Impact of genetic diversity and antibiotic-resistance of Salmonella isolated from feral cats: One Health approach.

Free-living cats usually live in colonies in urban areas, especially close to parks and neighbourhoods where people feed them without any sanitary control. This can pose a human, animal and environmental health concern due to the close contact between uncontrolled colonies, the population and other domestic and/or wild animals. Thus, this study aimed to assess the genetic diversity and antimicrobial resistance (AMR) among Salmonella enterica subsp. enterica strains isolated from feral cats in a previous epidemiological study in the Gran Canaria island (Spain). A total of nineteen Salmonella isolates were obtained from November 2018 to January 2019 in a Salmonella epidemiological study in feral cats. All isolates obtained were genotyped by pulsed-field gel electrophoresis (PGFE) and were tested for antimicrobial susceptibility, in accordance with Decision 2013/652/EU. PFGE analysis revealed isolates clustering by serovar, with identical clones for serovars Bredeney and Grancanaria, while differing pulsotypes were observed for serovars Florida (88.89 % similarity) and Nima (83.23 % similarity). All but two isolates were resistant to at least one antimicrobial. The results obtained demonstrate that feral cats in the region investigated are a reservoir of Salmonella strains resistant to gentamicin (94.1 %) and of the critically important antimicrobial tigecycline (23.5 %). Hence, they could excrete AMR strains through their faeces and contaminate the environment, favoring the spread of such bacteria to cohabiting pets. Moreover, this widespread presence of AMR Salmonella clones across various serovars highlights the urgent need to implement efficient antimicrobial stewardship and control programs by the local governments due to the ongoing need to protect human and animal health under a One Health concept.

Research Note: Persistent Salmonella problems in slaughterhouses related to clones linked to poultry companies.

Salmonellosis remains one of the main foodborne zoonoses in Europe, with poultry products as the main source of human infections. The slaughterhouse has been identified as a potential source for Salmonella contamination of poultry meat. Despite the mandatory programme of the EU, there are companies with persistent Salmonella that are unable to remove the bacteria from their processing environment, compromising the entire production line. In this context, an intensive sampling study was conducted to investigate a slaughterhouse with persistent Salmonella problems, establishing the genetic relationship among Salmonella strains isolated during the slaughter process. A total of 36 broiler flocks were sampled during processing at the slaughterhouse. Salmonella was identified based on ISO 6579-1:2017 (Annex D), serotyped by Kauffman-White-Le-Minor technique, and the genetic relationship was assessed with ERIC-PCR followed by PFGE. The outcomes showed that 69.4% of the batches sampled carried Salmonella upon arrival at the slaughterhouse and that 46.3% of the different samples from carcasses were contaminated with Salmonella. The two serovars isolated at the different steps in the slaughterhouse were Enteritidis (98.2%) and Kentucky (1.8%). Pulsed-field gel electrophoresis analysis revealed a low genetic diversity, with all S. Enteritidis isolates showing a nearly identical pulsotype (similarity >85%) and S. Kentucky strains showed the same XbaI PFGE profile (95.0% genetic similarity). The results of this study showed a high genetic relationship among isolates recovered from carcasses and environmental samples in the slaughterhouse from both Salmonella-positive and Salmonella-free flocks. Salmonella strains re-circulated across to poultry flocks and re-entered the slaughterhouse to survive on the processing line. Thus, it is necessary to implement molecular diagnosis methods in time at the field level to determine the Salmonella epidemiology of the flock, to make rapid decisions for the control of Salmonella and prevent entry into the slaughterhouse environment.

Dynamics of Haemophilus parasuis genotypes in a farm recovered from an outbreak of Glässer's disease.

Haemophilus parasuis is a colonizer of the upper respiratory tract of pigs, although it is better known as the etiological agent of Glässer's disease. Interestingly, several strains can be isolated from a single farm, as determined by both genotyping and serotyping. However, it is not known how an outbreak and the subsequent treatment affect the population of H. parasuis strains. In this study, a farm was studied during an outbreak of Glässer's disease and 1 year after antimicrobial treatment and elimination of clinical signs. Bacterial isolation was attempted from nasal swabs and lesions. After isolation, antimicrobial susceptibility, serotype and genotype were determined. Two different genotyping techniques, enterobacterial repetitive intergenic consensus (ERIC)-PCR and multilocus sequence typing (MLST) were used. The H. parasuis strain that was isolated from lesions during the disease outbreak clustered with other virulent strains by both MLST and serotyping analysis. Nasal isolates were included in the corresponding nasal cluster by MLST, but they presented high variability by serotyping. These nasal isolates included serotypes previously classified as virulent and non-virulent. Finally, we found that during the antimicrobial treatment the diversity of strains isolated in the farm was affected and just one strain, which was resistant to the treatment, was detected. One year after the treatment strain diversity was back to normal (three strains).

