Introduction

Urinary tract infections (UTIs) are the most common bacterial infections affecting approximately 11% of adult women each year globally, with approximately 60% of women experiencing UTI during their lifetime1, 2. Sporadic studies done on the prevalence of UTIs in Bangladesh and an investigation of 200 UTI patients, including men and women of various age groups, found females to be more susceptible to UTIs (80% positive) than males. In both genders, the prevalence rate was highest among those in the age group of 21–40 years (33%)3. The study also showed E. coli to be the predominant etiological agent, contributing to 57.38% of infections3.

Escherichia coli is an extremely diverse bacterial species which can be categorized into three major groups based on disease causing capability: commensal or nonpathogenic E. coli; intestinal pathogenic E. coli causing diarrhea; and extraintestinal pathogenic E. coli (ExPEC). The ExPEC term was described by Johnson et al. in 20004 and further subclassified as uropathogenic E. coli (UPEC), sepsis-associated E. coli (SEPEC), and neonatal meningitis-associated E. coli (MNEC)5. ExPECs are known to invade extraintestinal tissue and cause pathogenesis by harboring a variety of virulence factors, either present in the chromosome or carried in mobile genetic elements such as plasmids, thereby conferring greater diversity among ExPEC strains5,6.

Traditionally, E. coli phylogroups B2 and D have been understood to cause the majority of ExPEC infections, while phylogroups A and B1 were associated with comensal extraintestinal strains7. However, recent reports have revealed higher percentages of phylogroup A strains in UTI cases8. A strong association has also often been detected between a particular multilocus sequence type (MLST) with a pathology, such as the correlation of globally dominant E. coli ST131 and extraintestinal infections, especially in India9. Like ST131, many other successful clonal lineages of different sequence types (ST), including 410, 95 and 10 have disseminated globally due to their relatively higher virulence, fitness, and metabolic capabilities, along with acquisition of antibiotic resistance genes10,11,12,13.

Carbapenems are considered last-resort antibiotics, therefore resistance to this group of antibiotics is a greater health concern in treating infections caused by extended-spectrum-β-lactamase (ESBL)- or AmpC-producing bacteria14. New Delhi metallo-β-lactamase (NDM) is a relatively recent group of metallo-β-lactamase (MBL) that, over the last decade, has undergone rapid spread in the South-Asian continent15. While NDM producers have been found to be susceptible to a few antibiotics including colistin, several recent studies have reported that this treatment approach might not be sustainable and could become a very serious public health concern16,17. NDM genes are found both in plasmids and chromosomally integrated in various bacterial pathogens18,19. Reports of E. coli harboring NDM-1 and other ESBL genes such as CTX-M and OXA-48 have emerged from various parts of the world, including Japan, the Netherlands, South Korea, and Tanzania10,11,12,20,21,22,23. In addition, many such studies have detected the chromosomal integration of NDM genes in various ExPEC sequence types, such as ST38, ST410, ST131 and ST64824.

Association of different virulence factors, e.g., sat (secreted autotransporter toxin), iutA (aerobactin (siderophore) receptor), malX (pathogenicity island marker) and ompT (outer-membrane protease T), has also been reported, with specific sequence types such as ST38, ST131, ST405, and ST648 isolated25. In general, however, few genomic investigations have been done that could shed light on molecular mechanisms of pathogenesis and antibiotic resistance mechanisms and correlate those traits with the genotypes of local pathogens, especially in a developing country like Bangladesh.

To the best of our knowledge, there is no genomic information available on UPEC isolates circulating in Bangladesh. This study represents an initial effort to obtain genomic information on Bangladeshi UPEC isolates and to analyze genomic variations between Bangladeshi isolates and ExPECs from different parts of the world. Eleven strains representing different ExPEC phylogroups and antibiotic resistance were selected and their genomes determined using next-generation sequencing. Genotype–phenotype correlation analyses were also done on the isolates to determine virulence properties, e.g., biofilm formation, serum resistance, hemolysis, and antibiotic resistance.

