistance function, and plays an important role in self-protection by drug efflux mechanisms. This is the first report of such a protein in S. rimosus, and otrC could be a valuable target for genetic manipulation to improve the production of industrial antibiotics.

Distribution of Quinolones, Sulfonamides, Tetracyclines in Aquatic Environment and Antibiotic Resistance in Indochina

Southeast Asia has become the center of rapid industrial development and economic growth. However, this growth has far outpaced investment in public infrastructure, leading to the unregulated release of many pollutants, including wastewater-related contaminants such as antibiotics. Antibiotics are of major concern because they can easily be released into the environment from numerous sources, and can subsequently induce development of antibiotic-resistant bacteria. Recent studies have shown that for some categories of drugs this source-to-environment antibiotic resistance relationship is more complex. This review summarizes current understanding regarding the presence of quinolones, sulfonamides, and tetracyclines in aquatic environments of Indochina and the prevalence of bacteria resistant to them. Several noteworthy findings are discussed: (1) quinolone contamination and the occurrence of quinolone resistance are not correlated; (2) occurrence of the sul sulfonamide resistance gene varies geographically; and (3) microbial diversity might be related to the rate of oxytetracycline resistance.

Introduction of Extra Copy of Oxytetracycline Resistance Gene otrB Enhances the Biosynthesis of Oxytetracycline in Streptomyces rimosus

The aromatic polyketide antibiotic oxytetracycline (OTC) is produced by Streptomyces rimosus as an important secondary metabolite. Enhancement of self-resistance is one effective way to improve antibiotic production in Streptomyces spp. In the present study, we aimed to improve the production of OTC by introducing extra copies of the OTC resistance genes, otrA and otrB, into the chromosome of the industrial strain of S. rimosus (SRI). First, otrA and otrB were cloned and ligated with pSET152 to generate the recombinant plasmids pSET152-otrA/otrB, the demethylated pSET152-otrA/otrB by Escherichia coli ET12567(pUZ8002) were then introduced into SRI to yield otrA/ otrB knock-in mutants: SRI-A (otrA) and SRI-B (otrB). Ten selected mutants and the parent SRI strain were cultured in shake flasks. Production of OTC was increased by 67% in one SRI-B mutant compared with the parent strain, suggesting that the enhancement of resistance gene otrB in the antibiotic producer is an effective way to improve OTC biosynthesis. However, introduction of extra copy of otrB could retard growth of mutant cells.

Diversity and characterization of oxytetracycline-resistant bacteria associated with non-native species, white-leg shrimp (Litopenaeus vannamei), and native species, black tiger shrimp (Penaeus monodon), intensively cultured in Thailand

This study aimed at surveying prevalence of oxytetracycline (OTC)-resistant bacteria in the white-leg shrimp Litopenaeus vannamei, and the black tiger shrimp Penaeus monodon, intensively cultured in Thailand. We investigated the phylogenetic diversity of the bacterial isolates, as well as the minimum inhibitory concentration (MIC) of OTC, the occurrence of major OTC-resistant genes and multiple-antibiotic resistance in the isolates. Shrimps were collected from culture ponds, and the homogenates of whole bodies were plated on tryptic soy agar supplemented with or without OTC. Percentages of OTC-resistant bacteria were 0·3-52·1% in white-leg samples and 0·008-22·3% in black tiger samples. Analyses of 16S rDNA sequences indicated that most OTC-resistant isolates were closely related to Aeromonas spp. and Lactococcus garvieae. MICs of OTC were 4-128 μg ml(-1) in the OTC-resistant aeromonads and 128-256 μg ml(-1) in OTC-resistant L. garvieae. OTC resistance was found to be conferred by the genes tet(A), tet(C), tet(D), tet(E), tet(M) and tet(S), detected either singly or in pairs. No resistance to ceftazidime, imipenem or chloramphenicol was observed in any isolate. Both species of shrimp are associated with OTC-resistant bacteria, occasionally at high densities exceeding 10(6) cfu g(-1). The associated bacteria, predominantly Lactococcus and Aeromonas genera, are potential pathogens and are reservoirs of a variety of OTC-resistant genes. Cultured shrimps can be vehicle to carry OTC-resistant bacteria to domestic and foreign consumers via the food chain. Very low populations of OTC-resistant bacteria observed in the several ponds suggest that levels of the resistant bacteria are artificially high and should be reduced in farmed shrimps.

Antimicrobial resistance profile of Vibrio species isolated from marine shrimp farming environments ( Litopenaeus vannamei) at Ceará, Brazil

