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INTRODUCTION
Many pathogens utilize type III secretion systems (T3SSs) to deliver effectors into eukaryotic cells, which facilitates infections [1, 2]. Most bacteria contain only one T3SS, but some pathogens harbour multiple T3SSs, which function independently in different aspects of pathogenesis. Salmonella utilize the Salmonella pathogenicity island 1 (SPI-1) T3SS to invade host cells, while the SPI-2 T3SS is required for intracellular survival [3]. Intestinal pathogenic Escherichia coli, such as enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC), use the locus of enterocyte effacement (LEE)-encoded T3SS to cause attaching/effacing lesions and diarrhoeal disease [4]. A second T3SS, designated the E. coli type III secretion system 2 (ETT2), was identified by sequencing the genome of an EHEC O157 strain [5]; however, the role of ETT2 in E. coli infections is less clear.
ETT2 is unable to encode a functional secretion system in most E. coli because it has undergone widespread mutational attrition, but it plays a role in regulating bacterial virulence [6, 7]. ETT2 can affect the expression of virulence genes outside the ETT2 cluster, and it is indirectly involved in the virulence of intestinal pathogenic E. coli [EHEC and enteroaggregative E. coli (EAEC)] [8, 9]. However, ETT2 is crucial for the virulence of extraintestinal pathogenic E. coli (ExPEC) strains. The ETT2 locus in avian pathogenic E. coli (APEC), although degenerate, contributes to virulence and reduced serum survival [10]. Moreover, some evidence suggests that intact ETT2 might deliver effector proteins into human brain microvascular endothelial cells (HBMECs) to enhance bacterial invasion, intracellular survival and virulence of newborn meningitis E. coli (NMEC) strains [11].
ETT2 is known to be present, in whole or in part, in the majority of E. coli strains; it is most frequent in intestinal pathogenic E. coli strains [12-14], but of low prevalence in uropathogenic E. coli (UPEC) strains [15]. ExPEC strains (NMEC, UPEC, APEC) share a broad range of similar virulence factors and pathogenic mechanisms and APEC is thought to be a potential reservoir for human ExPEC [16-18]. Thus, this study investigated the prevalence and characteristics of ETT2 in APEC isolates to elucidate the potential zoonotic risk of APEC.
MATERIALS AND METHODS
Bacterial strains, growth conditions and...