The diversity of coliphages and coliforms in horse feces reveals a complex pattern of ecological interactions

Abstract

The diversity of coliphages and indigenous coliform strains (ICSs) simultaneously present in horse feces was investigated by culture-based and molecular methods. The richness of coliforms (as estimated by the Chao1 method) is about 1,000 individual ICSs distinguishable by genomic fingerprinting present in a single sample of feces. This unexpectedly high value indicates that some factor limits the competition of coliform bacteria in the horse gut microbial system. In contrast, the diversity of phages active against any selected ICS is generally limited to one to three viral genotypes present in the sample. The sensitivities of different ICSs to simultaneously present coliphages overlap only slightly; the phages isolated from the same sample on different ICSs are usually unrelated. As a result, the titers of phages in fecal extract as determined for different Escherichia coli strains and ICSs may differ by several orders of magnitude. Summarizing all the data, we propose that coliphage infection may provide a selection pressure that maintains the high level of coliform diversity, restricting the possibility of a few best competitors outgrowing other ICSs. We also observed high-magnitude temporal variations of coliphage titers as determined using an E. coli C600 test culture in the same animal during a 16-day period of monitoring. No correlation with total coliform count was observed. These results are in good agreement with our hypothesis.

FIG. 1.

Dynamics of coliforms (white circles) and coliphages (black dots) plated on E. coli C600 in animals A, B, C, and D.

FIG. 2.

Examples of IS1 patterns of 10 ICSs isolated from animal B on day 4.

FIG. 3.

RFLP analysis of DNA of phages isolated from E. coli C600. (A) EcoRV digestion. The letter in the isolate code indicates the animal host. M, DNA size marker. (B) Morphology of C600 phages of all identified RFLP types (Table 1). C3, type I; D1, type II; A2, type III; and B1, type IV. The morphologies of phages of different subtypes within one genotype are identical (not shown). Bar, 100 nm.

FIG. 4.

RFLP analysis of DNA of phages isolated from five different ICSs from a fecal sample from animal C. EcoRV enzyme digestion is shown. Isolates on lines 1 and 2 were obtained from the same strain, isolates on lines 6 and 7 were obtained from another, and isolates on lines 3, 4, and 5 were each obtained from a particular ICS.