A previous study conducted in Nha Trang, a medium-sized coastal city in southern Vietnam, showed that the prevalence of ESBL-E. coli in poultry, pork, and shrimp products was 58.7, 32, and 18.3 %, respectively (Le et al. 2015). The results of both the present and previous studies showed a higher prevalence of ESBL-E. coli in poultry products than in other food products in both areas of Vietnam. In addition, the results also showed that the overall prevalence of ESBL-E. coli is higher in Thai Binh than in Nha Trang. A study conducted in Denmark reported that 36 % of samples from imported broiler chicken were contaminated with extended-spectrum cephalosporinase-producing E. coli (Agersø et al. 2012), while a similar study conducted in the Netherlands showed that 76.8–94 % of poultry products were contaminated with ESBL-E. coli (Leverstein-van Hall et al. 2011; Overdevest et al. 2011). Thus, the prevalence of ESBL-producing bacteria in food products varies in different geographical areas.
Southeast Asian countries commonly use an integrated recycling farm system called VAC (vegetable, aquaculture, and caged animal) for food production. This system is effective for small-scale food animal breeding and agricultural production. Stool and waste from humans, animals, and plants are mixed into the soil or put into ponds as food for fish. In this system, antibiotic-resistant bacteria from improperly treated waste could easily enter the ecological cycle, especially in aquatic environments either directly or via sewage ditches. Unhygienic food-handling practices at retail markets may also contribute to the prevalence of antibiotic-resistant bacteria in livestock products. Notably, residual antibiotics are commonly detected in aquatic environments in Vietnam (Hoa et al. 2011). The heavy use of antibiotics in this area can lead to antibiotic contamination of the environment, which likely promotes the development of antibiotic-resistant bacteria (Suzuki and Hoa 2012). In Thai Binh, a typical rural area in northern Vietnam, retail poultry products from different chicken farms are brought to retailers’ houses where they are processed before being sold in markets. In addition, most retail pork products originate from households in the community. Nearly 100 % of the freshwater shrimp products are from small domestic rivers near fields or from private pools next to sewage ditches. Almost all fish products are from household fish pools, domestic rivers, or rice fields. Therefore, differences among the sources of the food products and the environments in which they are processed may be the main reasons for the variations in the prevalence of ESBL-E. coli in food samples tested from different geographical areas.
Our results showed that 96.5 % of the ESBL-E. coli isolates encoded genes of the CTX-M group with or without TEM-encoding genes, suggesting the emergence of the CTX-M gene group as the main contributor to the antibiotic resistance of ESBL-E. coli in food, food-producing animals, healthy people, and patients not only in developed countries but also in underdeveloped countries (Sasaki et al. 2010; Overdevest et al. 2011; Börjesson et al. 2013; Chen et al. 2014; Nakayama et al. 2015).
CTX-M-1 and CTX-M-9 (with or without TEM) were the major subgroups of CTX-M genotypes detected in the ESBL-E. coli isolates from the food samples tested in this study. The results of this study are similar to those of previous studies conducted in other regions of Vietnam (Cortés et al. 2010; Leverstein-van Hall et al. 2011; Le et al. 2015). There was a significant difference in the multi-ESBL genes encoded by the isolates obtained from poultry compared to those encoded by isolates from pork, shrimp, and fish samples (p < 0.001). Antibiotic usage at poultry farms, particularly the long-term antibiotic treatment from hatching to grow-out, may contribute to the acquisition of multi-ESBL genes, leading to a high prevalence of ESBL-E. coli in poultry products.
ESBL-E. coli belonging to phylogenetic groups A, B1, B2, and D were present in the tested food samples, and there were no significant differences among the food sources. ESBL-E. coli belonging to groups B1 and A accounted for approximately two-thirds of all ESBL-E. coli isolates from the food samples, followed by those belonging to group D. In contrast, ESBL-E. coli belonging to group B2 accounted for only 8.6 % of all ESBL-E. coli isolates from the food samples. This order of the most to least prevalent phylogenetic groups observed in the present study was similar to that observed in a recent study conducted in Nha Trang, Vietnam (Le et al. 2015). In addition, a similar phylogenetic profile was observed for ESBL-E. coli isolates from chicken samples in the Netherlands, with the highest percentage (44 %) of isolates belonging to group B1, followed by groups A (28 %), D (23 %), and B2 (2 %) (Kluytmans et al. 2013). Therefore, studies performed in the Netherlands and other regions of Vietnam showed that the prevalence of ESBL-E. coli isolates belonging to group B2 was significantly lower than the prevalence of ESBL-E. coli isolates belonging to other phylogenetic groups. However, the results of the present study showed a much higher prevalence of ESBL-E. coli isolates belonging to group B2 than reported in previous studies performed in other areas of Vietnam or in other countries. Since group B2 usually includes bacteria isolated from humans (Jakobsen et al. 2010), our findings imply a close relationship between ESBL-E. coli isolates from food samples and isolates from humans.
A high percentage (89.2 %) of the ESBL-E. coli isolates from the food samples tested in this study showed MDR. The prevalence of extensively MDR ESBL-E. coli was especially high in fishery products (56–62 % isolates). Quantitative and qualitative analysis of antibiotic-resistant bacteria in fishery products (Van et al. 2007a, 2007b; Le et al. 2015) showed that up to 61 % of E. coli isolates exhibit MDR. The prevalence of aquaculture as well as the unique distribution method for fishery products in rural areas of Vietnam (described above) could lead to cross contamination with antibiotic-resistant bacteria, particularly in markets, resulting in a high prevalence of these bacteria.
Since the spread of carbapenem-resistant bacteria in the community is a threat to public health due to the increased risk of intractable infection, the isolation of MEM-resistant ESBL-E. coli from pork samples is notable. Even though only one strain was isolated from the food samples tested in this study, possible dissemination of these bacteria in food should be monitored. The high rate of quinolone resistance in ESBL-E. coli isolates from food is also important. The heavy use of quinolone antibiotics in agriculture may be responsible for this high prevalence.
Food contaminated with ESBL-producing bacteria is a potential source for widespread dissemination of these bacteria in humans (Lazarus et al. 2015). The results of our study, combined with data from previous studies in different regions of Vietnam and other countries, suggests that rural areas are major contributors to the dissemination of not only ESBL-E. coli but also extensively MDR bacteria. Rural areas with livestock and aqua-agriculture activities are one of the largest reservoirs of MDR bacteria and are therefore a major threat to public health. Thus, the development of stringent monitoring strategies and the promotion of hygienic food distribution practices are needed to control the spread of these antibiotic-resistant bacteria.