First isolation of Haemophilus parasuis and other NAD-dependent Pasteurellaceae of swine from European wild boars.

Haemophilus parasuis is a colonizer of the upper respiratory tract of pigs and the etiological agent of Glässer's disease, which is characterized by a fibrinous polyserositis, meningitis and arthritis. Glässer's disease has never been reported in wild boar (Sus scrofa), although antibodies against H. parasuis have been detected. The goal of this study was to confirm the presence of this bacterium in wild boar by bacterial isolation and to compare the strains to H. parasuis from domesticated pigs. Therefore, nasal swabs from 42 hunted wild boars were processed for bacterial isolation and subsequent H. parasuis identification by specific PCR, biochemical tests and 16S rRNA gene sequencing. Two different strains of H. parasuis from two wild boars were isolated. These strains belonged to serotype 2 and were included by 16S rRNA gene sequencing and MLST analysis in a cluster with other H. parasuis strains of nasal origin from domestic pigs. During this study, Actinobacillus minor and Actinobacillus indolicus, which are NAD-dependent Pasteurellaceae closely related to H. parasuis, were also isolated. Our results indicate similarities in the respiratory microbiota of wild boars and domestic pigs, and although H. parasuis was isolated from wild boars, more studies are needed to determine if this could be a source of H. parasuis infection for domestic pigs.

Genetic diversity and antimicrobial resistance of Campylobacter and Salmonella strains isolated from decoys and raptors.

Infections caused by thermotolerant Campylobacter spp. and Salmonella spp. are the leading causes of human gastroenteritis worldwide. Wild birds can act as reservoirs of both pathogens. A survey was carried out to determine the prevalence, genetic diversity and antimicrobial resistance of thermotolerant Campylobacter and Salmonella in waterfowl used as decoys and wild raptors in Andalusia (Southern Spain). The overall prevalence detected for Campylobacter was 5.9% (18/306; CI95%: 3.25-8.52) in decoys and 2.3% (9/387; CI95%: 0.82-3.83) in wild raptors. Isolates were identified as C. jejuni, C. coli and C. lari in both bird groups. Salmonella was isolated in 3.3% (10/306; CI95%: 2.3-4.3) and 4.6% (18/394; CI95%: 3.5-5.6) of the decoys and raptors, respectively. Salmonella Enteritidis and Typhimurium were the most frequently identified serovars, although Salmonella serovars Anatum, Bredeney, London and Mikawasima were also isolated. Pulsed-field gel electrophoresis analysis of isolates showed higher genetic diversity within Campylobacter species compared to Salmonella serovars. Campylobacter isolates showed resistance to gentamicin, ciprofloxacin and tetracycline, while resistance to erythromycin and tetracycline was found in Salmonella isolates. The results indicate that both decoys and raptors can act as natural carriers of Campylobacter and Salmonella in Spain, which may have important implications for public and animal health.

Monitoring of West Nile virus, Usutu virus and Meaban virus in waterfowl used as decoys and wild raptors in southern Spain.

In the last decade, the number of emerging flaviviruses described worldwide has increased considerably, with wild birds acting as the main reservoir hosts of these viruses. We carried out an epidemiological survey to determine the seroprevalence of antigenically related flaviviruses, particularly West Nile virus (WNV), Usutu virus (USUV) and Meaban virus (MBV), in waterfowl used as decoys and wild raptors in Andalusia (southern Spain), the region considered to have the highest risk of flaviviruses circulation in Spain. The overall flaviviruses seroprevalence according to bELISA was 13.0% in both in decoys (n=1052) and wild raptors (n=123). Specific antibodies against WNV, USUV and MBV were confirmed by micro virus neutralization tests in 12, 38 and 4 of the seropositive decoys, respectively. This is the first study on WNV and USUV infections in decoys and the first report of MBV infections in waterfowl and raptors. Moreover we report the first description of WNV infections in short-toed snake eagle (Circaetus gallicus) and Montagu's harrier (Circus pygargus). The seropositivity obtained indicates widespread but not homogeneous distribution of WNV and USUV in Andalusia. The results also confirm endemic circulation of WNV, USUV and MBV in both decoys and wild raptors in southern Spain. Our results highlight the need to implement surveillance and control programs not only for WNV but also for other related flaviviruses. Further research is needed to determine the eco-epidemiological role that waterfowl and wild raptors play in the transmission of emerging flaviviruses, especially in decoys, given their close interactions with humans.

