E. coli ST131 is a universal clone of antimicrobial-resistant
E. coli isolated in most clinical samples [
39]. In the current study, based on the presence of
pabB gene using allele-specific PCR, 31 (25.83%) strains were subsequently assigned as O25-B2-ST131
E. coli. However, in line with the results of our study, Rasoulinasab et al. reported a prevalence of 26.9% of
E. coli O25b/ST131 in patients with urinary tract infection in Iran [
40]. However, the ST131 outbreak has occasionally been reported around the world, including Japan (10%), Denmark (38%) [
41], and Australia (51%) [
42]. Various research reports have inconsistencies that can be related to differences in the study population, sample size, age groups (children versus the elderly), type of samples (urine versus a diverse range of clinical specimens), and detected O-serogroups [
43]. Our result showed that in antibiogram test the highest resistance was against ampicillin and the lowest resistance was against imipenem. Similar to the results of our study, studies have shown that resistance to ampicillin is the most common among antibiotics, but resistance to imipenem is very low [
39,
44]. As a result, to prevent the incidence of hospital and community infection outbreaks, investigating the international distribution of
E. coli ST131 could be a helpful strategy. In the present research, the prevalence rate of ampicillin resistance in
E. coli was the highest. The observed resistance pattern to ampicillin in
E. coli ST131 clones in the current research was consistent with former studies in different provinces of Iran [
45,
46]. The highest prevalence of ampicillin resistance was found in isolates of patients ≤ 12 years old, and resistance rate to this antibiotic was low in the current study [
47]. Another study in the USA indicated that
E. coli ST131 clones had a resistance rate of 97.8% to ampicillin [
48]. In the United Kingdom, high levels of ampicillin resistance (55%) were found in isolates of
E. coli ST131 clones and are consistent with the findings of our study [
49]. Interestingly, the antibiotic resistance rate was higher in the O25-B2-ST131 strains in comparison with non-O25-B2-ST131 strains. Consistent with our results, previous studies reported a high frequency of antibiotic resistance in O25-B2-ST131 strains which turned them into a clinical challenge [
40,
44,
50]. The MIC results showed that none of the isolates, even
E. coli O25-B2-ST131 strains, were resistant to colistin, and colistin was the most effective antibiotic against
E. coli isolates from UTI. Although a small number of colistin-resistant
E. coli ST131 strains have been isolated from different samples in studies, colistin has still been identified as the most effective antibiotic in previous studies [
2,
51]. Altogether, colistin is probably more effective than other antibiotics in treating UTIs. However, the nephrotoxic properties of this antibiotic limit its usage as the drug of choice for the treatment of urinary tract infections [
52]. In our study, not only the frequency of MDR was high among 120 isolates (48.3%) but also the prevalence of MDR among O25-B2-ST131 strains (87.09%) was higher than non-O25-B2-ST131.
E. coli ST131 clone as an MDR pathogen that has recently been considered a huge public health issue. In line with our study, it was found that the rate of MDR in
E. coli ST131 is high as one of the high risk clones and is defined as one of the clones with a global distribution which has a high ability to survive, clone and spread in different types of niches. On the other hand,
E. coli ST131 has been identified as an O25b: H4 serotype and the highly dangerous phylogenetic group B2, which carries high amounts of MDR IncFII plasmids containing
blaCTXM-15 [
53,
54]. In total, 56.6% of isolates were ESBL producers and ESBL production was more common in O25-B2-ST131 isolates compared to the non-O25-B2-ST131 isolates. In a study, 31–36% of
E. coli strains produced ESBL in Korea and in consistent with our study, ST131 isolates were significantly associated with ESBL, specifically CTX-M-15, and were mostly MDR [
55]. This suggests that the frequency of ESBL can vary in different geographical regions in O25-B2-ST131 strains. Phenotypic characteristics play an essential role in the pathogenicity of UTIs caused by
E. coli. Also, the determination and analysis of these characteristics seem to be necessary for epidemiological studies. The O25-B2-ST131 isolates formed a more robust biofilm compared to the non-O25-B2-ST131 isolates. This result is in accordance with the study by Mostafavi et al. [
50]. The potency of
E. coli to evade the bactericidal effect of serum compounds, such as complement and antimicrobial peptides, makes it an advantage for extraintestinal
E. coli that enters the bloodstream. Serum bactericidal resistance was mostly observed in O25-B2-ST131 isolates compared with the non-O25-B2-ST131 isolates. Previous results reported high levels of serum bactericidal resistance among O25-B2-ST131 strains [
50,
56]. Consistent with our study, Duprllot et al. reported high rates of curli production in O25-B2-ST131 isolates [
57]. Olsen et al. showed that curli is not produced by the most pathogenic
E. coli strains when grown at 37 °C [
58]. One explanation for this contradiction in several studies may be the genetic differences among strains and growth conditions (media and temperature) [
