The prevalence of four important members of the Enterobacteriaceae family in this study followed the order E. coli (45%), Enterobacter spp. (24%), Citrobacter spp. (20%) and Klebsiella spp. (11%). The occurrence of these organisms in the different samples suggests exposure to faecal contamination, and may also indicate the possible presence of other pathogens such as Vibrio spp. and Campylobacter spp. . The occurrence of the Enterobacteriaceae in the samples raise a particular concern as these microorganisms have a potential for causing severe infections such as diarrhoea, pneumonia, urinary tract infection, meningitis and many more . Also, the occurrence of the organisms in vegetable samples presents a problem to public health because some of these fresh produce are usually consumed with little or no cooking. The presence of Enterobacteriaceae in rivers and hospital effluents is worrisome. The bacteria could persist and subsequently be transferred along the human food chain . Furthermore, the presence of these microbes in river water might be due to the discharge of untreated hospital sewage systems , or discharge of agricultural, animal, domestic, human and industrial wastes [24, 25].
Several factors are driving the contamination of fresh produce by Enterobacteriaceae. Sources of contamination in fresh produce might include application of animal manure, direct contact with infected worker during harvesting, unhygienic handling and processing. Also, untreated irrigation water may contribute to contamination of vegetables , and transportation vehicle have also been implicated as a source of contamination of vegetables by microorganisms. Also, cultivation of fresh produce on contaminated soil could lead to contamination of the produce .
A relatively large number of the isolates recovered in this study exhibited resistance against one or more test antibiotics, suggesting high frequency of antimicrobial resistance in the host communities. All the bacterial isolates showed high frequency of resistance against ampicillin, tetracyclines, beta-lactams and sulphonamides (Fig. 2). Tetracycline resistance has been frequently reported from environmental samples [28, 29, 30], and could be due to the abuse of tetracycline in animal feeds as a growth promoter .
Also, there is high frequency of resistance against ampicillin, tetracycline, cefuroxime and cefotaxime in all the four bacterial types and corroborates reports from previous studies [32, 33, 34]. High susceptibility of Klebsiella spp. (83%) against imipenem was observed in this present study, which is contrary to the report of  who reported high resistance of Klebsiella spp. against imipenem, colistin and polymyxin B. These antimicrobials are recommended to be antimicrobials of last resort for treating bacterial infections. Also,  reported that all their E. coli isolates were resistant against colistin. According to , for many decades, colistin was not recommended for use due to its toxicity and the availability of other safer antimicrobials such as penicillins. However, the use of colistin has now been included in therapeutic options due to the rise in antimicrobial resistance.
The health risk associated with the transmission of antimicrobial resistance in the environment was evaluated using multiple antibiotic resistance index (MARI). MARI values ranged between 0.16 and 0.94 (Table 1). The MARI value greater than 0.2 suggests that isolates are recovered from an environment with high usage of antibiotics . About 97% of the test bacterial isolates had a MARI estimate greater than 0.2 and this suggests that these isolates were exposed to high antibiotic pressure, which might have stemmed from misuse of antibiotics in the selected study areas [39, 1]. The test bacterial isolates were resistant to more than one test antibiotics and this indicates that these isolates are multi-drug resistant. Multidrug-resistance raises a health concern because it limits the treatment options available for bacterial infections . Multidrug-resistance leads to re-emerging of certain diseases, which are associated with health implications such as prolonged illness period, higher cost for therapy, and increased risk of death .
Antibiogram signature of the bacterial isolates revealed the occurrence of eighteen antibiotic resistance genes conferring resistance against different antibiotic classes. The most prevalent antibiotic resistant gene detected among the isolates was blaTEM, among the other genes conferring beta-lactam resistance. The findings from this study are inline previous studies regarding the frequency of blaTEM in members of Enterobacteriaceae family [40, 41, 42]. The isolates also exhibited frequent occurrence of other antibiotic resistant genes including aacA2, aadA, strA and strB which confer against aminoglycosides. This raises a particular concern, as aminoglycosides are one of the highly potent, broad-spectrum antimicrobials that have been frequently prescribed for treating life-threatening infections for several decades. The commonest mechanism of aminoglycosides resistance document include the production of N-acetyltransferases (AAC), nucleotidyltransferases (ANT) and O-phosphotransferases (APH), which are enzymes that modify the antibiotic, thus rendering the antibiotic inactive [43, 44, 45]. For instance the AAC enzyme acetylate the amino group at the 6′-position of the aminoglycoside antibiotic while APH, phosphorylate the hydroxyl group at the 3′-position of aminoglycosides and disrupt the binding of the aminoglycoside antibiotic to the 16S rRNA molecule of the target organism .
The tetA and tetM resistant genes conferring tetracycline resistance were detected in the recovered isolates. Tetracycline resistance genes like tetM disallow the binding of tetracycline antibiotic to the ribosome by producing elongation factor-like ribosomal protection proteins that stabilize ribosome transfer RNA interactions in the presence of tetracycline molecules . Occurrence of tetracycline resistance genes in Enterobacteriaceae have been previously reported [47, 48, 49, 50]. Interestingly, only Klebsiella species that demonstrated high occurrence of sul2 gene at 70% proportion, while sul1 was not detected in this organism. Surprisingly, E. coli, Citrobacter species and Enterobacter spp. displayed low occurrence of both sul1 and sul2 genes even though they displayed high resistance against trimethoprim antibiotic. This might suggest that resistance against trimethoprim was mediated by other resistance mechanisms such as efflux pump, the ability to form biofilm and possession of integrons, which were not assayed in the present study.