Human infections caused by STEC O157:H7 have particularly been distinguished to be originated from foods that come from animals. Particularly, cattle, sheep, and goats have been demonstraded as main natural reservoirs for STEC O157:H7 and play an important role in the public health concern (Atnafie et al. 2017).
The high morbidity of this serotype around the world has been focused as a major public health threat. It can cause acute human infections and outbreaks. The STEC O157 infection might involve abdominal pain, bloody diarrhea, hemorrhagic colitis (HC) and haemolytic uremic syndrome (HUS) (Zhang et al. 2006). The majority of E. coli O157 infections in human are food borne and concerning with cattle sources.
A total of 236 E. coli isolates were collected from faecal samples of healthy cattle in Tunisia during a five-month time period in 2017 and nine months in 2018, and were evaluated for the incidence of E. coli O157 and antimicrobial profiles. This is the first report concerning the presence of E. coli O157:H7 in cattle in Tunisia.
Our finding exhibited that among 236 E. coli isolates, ten E. coli O157:H7 were detected with a rate of 4.2 %. These isolates were cultured on CT-SMAC agar as non-sorbitol fermenters and were confirmed as STEC O157 by using latex agglutination and PCR. This is in agreement with other studies investigating E. coli O157:H7 among cattle feces samples and carcass swabs in slaughterhouses where the prevalences were reported as 4.7% and 2.7% respectively in Ethiopia (Atnafie et al. 2017). In a study in United Arab Emirates, the prevalence of E. coli O157:H7 among slaughtered cattle was 1.4% (Al-Ajmi et al. 2020). An Algerian study reported an occurence of E. coli O157 inmore than 7% of bovine carcasses (Chahed et al. 2006). In Morocco, the incidence of E. coli O157:H7 in dairy products and marketed meat products was 9.1% and 11.1% respectively (Benkerroum et al. 2004). In Tunisia, 327 E. coli strains were isolated from diarrheic and non-diarrheic people. By using PCR techniques it has been demonstraed that 11 isolates (3.4%) express the stx gene encoding for STEC (EHEC) and only one (0.3%) was confirmed as E. coli O157:H7 (Al-Gallas et al. 2006).
In Africa, the highest incidence in cattle was 31.2% representive in four studies. In Asian countries, the highest rates was 12.22% in Jordian cattle and the lowest (0.13%) was evaluated in Taiwan. In Europe, the highest estimated occurrence was demonstrated from Italy (10.45%) and the lowest from Norway (0.25%). Furthermore, the USA incidence estimate was 7.60% among fourty studies (Islam et al. 2014).
Healthy cattle can be a main reservoir for prospect human infection, and it plays an important role in the epidemiology of STEC infections. Moreover, most human diseases by STEC bacteria orgininate from cattle (Mead, Griffin 1998). The existence of STEC O157:H7 in our study among animal feces in slaughterhouses highlighted the possible contamination of meat products prepared for human consumption. On the other hand, identifying the STEC O157:H7 in human is very important for public health objective, like finding outbreaks.
Antimicrobial resistance is considered as a global health threat. Food animals products have been demonstrated as reservoirs of antimicrobial resistant bacteria because the same genes encoded for antimicrobial resistance were demonstrated in the bacteria of animal food and in humans (Founou et al. 2016).
Our results show that all E. coli O157:H7 isolates were susceptible to amoxicillin/clavulanic acid, cefotaxime, cefepime, aztreonam, colistin and sulfamethoxazole/trimethoprim. Previous studies in animals reported different antibiotics resistance profiles of E. coli O157:H7 isolates. One study found that all E. coli O157:H7 isolates were susceptible to cefotaxime, ceftriaxone, gentamycin, kanamycin and nalidixic acid (Atnafie et al. 2017). Further report showed that all isolates were susceptible to cefotaxime, chloramphenicol, ciprofloxacin, norfloxacin, and polymyxin B (Al-Ajmi et al. 2020). However, a Saudian study reported that the isolates were resistant to all used antibiotics (Al-Wabel 2007). One study in Iran revealed that resistance rate to gentamycin, ampicillin, erythromycin, amoxicillin and tetracycline was 56.0%, 48.0%, 40.0%, 16.0% and 12.0% respectively (Rahimi, Nayebpour 2012). A UK study in human showed that resistance profile among 327 STEC O157 to ampicillin, streptomycin, trimethoprim/sulphonamide and tetracycline was 5.8% followed by the resistance rate in ciprofloxacin (2.6%) and chloramphenicol (2.1%) (Day et al. 2016).
A study conducted in Latin American countries has documented 78.5% sensitivity to all the antimicrobial agents in 14 O157 STEC strains from cattle. Three strains were resistant to streptomycin, trimethoprim and sulfonamide (Bastos et al. 2006).
Antimicrobial resistance variation might be due to expression of resistance genes among bacteria in animals, environment or humans (Reuben, Owuna 2013).
On the other hand, more than 40% of the isolates were resistant to cefuroxime and streptomycin, perhaps via inappropriate or wide use of drug for prophylactic purpose and treating infections.
In our study, most strains exihibed an intermediate resistance pattern, suggesting the possibility for future resistance. The intermediate susceptibility profiles should be elevated and take in consideration with resistance results because it means the organism may be at the way to resistant.
Shiga toxins (stx genotypes) are important factors of the clinical outcome which correlate with HC and HUS and the pathogenicity higher in the strains harbouring stx2 genotype (Kawano et al. 2008). The eae gene encoding for an intimin protein, which is important for attaching and effacing activity in host intestinal cells and cause severe human illnesses particularly HUS (Cornick et al. 2002). Furthermore, a hemolysin produced by STEC called enterohemolysin is encoded by hlyA gene and cause erythrocyte lysis which participate in iron intake in the intestine. This gene is commonly used as epidemiological marker of STEC strains (Schwidder et al. 2019).
In this study, stx2 gene was present in most isolates, eae and ehxA were found in more than half of isolates. Many studies mentioned that virulence factors stx2 and eaeA are clinically significant and linked with the acuteness of human disease, particularly HUS (Friedrich et al. 2002,Beutin et al. 2004). In UAE, shiga toxin gene (stx2) were confirmed in all twenty four E. coli O157 from camels, cattle and goats. The eaeA and hlyA genes were present in 79.2% and 66.7% respectively,whereas stx1 was absent in all isolates (Al-Ajmi et al. 2020).
An Ethiopian study revealed that prevalence of stx1, eae, hly and stx2 among 157 isolates E. coli were 11 (78.5%), 6 (42.8%), 3 (21.4%) and 11 (78.5%) respectively (Atnafie et al. 2017).
Our study showed that 9 STEC strains harbored fimH and half isolates harbored sfa/focDE, cdt3, traT, and iutA. These factors were identified in a previous study among E. coli from dairy farms in America (Pereira et al. 2011). In an Iraian study of STEC, they found papA, cnf1, traT and cnf2 the highest virulence genes (Momtaz et al. 2012). The detected factors contribute to virulence which affect of host cell processes and contribute to bacterial pathogenesis. The findings of these virulence factors in our isolates in associated with high prevalence of stx1, stx2 and ehxA suggest that STEC O157 in Tunisian calves may pose a serious public health concern.
The findings of our study revealed that all E. coli O157 isolates belonged to phylogroup E. This was identical to the report of Tenaillon et al. (Tenaillon et al. 2010). A study in Brazil demonstrated that E. coli belonging to phylogroups E and B1 were isolated from cattle, wherease phylogroups A and F were from poultry and B2 and D were associated with isolates from water buffalo (Morcatti Coura et al. 2015).