Results

Antibiogram and phylogroup analysis

Presumptive identification using colony morphology revealed 47 of 74 (63.5%) bacterial isolates obtained from the Dhaka Central International Medical College and Hospital (DCIMCH) and 19 of 32 isolates (59.4%) from the Ibn Sina Hospital, Sylhet (ISH) were E. coli. All isolates from DCIMCH exhibited increased resistance to commonly used antimicrobials, including β-lactams (third- and fourth-generation cephalosporins), fluoroquinolones, and aminoglycosides (Table 1; Supplementary Table S1). Antibiotic sensitivity patterns differed between isolates collected from the two hospitals, with isolates from DCIMCH showing resistance to a larger number of antibiotics. For example, while most of the DCIMCH isolates were resistant to cefixime (85.1%), only 26.3% from ISH showed resistance to this antibiotic. DCIMCH isolates showed high frequency of resistance to second generation cephalosporin cefuroxime (83%), third generation cephalosporins ceftriaxone (80.9%) and ciprofloxacin (72.3%), and the monobactam, aztreonam (72.3%). ISH isolates, in contrast, showed resistance mainly to doxycycline (42.1%) and amoxicillin (36.8%). While ca. 17% (n = 8) of DCIMCH isolates conferred resistance to the carbapenem, imipenem, all of the ISH isolates were sensitive to carbapenems and all isolates included in this study were sensitive to colistin. Five isolates from DCIMCH were ESBL (Extended Spectrum β-lactamase) positive, but none from ISH were positive (Table S1).

Table 1 Percentage of DCIMCH and ISH strains resistant to different antibiotics.
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Phylogroup determination based on PCR detection of chuA, yjaA and TspE4.C226, showed phylogroup B2 and phylogroup A to be most abundant, with B2 comprising 19 (40.4%) and A 14 (29.8%) of the 47 isolates (Table S2). A total of 11 strains (23.4%) were classified in phylogroup B1, while three (6.4%) were phylogroup D. Seven of 66 isolates harboured NDM-1 gene and all NDM positive strains were from DCIMCH. No association was observed between NDM and a particular phylogroup, with three strains from phylogroup A, two strains from B1, and one strain from B2 carrying the NDM gene.

Genomic features and strain characterization

Eleven isolates from the phylogroups were selected with the number of isolates from each phylogroup roughly proportional to prevalence of that phylogroup within the set of 47 isolates in this study. These were selected based on resistance patterns. Four isolates were selected from phylogroups A and B2, two from phylogroup B1 (NGE5 and NGCE100), and one from phylogroup D (NGE3). Combined length of contigs of the assembled genomes of each of the 11 strains ranged from ~ 4.3 to 5.4 Mbp, with N50 value (the minimum contig length required to cover 50% of the genome) ranging between 58,987 and 363,834 bp (Table 2). Size of the pangenome (i.e. total gene repertoire) was 16,797 genes and core genome 2,945 genes.

Table 2 Genome assembly statistics of the 11 sequenced UPEC isolates.
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De novo analysis was used to confirm phylogroups of the assembled genomes and MLST analysis showed that, while strains belonging to a phylogroup were heterogenous in MLST types, there was direct correlation between serotype and MLST, with ST131 strains NGE7 and NGE16 both serotype O25:H4 (Table 3).

Table 3 De novo prediction of phylogroups, MLST types and serotypes of the sequenced UPEC isolates.
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Phylogenetic and cluster dendrogram analysis

The 11 strains were compared to obtain the number of SNPs shared between any two strains. From the SNP matrix shown in Supplementary Table S3, isolates with the same ST shared a low SNP count, while isolates within the same phylogroup but different STs had high SNPs. For example, NGE7, NGE16, and reference strain (NA114) belong to ST131 and share a low SNP count of 411 bp, whereas ST219 strains (NGE9 and NGE6) and ST131 strains had high SNP count but were the same phylogroup.

Core alignment using parSNP aligned 189 of 402 UPEC strains available online (list of strains is given in Supplementary Table S4). A total of the 60,815 SNPs identified was extracted and linked to construct a midpoint rooted phylogenetic tree (Fig. 1), showing two major clades, Clade A and Clade B. Clade A branched into subclades, with strains from phylogroup A and B1 in one subclade and ST219 strain of phylogroup B2 in another. NGCE33 is an ESBL-containing, highly virulent strain of ST410 which, despite belonging to phylogroup A, clustered distantly from the rest of phylogroup A strains (NGE22, NGE4 and NGCE94) and closer to phylogroup B1 (NGE5, NGCE100 and SE11). Clade B also branched phylogroup D and B2 away from each other. It was observed that isolates obtained from urine and blood samples interleaved, without significant clustering of infection type. However, NGE5 (proposed as a new ST) in clade A, NGE7, and NGE16 (ST131) in clade B joined strains isolated from blood. Strains belonging to the same MLST were placed together in the phylogenetic tree.

