Extraintestinal pathogenic Escherichia coli (ExPEC) comprises a pathogenic group of E. coli, which can colonize non-intestinal normally sterile body sites and cause invasive E. coli disease (IED) [1]. ExPEC, which is the etiologic source of the majority of urinary tract infections and abdominal infections [2,3], is the most common human pathogen. ExPEC is a primary cause of neonatal sepsis and meningitis [1,[4], [5], [6]] and in adults, one of the leading causes of community-acquired bacteremia and sepsis [[7], [8], [9]]. The risk of developing ExPEC infections increases considerably with age, which contributes to the substantial burden of IED in older adults [10,11].

Recently, several studies have confirmed the association of specific clones with numerous extra-intestinal virulence genes, encoding adhesins, toxins, protectins, and iron capture systems, which play an essential role in the process of ExPEC invasion of the host, immune defense, and causing host damage [3,12,13]. Antibiotic resistance in E. coli is increasingly common. E. coli can express a variety of beta-lactamases, including AmpC enzymes, extended-spectrum beta-lactamases (ESBLs), and carbapenemases [14]. The increase in multidrug resistance (MDR; i.e., resistance to 3 or more antibiotic classes) among ExPEC strains constitutes a major obstacle to successful treatment [1,15]. Moreover, aging populations and the continuing rise in MDR are implicated in increasing numbers of infection-related hospitalizations and deaths and are associated with increasing healthcare costs [1,10].

Classical serotyping of E. coli differentiates E. coli by its surface antigens O, H, and K [16]. The O antigen, which forms part of the E. coli lipopolysaccharide (LPS), is highly variable among strains, with greater than 180 distinct O antigens described [17]. As O antigens are surface-exposed and have remained remarkably stable over time, the O antigen is a promising target for a prophylactic vaccine [1]. In a surveillance study that analysed 3,217 blood isolates from adults ≥ 60 years in Europe, North America, Asia-Pacific, and South America, the nine O-serotypes included in the ExPEC9V conjugate vaccine (O1, O2, O4, O6, O15, O16, O18, O25, and O75) accounted for 64.6 % of all bloodstream isolates globally. Region-specific data from the same dataset show that these nine serotypes together made up >60 % of isolates in Europe (62.4 %), North America (68.9 %) and Asia-Pacific (68.0 %), with minimal inter-regional variation. The three leading serotypes (O25, O2, O6) alone represented roughly 40 % of bloodstream isolates in each of those regions. ExPEC9V is a nine-valent O-antigen–exotoxin A glycoconjugate now in a double-blind Phase III efficacy trial in adults ≥60 y (NCT04899336) [18].

In China, E. coli is the most common pathogenic Enterobacterales according to the China Antimicrobial Surveillance Network (CHINET, a nation-wide antimicrobial resistance surveillance system of China) (http://www.chinets.com/Data/AntibioticDrugFast). However, despite the clinical significance of ExPEC infections among older adults, the distribution of ExPEC O serotypes that cause blood and sterile site infections among older adults in China is not well described. As there are known geographic differences in the distribution of E. coli O-serotypes and antibiotic resistance, this study aimed to estimate the O-serotype distribution of ExPEC in China among older adults and its correlation with the phenotypic and genotypic antibiotic resistance.