E. coli as an important zoonotic pathogen

Most of the pathogens that cause foodborne diseases are referred to as emerging pathogens, including E. coli. In fact, E. coli manifest an excellent ability to adapt to variable environmental niches (Milanov et al., Reference MILANOV, PRUNIĆ, VELHNER, TODOROVIĆ and POLAČEK2015). The emergence of some pathogens could be explained by the fact that detection methods and techniques have been improved and that the market of fresh, frozen and ready-to-eat foods has increased that has led to a worldwide availability of high-risk food (EFSA, 2011).

E. coli are genetically highly adaptable bacterial species, which can cause a range of diseases in both humans and animals. Virulent strains of E. coli cause a series of human diseases, such as gastroenteritis, urinary tract infection and neonatal meningitis as well as haemolytic uremic syndrome, peritonises, mastitis, and septicaemia in rare cases (Ewers et al., Reference EWERS, JANßEN, KIEßLING, PHILIPP and WIELER2004). E. coli is a commensal bacteria in the intestinal tract of the chicken, but some serotypes are pathogenic and the main route for their entry is the respiratory tract after inhalation of dust contaminated with faeces, where pathogenic E. coli target the air sac system and cause systemic infection of vital organs. The intestine of poultry is the most important reservoir of avian pathogenic E. coli and probably the most frequent and economically significant cause of bacterial diseases, particularly colisepticaemia, in broiler chickens (Edelman et al., Reference EDELMAN, LESKELÄ, RON, APAJALAHTI and KORHONEN2003). This reservoir of E. coli has a zoonotic potential that could be transferred directly from poultry to humans (Ewers et al., Reference EWERS, ANTÃO, DIEHL, PHILIPP and WIELER2009). The use of antibiotics in poultry feed may modify the intestinal flora by favouring resistant E. coli growth, that could then find their way into the environment and food chain (Diarra et al., Reference DIARRA, SILVERSIDES, DIARRASSOUBA, PRITCHARD, MASSON, BROUSSEAU, BONNET, DELAQUIS, BACH, SKURA and TOPP2007). It is important to understand that most of the antibiotics used in poultry feed are not fully absorbed in the chicken gut and up to 90% of the administered dose can be excreted in the faeces (Kumar et al., Reference KUMAR, GUPTA, CHANDER and SINGH2005).

Poultry litter as a source of pathogenic and multi-resistant strains of E. coli

Poultry litter represents a mixture of poultry faeces, feed, contaminated water and different bedding materials and other materials which accumulate during farming. Poultry litter may become a potential source of infectious agents and E. coli is used as an indicator bacterium to detect their possible presence (McCrea et al., Reference MCCREA, MACKLIN, NORTON, HESS and BILGILI2008). Poultry litter is commonly used as organic fertiliser and it can be also be used as a fuel (Kelleher et al., Reference KELLEHER, LEAHY, HENIHAN, O'DWYER, SUTTON and LEAHY2002). Instead of serving as a problem of waste, poultry litter can now be used as a source of energy and nutrients, so it is a useful by-product of poultry farming. However there is a need to examine its microbiological safety before further use.

Antibiotics have been, and are sometimes still used as growth promoters on poultry farms, so it is not surprising that resistance of E. coli to antibiotics has spread (Velhner and Milanov, Reference VELHNER and MILANOV2015). It is known that the use of large amounts of antibiotics in poultry farming may result in an increased amount of resistant bacteria in poultry, their excreta and consequently in the litter and environment (Heuer and Smalla, Reference HEUER and SMALLA2007). Khan et al. (Reference KHAN, NAWAZ, WEST, KHAN and LIN2005) and Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) detected antibiotic resistant isolates of E. coli in poultry litter and in the production environment while Van den Bogaard et al. (Reference VAN DEN BOGAARD, LONDON, DRIESSEN and STOBBERINGH2001), Sáenz et al. (Reference SÁENZ, ZARAZAGA, BRIÑAS, LANTERO, RUIZ-LARREA and TORRES2001) and Johnson et al. (Reference JOHNSON, MURRAY, GAJEWSKI, SULLIVAN, SNIPPES, KUSKOWSKI and SMITH2003) isolated antibiotic resistance E. coli in poultry products.