Figure 1
figure 1

Phylogenomic organization of publicly accessible UPEC isolates with sequenced Bangladeshi isolates in this study. Mid-point rooted SNPtree demonstrates the phylogenetic distribution of 11 UPEC genomes of Bangladeshi UPEC isolates amongst 5 UPEC reference genomes and 386 UPEC genomes (isolated from both urine and blood) available online. The well characterized reference genomes and UPEC isolates of this study have been labeled in blue and red respectively.

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Hierarchical clustering of the 11 UPEC strain sequences represents similarity of shared accessory genome content, yielding three major clusters, C1, C2, and C3, respectively (Fig. 2). Phylogroup D out grouped, forming a distinct cluster, C1, while a combination of phylogroups A, B1, and B2 joined the remaining two clusters (C2 and C3). C2 further divided into two distinguishable clusters comprising strains of ST131 family in one group and four multidrug resistant, highly virulent strains of various MLSTs in another. These four isolates included one ST448 strain, one ST361 strain carrying both blaNDM and ESBL genes, one “high-risk” clone of ST410 lineage27, and one resistant strain of the ST4204 family. The clustering pattern of C2 suggests sharing of accessory genes between highly virulent strains, irrespective of phylogroups and ST. C3 also separated into two clusters, comprising two less virulent phylogroup B2 strains of the ST219 family in one clade, and the moderately virulent ST4204 strain of phylogroup A (NGE4) and the phylogroup B1 strain of unknown ST (NGE5) in the other.

Figure 2
figure 2

Dendrogram illustrating pan genome clustering of sequenced UPEC isolates. Dendrogram branches into three major clusters (C1, C2 and C3) based on the presence and absence of characterized accessory genes in the pan genome.

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Accessory gene distribution responsible for this hierarchical clustering pattern is listed in Supplementary Table S5. Strains in cluster C2 share genes including OriC-binding nucleoid-associated protein (cnu), hemolysin expression-modulating protein (hha), suppressor of T4 td mutant (stpA) responsible for regulation of hemolysin (hly) gene expression and genes such as periplasmic inhibitor of g-type lysozyme (pliG) which provides lysozyme tolerance. Similarly, shared genomic content in cluster C3 involves genes RenD, ybcN and ybcK coding for uncharacterized prophage related proteins absent in C1 and C2 strains. Strains ST131 and ST219 belong to phylogroup B2 yet are located in two different clusters as ST131 strains contain 223 unique genes with significant enrichment in genes involved in biosynthetic and metabolic processes absent in ST219 strains, thus explaining the high numbers of SNPs between strains of the two STs.

De novo identification of antibiotic resistance markers

Genome annotation revealed that chromosomes of all strains sequenced had previously been reported to carry intrinsic antibiotic resistance genes, such as marA, gyrA, parC and parE28, as well as plasmid-mediated resistance genes belonging to AMR families, including β-lactamases, fluoroquinolones, aminoglycosides, macrolides, tetracyclines, trimethoprims, and sulfonamides. Genes associated with antibiotic resistance are shown in Fig. 3a.

Figure 3
figure 3

(a) Distribution of key antimicrobial resistance conferring genes within the 11 strains. (b) Distribution of key virulence factors within the 11 strains. Black: present, grey: absent.

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Eleven isolates, irrespective of phylogroup and sequence type (ST), showed resistance to antibiotics according to presence of resistance genes. ESBL genotype blaCTX-M-15 is a predominant gene present in two NDM positive strains (NGCE100, NGCE94) and two ST131 strains (NGE16 and NGE7). NDM positive strains possessing blaNDM-5 and blaNDM-7 belong to phylogroup A and B1, and MLST groups ST361 and ST448, respectively. In addition, NGCE94 (ST361) contained β-lactamases blaTEM-1B and blaOXA-1, tetracycline-resistance tet(B), quinolone-resistance gene qepA, trimethoprim-resistance gene dfrA12 and chloramphenicol-resistance gene catA1. Variants of blaTEM and dfrA were also present in the highly resistant ST4204 strains (NGE4 and NGE22) belonging to the same phylogroup as ST361. Relatively less resistant ST219 strains (NGE6 and NGE9) of phylogroup B2 present a similar resistance pattern and contain resistance markers (tet(A) and qnrS1) common to phylogroup A rather than phylogroup B. Another member of phylogroup A (NGCE33) included highly resistant ST410 containing an array of β-lactamase genes including blaOXA-1B, blaCMY-2, blaDHA-1 and blaSHV-12. In addition, it contained a number of aminoglycosidase genes, fluoroquinolone-resistance gene qnrB4, macrolide gene mph(A) and trimethoprim-resistance genes drfA1 and drfA17. Genes strB and mph(A) were also shared by ST4204 isolates. Moreover, genes rmtB, aadA1, mph(A) and erythromycin-resistance gene erm(B) were harboured by NDM positive strains. Only a few resistance genes such as acc(6′)lb-cr, catB3 and blaOXA-1 were shared between the ST131 and ST410 strains.

