Antibacterial agents and heavy metal resistance in Gram-negative bacteria isolated from seawater, shrimp and sediment in Iskenderun Bay, Turkey

The aim of the present study was to determine the level of antibiotic resistance patterns and distribution of heavy metal resistance of bacterial isolates from seawater, sediment and shrimps, and to determine if there is a relationship between antibiotic and heavy metal resistance. We undertook studies in 2007 in the industrially polluted Iskenderun Bay, on the south coast of Turkey. The resistance of 236 Gram-negative bacterial isolates (49 from seawater, 90 from sediment and 97 from shrimp) to 16 different antibiotics, and to 5 heavy metals, was investigated by agar diffusion and agar dilution methods, respectively. A total of 31 species of bacteria were isolated: the most common strains isolated from all samples were Escherichia coli (11.4%), Aeromonas hydrophila (9.7%) and Stenotrophomonas maltophilia (9.3%). There was a high incidence of resistance to ampicillin (93.2%), streptomycin (90.2%) and cefazolin (81.3%), and a low incidence of resistance to imipenem (16.5%), meropenem (13.9%) and cefepime (8.0%). Some 56.8% of all bacteria isolated from seawater, sediment and shrimp were resistant to 7 or more antibiotics. Most isolates showed tolerance to different concentrations of heavy metals, and minimal inhibition concentrations ranged from 12.5 μg/ml to > 3200 μg/ml. The bacteria from seawater, sediment and shrimp showed high resistance to cadmium of 69.4%, 88.9%, and 81.1% respectively, and low resistance to manganese of 2%, 6.7% and 11.3% respectively. The seawater and sediment isolates which were metal resistant also showed a high resistance to three antibiotics: streptomycin, ampicillin and trimethoprim-sulphamethoxazole. In contrast, the shrimp isolates which were metal resistant were resistant to four antibiotics: cefazolin, nitrofurantoin, cefuroxime and ampicillin. Our results show that Iskenderun Bay has a significant proportion of antibiotic and heavy metal resistant Gram-negative bacteria, and these bacteria constitute a potential risk for public health.     

Prevalence of antibiotic and heavy metal resistance in Gram negative bacteria isolated from rivers in northern Pakistan

Antibiotic and metal resistance is a global occurring phenomenon and a universal threat. The release of antibiotics and metals into the environment like natural water bodies stimulates resistant microbes that could become a reservoir for a resistant gene pool for other pathogenic bacteria. The aim of the study was to determine microbial diversity, distribution of antibiotic and metal resistance bacteria from five rivers. The resistance of 168 Gram negative isolates was tested against 10 antibiotics and five metals. The study revealed a high incidence of bacterial isolates resistant to ampicillin (44.7%), streptomycin (37.3%) and chloramphenicol (33.9%). The highest level of antibiotic resistance was detected in the River Ravi. The multiple antibiotic resistance (MAR) index of the five rivers ranged from <0.2 to 0.6. Isolates displayed tolerance to different concentrations of heavy metals. High antibiotic and metal‐resistances in samples from the Rivers Ravi and Soan suggest co‐resistance amongst the bacterial populations.     

Biodegradation of low density polyethylene (LDPE) by certain indigenous bacteria and fungi

Low-density polyethylene (LDPE) is an environmental problem because it is discarded randomly, and because in dumpsites, it does not readily degrade. This study reports evidence of successful biodegradation by two fungal species (Aspergillus flavus MCP₅ and Aspergillus flavus MMP₁₀) and eight bacterial species (Acinetobacter sp. MGP₁, Bacillus sp. MGP₁, Pseudomonas sp. MMP₁, Bacillus sp. MMP₅, Staphylococcus sp. MMP₁₀, Bacillus sp. MGP₁, Micrococcus sp. MMP₅ and Bacillus sp. MMP₁₀). These were demonstrated to have used LDPE as both nitrogen and carbon source. LDPE-users were then characterised and identified using standard microbiological procedures. Biodegradation study was done using selected bacterial and fungal isolates, singly and in consortia, to degrade heat-sterilised ground LDPE in media devoid of carbon source, and carbon and nitrogen source. Biodegradation was monitored using gravimetric methods and Fourier transform infrared spectroscopy. This study revealed some microorganisms can use LDPE as both nitrogen and carbon source in the absence of additives.     

