Streptomycin Sensitivity of Oral Lactobacilli

Abstract

IN an attempt to find markers for transduction experiments with temperature Lactobacillus fermenti phages¹ the sensitivity of Lactobacilli to various antibiotics was determined. The strains comprised 112 L. acidophilus, 25 L. casei, 19 L. plantarum, 25 L. casei var. rhamnosus, 10 L. salivarius, 120 L. fermenti and 12 L. brevis. These were isolated from, human saliva and were identified by the methods of Rogosa et al.². Each species included one type-strain kindly supplied by Dr. M. E. Sharpe, of the National Institute for Research in Dairying, Reading, England. The composition of the tomato dextrose agar and broth used has been described³. Antibiotics were incorporated in the molten agar at 45° C. The concentrations used were: 1, 2.5, 5 and 10 µgm./ml. for penicillin G, chloramphenicol, tetracycline, erythro-mycin and novobiocin and 10, 100 and 500 µgm./ml. for polymyxin B and kanamycin. Streptomycin sulphate was used in 10, 50, 100, 200, 500 and 1,000 µgm./ml. The inoculum consisted of a standard loopful of a vigorously growing broth culture. Four inocula were made per plate. Plates were incubated at 37° C. in containers in which most of the air had been replaced by carbon dioxide. Cultures were incubated for 48 hr. before determining the presence of growth. Similar inocula on the medium without antibiotics served as controls.