Pathogenic Escherichia coli-Specific Bacteriophages and Polyvalent Bacteriophages in Piglet Guts with Increasing Coliphage Numbers after Weaning.

Postweaning diarrhea in pigs is mainly caused by pathogenic Escherichia coli and is a major source of revenue loss to the livestock industry. Bacteriophages dominate the gut virome and have the potential to regulate bacterial communities and thus influence the intestinal physiology. To determine the biological characterization of intestinal coliphages, we isolated and identified the fecal coliphages of healthy preweaned and postweaned piglets from the Nanjing and Chuzhou pig farms. First, ahead of coliphage isolation, 87 E. coli strains were isolated from healthy or diarrheal fecal samples from three pig farms, of which 8 were pathogenic strains, including enterotoxigenic E. coli (ETEC) and enteropathogenic E. coli (EPEC). Of the E. coli strains, 87.3% possessed drug resistance to three antibiotics. Using these 87 E. coli strains as indicator hosts, we isolated 45 coliphages and found a higher abundance in the postweaning stage than in the preweaning stage (24 versus 17 in the Nanjing and 13 versus 4 in the Chuzhou farm). Furthermore, each farm had a single most-prevalent coliphage strain. Pathogenic E. coli-specific bacteriophages were commonly detected (9/10 samples in the Nanjing farm and 7/10 in the Chuzhou farm) in guts of sampled piglets, and most had significant bacteriostatic effects (P < 0.05) on pathogenic E. coli strains. Three polyvalent bacteriophages (N24, N30, and C5) were identified. The N30 and C5 strains showed a genetic identity of 89.67%, with mild differences in infection characteristics. Our findings suggest that pathogenic E. coli-specific bacteriophages as well as polyvalent bacteriophages are commonly present in piglet guts and that weaning is an important event that affects coliphage numbers. IMPORTANCE Previous studies based on metagenomic sequencing reported that gut bacteriophages profoundly influence gut physiology but did not provide information regarding the host range and biological significance. Here, we screened coliphages from the guts of preweaned and postweaned piglets against indicator hosts, which allowed us to identify the pathogenic E. coli-specific bacteriophages and polyvalent bacteriophages in pig farms and quantify their abundance. Our approach complements sequencing methods and provides new insights into the biological characterizations of bacteriophage in the gut along with the ecological effects of intestinal bacteriophages.