Novel homologs of the multiple resistance regulator marA in antibiotic-contaminated environments

Antibiotics are commonly detected in the environment as contaminants. Exposure to antibiotics may induce antimicrobial-resistance, as well as the horizontal transfer of resistance genes in bacterial populations. We selected the resistance gene marA, mediating resistance to multiple antibiotics, and explored its distribution in sediment and water samples from surface and sewage treatment waters. Ciprofloxacin and ofloxacin (fluoroquinolones), sulphamethoxazole (sulphonamide), erythromycin, clarythromycin, and spiramycin (macrolides), lincomycin (lincosamide), and oxytetracycline (tetracycline) were measured in the same samples to determine antibiotic contamination. Bacterial populations from environmental samples were challenged with antibiotics to identify resistant isolates. The gene marA was found in almost all environmental samples and was confirmed by PCR amplification in antibiotic-resistant colonies. 16S rDNA sequencing revealed that the majority of resistant isolates belonged to the Gram-positive genus Bacillus, not previously known to possess the regulator marA. We assayed the incidence of marA in environmental bacterial populations of Escherichia coli and Bacillus by quantitative real-time PCR in correlation with the levels of antibiotics. Phylogenetic analysis indicated the possible lateral acquisition of marA by Bacillus from Gram-negative Enterobacteriaceae revealing a novel marA homolog in Bacillus. Quantitative PCR assays indicate that the frequency of this gene in antropised environments seems to be related to bacterial exposure to water-borne antibiotics.     

Occurrence of sulfonamide and tetracycline-resistant bacteria and resistance genes in aquaculture environment

The occurrence of sulfonamide and tetracycline resistance and their pollution profile in the aquaculture environment of Tianjin, northern China, were investigated. The presence of antibiotic-resistant bacteria was identified and the corresponding antibiotic resistance genes (ARGs) were quantified at 6 aquaculture farms in Tianjin. Sulfonamide-resistance genes were prevalent and their concentrations were the highest detected (3.0 × 10⁻⁵ to 3.3 × 10⁻⁴ for sul1/16S rDNA, 2.0 × 10⁻⁴ to 1.8 × 10⁻³ for sul2/16S rDNA) among the various ARGs, most likely because the use of sulfonamides is more prevalent than tetracyclines in this area. Bacillus was the most dominant bacterial genus in both sulfamethoxazole resistant bacteria (63.27% of the total resistant bacteria) and tetracycline-resistant bacteria (57.14% of the total resistant bacteria). At least two of those genes (tetM, tetO, tetT, tetW, sul1 and sul2) were detected in the isolates of Bacillus cereus, Bacillus subtilis, Bacillus megaterium and Acinetobacter lwofii, and all of the above genes were detected in B. cereus, suggesting the occurrence of multi-resistance in the studied area. The genetic transfer of sul1 between intestinal bacteria (e.g., Enterococcus spp.) and indigenous bacteria (e.g., Bacillus spp.) was implied by phylogenetic analysis. Several strains of resistant opportunistic pathogens (e.g., Acinetobacter spp.) were found in indigenous bacteria, which increase the risk of ARGs to public health. Overall, this is the first study to comprehensively investigate the antibiotic resistance profile by analyzing the species of antibiotic-resistant bacteria and adopting qualitative and quantitative methods to investigate ARGs at a typical aquaculture area in northern China.     

Microbial contamination of main contact surfaces of Automated Teller Machines from Metropolitan Area of Porto

ABSTRACT

Automated Teller Machines (ATM) are likely to be contaminated with various microorganisms because of their contact with the hands of many users daily. The main objective of this study was to investigate ATMs as a potential source of bacterial contamination. This study was conducted in the Metropolitan Area of Porto, in which 50 swab samples were cultured on selective media. Some isolates were identified based on colonial, morphological and biochemical characteristics. Susceptibility to several antibiotics was also evaluated for each isolated organism. Most contaminated ATMs had high numbers of Enterobacteriaceae and Enterococci. Staphylococci and Enterobacteriaceae were the most prevalent bacteria found. The recovered bacterial isolates varied in their antibiotic resistance pattern; Staphylococcus spp. demonstrated the most resistant profiles. With this pioneering study in Portugal, it was demonstrated that although ATMs in metropolitan area of Porto were not highly contaminated, some potentially pathogenic bacteria were present and resistant to some commonly used antibiotics.     

