THE ROLE OF PLASMIDS IN APEC´S BASIC BIOLOGY
We have found that large plasmids (extrachromosomal ´accessory´ DNA present in some bacteria) are a defining trait of the APEC pathotype, occurring in ~80% of APEC and harboring many of the genes thought to be linked to APEC´s abilities to cause disease and resist therapy. Such plasmids are often transmissible to recipient bacteria, and acquisition of these plasmids results in an enhancement of the recipient´s virulence and resistance (Rodriguez-Siek et al 2005a; Johnson et al 2005, Johnson et al 2006 a, b,c; Skyberg et al 2006, Skyberg et al 2008; Johnson et al 2010). For these reasons alone, study of these plasmids seems justifiable, but other reasons exist to study these plasmids. For instance, E. coli, containing large R (resistance) plasmids, have been found in rivers, irrigation waters, and sediments polluted by agricultural and municipal run-offs, where they may enter the water and food supply (Cernat 2002 a,b; Roe 2003). Once there, they may become involved in human disease or serve as reservoirs of virulence and resistance genes for human pathogens. Too, analysis of bacterial chromosomes has revealed a more prominent role of plasmids in bacterial evolution than previously recognized (Smets and Barclay 2005). Despite this new appreciation for the role of plasmids in bacterial evolution, plasmid genomics continues to lag behind that of chromosomal genomics. Given the importance of plasmids to animal and human disease, environmental processes, antimicrobial resistance, and microbial evolution, this paucity of useable plasmid genome data is a serious oversight. Therefore, in order to clarify the role of APEC-like plasmids in disease, in the evolution of APEC and other pathogens, and as reservoirs of virulence and resistance genes for human disease, we are working to increase the numbers of plasmid genomes in public databases and enhance the tools for their analysis. To date, we have sequenced a number of APEC plasmids (Johnson et al 2005, 2006 a, b, c; 2010), and several have been characterized for their contributions to disease (e.g., Skyberg et al 2006, 2008; Johnson et al 2010). We are exploiting their presence to track APEC in the environment (Johnson et al 2009), assess bacterial changes over time (unpublished data), and improve APEC diagnostics and control (Lynne et al 2006; Johnson et al 2008). Also, by studying APEC isolated over the past 40 years for certain plasmid genes, we have demonstrated that APEC have become better able to cause disease and resist therapy over time (unpublished data).
APEC AS A POULTRY PATHOGEN
We seek to enhance food security through improved understanding of APEC pathogenesis with the ultimate goal of improving control of avian colibacillosis. Colibacillosis is an economically devastating disease for the world´s poultry industries due to mortality, morbidity, condemnations at processing, and decreased production. Recent completion of the APEC and chicken genomes and generation of APEC and chicken DNA microarrays and other expression technologies has provided an unprecedented opportunity to understand the mechanisms of APEC pathogenesis, the relationship between APEC and its host during infection, and the traits that underlie effective host resistance to APEC infection. Such "systems approaches" will provide a degree of insight into the APEC: chicken interaction that has not been previously achievable. As a result of such studies, it is anticipated that designers of future colibacillosis control strategies can target both host and pathogen, which should result in improved control of this important disease.
