The in-vitro inactivation of metronidazole by different clinical isolates of Enterococcus faecalis was investigated by means of association experiments in which Ent. faecalis strains and Bacteroides fragilis group strains were cultured in the same liquid medium. All of the tested Ent. faecalis strains (20 isolates) were able to protect the B. fragilis group strains against the killing effect of metronidazole at a concentration four or eight times higher than the normal MIC. Different strains of Streptococcus (14), Staphylococcus aureus (10), Staph, epidermidis (10) and Escherichia coli (8) failed to exhibit the same effect. When Ent. faecalis strains were cultured anaerobically for 24 h in the presence of 4 mg metronidazole/l, either alone or together with different B. fragilis group strains, no metronidazole could be detected subsequently in the culture supernatants by HPLC. Concomitantly an increase of four or five logs in the viable counts of the co-cultured Bacteroides strains was observed compared with Bacteroides strains cultured alone. Sonicated cell extracts of Ent. faecalis cultured either aerobically or anaerobically were found to inactivate metronidazole to the same extent, whereas the culture supernatants had no such effect.

Electron microscopic investigation of lysogeny of Clostridium dfficile strains isolated from antibiotic-associated diarrhea cases and from healthy carriers APMIS YY: 321-326, 1991.An electron microscopic investigation was performed on 28 Clostridium dificile strains isolated from 15 antibiotic-associated diarrhea cases and from 13 healthy infants. Through the use of supernatants of the cultures induced by mitomycin C (1 or 3 pg/ml), 18 of the 28 C. diJjcicile strains proved to harbor phage particles with a different morphology and size. Most of the phages revealed belonged in phage group B1 with a non-contractile tail. Seven of the 11 toxin-producing, lysogenic C. dificile strains carried a defective phage structure (120 nm-long tail with an incomplete head capsule) alone or together with other normal phages. With different C. diflicile strains as indicator, plaque formation could not be detected in any of the lysates.

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