Human E.coli O157:H7 infections primarily originate from animal food sources [20]. Specifically, domestic ruminants, including cattle, sheep and goats have been reported to be major natural reservoirs for E. coli O157 and contribute significantly to the epidemiology of human infections [21]. In the present study, we completed a survey on the occurrence and concentration of E. coli O157 in feces of meat animals from a slaughterhouse in Al Ain, UAE. We successfully identified E. coli O157 in 1.5%, 2% and 4.3% of cattle, goat and camel samples, respectively, with concentration ranges from too few to count (TFTC) to 4 × 104 CFUs/mL. Hence, our observed E.coli O157:H7 prevalence in the UAE slaughterhouse is in agreement with previous studies carried out in various countries like Ethiopia, South Africa, United Kingdom and Ireland where the prevalence of E. coli O157:H7 at abattoir level was reported as 2.7, 2.8, 2.9, 3.2 and 3.0% [22, 23, 24, 25, 26]. Further, the highest prevalence of E. coli O157 (4.3%) was identified in camel feces compared to the other animals, which was similar to reports carried out in neighboring countries including Riyadh, Saudi Arabia, where the prevalence in camel feces was 2.4% [27]. Alternatively, our reported prevalence was found to be slightly lower compared to those reported in Ethiopia (8%) and Iran (6.4–9.6%) [28, 29]. Moreover, reports from Qatar showed that camel fecal samples contained E. coli O157 at a ten times higher prevalence than that of cattle [30]. Interestingly, in Iraq, Al-Gburi (2016) [31] reported the highest number of camels infected with E. coli O157 (19%), however, these isolates were found to be multi-drug resistant.
Previous studies carried out in UAE camels were unable to isolate any E.coli O157. Furthermore, a study conducted by Moore et al [32] on racing camel calves failed to detect E. coli O157 from UAE, with similar results obtained by El-Sayed et al. [33] when large herds of camels were investigated. However, this discrepancy with our results may be due to the absence of a reliable ‘‘gold standard’’ for detecting the presence of STEC O157 in samples as well as the inherent difficulty associated with direct comparison of results between the methods employed in this study and those obtained using other culture techniques. The results from these other studies were primarily based on characterization of very few colonies picked from the sample while lacking a specific screening and isolation technique. Alternatively, rather than using a direct culturing method, we employed enriched buffered peptone water (BPW) followed by immunomagnetic separation of E. coli O157 (IMS) and culturing on cefixime-tellurite sorbitol MacConkey (CT-SMAC) agar to improve isolation efficiency of E. coli O157 from fecal samples [34, 35]. All isolates were confirmed as E. coli O157 using the latex agglutination test as well as via PCR techniques.
Conversely, from sheep fecal samples, we were unable to identify E. coli O157 strains. Similar results were published by Alhelfi et al. [36] where no E. coli O157 was detected in either cattle or sheep fecal samples within the target area studied in UK. Failure to detect positive samples may be indicative of several factor, study design, sample size, geographical origin, age and sex of animals and/or abattoir conditions, animal husbandry as well as diet have been shown to impact prevalence rates within livestock [37–42]. In our study, save for the age and animal breeds, we did not have access to sufficient information regarding the local sheep population including husbandry and diet history. Hence, additional studies on larger herds, using a meta-analysis approach are required to confirm the absence of E.coli O157 in local sheep samples.
Although previous studies have reported cattle to be the most common E.coli O157 reservoir [43], in our study we only observed a prevalence of approximately 1.5%. Alternatively reports carried out in Riyad, Saudi Arabia, determined that 10.7% of cattle feces samples contained E.coli O157:H7. Moreover, in comparison with other countries in Asia, cattle in Jordan had the highest prevalence of E.coli O157:H7 (12.22%) [44], whereas the lowest was reported in Taiwan (0.13%) [45]. Other studies from across the world have reported the prevalence rate of E.coli O157:H7 in cattle fecal samples to range from 2.4–24% [46–48].
We also examined whether there was seasonal variation in E. coli O157 prevalence, as seasonal variations in cattle and meat products have previously been reported to affect human E.coli O157 infections [49–51]. Specifically, cattle feces have been reported to have low prevalence in winter and higher prevalence in spring reaching peak levels in summer [52]. The warmer summer months may provide more suitable environments outside of the host in soil, feed, and water for E. coli O157:H7, resulting in a continual source of infection or re-infection for cattle populations. Al Ain, UAE is located in a tropical dry area where the average temperature during different seasons can serve to enhance bacterial growth. In fact, studies have shown that the prevalence of E. coli O157 increased during summer months and declined in the winter as summer temperatures provided favorable conditions for bacteria to survive and potentially multiply [53, 54]. Our results showed that the highest prevalence occurred in spring and early summer months (February to April) which was likely due to the intense heat during summer months in UAE when daytime temperatures may reach as high as 50 °C which causes E. coli 0157:H7 prevalence to decline due to its inability to persist in the extreme environment. These results are in agreement with previous studies carried out in Riyadh, Saudi Arabia, where the highest prevalence was reported in spring and early autumn months [27]. In fact we were unable to detect E. coli O157:H7 in any other months as was also reported in other parts of the world.