Poultry and their production environments are reservoirs of both resistant bacteria and resistance genes that could be transferred to humans by direct contact between poultry and humans, indirectly via the food chain or as a result of fertiliser use (Marshall and Levy, Reference MARSHALL and LEVY2011). The results reported by Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) suggest that poultry litter could enhance the spread of antibiotic resistances in soil bacterial communities and could be a hot spot for antibiotic resistance genes from mobile genetic elements. When poultry litter which contains antibiotic residues and resistant bacteria strains is used as fertiliser, the level and effects of these antibiotics and antimicrobial-resistant bacteria in the environment are of significance public health (Furtula et al., Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010). The epidemiological importance of poultry litter is well documented in a number of research papers (Nandi et al., Reference NANDI, MAURER, HOFACRE and SUMMERS2004; Islam et al., Reference ISLAM, SULTANA, DAS, SHARMIN and HASAN2008; Adelowo et al., Reference ADELOWO, OJO and FAGADE2009; Reference ADELOWO, FAGADE and AGERSØ2014) and the presence of veterinary antibiotics in the environment has been documented.

Antibiotic resistance in E. coli isolated from poultry litter

Nandi et al. (Reference NANDI, MAURER, HOFACRE and SUMMERS2004), Khan et al. (Reference KHAN, NAWAZ, WEST, KHAN and LIN2005) and Islam et al. (Reference ISLAM, SULTANA, DAS, SHARMIN and HASAN2008) isolated antibiotic resistant E. coli from poultry waste and litter and farm environments. The tested pathogens were found to be resistant to multiple antibiotics. Nandi et al. (Reference NANDI, MAURER, HOFACRE and SUMMERS2004) quantified antibiotic resistance genes and mobile elements called integrons in poultry litter from commercial poultry farms using traditional cultivation as well as independent molecular techniques. Unexpectedly, they found that the major reservoir for Class 1 integrons in poultry litter is not Gram-negative Enterobacteriaceae such as E. coli. Contrary, integrons and associated resistance genes exist in large amounts in several genera of Gram-positive bacteria that constitute >85% of the litter community compared with Enterobacteriaceae that comprise <2% of this ecosystem. They sampled litter from two sequential flocks in a single house at each farm over a 13-week period during the summer. In order to prevent E. coli peritonitis, gentamicin was administered either in ovo or by injection of the chickens just before placement in the broiler house. Other antibiotics, antibacterials, and coccidiostats were used to prevent necrotic enteritis. The therapeutic use of antibiotics was not required on farms.

Khan et al. (Reference KHAN, NAWAZ, WEST, KHAN and LIN2005) isolated nineteen fluoroquinolone-resistant E. coli isolates in 2002 and 2003 from 200 litter samples collected from chicken and turkey farms in Arkansas and found that all isolates were resistant to multiple antibiotics. The majority of strains were resistant to gentamicin, kanamycin, chloramphenicol and streptomycin. Their findings suggested that individual development of resistance occurs in most isolates rather than clonal selection by horizontal transfer. The authors concluded that fluoroquinolone-resistant E. coli isolated from poultry litter belong to different serotypes and ribogroups.

Islam et al. (Reference ISLAM, SULTANA, DAS, SHARMIN and HASAN2008) conducted their research in Chittagong University, Bangladesh to investigate the relationship between the use of antibiotics and the development of resistance. They isolated 30 strains of E. coli from poultry litter from Denm poultry farm, Chittagong, Bangladesh. The authors analysed susceptibility patterns to seven antimicrobial agents of the E. coli strains, by agar disc diffusion method. Results showed that all of the isolates were multi-drug resistant, and the resistance to tetracycline (96.6%), ciprofloxacin (30%), penicillin (100%), erythromycin (66.6%), gentamycin (50%) and chloramphenicol (100%) was the most abundant. All isolates were sensitive to imipenem. Their research highlighted that resistance development is directly related to the use of antibiotics.

Adelowo et al. (Reference ADELOWO, OJO and FAGADE2009) stated that antimicrobial agents are routinely used in livestock production in Nigeria. Results of their study showed that poultry waste can serve as environmental reservoirs of multiple antibiotic resistant bacteria and their spreading in the environment presented major health risks for the human population. They collected faecal material and poultry litter from the poultry farms. Most of the isolated organisms including E. coli were of public health importance and showed multiple resistances to the tested antimicrobial agents (nalidixic acid, ampicillin, colistin, gentamicin, nitrofurantoin, cotrimoxazole, streptomycin, and tetracycline) which were chosen for the susceptibility test because they are important in livestock production or in human clinical medicine in Nigeria. Resistance of the bacteria isolated from the poultry waste samples ranged between 0% for gentamicin to 100% for tetracycline and ampicillin. High level of resistances were observed for nalidixic acid, nitrofurantoin, cotrimoxazole, streptomycin, ampicillin and tetracycline. The results obtained by Adelowo et al. (Reference ADELOWO, OJO and FAGADE2009) showed that poultry litter could be an environmental reservoir of multiple antibiotics resistant bacteria, including E. coli and consequently it have a potential for spreading of multidrug resistant zoonotic pathogens.

Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) investigated poultry litter collected from controlled feeding trials and from commercial farms for the presence of antibiotics and resistance patterns of E. coli. Their research confirmed the presence of antibiotic residues in broiler litter from controlled environments as well as commercial farms, noting that the levels of individual residues were higher in litter from controlled feeding trials than those from commercial farms. Moreover, all E. coli isolates from commercial farms were multiresistant to at least seven antibiotics. Resistance to β-lactam antibiotics (amoxicillin, ceftiofur), tetracyclines, and sulphonamides was the most prevalent. The authors concluded that broiler litter represents a significant source of antimicrobial residues and represents a reservoir of multiple antibiotic-resistant E. coli. Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) provided additional information on potential environmental and public health risks associated with the use of untreated poultry litter using the combination of antimicrobial residues analysis and the determination of antimicrobial-susceptible E. coli. The authors proved that the presence of the antibiotics in poultry litter followed the pattern of their addition to the feed. Furthermore, they suggested that it could be expected that the concentrations of antimicrobials found in the litter could persist in the environment. They also stated that, in the case of applying poultry litter to crops as fertiliser, the number of localised areas with very high concentrations of antibiotics would increase, exposing soil microbes to veterinary antibiotics. The presence of antibiotics, as well as the multiresistant strains of E. coli in the litter increases the need for the development of better management practices and regulations for the treatment of poultry litter before it is used as fertiliser.

Adelowo et al. (Reference ADELOWO, FAGADE and AGERSØ2014) investigated the mechanisms of resistance in 36 E. coli isolated from waste, litter, soil and water samples collected from poultry farms in Southwestern Nigeria. Results of their investigation showed that resistance to tetracycline was 81%, sulphamethoxazole 67%, streptomycin 56%, trimethoprim 47%, ciprofloxacin 42%, ampicillin 36%, spectinomycin 28%, nalidixic acid 25%, chloramphenicol 22%, neomycin 14% and resistance to gentamicin was 8%. All the isolates were sensitive to amoxicillin-clavulanate, ceftiofur, cefotaxime, colistin, florfenicol and apramycin. Their trial was not designed to investigate the relationship between antibiotic use and resistance on the study farms due primarily to the non-availability of antibiotic-use information, but the authors noted that tetracycline, streptomycin, sulphonamides, and ampicillin have been most likely used on all the study farms. The authors indicated that their investigation did not include isolates from farms without antimicrobial use, and consequently it was difficult to make a definite conclusion on the relationship between antibiotic use on the farms and the occurrence of resistance in the isolates. Their findings showed that the poultry farms in developing countries could be an important reservoir of antibiotic resistance genes that may spread from poultry farms to humans via manure and water. They highlighted the importance of proper regulation of antimicrobial usage in poultry production systems. The results obtained in the study conducted by Adelowo et al. (Reference ADELOWO, FAGADE and AGERSØ2014) emphasised the significance of routine screening for antibiotic resistance in commensal bacteria from poultry farms using both phenotypic and genotypic methods, especially in developing countries which have limited studies on occurrence of resistance genes in food animal production systems.

The few studies that have investigated antibiotic resistance in E. coli from poultry production in the Republic of Serbia stopped at the phenotypic level without a corresponding investigation of the molecular mechanisms of resistance (Krnjaić et al., Reference KRNJAIĆ, MIŠIĆ and AŠANIN2005; Filipović et al., Reference FILIPOVIĆ, MIŠIĆ and AŠANIN2007; Knežević and Petrović, Reference KNEŽEVIĆ and PETROVIĆ2008). There are currently no reports from investigations of the E. coli which originated from the poultry litter.