Overall, the most resistant strain was NGCE33, based on genes coding for β-lactamase resistance, including the ESBL, blaCTX-M-15. This strain and NGCE100 were resistant to nitrofurantoin, a last resort nephrotoxic antibiotic that recently is more commonly used to treat carbapenem resistant UTIs.

Analysis of virulence profiles and genotype–phenotype correlation

UPEC pathogenesis encompasses a range of mechanisms including colonization of the urinary tract, protection against host defenses, and toxin production. Hemolysin production of 11 strains was tested, using blood agar and, while alpha hemolytic activity was observed only for ST131 strains NGE7 and NGE16, mild hemolysis was detected for NGE3, NGE4, and NGE22.

An important pathogenic determinant of UPEC is ability to form biofilm29. The biofilm formation assay results showed variation in biofilm formation both between and within the phylogroups (Fig. 4). Three strains (NGE22, NGCE94 and NGCE100) were classified as strong biofilm formers and four (NGE4, NGE7, NGE9 and NGE16) were moderate biofilm formers, based on specific biofilm formation value (SBF). Differences in biofilm forming ability were observed by sequence type, as exemplified by NGE7 and NGE16, both in the pandemic ST131 family, as well as ST219 strains (NGE6 and NGE9) and ST4204 strains (NGE4 and NGE22).

Figure 4
figure 4

Serum resistance and biofilm forming propensities of 11 E. coli isolates. Pooled NHS was inoculated with overnight LB culture in 1:10 ratio. Bacteria were enumerated at 0 and 3 h of incubation at 37 °C, and percentage survival in serum was calculated. For biofilm assay, bacteria were grown in M63 media and specific biofilm formation (SBF) was calculated. Error bars represent standard error (SE).

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Bactericidal activity of serum complement acts as a major first-line defense against bacterial infection infiltrated into tissue30. In vitro serum resistance assay results showed all strains were susceptible to human serum bactericidal activity to varying degrees (Fig. 3). Except for NGE3, NGE5, and NGE6, they showed survival capacity equal to or greater than that of DH5α. While serum has a more pronounced bactericidal activity for strains belonging to phylogroup B1 and D, most strains of phylogroups A and B2 exhibited better survival in serum. Although little correlation was observed between degree of biofilm formation and serum resistance (r = 0.188) (Fig. 4), moderate/strong biofilm formers all showed greater survival in serum compared to weak biofilm formers.

Pan-genome analyses revealed virulomes of the 11 strains (Fig. 3b). In general, ST131 strains NGE7 and NGE16 carry an extensive repertoire of virulence genes. While the core genome encompasses gene families from different classes of virulence factors, not all are conserved, according to the SNP/bp ratio (Table 4).

Table 4 SNP distribution of core virulence genes.
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Comparison with VFDB indicated adhesion factors belonging to the ecp (Escherichia coli common pilus), csg (Curli fibers) and fim (type 1 fimbriae) gene families are part of the core genetic pool, while afaB/C (adherence fibrillar adhesion) and sfa (S fimbrial adhesion) are absent. Variation was noted in presence of the foc (F1C fimbriae) and pap (pyelonephritis associated pili) family genes. SNP distributions show that while other members of the fim genes are moderately conserved, fimA displays high genetic variability with SNP/bp of 0.383. Among the conserved usp (universal stress protein) genes that are involved in bacterial adhesion, uspA also indicated greater variation with SNP/bp of 0.181. Among toxicity conferring genes, hemolysin toxin hlyA was detected in five strains, including the two ST131 strains. Other genes, such as hlyB, hlyC, hlyD, cnf-1 and sat, were present exclusively in only ST131 strains.