Antibiotic-induced dysbiosis alters host-bacterial interactions and leads to colonic sensory and motor changes in mice.

Alterations in the composition of the commensal microbiota (dysbiosis) seem to be a pathogenic component of functional gastrointestinal disorders, mainly irritable bowel syndrome (IBS), and might participate in the secretomotor and sensory alterations observed in these patients.We determined if a state antibiotics-induced intestinal dysbiosis is able to modify colonic pain-related and motor responses and characterized the neuro-immune mechanisms implicated in mice. A 2-week antibiotics treatment induced a colonic dysbiosis (increments in Bacteroides spp, Clostridium coccoides and Lactobacillus spp and reduction in Bifidobacterium spp). Bacterial adherence was not affected. Dysbiosis was associated with increased levels of secretory-IgA, up-regulation of the antimicrobial lectin RegIIIγ, and toll-like receptors (TLR) 4 and 7 and down-regulation of the antimicrobial-peptide Resistin-Like Molecule-ß and TLR5. Dysbiotic mice showed less goblet cells, without changes in the thickness of the mucus layer. Neither macroscopical nor microscopical signs of inflammation were observed. In dysbiotic mice, expression of the cannabinoid receptor 2 was up-regulated, while the cannabinoid 1 and the mu-opioid receptors were down-regulated. In antibiotic-treated mice, visceral pain-related responses elicited by intraperitoneal acetic acid or intracolonic capsaicin were significantly attenuated. Colonic contractility was enhanced during dysbiosis. Intestinal dysbiosis induce changes in the innate intestinal immune system and modulate the expression of pain-related sensory systems, an effect associated with a reduction in visceral pain-related responses. Commensal microbiota modulates gut neuro-immune sensory systems, leading to functional changes, at least as it relates to viscerosensitivity. Similar mechanisms might explain the beneficial effects of antibiotics or certain probiotics in the treatment of IBS.

Free-living Waterfowl as a Source of Zoonotic Bacteria in a Dense Wild Bird Population Area in Northeastern Spain.

Salmonella spp. and Campylobacter spp. are zoonotic bacteria that represent an economic and public health concern worldwide. Due to the difficulty to collect samples from free-living waterfowl, little is known on their importance as a reservoir of zoonotic agents. Thus, a study was conducted to determine the prevalence, genotypic diversity and antimicrobial susceptibility of Salmonella and Campylobacter from waterfowl in Ebro Delta (northeastern Spain), a geographical area with a dense wild bird population. Samples were collected from 318 adult waterfowl belonging to nine fowl species. All the samples were taken during the hunting season from 2008 to 2010. None of the birds were positive for Salmonella, while the overall Campylobacter prevalence was 12.58% (40/318). A much higher Campylobacter coli prevalence than Campylobacter jejuni was found (11.64% versus 0.94%). The species Fulica atra showed the highest Campylobacter prevalence (78.05%). ERIC-PCR of the isolates showed a high diversity of strains. Antimicrobial susceptibility testing of Campylobacter isolates showed that all the isolates were susceptible to the seven antibiotics tested.

Effect of feeding piglets with different extruded and nonextruded cereals on the gut mucosa and microbiota during the first postweaning week.

Two trials were conducted to evaluate the effect of different cereals in piglet diets on the jejunal mucosa and the ileal and cecal microbiota during the first postweaning days. In Trial 1, 48 newly weaned pigs (7.95 kg BW; 26 d of age) were individually housed and distributed among 3 experimental diets containing white rice (Oryza sativa), naked oats (Avena sativa), or barley (Hordeum vulgare) as the cereal source. At the start of the trial (weaning; day 0), 12 piglets were slaughtered and sampled to obtain initial reference values for histology and microbiology determinations. Additionally, 4 pigs per treatment per day were slaughtered and sampled at days 1, 2, and 6 postweaning. Villus height (VH), crypt depth (CD), and intraepithelial lymphocytes (IEL) in jejunal mucosa were measured, and microbiota in ileal and cecal digesta were evaluated by RFLP. The Manhattan distances between RFLP profiles were calculated and, for each treatment and sampling day, intragroup similarities (IGS) were estimated. In Trial 2, an additional 48 piglets were used (7.56 kg BW; 26 d of age), and the same experimental procedures were performed except that the 3 experimental diets contained extruded white rice, extruded naked oats, or extruded barley as the cereal source. A reduction in VH was observed in both trials from day 0 to 6 (P < 0.05). In Trial 1 (raw cereals), more IEL and deeper crypts were observed for the barley than for the naked oats based diets (P < 0.05). In Trial 2, no differences among extruded cereals were observed for the histological parameters. In Trial 1, feeding naked oats resulted in lower IGS (increased heterogeneity) of the microbiota in the ileum and in the cecum compared to rice and barley (P < 0.05). In Trial 2, the pigs fed extruded barley had lower IGS in the ileum than those fed extruded naked oats and extruded rice whereas in the cecum, both extruded barley and extruded rice resulted in lower IGS than extruded naked oats (P < 0.05). It is concluded that cereal nature affects the composition of the microbiota and the morphology of the gut mucosa in newly weaned pigs.