Krnjaić et al. (Reference KRNJAIĆ, MIŠIĆ and AŠANIN2005) carried out sampling on 42 cattle, pig and poultry farms. The samples were faeces from young healthy, young unhealthy and adult individuals or internal organs, cloacal swabs and eggs in case of dead animals. Prevalence of resistance of the isolated E. coli strains isolated from healthy chicken showed a 100% resistance to tetracycline, 95% to ampicillin, 95% to streptomycin, 70% to sulphemethosazol, 60% to trimethoprim, 60% to sulphamethosazol-trimethoprim, 60% to neomycin, 50% to nalidixic acid, 45% to amoxicillin-clavulanate, 20% to cephalexin, 15% to spectinomycin, 15% to cypropfoxacin/enropfoxacin and 5% to gentamycin. No resistance was observed to cephachlor, cephtriaxon, chloramphenicol and colistin. The level of multi-resistant strains in chickens was 85%. In the case of dead chickens, the isolated E. coli strains showed a 95% resistance to streptomycin, 95% to sulphemethosazol, 90% to ampicillin, 90% to amoxicillin-clavulanate, 85% to tetracycline, 85% to trimethoprim, 85% to sulphamethosazol-trimetoprim, 45% to nalidixic acid, 40% to neomycin, 35% to spectinomycin, 35% to cypropfoxacin/enropfoxacin, 30% to chloramphenicol, 25% to cephalexin, 10% to cephachlor and 10% to gentamycin. No resistance was found to cephtriaxon and colistin. The multiple resistance to antibiotics was found out in 95% of E. coli strains originating from the dead chickens. The prevalence of resistance of the E. coli strains from hens was 75% to tetracycline, 75% to streptomycin, 50% to neomycin, 40% to ampicillin, 30% to sulphemethosazol, 30% to nalidixic acid, 15% to spectinomycin, 10% to amoxicillin-clavulanate, 5% to chloramphenicol and 5% to cephalexin. Isolates were sensitive to cephachlor, cephtriaxon, colistin, gentamycin, cypropfoxacin/enropfoxacin, trimethoprim and sulphamethosazol-trimethoprim. The level of multi resistant strains from hens was 45%.

Knežević and Petrović (Reference KNEŽEVIĆ and PETROVIĆ2008) examined the resistance in E. coli from three farms from Vojvodina and they chose commensal non-pathogenic E. coli because it represents a good indicator of commensal resistance and as an opportunistic pathogens may spread resistance genes by vertical gene transfer to other bacteria, including pathogens. They took sixty rectal or cloacal swabs from randomly selected animals settled in different objects without direct contact: 10 dairy cows, 10 calves, 10 sows, 10 pigs and 20 broilers (aged six week old) from three different farms. They reported that the antibiotics which were regularly used on the cattle farms including penicillin G, ampicillin, tetracycline and dihydrostreptomycin for therapy of mastitis. Also, ampicillin, neomycin and tetracycline were used on the swine farms for therapy purposes. The sows and the broilers were not treated with antibiotics. Among broilers E. coli isolates the 94.7% isolates were resistant to tetracycline, 44.7% isolates were resistant to ampicillin, 39.5% to streptomycin, 26.3% to kanamycin, 21.1% to neomycin, 21.1% of isolates were resistant to cefalothin, 21.1% to trimetoprim/sulphamethoxazole, 15.8% to amoxicillin-clavulanate, 15.8% to nalidixic acid, and 7.9% to ciprofloxacin. No resistance was observed for ceftazidime, cefotaxime, ceftriaxone, cefoxitin, gentamicin, amikacin, netilmicin, tobramycin. Among the isolates originated from broilers 36.8% were resistant to one antibiotic, 23.7% to two antibiotics; 10.5% to three; 23.7% to four; 5.3% to five, so the prevalence of multiple resistance among E. coli isolates from broiler cloacal swabs were 63.2%. The authors stated that the high resistance rate to tetracycline of broiler isolates is not associated with its usage. The authors found that commensals from these animals, including broilers from Vojvodina region, were important reservoirs of resistance to older-generation antibiotics.

Filipović et al. (Reference FILIPOVIĆ, MIŠIĆ and AŠANIN2007) investigated the presence of extended spectrum beta-lactamases in multi-resistant strains of E. coli and Salmonella species from dogs, cats, cattle, sheep, goats, pigs and poultry. The examined strains were isolated from faecal, egg and eggshell swabs from healthy and diseased poultry of various ages and breed categories. In multi-resistant E. coli strains originating from poultry, no resistance to amoxicillin with clavulanic acid was found. Gavrović et al. (Reference GAVROVIĆ, AŠANIN, MIŠIĆ, JEZDIMIROVIĆ and ŽUTIĆ2011) investigated samples of internal organs, faeces and swabs from ill animals (cattle, pigs, poultry, dogs and cats) and found that the E. coli isolates were resistant to all tested antibiotics with the exception of ceftriaxone and florfenicol. The incidence of resistance to tetracycline in E. coli isolated from poultry was 56%, while the incidence of resistance to ampicillin in E. coli strains originated from poultry was 78%. In Serbia, there is no systematic monitoring programme and hence no data on antibiotic resistance for isolates originated from the poultry litter.

