Bacteria sensitive to nutrient broth medium in terrestrial environments

Effect of dilution of plate counting media were studied on various terrestrial samples. It was noticed that nutrient broth (NB) medium always gave less counts than its 100-fold dilution (DNB). Many isolates on diluted media did not show appreciable growth on NB agar medium. Such isolates whose growth was suppressed in NB medium but not in DNB medium were called “DNB-organisms.” Percentages of DNB-organisms among isolates were 58% with Japan Alps soil, 30–40% with river and field samples, and 5–10% with avenue samples. About half the DNB-organisms were gram-negative, 10% were gram-positive, and the remainder were irregularly gram-stained organisms. About 65% of the DNB organisms from Japan Alps soil and 80% of those from the field soil showed irregularly rodshaped forms. Almost all irregularly gram-stained organisms were also irregularly rod-shaped. Those from river samples were either rod-shaped or filamentous. All the DNB-organisms were non-sporeformers.