Exposure to culturable airborne bioaerosols during noodle manufacturing in central Taiwan

Biological hazards associated with the manufacturing of noodles have not been well characterized in Taiwan. This is an issue that flour workers can be exposed to bioaerosols (airborne fungi and bacteria) resulting flour-induced occupational asthma or allergic diseases. This study is to survey the species and concentrations of bioaerosols at different sites within a noodle factory for one year, and to investigate the effects of environmental factors on concentrations of bioaerosols. Air samples were taken twice a day, one day each month using a MAS-100 bioaerosol sampler.Nine species of culturable fungi were identified, with the main airborne fungi being Cladosporium, Penicillium, Aspergillus spp., non-sporing isolates and yeasts. Cladosporium, Penicillium and Aspergillus were the dominant fungal isolates in the indoor and outdoor air samples. Micrococcus spp. and Staphylococcus xylosus were the dominant bacterial isolates. Peak fungal and bacterial concentrations occurred at the crushing site, with mean values of 3082 and 12,616 CFU/m³. Meanwhile, the most prevalent fungi and bacteria at the crushing site were in ranges of 2.1-1.1 μm and 1.1-0.65 μm, respectively. Significant seasonal differences in total bacterial concentration were observed at all sampling sites (?P < 0.05). Moreover, significant seasonal differences were observed for most of the fungal genera except Fusarium. Levels of Aspergillus and Rhizopus differed significantly during the two sampling times, as did levels of Micrococcus spp. and Staphylococcus arlettae.Regarding the same operation procedures, relative humidity affected fungi levels more than temperature did. However, crushing generated the highest concentration of bioaerosols among all operation procedures. Furthermore, levels of bacteria at sites fitted with ventilation systems were lower than at sites without ventilation systems, especially at the crushing site. Therefore, we recommend these workers at the crushing site wear breathing protection and improve the local ventilation systems to minimize the biological hazards.     

Airborne microbiological characteristics in public buildings of Korea

Characteristics of airborne bacteria and fungi were surveyed in the public buildings regulated in Korea, with the six-stage cascade impactor. The total concentrations of airborne bacteria and fungi were averaged to 404 and 382 cfu m⁻³ in hospital, 931 and 536 cfu m⁻³ in kindergarten, 294 and 334 cfu m⁻³ in elderly welfare facility, and 586 and 371 cfu m⁻³ in postpartum nurse center. Mean respirable concentrations of airborne bacteria and fungi were 194 and 292 cfu m⁻³ in hospital, 358 and 347 cfu m⁻³ in kindergarten, 134 and 266 cfu m⁻³ in elderly welfare facility, and 254 and 289 cfu m⁻³ in postpartum nurse center, respectively. Based on this results, total and respirable concentrations of airborne bacteria and fungi were significantly highest in kindergarten and lowest in elderly welfare facility (p<0.05$p<0.05$). The ratios of indoor and outdoor concentration for airborne bacteria and fungi were below 1.0 in all the investigated public buildings regardless of size distribution. The dominant genera identified in the public buildings were Staphylococcus spp., Micrococcus spp., Corynebacterium spp., and Bacillus spp., for airborne bacteria and Penicillium spp., Cladosporium spp., and Aspergillus spp., for airborne fungi, respectively. Size distributions of airborne bacteria and fungi in terms of the dominant genera were not observed consistently except for Staphylococcus spp., which was detected mainly on the first stage (>7.0 μm) and second stage(4.7–7.0 μm), and Penicillium spp., and Cladosporium spp., showing the highest collection rate at stage 3 (3.3–4.7 μm) regardless of the kind of the public buildings.     

Antibiotic resistance and tolerance to heavy metals demonstrated by environmental pathogenic bacteria isolated from the Kahwa River,Bukavu Town,Democratic Republic of the Congo

Increasing concentration of heavy metals and antibiotics within bacterial microenvironments contribute to medical problems. This research confirmed the presence of enteropathogenic bacteria in the sediment and water of Kahwa River. Vibrio cholerae, Shigella dysenteriae, Salmonella typhi and Salmonella paratyphi showed resistance to antibiotics and tolerance to heavy metals. The bacteria, particularly Vibrio, were extensively resistant to Ampicillin; less so to Chloramphenicol, Co-trimoxazole and Tetracycline with a relatively reduced resistance shown to Ciprofloxacin. Higher levels of antibiotic resistance were noticed in bacteria isolated from river sediment when compared to bacteria isolated from river water (p < 0.05). Tolerance to cadmium and lead was also more obvious in bacteria isolated from river sediment. An increasing association was observed between multiple antibiotic resistance and tolerance to heavy metals. Bacterial growth in the presence of cadmium was restricted when compared to lead.     

