Direct screening of bacterial genes expressed during infection in the host is limited, because isolation of bacterial transcripts from host tissues necessitates separation from the abundance of host RNA. Selective capture of transcribed sequences (SCOTS) allows the selective capture of bacterial cDNA derived from infected tissues using hybridization to biotinylated bacterial genomic DNA. Avian pathogenic E. coli strain E037 (serogroup O78) was used in a chicken infection model to identify bacterial genes that are expressed in infected tissues. Three-week-old white leghorn specific-pathogen-free chickens were inoculated into the right thoracic air sac with a 0.1 mL suspension containing 10(7) CFU of APEC strain E037. Total RNA was isolated from infected tissues (pericardium and air sacs) 6 or 24h postinfection and converted to cDNAs. By using the cDNA selection method of selective capture of transcribed sequences and enrichment for the isolation of pathogen-specific (non-pathogenic E. coli K-12 strain ) transcripts, pathogen-specific cDNAs were identified. Randomly chosen cDNA clones derived from transcripts in the air sacs or pericardium were selected and sequenced. The clones, termed aec, contained numerous APEC-specific sequences. Among the distinct 31 aec clones, pathogen-specific clones contained sequences homologous to known and novel putative bacterial virulence gene products involved in adherence, iron transport, lipopolysaccharide (LPS) synthesis, plasmid replication and conjugation, putative phage encoded products, and gene products of unknown function. Overall, the current study provided a means to identify novel pathogen-specific genes expressed in vivo and insight regarding the global gene expression of a pathogenic E. coli strain in a natural animal host during the infectious process.