From our own work (unpublished) the situation in the two poultry farms was examined and there were found a large number of multi-resistant strains of E. coli where the improper and continuous use of antibiotics is common practice. However, on a farm that did not use antibiotics, but rather applied herbal products which have antimicrobial activity, resistance of E. coli to cephalosporins - cefpodoxime, ceftazidime and cefotaxime in one isolate only was observed and it was suspected that such resistance may have been transferred from farm workers.

The relationship between antibiotic use on farms and the occurrence of resistance in the isolates

The microbial ecosystem of poultry litter is influenced by management practices including feeding and antibiotic use on the poultry farms. Baurhoo et al. (Reference BAURHOO, PHILLIP and RUIZ-FERIA2007) reported that the addition of purified lignin and mannan oligosaccharides to broiler feed decreased the number of E. coli in the litter. Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) explained the high resistance to tetracycline which was found in poultry litter in the majority of the studies by the common and long term use of this antibiotic (Chander et al., Reference CHANDER, GUPTA, KUMAR, GOYAL and MURRAY2008; Islam et al., Reference ISLAM, SULTANA, DAS, SHARMIN and HASAN2008; Thibodeau et al., Reference THIBODEAU, QUESSY, GUÉVREMONT, HOUDE, TOPP, DIARRA and LETELLIER2008; Adelowo et al., Reference ADELOWO, OJO and FAGADE2009; Reference ADELOWO, FAGADE and AGERSØ2014). Diarra et al. (Reference DIARRA, SILVERSIDES, DIARRASSOUBA, PRITCHARD, MASSON, BROUSSEAU, BONNET, DELAQUIS, BACH, SKURA and TOPP2007) reported that multidrug-resistant E. coli was found in the chicken digestive tract and that feed supplementation with certain antibiotics like salinomycin or bacitracin could influence the distribution of the resistance phenotype in E. coli.

The high level of resistance to sulphonamides in a number of studies (Furtula et al., Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010; Adelowo et al., Reference ADELOWO, FAGADE and AGERSØ2014) could be explained by the fact that sulphonamides degrade very slowly in the environment (García-Galán et al., Reference GARCÍA-GALÁN, DÍAZ-CRUZ and BARCELÓ2008). Diarrassouba et al. (Reference DIARRASSOUBA, DIARRA, BACH, DELAQUIS, PRITCHARD, TOPP and SKURA2007) suggested that sulphonamide resistance genes have been maintained due to their association with other resistance genes. This theory is supported by the fact that each of the isolates in the research conducted by Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) that were resistant to sulphonamides were also resistant to amoxicillin and tetracycline.

The observed increase in resistance to trimethoprim-sulphamethoxazole on commercial farms in the research conducted by Chander et al. (Reference CHANDER, GUPTA, KUMAR, GOYAL and MURRAY2008), Thibodeau et al. (Reference THIBODEAU, QUESSY, GUÉVREMONT, HOUDE, TOPP, DIARRA and LETELLIER2008), Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) and Adelowo et al. (Reference ADELOWO, FAGADE and AGERSØ2014) is of concern because this is the drug of choice for treating urinary tract infections caused by E. coli in humans (Karlowsky et al., Reference KARLOWSKY, KELLY, THORNSBERRY, JONES and SAHM2002). Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) noted that the resistance to ceftiofur found in the isolates was surprising and of important concern because ceftiofur is an important cephalosporin antibiotic for therapy in veterinary medicine and for the therapy of salmonellosis in children (Alcaine et al., Reference ALCAINE, SUKHNANAND, WARNICK, SU, MCGANN, MCDONOUGH and WIEDMANN2005).

The higher resistance to amoxicillin, spectinomycin, streptomycin, and sulphonamides found by Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) in isolates from commercial farms compared with those from the controlled feeding trials imply that the use of commercially available feeds containing multiple antibiotics could contribute to the increase in resistance. The high resistance to the antibiotics in isolates of E. coli from commercial farms were also observed by Islam et al. (Reference ISLAM, SULTANA, DAS, SHARMIN and HASAN2008) and Adelowo et al. (Reference ADELOWO, OJO and FAGADE2009, Reference ADELOWO, FAGADE and AGERSØ2014).