Bottled mineral water as a potential source of antibiotic resistant bacteria

The antibiotic resistance phenotypes of the cultivable bacteria present in nine batches of two Portuguese and one French brands of commercially available mineral waters were examined. Most of the 238 isolates recovered on R2A, Pseudomonas Isolation agar or on these culture media supplemented with amoxicillin or ciprofloxacin, were identified (based on 16S rRNA gene sequence analysis) as Proteobacteria of the divisions Beta, Gamma and Alpha. Bacteria resistant to more than three distinct classes of antibiotics were detected in all the batches of the three water brands in counts up to 10² CFU/ml. In the whole set of isolates, it was observed resistance against all the 22 antimicrobials tested (ATB, bioMérieux and disc diffusion), with most of the bacteria showing resistance to three or more classes of antibiotics. Bacteria with the highest multi-resistance indices were members of the genera Variovorax, Bosea, Ralstonia, Curvibacter, Afipia and Pedobacter. Some of these bacteria are related with confirmed or suspected nosocomial agents. Presumable acquired resistance may be suggested by the observation of bacteria taxonomically related but isolated from different brands, exhibiting distinct antibiotic resistance profiles. Bottled mineral water was confirmed as a possible source of antibiotic resistant bacteria, with the potential to be transmitted to humans.     

Resistance transfer fecal coliforms isolated from the whippany river

Three sites on the Whippany River, a tributary of the Passaic Watershed in Morris County, N.J., were monitored for resistance transfer coliforms. A high percentage of fecal coliform isolates showed multiple antibiotic resistance. A significant number of lac⁻ and lac⁻ isolates proved to be donors of transmissible antibiotic resistance plasmids (R donors), by conjugation with a recipient strain of Salmonella gallinarum. Re-examination of testing methods used for recreational and potable water sources is suggested by these findings.     

Assessment of heavy metals contamination and its effects on oyster (Saccostrea cucullata) biometry parameters in the Asaluyeh port coasts,Persian Gulf,Iran

The current study examines biometric parameters and concentration of some heavy metals in soft tissues of Saccostrea cucullata in the Asaluyeh coast, Persian Gulf, Iran. The metals concentration in the soft tissue of the Saccostrea cucullata ranged as: Zn: 765–3411 mg kg^?1, Cu: 120.3–580.6 mg kg^?1, Cd: 0.13–3.5 mg kg^?1, As: 0.39–3.3 mg kg^?1, Pb: 0.05–1.64 mg kg^?1, Ni: 0.02–0.44 mg kg^?1, V: 0.06–0.32 mg kg^?1, Cr: 0.02–0.16 mg kg^?1 and Hg: 0.002–0.068 mg kg^?1. The results showed that the concentration of total heavy metals in oysters of the control station (CA) is usually lower. The results also indicated lack of correlation between oyster biometric parameters and bioaccumulated heavy metals, except between Cu and longitudinal diameter (R = 0.77). The concentration of heavy metals is higher than limits recommended by the World Health Organization, except for Cd and Pb.     

Heavy metal accumulation by aquatic fungi and reduction in viability of Gammarus pulex fed Cd2+ contaminated mycelium