Effect of feeding different cereal-based diets on the performance and gut health of weaned piglets with or without previous access to creep feed during lactation.

A trial was conducted to evaluate the effect of different cereals on the performance, gut mucosa, and microbiota of weanling pigs with or without previous access to creep feed during lactation. A total of 108 newly weaned pigs (7.4 kg BW; 26 d of age; half with and half without creep feed) were used. Piglets were distributed by BW into 36 pens according to a 2 × 6 factorial arrangement of treatments with previous access to creep feed (with or without) and cereal source in the experimental diet [barley (Hordeum vulgare), rice (Oryza sativa)-wheat (Triticum aestivum) bran, corn (Zea mays), naked oats (Avena sativa), oats, or rice] as main factors. Pigs were offered the experimental diets for 21 d and performance was monitored. At day 21, 4 piglets from each treatment were killed and sampled for the histological evaluation of jejunal mucosa and the study of ileal and cecal microbiota by RFLP. The Manhattan distances between RFLP profiles were calculated and intragroup similarities (IGS) were estimated for each treatment. An interaction between cereal source and previous creep feeding was observed for ADFI (P < 0.05), indicating that whereas creep feeding increased ADFI for the rice-wheat bran diet it reduced it for naked oats. No differences in mucosal morphology were observed except for deeper crypts in pigs that did not have previous access to creep feed (P < 0.05). Cereal source had a significant effect on IGS of ileal and cecal microbiota (P < 0.01). In the ileum oats and corn had the highest IGS (i.e., lowest heterogeneity of the microbiota) followed by rice, naked oats, barley, and rice-wheat bran whereas in the cecum, IGS was highest for rice and oats followed by corn, barley, rice-wheat bran, and naked oats. An interaction between creep feeding and cereal was also observed for the IGS of the cecal microbiota at day 21 (P < 0.05). Access to creep feed reduced IGS in the piglets fed oats or barley but no differences were observed for the other cereal sources. It is concluded that the effect of creep feeding during lactation on the performance and the microbiota of piglets after weaning is dependent on the nature of the cereal in the postweaning diet.

Sow vaccination modulates the colonization of piglets by Haemophilus parasuis.

Haemophilus parasuis is the etiologic agent of Glässer's disease in pigs and a colonizer of the upper respiratory tract of healthy pigs. A good balance between colonization and immunity is important to avoid a disease outbreak. This work studied the colonization of H. parasuis in healthy piglets coming from vaccinated and non-vaccinated sows. Piglets from vaccinated sows had higher IgG levels at early time points and subsequently were colonized later and to a lower degree than piglets from non-vaccinated ones. The variability of H. parasuis isolates was investigated by 2 genotyping methods: enterobacterial repetitive intergenic consensus (ERIC)-PCR and multilocus sequence typing (MLST). A high turnover of strains was found in both groups of piglets, with few strains found on more than one sampling occasion. We found a higher number of H. parasuis strains (16 strains) within a given farm than previously thought. Overall, more H. parasuis diversity was found in piglets from non-vaccinated sows than in those from vaccinated sows. These results indicate that vaccination of sows in a farm delays the colonization of piglets and reduces the carriage and heterogeneity of H. parasuis strains.

Moraxella pluranimalium sp. nov., isolated from animal specimens.