All 11 strains carried the well-characterized serum resistance gene ompA, while traT and bor were missing from their genomes. Only ST131 strains harbored iss (increased serum survival) another important gene in the serum resistome of E. coli. The genomes were also analyzed for presence of 56 genes recently characterized as belonging to the serum resistome of EC985 by transposon-directed insertion site sequencing (TraDIS)31. Most of these genes, including tol(A,B,Q) and rfaH were detected in the core genome. However, some essential genes were either completely absent (hyxA and hyxR) or not identical to the reference strain from the VFDB database (waaP and waaG).

Analysis of pathogenicity islands (PAIs)

CFT073, a well characterized pyelonephritogenic strain, harbours PAIs that suggest strong virulence when present in UPEC isolates32,33. Many UTI associated strains, as well as commensal E. coli, carry PAIs that were first identified in strain 53634. To determine genetic composition of PAIs, genomes of the 11 UPEC isolates were analyzed for presence of the CFT073 and 536 associated PAI genes as retrieved from the PAthogenisity Island DataBase (https://www.paidb.re.kr/) (Supplementary Table S6).

Analysis of PAI ICFT073 (Fig. 5a) showed that only two ST131 strains (NGE7 and NGE16) and NGE3 carried malX, a phosphotransferase system enzyme coding gene linked with occurrence of extraintestinal infections. The PAI ICFT073 gene dadX, however, was detected in all strains, while other genes, such as those coding for the motility regulating factors ycgR and emtA, a peptidogycan recycling enzyme ldcA (L, D-carboxypeptidase A) and cvrA (putative K+:H+ antiporter), were detected in all strains except NGE9. Apart from cad (BAC) which was present in all strains, most of the PAI IICFT073 associated genes (Fig. 5b) were found only in the ST131 isolates, NGE7 and NGE16. However, a phylogroup A strain NGCE94, was found to carry several genes of the pap (P fimbriae coding) operon indicating the possibility of acquisition of PAI IICFT073 from more virulent strains in the genitourinary microenvironment.

Figure 5
figure 5

Distribution of genes associated with a. PAI ICFT073 and b. PAI IICFT073 in 11 UPEC genomes. Relational diagrams were generated using CIRCOS software. Bands were created for each gene of CFT073 PAI-I and PAI-II along with the 11 sequenced strains to depict the presence of CFT073 associated PAI genes in the strains.

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Analysis of Strain 536 associated genetic elements showed that PAI I536 (containing hemolysin genes hlyA, B, C, and D) as well as PAI II536 were exclusively found in NGE7 and NGE16. PAI III536 gene yciC (UPF0259 membrane protein) was only present in NGE3, NGE7 and NGE16. Furthermore, all strains apart from NGE3, NGE4 and NGE22 were enriched with genes from PAI V536. PAI IV536 genes on the contrary, had more diffuse distribution among the strains with only mtfA (mnemonic for Mlc titration factor A). That PAI IV536 is considered more stable than other PAIs of strain 53635 may explain these results.

Discussion

High-throughput sequencing technology development has led to a significant decrease in the cost of whole genome sequencing of bacterial pathogens so that sequencing is routine in developed countries. Infrastructure for next generation sequencing is now being developed in research centers located in developing countries including Bangladesh. The NSU Genome Research Institute (NGRI) at North South University, Bangladesh, has been established and aims to decipher whole genomes of bacterial pathogens of public health concern to Bangladesh. This initial study was carried out to determine genome characteristics of UPEC strains circulating in the country by sequencing 11 UPEC strains representing prevalent ExPEC phylogroups and antibiotic resistance profiles.

ST410 has been previously detected in Southeast Europe, Middle East and Greece28 and classified as a “high-risk clone” with essential regular monitoring due to its enhanced resistance mechanisms and moderate virulence36. The resistance and virulence patterns of ST410 strain NGCE33 validates this finding, and its very first identification in Bangladesh emphasizes the extent of its spread across continents. This study also reports an unknown MLST type i.e. NGE5, an observation that highlights the rapidly mutating nature of the UPEC genome and indicates the value of genomic characterization of local isolates.