To date, some of the things we know about APEC virulence is that these organisms possess various virulence traits enabling their extraintestinal lifestyle. APEC have acquired genes by horizontal gene transfer that encode virulence traits and distinguish APEC from avian fecal E. coli commensals (Rodriguez-Siek et al 2005a). These genes are often clustered into chromosomal or plasmid pathogenicity islands (Barnes et al 2008). Portions of plasmid-located pathogenicity islands occur widely among APEC isolated from different parts of the world (e.g., Maurer et al 1998; Janssen et al 2001; Altekruse et al 2002; Delicato et al 2002; Delicato et al 2003; Ewers et al 2004; Yang et al 2004; McPeake et al 2005; Rodriguez Siek et al 2005a, b; Vandkerchove et al 2005; Zhao et al 2005), various avian host species (e.g., Altekruse et al 2002; McPeake et al 2005; Rodriguez Siek et al 2005 a, b), and different syndromes (e.g., De Brito et al 2003; Rodriguez Siek et al 2005a) and are considered a defining trait of the APEC pathotype (Rodriguez Siek et al 2005a; Johnson et al 2008). Other virulence factors have been described in APEC, including adhesins (e.g., Naveh et al 1984; Yerushalmi et al 1990; Arne et al 2000, Babai et al 2000; Gophna et al 2001a; La Ragione et al 2002; Lymberopoulos et al 2006), toxins (e.g., Parreira et al 1998; Salvadori et al 2001a,b; Parreira et al 2002; La Ragione et al 2002, Rodriguez Siek et al 2005 a,b; Rosario et al 2004), and hemolysins (e.g., Nagai et al 1998; Reingold et al 1999; Altekruse et al 2002; Morales et al 2004; Wyborn et al 2004; Rodriguez Siek et al 2005 a,b; McPeake et al 2005; Johnson et al 2006a,b). Also, the ability of APEC to obtain iron is well documented and likely due to several iron acquisition mechanisms (e.g., Dho and Lafont 1984; Lafont et al 1987; Wittig et al 1988; Gophna et al 2001b; Dozois et al 2003; Sabri et al 2006). In addition, ability to resist serum is a common trait among APEC, regardless of the syndrome or host of origin (Nolan et al 2003). Though a recent transcriptome analysis of an APEC following growth in serum failed to confirm the role of certain traits expected to contribute to serum resistance, several other virulence genes; genes involved in adaptive metabolism, protein transport, biosynthesis pathways, stress resistance, virulence regulation; and genes of unknown function, including those found in a plasmid pathogenicity island were identified as contributing to survival in serum (Li et al 2010). In addition, APEC´s ability to resist phagocytes is another important virulence determinant (e.g., Kottom et al 1997; Mellata et al 2003).
APEC´S POTENTIAL AS A HUMAN PATHOGEN
Extraintestinal pathogenic Escherichia coli (ExPEC), including uropathogenic E. coli, neonatal meningitis E. coli, and septicemia-causing E. coli, are responsible for significant morbidity and mortality in human beings, resulting in hundreds of thousands of deaths and millions of days of lost productivity annually (Russo and Johnson 2003). Every year in the US alone, it is estimated that ExPECcaused diseases result in over $4 billion in direct healthcare costs (Russo and Johnson 2003). Thus, no matter how we measure the costs of ExPEC-caused diseases, they are unacceptably high. While it is generally agreed that the source of ExPEC causing human disease is the victim´s own colonic flora, it is not known how ExPEC come to inhabit the colon. Similarities in APEC and ExPEC in the diseases they cause, content of virulence traits, and genomic sequences have led to the hypothesis that ExPEC enter the colon following ingestion of APECcontaminated food. This contention is based on the similarities between human ExPEC and APEC in terms of genomic sequences, virulence genotypes, serogroups, phylogenetic groups, antimicrobial susceptibilities, diseases caused, and a documented history of transmission of E. coli and their plasmids from poultry to humans (Levy et al 1976; Linton et al 1977; Ojeniyi 1989; van den bogaard et al 2001; Rodriguez Siek et al 2005b; Johnson et al 2007). To date, we have shown that some APEC are more like human ExPEC than human ExPEC are like each other (Johnson et al 2007), APEC-like organisms enter the human food chain on contaminated poultry (Johnson et al 2009), APEC cause urinary tract infections and neonatal meningitis in models of human disease (Tivendale et al 2010; unpublished data), and APEC plasmids are transmissible to human ExPEC and enable E. coli to grow in human urine (Skyberg et al 2006) and cross the blood brain barrier (a trait required for a bacterium to cause meningitis) (Johnson et al 2010). Thus, we still cannot eliminate the possibility that APEC-contaminated poultry is a food-borne source of human ExPEC.
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