Four unusual Gram-negative, catalase-positive, oxidase-positive, coccus-shaped bacteria isolated from one sheep and three pigs were characterized using phenotypic and molecular genetic methods. On the basis of cellular morphology and biochemical criteria, the isolates were tentatively assigned to the genus Moraxella, although the organisms did not appear to correspond to any recognized species. Comparative 16S rRNA gene sequencing studies demonstrated that the isolates represent a novel subline within the genus Moraxella. The most closely related species in phylogenetic terms was Moraxella cuniculi, with 16S rRNA gene sequence similarity of 97.9 % to the type strain CCUG 2154(T), although the DNA-DNA relatedness value was only 29 %. The novel isolates were readily distinguished from all recognized Moraxella species by means of physiological and biochemical tests. On the basis of molecular genetic and phenotypic evidence, therefore, the four isolates represent a novel species of the genus Moraxella, for which the name Moraxella pluranimalium sp. nov. is proposed. The type strain is 248-01(T) (=CECT 7295(T) =CCUG 54913(T)).

Detection and identification of Vibrio scophthalmi in the intestinal microbiota of fish and evaluation of host specificity.

AIMS: To develop a species-specific probe (VSV3) for the detection of Vibrio scophthalmi in fish intestine and to apply this probe to study the host specificity of V. scophthalmi. METHODS AND RESULTS: A specific probe (VSV3) based on the variable region V3 of the 16S rRNA gene (rDNA) was designed. Its specificity was tested by DNA-DNA hybridization and by colony hybridization. No cross-hybridization was found. The sensitivity of the probe was tested both by DNA-DNA hybridization and by colony hybridization. The detection limit of V. scophthalmi 16S rDNA was 150 pg or 10 cfu. Vibrio scophthalmi cells were detected in experimental samples constituted by mixed cultures when present in proportions of 1 : 10 and 1 : 100. The VSV3 probe also proved to be reliable for the detection of V. scophthalmi in samples of fish intestine. CONCLUSIONS: The VSV3 probe can be used for the detection of V. scophthalmi in colony hybridization or DNA-DNA hybridization of amplified 16S rDNA. Preliminary results indicate that V. scophthalmi may present certain host specificity for turbot. SIGNIFICANCE AND IMPACT OF THE STUDY: The VSV3 probe provides a useful tool for ecological studies.

Diversity of Vibrio spp. populations in several exhibition aquaria with a shared water supply.

AIMS: Abiotic factors may influence the settlement of bacterial populations in similar marine environments. Exhibition aquaria are a model for the study of the settlement of bacteria in different environment. Vibrio populations in the seawater reservoir, the Mediterranean tank and the Tropical tank from an exhibition aquarium on the western coast of the Mediterranean were compared and the effect of abiotic factors on the structure of these populations was considered. METHODS AND RESULTS: High diversity indexes and similar Vibrio populations were found in the water of the reservoir and of the Mediterranean tank, whereas a lower diversity and different main populations were found in the water of the Tropical tank. The antibiotic resistance profiles of the most representative strains, presented a number of differences depending on the origin of the sample. CONCLUSION: Abiotic conditions, mainly temperature, may determine the structure and composition of Vibrio populations in exhibition aquaria. SIGNIFICANCE AND IMPACT OF THE STUDY: Bacterial monitoring of water could be useful for health management of aquatic environments.

A selective medium and a specific probe for detection of Vibrio vulnificus.

A selective medium (VVM) and a specific 16S rRNA gene (rDNA) probe (V3VV) for the detection of Vibrio vulnificus were developed. The medium contains D-(+)-cellobiose as the main carbon source and electrolytes (MgCl(2)-6H(2)O and KCl), which stimulate bacterial growth. Polymyxin B, colistin, and moderate alkalinity and salinity provide selectivity properties. V. vulnificus grows on VVM as flat, bright yellow colonies. Other Vibrio species tested either did not grow or showed green-bluish colonies, with the exception of V. campbelli, V. carchariae, and V. navarrensis. There is a higher colony count on VVM agar than on cellobiose-colistin agar or on modified cellobiose-polymyxin B-colistin agar. The specific probe was evaluated by colony hybridization and dot blot hybridization with PCR-amplified 16S rDNA using collection strains and environmental isolates. No strain studied other than V. vulnificus showed positive hybridization with this oligonucleotide. The combined use of VVM agar and the V3VV probe provided the recovery of V. vulnificus from mixed bacterial suspensions and spiked mussels.

Comparison of selective media for the detection of Vibrio vulnificus in environmental samples.