The phylogenetic tree based on difference in SNPs (Fig. 1) showed phylogroups A and B1 branching separately from phylogroups B2 and D. This finding supports previous reports of phylogroup A and B1 belonging to sister lineages36,37. Phylogroup D and B2 can be concluded to have the same ancestral origin since most of the strains of these phylogroups are located in the same clade (Clade B) in SNP tree. A lack of clear phylogenomic separation of strains isolated from urine and blood coincides with results of previous related work28,38. The difference in SNP counts observed in this study further elucidates genomic relatedness since these sites account for the variation in nucleotide sequence. It was observed that ST219 and ST131 strains have an increased number of SNP difference despite belonging to the same phylogroup and thus cluster in separate clades in the phylogenetic tree. This may be due to difference in pathogenic capabilities and resistance potentials between STs, i.e., gain and loss of accessory genome in genomic diversity, with strains harbouring different fitness factors and MDR genotypes, irrespective of ST and phylogroup. Comparative genomic analysis of the sequenced UPEC strains showed that the phylogenetic analysis is congruous with serotype, sequence type, virulence and AMR pattern.

This is further supported by results of the genomic and phenotypic analyses (Fig. 4). The genetic architectures of strains included in this study are concordant with previous findings, with well known pandemic strains such as ST131 possessing many virulence genes and enriched PAIs9. However, as supported by the results of this study, although ST131 strains exhibit a notably dense virulome compared to other sequenced strains, a stark difference was not observed between the phenotypic virulence of ST131 and non ST131 strains (Fig. 4). Strains within the same phylogroup also displayed varied levels of virulence. While ST131 strains of phylogroup B2 (NGE7 and NGE16) displayed relatively strong phenotypic virulence, another strain NGE6, also classified under phylogroup B2 but of ST219 origin, displayed a much weaker pathogenic potential, such as weak biofilm formation.

Closer scrutiny of virulomes of the sequenced strains reveal certain virulence factors pertaining to resistance against serum bactericidal activity were either completely missing or present in degraded form within the bacterial genomes. These discrepancies explain that while most strains are capable of enduring serum bactericidal activity, they do not possess the robust serum resistome required to overcome complement action and proliferate in serum. The lack of any clear association between a particular gene and a given virulence phenotype suggests that likely there is a combinatorial effect of genes on pathogenic potential. E. coli possesses an open pan-genome by continuing gene acquisition, as found in other studies38,39. This characteristic may also explain phenotypic results as mentioned above and acquisition of yet-to-be characterized new genes may determine pathogenicity of strains. Several recent studies report an altered pathogenic potential of commensal E. coli40 and a similar observation can be made from this study. Emergence of highly virulent strains belonging to phylogroup A may be due to the open pan genome nature of E. coli, allowing it to acquire new virulence factors and resistance markers. SNP analyses of core virulence factors reveal that certain genes within a given gene family, such as fimA, are prominently more polymorphic compared with other members.

Several findings have indicated that septicemic/pyelonephritogenic strains carry certain virulent genes located in mobile genetic elements, called pathogenicity islands (PAI), usually absent in avirulent or less virulent strains41. Of all the strains sequenced in this study, ST131 strains NGE7 and NGE16 predictably possessed the most gene-dense CFT073 and 536 associated PAIs (Fig. 5). However, the presence of a large number of PAI IICFT073 mediated adhesion factors in the genome of NGCE94 indicates dissemination of virulence traits via horizontal transfer of PAIs from commonly virulent phylogroup B2 strains to less virulent phylogroup A strains. Transfer and evolution of genetic elements like PAIs contribute to fitness and pathogenic properties of UPEC.

Rapid emergence of multidrug resistant (MDR) strains of E. coli is a very serious concern, especially in a developing country like Bangladesh which experiences antibiotic misuse. Most resistance properties emerge via intra-species horizontal resistance gene transfer42. Clearly, however, excessive use of antibiotics in the global community creates evolutionary pressure towards enhanced resistance of UPEC28. Reported spread of the pan beta-lactam antibiotic resistance blaNDM family of genes is a major cause for concern, because of resistance conferred against penems, cephamycins, cephalosporins, and carbapenems, as well as horizontal transfer since the gene is located in a plasmid. This study reveals the presence of NDM gene varieties and a number of other ESBL genes in phylogroup A and B1 strains. In addition, the spread of virulence properties to strains like NGCE94, which is both NDM and ESBL positive, can have a profound effect on healthcare and spread of disease in Bangladesh.