The concentration of E.coli O157 within the animal fecal samples were also quantified. In camels the CFU ranged from TFTC to 4 × 104 CFU/mL, whereas goat and cattle samples contained 4 × 104 CFU/mL and 2 × 103 CFU/mL bacteria, respectively. These results demonstrate that the animals containing E. coli O157 were considered super shedders and they maybe colonized with E. coli O157. These high levels of fecal E. coli O157 would cause the carcasses of these animals to be considered a high risk for contamination if the proper precautions were not taken. Previous work carried out in Scottish slaughterhouses reported animals shedding > 104 CFU/g; these animals accounted for > 96% of the bacteria that were shed by all animals tested [55]. These findings underscore that the presence of high-shedding animals in a herd could prove to be more important than the prevalence of colonization in the entire cattle population [55].
Shiga toxins are associated with HC and HUS, while intimin is responsible for attaching/effacing (A/E) lesions on intestinal epithelial cells. As such enterohemolysins have been proposed as potential epidemiological markers for STEC strains [56]. Intimin, encoded by the eaeA gene, adheres to intestinal mucosa and causes formation of intestinal lesions [57, 58]. In addition, enterohemolysin, encoded by hlyA, leads to lysis of erythrocytes, which may contribute to the iron intake of bacterium residing in the intestine [59]. Previous studies have reported that E. coli O157:H7/H- strains isolated from the feces of slaughtered ruminants exhibited Stx2 gene prominence over the Stx1 gene [39, 42, 45, 60, 61]. In our study, Stx1 was absent in all the strains irrespective of the species. Various studies have found that Stx2 and eaeA are clinically important virulence genes; in fact carriage of these genes have been shown to be associated with the severity of human disease, especially for HUS [62, 63]. Hence, in this study E. coli O157 strains harboring the major virulence genes may be considered to be more virulent to humans than those without. Nevertheless, it has been reported that the production of major virulence genes was not essential for pathogenesis, as a number of sporadic cases of HUS were induced by Stx and eaeA-negative strains [64–66].
Antimicrobial resistance has been recognized as a global health issue for many decades. Food animals are considered to be key reservoirs of antibiotic resistant bacteria as certain antibiotic resistance genes identified in the bacteria of animal food products have also been identified in humans [67]. However, effectively treating E. coli O157:H7/H- infections is challenging due to differing opinions presented by various investigators [68–70]. As a result, many studies have reported increasing incidence of multi-drug resistant E. coli O157:H7/H- strains isolated from the feces of slaughtered ruminants [42, 71–73].
Our results show susceptibility of all isolated strains for three antibiotics i.e., norfloxacin, chloramphenicol and polymyxin B. Further, cefotaxime and ciprofloxacin only elicited a significant effect on some of the E.coli O157 strains and only intermediate effects on others. This is in agreement with previous reports on antimicrobial resistance patterns of E. coli O157:H7 isolates from animal and human sources [74–76]. Further, previous studies on antibiotic resistance of bacteria in the feces of slaughtered ruminants have reported that isolates showed higher resistance or multiple resistance [42, 72, 73]. In fact, a study found that gentamycin resistance was the most common (56.0%), followed by ampicillin (48.0%), erythromycin (40.0%), amoxicillin (16.0%), tetracycline (12.0%), chloramphenicol (8.0%), nalidixic acid (8.0%) and streptomycin (4.0%) [72]. Although studies concerning antibiotic resistance of E. coli O157 strains isolated from ruminant feces samples are limited in the UAE geographical variations, testing of only a small proportion of a study population for susceptibility, variability in resistant genes within isolates for particular antimicrobials, as well as differences in preferred antibiotics and the origins of strains, may all impact observed resistance or even conclude higher resistance rates.
Recently we conducted a whole genome sequencing analysis of E.coli O157:H7 isolates from camel feces and deposited the results in GenBank [77]. These sequences allow for potential identification of 5,444,610 bp containing 5,399 coding sequences (CDSs) from these isolates. The data obtained from whole genome analysis will advance the current understanding regarding the evolution of these particular strains in relation to other isolates, thereby serving to improve our understanding of these specific pathogenic strains.