AIMS: To compare two selective agars, cellobiose-colistin (CC) agar and a modification of the Vibrio vulnificus medium (VVMc agar), for the isolation of Vibrio vulnificus from environmental samples. METHODS AND RESULTS: The efficiencies of recovery of V. vulnificus collection strains on CC, VVM, VVMc and on thiosulphate-citrate-bile salts-sucrose (TCBS) agar were compared and similar efficiencies were obtained. A slightly higher recovery was observed on VVMc agar. The detection of V. vulnificus in environmental samples (eels and water) was performed by combining culture-based methods (CC and VVMc agars) with DNA-based methods using species-specific probes based on the cytolysin-haemolysin and the 16S rDNA genes. A lower accompanying microbiota was found on CC agar than on VVMc agar. CONCLUSION: The comparison between CC and VVMc agars confirms that both are useful for the detection of V. vulnificus in environmental samples. However, the use of any of these media should be combined with a species-specific probe. SIGNIFICANCE AND IMPACT OF THE STUDY: The combined use of a selective medium and a specific probe provides a feasible method for the detection of V. vulnificus for epidemiological and ecological studies.

Vibrio scophthalmi sp. nov., a new species from turbot (Scophthalmus maximus).

Six strains isolated from the intestines of juvenile turbot in a fish hatchery in the north of Spain were found to be phenotypically members of the genus Vibrio. However, the phenotypic traits of these organisms did not place them in any of the currently known Vibrio species. These isolates formed an homogeneous group in which the DNA-DNA similarity values (the differences between the thermal denaturation midpoints of the homologous and heterologous duplexes) with reference strain A089T (T = type strain) ranged from 0 to 1.7 degrees C. The results of a 16S rRNA sequence analysis of A089T placed this strain in the genus Vibrio in the gamma subclass of the Proteobacteria. The closest relative is Vibrio aestuarianus, with a sequence similarity of 97.8%. This group of strains can be easily differentiated from the other Vibrio species by their clear phenotype. We propose the name Vibrio scophthalmi sp. nov. for these strains; the type strain is strain A089 (= CECT 4638).

The effect of a sewage treatment plant effluent on the faecal coliforms and enterococci populations of the reception river waters.

AIMS: A rural sewage treatment plant and the effect of its effluent on the enterococci and faecal coliforms populations of the receiving river waters was evaluated. METHODS AND RESULTS: The enumeration of bacteria was performed by membrane filtration. Diversity and population similarity were analysed using the PhP-plates system. The treatment plant reduces the number of enterococci and faecal coliforms to values similar to those observed upstream. All water samples showed a high diversity for both bacterial populations. A high similarity in the composition and structure, was detected among all the samples. CONCLUSIONS: The impact of the disposal of treated sewage on the river did not modify the composition of either bacterial populations in the river water. SIGNIFICANCE AND IMPACT OF THE STUDY: The biochemical phenotyping of bacterial populations is a reliable tool for ecological and biodiversity studies. The obtained results provide a better understanding of the sewage treatment process and the impact of the treated sewage effluents in the environment.

The composition and persistence of faecal coliforms and enterococcal populations in sewage treatment plants.

AIMS: The changes in structure and composition of faecal coliforms and enterococcal populations in sewage from different treatment plants, and the elimination of vancomycin- and erythromycin-resistant enterococci (VRE and ERE, respectively) in these treatment plants was analysed to determine any selective reduction. METHODS AND RESULTS: Faecal coliforms, enterococci, VRE, ERE and spores of sulphite-reducing bacteria were enumerated using standard methods. Samples were enriched where necessary in order to isolate antibiotic resistant strains. The structure and composition of these bacterial populations were determined by biochemical fingerprinting and clustering analysis. High diversity and similarity indexes were detected among all the bacterial populations in raw and treated sewage, independently of their origin and the treatment processes employed. Antibiotic resistant strains were detected in all sewage tested and no selective reduction was observed. CONCLUSIONS: The faecal coliforms and enterococci populations did not differ in the sewage samples studied. The vancomycin and erythromycin resistances of the enterococcal populations were similar in the sewage samples. Resistance to both antibiotics persisted after the treatment process independently of raw sewage flow, faecal origin or size of the human population contributing to sewage. However, sewage of mixed origin (human and animal) presented a lower similarity index for the two bacterial populations compared with that of the other human sewage analysed. SIGNIFICANCE AND IMPACT OF THE STUDY: Although a significant reduction in bacterial populations was observed, the persistence of VRE and ERE strains in the same proportions in sewage suggests that there is no selective elimination of bacterial populations during the treatment processes. The ability of antibiotic resistance strains to survive sewage treatment systems should be considered in certain water reuse programmes.