The addition of Pb²⁺ (1.0 and 2.5 μg ml^?1) and Cd²⁺ (2.5 and 5.0 μg ml^?1) to growth medium decreased the growth of aquatic fungi, while Zn²⁺ even at a concentration of 10 μg ml^?1 had no inhibitory effect. Low concentrations of Cd²⁺ and Zn²⁺ on the other hand stimulated mycelial growth. The fungi accumulated considerable amounts of metal from the growth medium, generally in the order Zn²⁺ > Pb²⁺ > Cd²⁺ · Cd²⁺ was also accumulated by fungi from successive changes of medium containing low concentrations of the metal. Application of Langmuir and Freundlich isotherms to the results showed that the metals were accumulated largely by adsorption to the surface of the mycelium.Three species of aquatic fungi supported the growth of the freshwater shrimp. Gammarus pulex when provided as sole food source, sustaining from 30 to 60% of shrimps fed for a period of 21 days. A marked reduction in shrimp viability occurred however, when G. pulex was fed Pythium sp. containing Cd²⁺ (150–170 μg g^?1), with none of the shrimps surviving beyond 13 days, compared with a survival rate of 60% after 21 days for shrimps fed uncontaminated mycelium. Bodies of poisoned shrimps sampled on day 13 were found to contain Cd²⁺ (22.03 μg g^?1) showing that the metal can be transferred from aquatic fungi to G. pulex, the first step in a food chain involving freshwater fish and higher organisms.     

Bacterial suppression of chlorella by hydroxylamine production

An actively nitrifying Arthrobacter sp. isolated from a lake inhibited the growth of Chlorella vulgaris. This was found to be due to hydroxylamine released by the bacterium during the oxidation of ammonium or other reduced nitrogen compounds. Plates containing Plate Count Agar streaked with Arthrobacter, accumulated 5 μg ml⁻¹ hydroxylamine-N. Chlorella was sensitive to less than 0.24 μg ml⁻¹ hydroxylamine-N. Non-nitrifying bacterial isolates of lacustrine origin did not demonstrate inhibiting activity. Our results indicate that nitrifying microorganisms may thus affect the population dynamics of algae in eutrophic lakes.     

Biomonitoring of maritime pollution by heavy metals (Pb and Zn) in the coast of Jijel (Algeria)

This paper reports a study of pollution in the coastal waters of Jijel, Algeria, using algae Ulva lactuca and Corallina officinalis as bioindicators. Samples of seawater and algae were collected at four different stations from the coast of Jijel, during the period of April–June 2014. The heavy metal content (Pb and Zn) was determined in seawater and in the algae tissue by the technique of atomic absorption spectrophotometry. In seawater, the contents of heavy metals vary from 0.017 to 0.03 mg/l (Pb) and 0.235 to 0.873 mg/l. In the algae tissues, metals concentrations vary between 1.88 to 6.25 μg g^?1 dry weight (Pb), and from 92 to 178.9 μg g^?1 dry weight (Zn). These levels differ by site and species. The calculation of the bioconcentration factor (BCF) leads us to conclude that algae bioaccumulate significant levels of Pb and Zn metals in their tissues. Our results shows that the species of C. officinalis bioaccumulate the metals Pb and Zn more than U. lactuca, where, high biosorption of Zn was observed with BCF values between 203.21 and 238.40. Zn content in seawater and algae tissues appear higher than standards set by USEPA and guides values of Certified Reference Materials, but Pb levels appear lower than this standard.     

Occurrence of metal and antibiotic resistant Escherichia coli harbouring zntA and copA genes in selected surface water in Ibadan,South-west Nigeria

ABSTRACT

Escherichia coli strains were isolated from surface water collected from three sampling points and cultured on CHROMAgar E. coli. The zinc and copper concentrations of the water samples were determined using Atomic Absorption Spectrophotometer. Colonies of Escherichia coli were subjected to increasing concentration (0.1–1.0 mM) of copper and zinc in the isolation medium. Isolates showing resistance to both metals at 1 mM were identified using conventional methods and by targeting the uidA gene, while PCR amplification of the copA and zntA genes was carried out using specific primers. The selected bacteria were subjected to 10 classes of antibiotics using the Kirby–Bauer disc diffusion technique. Five E. coli showing resistance to copper and zinc and possessing uidA gene were obtained. Of the five isolates, three possessed both zntA and copA genes, while zntA was absent in the remaining two. All the isolates peculiarly showed resistance to ampicillin and doxycycline, while three were multi-drug resistant (MDR).     