In conclusion, this study presents useful insight into the genomes of Bangladeshi UPEC isolates, notably reporting for the first time an emerging pandemic clone ST410 in Bangladesh, contributing to the global distribution of this lineage. The study also demonstrates that strains belonging to phylogroup A exhibit virulence characteristics comparable to globally predominant known virulent ST131 (phylogroup B2) isolates, while other phylogroup B2 strains, such as ST219, display lower pathogenic potential. It also substantiates classification based on sequence type being an improved measure of genomic relatedness and pathogenicity. The risks posed by emerging pathogenic strains within different phylogroups need further assessment using comparative genomics and larger sample size.

Methods

Selection of isolates and antibiotic resistance profiling

A total of 106 bacterial isolates were cultured from urine of patients suffering from UTI and admitted to either the intensive care or emergency unit of two tertiary hospitals in Bangladesh from the period of June, 2017 and July, 2018. A total of 74 isolates were from inpatients admitted to Dhaka Central International Medical College and Hospital (DCIMCH), Dhaka and 32 isolates were collected from inpatients at Ibn Sina Hospital, Sylhet (ISH). E. coli colonies were presumptively identified by their colony morphology on MacConkey agar. Antibiotic susceptibility was carried out using the disc diffusion method and included 22 antibiotics for the 47 strains isolated from DCIMCH and 16 antibiotics for the 19 ISH strains (Table S1). Results were interpreted according to the 27th edition of Clinical and Laboratory Standards Institute (CLSI) guidelines. The isolates were transferred to North South University Genome Research Institute (NGRI), Dhaka, Bangladesh for further analysis.

PCR amplification and gel electrophoresis

The 66 E. coli isolates were inoculated into LB broth (HiMedia, India), grown at 37 °C overnight, and DNA was extracted using GeneJET Genomic DNA Purification Kit (Cat. No K0721) (ThermoFisher Scientific, USA) according to manufacturer’s protocol. Conventional singleplex PCR was carried out to detect the NDM gene43, and to classify strains into phylogroups using previously described protocols (Table S2)26. In brief, 12.5 µl of reaction volume was used containing 6.25 µl DreamTaq Green PCR Master Mix (ThermoFisher Scientific, USA), 1.0 µl 25 nmol of MgCl2, 20 pmol of forward and reverse primers and ca. 100 ng DNA. Amplification was carried out using GeneAtlas (Astec Co, Ltd), with the following assay conditions: denaturation at 94 °C for 5 min; 30 cycles of 30 s at 94 °C, 30 s at annealing temperature, 30 s at 72 °C, and final extension at 72 °C for 5 min. Agarose gel electrophoresis was used to visualize banding patterns of the strains.

Genome assembly and annotation

Library preparation and sequencing of the 11 selected strains were conducted at NGRI. Ca. 1 μg of high molecular weight E. coli genomic DNA was used to prepare Illumina libraries and employing Nextera DNA Library Preparation Kit (Cat. No. FC-121-1030) according to manufacturer’s guideline. De novo assembly of good quality paired-end Illumina reads (Q ≥ 30) was done by running genome assembly software SPAdes (v3.12)44 with filters to decrease the number of mismatches and short indels. Assembled contigs were annotated using PROKKA pipeline45 with contiglength < 500 bp filtered out. Possible genomic contaminations were assessed using the ContEst16S tool46. Pan and core genome size of the 11 isolates and reference genome NA11447 were identified using the GF (Gene Family) method of pan-genome analysis pipeline (PGAP) (v1.2.1)48. Further pan and core genome analyses were performed using Roary49. Hierarchical clustering based on presence and absence of accessory genes, was performed using PVclust: R package50, based on bootstrap resampling to generate p-values. The bootstrap value was set to n = 1,000, to cross-validate the clustering pattern. The functions of the accessory genes were analysed via STRING database (https://string-db.org/).

Evolutionary relationship and phylogenomic analysis

Single Nucleotide Polymorphism (SNP) matrix was generated using CSIphylogeny 1.4 (Conserved Signature Indels) (Table S3)51. The 11 sequenced UPEC isolates from this study, 386 publicly available published genomes12,38, and a few reference strains of diverse STs and phylogroups, including NA11447, CFT07352, IAI3953, SE1154 and BW295255 (Table S4), were aligned to generate a core alignment in order to derive whole genome SNP using Parsnp v1.2 from the Harvest suite56. SNP file was processed by SNPRelate: R package57 and phylogenomic tree was visualized using FigTree (https://tree.bio.ed.ac.uk/software/figtree/).

