Fresh Steam-Flaked Corn in Cattle Feedlots Is an Important Site for Fecal Coliform Contamination by House Flies

House flies are a common pest at food animal facilities, including cattle feedlots. Previously, house flies were shown to play an important role in the ecology of Escherichia coli O157:H7; house flies in cattle feedlots carried this zoonotic pathogen and were able to contaminate cattle through direct contact and/or by contamination of drinking water and feed. Because house flies aggregate in large numbers on fresh (<= 6 h) steam-flaked corn (FSFC) used in cattle feed, the aim of this study was to assess FSFC in a cattle feedlot as a potentially important site of fecal conform contamination by house flies. House flies and FSFC samples were collected, homogenized, and processed for culturing of fecal conforms on membrane fecal conform agar. Selected isolates were identified by 16S rRNA gene sequencing, and representative isolates from each phylogenetic group were genotyped by pulsed-field gel electrophoresis. Fecal conforms were undetectable in FSFC shortly (0 h) after flaking; however, in summer, after 4 to 6 h, the concentrations of fecal conforms ranged from 1.9 x 10(3) to 3.7 x 10(4) CFU/g FSFC (mean, 1.1 +/- 3.0 x 10(4) CFU/g). House flies from FSFC carried between 7.6 x 10(2) and 4.1 x 10(6) CFU of fecal coliforms per fly (mean, 6.0 +/- 2.3 x 10(5) CFU per fly). Fecal coliforms were represented by E. coli (85.1%), Klebsiella spp. (10.6%), and Citrobacter spp. (4.3%). Pulsed-field gel electrophoresis demonstrated clonal matches of E. coli and Klebsiella spp. between house flies and FSFC. In contrast, in winter and in the absence of house flies, the contamination of corn by fecal conforms was significantly (similar to 10-fold) lower. These results indicate that FSFC is an important site for bacterial contamination by flies and possible exchange of E. coli and other bacteria among house flies. Further research is needed to evaluate the potential use of screens or blowers to limit the access of house flies to FSFC and therefore their effectiveness in preventing bacterial contamination.