Brazilian shrimp culture industry has a great economic importance mainly to the northeast region. However, the accelerated development of this activity has resulted in the emergency of outbreaks of diseases from farming shrimp, and as a consequence the use of antimicrobial drugs to minimize the potential adverse effect under the shrimp production. The inappropriate use of antibiotics in aquaculture is one of the causes for the high incidence of antimicrobial resistant bacteria isolated from aquatic environments that represent a danger for aquatic organisms and human health. There is little information available on the level of antimicrobial resistance in pathogenic bacteria from shrimp farming environment. Therefore, this study aimed to evaluate the phenotypic resistance profile among Vibrio isolates from hatcheries water samples and from cultivated marine shrimp hepatopancreas ( L. vannamei). Antimicrobial susceptibility testing was carried out by a standard disc diffusion method and the minimum inhibitory concentration (MIC) of oxytetracycline (OTC) for resistant Vibrio isolates was determinate by broth dilution method. The results showed a high incidence of resistance to ampicillin (45.2%) and to the tetracycline class (38.7%). Florfenicol and nitrofurantoin were 100% effective against Vibrio isolates. In this study, the OTC-resistant Vibrio spp. showed MIC values of more than 400 mg/L and the presence of seawater did not influence the oxytetracycline bioactivity. The occurrence of antimicrobial multiresistance patterns was observed in 29% of Vibrio isolates. Fifty-five percent of multiresistant isolates of Vibrio lost one or more antibiotic resistance phenotype after procedure to curing of resistance plasmids. The oxytetracycline resistance was the phenotype most often lost among plasmid-cured isolates.

English

In India antibiotics are frequently used for preventing and controlling bacterial pathogens in carp aquaculture system, yet no studies have been performed to evaluate the ecological impact of its intensive and prolonged use. In this work the frequency of oxytetracycline-resistant bacteria from water, palletized feed and different life stages of fish from Indian freshwater carp aquaculture system as well as the level of resistance of selected strains was investigated. Viable as well as antibiotic-resistant bacterial counts were performed by spread plate method in culture media supplemented with the oxytetracycline. Sixty two resistant Gram negative isolates which represented the oxytetracycline-resistant bacterial population, were randomly selected on nutrient agar supplemented with oxytetracycline (50μg/ml) from carp farms and feed pellet samples. Among these bacterial isolates Flavobacterium (21%), Alcaligenes (14.5%), Aeromonas (11%), Pseudomonas (10%) and Enterobacteriace (19%) were the most frequent. The Escherichia, Serratia, Citrobacter, Enterobacter, Shigella and Proteus from Enterobacteriace were recovered. Twelve isolates of oxytetracycline resistant bacteria were mainly dominated in adult fishes by the genus Flavobacterium (23%) and Enterobacteriace(41%). Selected strains exhibited high levels of oxytetracycline resistance with minimum inhibitory concentrations (MICs) ranging from 50 to 600μg/ml.This study shows the presence of an important population of oxytetracycline-resistant bacteria in the microflora of Indian carp aquaculture farms. Therefore the environment of these farms might play important roles as reservoirs of bacteria carrying genetic determinants for high level tetracycline resistance, prompting an important risk to public health.

Classification and Antimicrobial Susceptibilities ofEnterococcusSpecies Isolated from Apparently Healthy Food-Producing Animals in Japan

A total of 1024 enterococci were recovered from faeces of healthy animals from 178 cattle, 178 pig and 156 broiler farms. Enterococcus faecium was the predominant species recovered (35.8%), followed by E. faecalis (31.3%) and E. hirae (25.6%). Oxytetracycline resistance was most frequently found among E. faecalis (85.9%), E. faecium (58.8%) and E. hirae (48.1%). Resistance rates to almost all antimicrobials were higher in E. faecalis than E. faecium and E. hirae. Isolates from cattle were more susceptible to the antimicrobials studied than those from pigs and broilers. VanA- or VanB-type vancomycin-resistant enterococci have not been found since the ban of avoparcin use 5 years ago.

Identification and Antimicrobial Resistance of Streptococcus uberis and Streptococcus parauberis Isolated from Bovine Milk Samples

The conventional identification of Streptococcus uberis/parauberis group (n=137) in clinical and subclinical bovine mastitis samples originating from 111 different farms was compared with identification based on 16 and 23S rRNA gene HindIII RFLP patterns used as operational taxonomic units in numerical analyses. On the basis of ribopattern analysis only 2 isolates belonged to S. parauberis, which is thus not a frequent cause of bovine intramammary infections in Finland. According to in vitro antimicrobial susceptibility testing, Streptococcus uberis is susceptible to β-lactam antibiotics. The prevalence of erythromycin (15.6%) and oxytetracycline (40.6%) resistance of clinical S. uberis isolates was higher than reported previously among subclinical isolates. The 2 subclinical S. parauberis isolates were susceptible to all the antimicrobials tested.

A micro-dilution method for detecting oxytetracycline-resistant bacteria in marine sediments from salmon and mussel aquaculture sites and an urbanized harbour in Atlantic Canada

A micro-dilution technique with changes in optical density (OD) used to measure bacterial growth over 72 h in culture media containing 0, 5, 10, 20, 40, 80 and 160 μg OTC (oxytetracycline) ml −1 was applied to determine growth inhibition (GI) in mixed bacterial strains cultured from samples of marine sediments and salmon feed pellets. Growth of control cultures ( Aeromonas salmonicida) was inhibited at all OTC concentrations. Some feed pellet samples and under-cage sediments from salmon aquaculture sites in the Bay of Fundy showed GI up to ≥160 μg OTC ml −1. Lower values (40 to 80 μg OTC ml −1) occurred in subsurface (5–18 cm) sediments, >100 m from salmon pens and adjacent to sewage outfalls in Halifax Harbour. GI values <20 μg OTC ml −1 in sediment from reference locations in Halifax Harbour and mussel aquaculture sites are considered to reflect natural background levels for OTC resistance.