In silico analysis of UPEC genome sequences

Phylogroups were confirmed based on presence of marker genes arpA, chuA, yjaA and TspE4.C226 using local BLAST the scheme set by Clermont et al. in 201258. The ST of each annotated genome was extracted from the MLST 2.0 (Multi-locus sequence typing) database59, and serotypes were determined using SerotypeFinder 2.060. Similarly, antimicrobial resistance (AMR) genes and plasmids of each isolate was obtained from ResFinder 3.161 and PlasmidFinder 2.062 respectively. Furthermore, a virulence factor profile was generated by amalgamation of results obtained using BLASTp against the Virulence Factors of Bacterial Pathogens database (VFDB)63 that had been made available in 2016, and the tool VirulenceFinder 2.051. SNPs per gene were calculated using DnaSP v664. To study the genetic composition of possible PAIs, genomes of the 11 UPEC isolates were analysed for presence of CFT073 and 536 associated PAI genes retrieved from the PAthogenisity Island DataBase (https://www.paidb.re.kr/)65 and visualized using CIRCOS66.

Phenotypic virulence determination

Alpha and beta haemolytic reactions of the strains were demonstrated using blood agar (Oxoid, UK), prepared using sheep blood. A single isolated colony for each strain was streaked on a blood agar plate which was incubated overnight at 37 °C. Partially clear and completely clear zones around the colony were indicative of alpha and beta hemolytic activity respectively.

Biofilm formation assays were performed using previously described protocols with minor modifications13. Bacteria were grown overnight in M63 at 37 °C after which 2 µl aliquots were added to 198 µl fresh M63 medium in a sterile 96-well polystyrene microtitre plate with four replicate wells for each strain. M63 broth without inoculum served as negative control. The plates were incubated statically at 37 °C for 24 h and OD600 was measured both at the beginning of incubation (0 h) and end of incubation at 24 h (GloMax, Promega). The culture was then discarded and plates were gently washed twice with sterile saline and air dried. Ca. 250 µl 0.1% crystal violet was added to the wells and allowed to stain for 15 min. The plates were then washed thrice with distilled water and air dried. The stained bacterial cells were resolubilized in 200 µl of 33% glacial acetic acid and the plates were read at 560 nm to enumerate cells in the biofilm. Specific biofilm formation (SBF) was measured using the formula SBF = (AB – CW)/G, where AB is OD560 of stained cells, CW is OD560 of control wells, and G is bacterial cell growth calculated using the formula G = OD600nm(24 h) – OD600nm(0 h). The strains were classified as follows: SBF < 0.5 = weak biofilm former, 0.5 ≤ SBF < 1.0 = moderate biofilm former and SBF ≥ 1.0 = strong biofilm former67. The entire assay was performed at least twice for each strain.

Assay for serum resistance was performed using a slightly modified version of previously described protocols13. Ca. 5 µl from overnight cultures was added to 495 µl fresh LB broth (HiMedia, India) and inoculated statically for 2 h at 37 °C. The culture was then centrifuged at 5,000×g for 7 min and the pellet obtained was suspended in 500 µl of sterile saline. Ca. 20 µl aliquots from this mixture were transferred to 180 µl of normal human serum (NHS) in a sterile 96-well microtitre plate and incubated at 37 °C under static conditions for 3 h. Ca. 20 µl was removed from the culture at 0 h and after 3 h incubation and plated on LB plates after serial dilution. Bacteria were enumerated after the plates had incubated overnight at 37 °C. Resistance to serum was measured as percentage change in colony forming units (CFU) at the beginning and end of the incubation period. The entire experiment was run in duplicate.

Ethical statements

All isolates included in this study were collected for diagnostic purposes from two local tertiary hospitals where pathogens are isolated from clinical specimens as part of a routine diagnostic procedure and not for experimental purposes. All experiments and methods were carried out in accordance with relevant guidelines and regulations. All experimental protocols were approved by the North South University (NSU) Institutional Review Board (IRB) / Ethical Review Committee (ERC), protocol No. CTRG:NSU-RP-18-042. Clinical isolates used in this study were recovered for diagnostic purposes from local diagnostic centers or hospitals and were not experimental in nature. The clinical data were anonymized and unlinked and the requirement for informed consent was waived by the NSU IRB/ERC.