Characterization of Lactobacillus sp. strain 100-37 from the murine gastrointestinal tract: Ecology, plasmid content, and antagonistic activity toward Clostridium ramosum H1

Abstract

A gram-positive, nonsporulating, microaerophilic rod that had two colonial variants was obtained during a study in which anaerobic bacteria were isolated from murine gastrointestinal tracts and screened for cryptic plasmids. The rod (both colonial variants) was identified as a Lactobacillus sp. (strain 100-37) by seletive media, gas chromatography, and biochemical tests. In monoassociated, ex-germfree mice, the bacterium colonized the gastrointestinal tract and formed a thick, continuous layer on the keratinized squamous epithelium of the nonsecreting portion of the stomach. When lysate preparations of both colonial variants were electrophoresed in agarose gels, two bands which stained with ethidium bromide were detected with each lysate. When the DNA generations were exposed to UV light, the lower ethidium bromide band gradually disappeared while the top band became either broader or more intense. The approximate size of the lower band was 2.2 megadaltons, as determined by comparison with plasmid molecular weight standards. In a search for phenotypes which could be encoded by the cryptic 2.2-megadalton plasmid, we detected an antagonistic activity toward an obligate anaerobe isolated from mouse feces, Clostridium ramosum H1. The antagonistic factor was precipitated with (NH4)2SO4 (70% saturation) from supernatant solutions of broth cultures of strain 100-37. The factor was not inducible with mitomycin C or UV light, but was stable in flowing steam for up to 50 min, and in buffers of pHs over a range of 1.6 to 6.8. It was nondialyzable and inactivated by trypsin and papain. In an effort to demonstrate that this antagonistic property was encoded by the 2.2-megadalton plasmid, we attempted to cure the plasmid with a variety of methods. No cured variant was obtained. The characteristics of the antagonistic property are similar to those of previously described bacteriocins of lactobacilli; thus, the property may be a true bacteriocin. Lactobacillus sp. strain 100-37 and C. ramosum H1 may be useful for the study of bacteriocin involvement in the ecology of the murine gastrointestinal tract.