59]. As a high-risk pandemic strain,
E. coli sequence type (ST) 131 has been identified in human, food, environmental, and animal samples.
E. coli ST131 has been repeatedly reported to carry clinically important antimicrobial resistance genes and is associated with extraintestinal diseases, mainly UTI. In this study, except for
mcr-1 gene, other virulence genes including
iucD,
traT,
kpsMT11,
afa,
fos,
csgA,
vat,
hlyA, and
cnf1 were detected in O25-B2-ST131 and non- O25-B2-ST131 isolates. It was also found that the frequency of the majority of virulence factors in O25-B2-ST131 was higher compared to non-O25-B2-ST131 strains, which indicates the high pathogenicity of O25-B2-ST131. As with our results, previous studies have agreed that the virulence factors of O25-B2-ST131 strains are significant as a dangerous clone type and that these strains are considered as a therapeutic challenge [
40,
50]. Although the frequency of
mcr genes in
E. coli ST131 strains is very low, studies have reported the prevalence of this gene in
E. coli ST131 strains isolated from animal and environmental samples [
60,
61]. In contrast to our study, some studies have confirmed the presence of the
mcr gene in
E. coli strains isolated from human samples in America, Japan and Korea which indicates that the frequency of these plasmid genes can be different based on the isolation source and geographical environment [
62‐
64]. On the other hand,
E. coli isolates were found to carry antibiotic resistance genes such as
blaCTXM-15,
blaCTXM-27, blaTEM, and
blaSHV genes. The abundance of these important genes in antibiotic resistance was higher in O25-B2-ST131 strains compared to non-O25-B2-ST131 strains. Antibiotic resistance studies have been performed in O25-B2-ST131 strains which confirm our results. These studies have shown that resistance and the presence of genes involved in resistance are higher in O25-B2-ST131 strains [
40,
50,
62]. It is also shown that
E. coli ST131 has a wide range of virulence and resistance genes located on plasmids with high transmissibility that has a global spread [
39]. In preceding studies, the O25-B2-ST131
E. coli with resistance genes and high virulence potential has been described worldwide. A worrying elevation in the isolation of
E. coli isolates with the ability to produce CTX-M-15 from different countries has been described, and this phenomenon is associated with the development of the clonal ST131 [
65,
66]. Resistance in
E. coli ST131 has been commonly reported worldwide and associated with other resistance genes [
45]. In previous studies, the percentage of virulence factors among ST131 strains has been reported with occasional variations [
65,
67]. Although in the above studies different virulence genes were usually investigated in O25-B2-ST131
E. coli strains, the important point is that the amount of antibiotic resistance and virulence genes in these strains is high. Examination of all characteristics of
E. coli isolates from children with UTI revealed that phylogenetic group B2 was the most common in all isolates (
n = 101; 84.16%) and O25-B2-ST131 strains. Consistent with our study, Hojabri et al. showed that the existent ST131 strains were considerably more similar to the B2 group than the
E. coli and non-ST131 isolates [
68]. In other studies, the high prevalence of B2 group was reported among UPEC isolates that are well known worldwide [
69,
70]. The higher prevalence of group B2 among UPEC isolates is due to the antibiotic resistance genes and virulence factors existing within this group which can cause an increased survival fitness in the urinary tract [
70]. Another part of our results showed that O25-B2-ST131 and non-O25-B2-ST131 strains are located in fifteen different genetic clusters with 80% homology. The largest cluster consisted of eight strains, seven of which belong to the O25-B2-ST131 isolate. Of 31 O25-B2-ST131 isolates, 19 isolates were classified as various clusters. According to our study, several specific host sub-clusters were found in the McLellan study. Approximately 33% of the strains showed less than 65% similarity [
71].
The present study encountered several limitations. The study was done in one hospital, and due to budget limitation doing a multicenter study was not possible. Access to medical data of the children, like underlying diseases and recent medications, to correlate their links with colonization of E. coli O25-B2-ST131 strains and their antimicrobial resistance phenotypes was not possible, because of the lack of a registry system for outpatients. Moreover, no follow-up program at the time of study was considered to understand differences in the success or complications of UTI by E. coli O25-B2-ST131 strains in comparison to non-O25-B2-ST131 strains. Hence, many mechanisms and resistance genes that can prove higher resistance in E. coli O25-B2-ST131 strains were not investigated such as carbapenems, aminoglycosides, and fluoroquinolones due to the costs getting higher in this study.
In conclusion, obtained results showed a higher frequency of antibiotics resistance and virulence factors in O25-B2-ST131 strains compared with other E. coli isolates in children with CA-UTI. The high frequency of antibiotic resistance and virulence genes in O25-B2-ST131 strains, which can be the cause of increased pathogenicity and treatment failure, showed the importance of these strains in the children's infections. In this study, E. coli isolates with common rep-types presented a diversity in their clone types, virulence capacity and antibiotic-resistance patterns. Constant monitoring, due to the high prevalence of these strains and their involvement in UTI, should be done to control their spread in the community.