The additional resistance to amoxicillin, ceftiofur, gentamicin, spectinomycin, streptomycin, and sulphonamides found by Furtula et al. (Reference FURTULA, FARRELL, DIARRASSOUBA, REMPEL, PRITCHARD and DIARRA2010) in some of the isolates in poultry that were not provided with the medicated feed implied that E. coli could colonise in the chickens from birth. Keyes et al. (Reference KEYES, HUDSON, MAURER, THAYER, WHITE and LEE2000) discovered that E. coli originating from poultry was resistant to florfenicol, despite the fact that this antibiotic has never been used in these species. Chander et al. (Reference CHANDER, GUPTA, KUMAR, GOYAL and MURRAY2008) and Thibodeau et al. (Reference THIBODEAU, QUESSY, GUÉVREMONT, HOUDE, TOPP, DIARRA and LETELLIER2008) found resistance to antibiotics, including trimethoprim-sulphamethoxazole, amoxicillin, streptomycin, gentamycin, and tetracycline, in E. coli isolated from broilers in Canada and the United States, even when these flocks were not treated with antibiotics. Knežević and Petrović (Reference KNEŽEVIĆ and PETROVIĆ2008) reported resistance in broilers E. coli isolates to tetracycline, ampicillin, streptomycin, kanamycin, neomycin, cefalothin, trimetoprim/sulphamethoxazole, amoxicillin-clavulanate, nalidixic acid and ciprofloxacin from farm where no antibiotics were used during broilers rearing. It is known that microorganisms may acquire resistance mechanisms prior to application of antibiotics, but their use has caused the changes in the genetic structure of bacteria in order to adapt to a new conditions (Velhner et al., Reference VELHNER, POTKONJAK, STOJANOVIĆ, MITEVSKI, STOJANOV and PETROVIĆ2013). Regular use of antibiotics have contributed to maintaining resistant bacteria in the environment and they are able to survive in the presence of antibiotics in different ecological niches and to transfer mobile genetic elements to related species. According to Blake et al. (Reference BLAKE, HUMPHRY, SCOTT, HILLMAN, FENLON and LOW2003) transfer of plasmids between different genera of Enterobacteriaceae occurs readily in the intestine of humans and animals and stress such as antimicrobial treatment, can increase plasmid mobilisation and transfer.

Management practices on poultry farms and monitoring programmes

Nowadays, urbanised areas are relatively close to poultry farms presenting increasing public health concerns. Although management efficiency in poultry production is improving in developing countries, insufficient attention is paid to the management and utilisation of waste which may lead to environmental pollution. Slogans such as ‘from field to fork’, ‘from farm to table’, ‘from plow to plate’ and similar are used in the food industry in order to emphasise the fact that food safety begins on the field and farm. The technological advances with the aim to control potential foodborne pathogens include the development of antimicrobial products, vaccines and improved methods for the detection of infection or contamination in the animals and their environment. Biosecurity on farms should include measures which prevent recirculation of pathogens between the animals, animal facilities and environment. Infection of poultry does not always have to lead to a disease that would be clinically manifested. Asymptomatic carriers lead to the amplification of pathogenic agents which are further returned to the environment in a cyclic manner. Many producers and end users are not even aware that poultry litter contains pathogens. It is necessary to encourage processing and management practices in order to eliminate pathogens from poultry litter before further use. Briefly, risk management strategies on poultry farms include: prevention of recirculation of pathogens throughout management of soil (animal waste, irrigation), control of vectors (workers, rats, wild animals), good farming practice (GFP), good hygienic practices (GHP) and the cleanliness of animals; prevention of ingestion of pathogens throughout treatments of feed, treatments of drinking water, control of interactions between animals; suppression of ingested pathogens by manipulations of feed, use of probiotics, prebiotics, etc.; modifications of the immune response of animals by vaccination (Velhner et al., Reference VELHNER, POTKONJAK, STOJANOVIĆ, MITEVSKI, STOJANOV and PETROVIĆ2013).

The criteria for setting priorities in monitoring programmes should be defined by the principles set out in the directive on zoonosis (2003/99/EC). According to article 4 of the directive, the following criteria should be taken into account: occurrence or presence of pathogens in the population of animals and humans, feed and food; the gravity of their effects for humans; economic consequences as consequence of the protection of human and animal health, as well as economic consequences for feed and food manufacturers; and epidemiological trends in animal and human populations, feed and food.

Acknowledgments

This work was financially supported by a grant from the Ministry of Education, Science and Technological Development, Republic of Serbia, Project number TR 31071.