Microbial and antibiotic resistant constituents associated with biological aerosols and poultry litter within a commercial poultry house

Poultry are known to harbor antibiotic resistant and pathogenic bacteria, and as such poultry litter and poultry house air can be contaminated with these bacteria. However, the presence of antibiotic resistant bacteria in biological aerosols and litter is largely not understood. The purpose of this study was to determine the amount of aerosolized bacteria and endotoxin, particularly fecal indicators, staphylococci, and enterococci, associated with poultry house and outdoor air. Aerosol samples were collected at multiple locations on the farm and in the house. Antibiotic resistance was investigated using the Kirby Bauer method on selected isolates using twelve different antibiotics spanning both narrow to broad spectrums of effectiveness. Overall there was a cyclical increase in bacterial concentrations as flocks progressed from pre-flock to late-flock, with > 2 orders magnitude lower concentration during pre-flock periods (no chickens), in both the litter and aerosol samples. The house environment provided for significantly concentrated bacterial and endotoxin levels. It was estimated that Staphylococcus bacteria accounted for at least 90% of cultured aerobic bacteria and culture-independent 16S rRNA analyses demonstrated that significant population changes occurred from pre- to late-flock. Rarely was an isolate resistant to more than 4 antibiotic classes; however there was a trend upwards in overall resistance of enterococci as the flock cycle progressed. It appears that although levels of antibiotic resistant bacteria were highly concentrated within the house, levels were much lower outside of the house, and very little house escape occurred.     

CuFe2O4 as heterogeneous catalyst in degradation of p-nitrophenol with photoelectron-Fenton-like process

In this research, the heterogeneous photoelectron-Fenton-like process using copper ferrite (CuFe₂O₄) as catalyst was employed to remove p-nitrophenol (PNP). The CuFe₂O₄ catalyst was prepared by a co-precipitation process and characterised with X-ray fluorescence analysis, X-ray diffraction analysis and BET surface area. The results confirmed that CuFe₂O₄ was prepared successfully. Under the optimum conditions, namely 0.05?mol?L^?1 Na₂SO₄, the electrical potential of ?0.6?V, pH 3.0 and 0.067?g?L^?1 CuFe₂O₄, 92.8% of 0.1?mM?L^?1, the PNP removal efficiency could reach 92.8% for 0.1?mM?L^?1 PNP after 180-min treatment. This technology can be applied for the treatment of organic compounds which are not readily biodegradable.     

Impact of cement factory operations on air quality and human health in Ewekoro Local Government Area,South-Western Nigeria

The concentrations of pollutants associated with cement production and the effects on ambient air and community health were assessed in this study. Suspended particulates were monitored using a Negretti 1000^TM air sampler. Additionally, an Ogawa^TM sampler was employed to assess levels of selected gaseous pollutants at selected sites. To collect data on health status, a questionnaire survey was used for selected neighbouring communities and clinic records for industry workers. The data were analysed with the aid of percentage, variation test, correlation and regression statistics. Mean concentrations of particulate matter of 10?μ (74–338?μg/m³) and 2.5?μ (28–116?μg/m³) were significantly higher than permissible limits (50?μg/m³ and 10?μg/m³) within and around the production plant. At all sites, levels of sulphur dioxide (0.1–12?ppb), nitrogen dioxide (0.1–13?ppb) and carbon monoxide (0.1–1.7?ppm) were below the allowable limits for human exposure. Analysis of variance showed significant spatial variations (p?相似文献   

Antibiotic resistant enterococci and staphylococci isolated from flies collected near confined poultry feeding operations

Use of antibiotics as feed additives in poultry production has been linked to the presence of antibiotic resistant bacteria in farm workers, consumer poultry products and the environs of confined poultry operations. There are concerns that these resistant bacteria may be transferred to communities near these operations; however, environmental pathways of exposure are not well documented. We assessed the prevalence of antibiotic resistant enterococci and staphylococci in stored poultry litter and flies collected near broiler chicken houses. Drug resistant enterococci and staphylococci were isolated from flies caught near confined poultry feeding operations in the summer of 2006. Susceptibility testing was conducted on isolates using antibiotics selected on the basis of their importance to human medicine and use in poultry production. Resistant isolates were then screened for genetic determinants of antibiotic resistance. A total of 142 enterococcal isolates and 144 staphylococcal isolates from both fly and poultry litter samples were identified. Resistance genes erm(B), erm(A), msr(C), msr(A/B) and mobile genetic elements associated with the conjugative transposon Tn916, were found in isolates recovered from both poultry litter and flies. Erm(B) was the most common resistance gene in enterococci, while erm(A) was the most common in staphylococci. We report that flies collected near broiler poultry operations may be involved in the spread of drug resistant bacteria from these operations and may increase the potential for human exposure to drug resistant bacteria.