Effect of penicillin on the morphology and reproduction ofAzotobacter chroococcum

The effect of penicillin on the morphology and reproduction of some strains ofA. chroococcum was studied on a number of solid media. When the growth was not entirely suppressed by the penicillin, filamentous cells and spheroplasts were formed. The formation of spheroplasts was stimulated by peptone. Gonidia were sometimes formed inside the spheroplasts and also inside giant cells. They were released from the cell after disruption or after lysis of the cell wall. In some cases they produced dwarf cells. Under certain conditions groups of gonidia present in a cell fused and formed one or more normal-looking cells inside the mother cell. Sometimes one or moreAzotobacter cells developed inside a spheroplast or at the site of a spheroplast with a lysed cell wall. Microcolonies consisting of small cocci representing gonidia and dwarf cells were also observed occasionally at the sites of spheroplasts with lysed cell walls. Occasionally tiny groups of small elements with a less marked structure were found at such sites, probably representing debris of lysed cells. The production of normal-looking cells inside filamentous cells was greatly stimulated on a medium containing 10 percent horse serum, with a drop of sterile water containing 200 or 250 I.U. penicillin added in the centre of the plate. The growth ofA. chroococcum was greatly retarded when the medium contained 10 U/ml penicillin and seemed to be checked entirely at concentrations of 20 U/ml penicillin or higher. Occasionally, however, even at concentrations of 100 and 300 U/ml penicillin, a few filamentous cells were found and also a few microcolonies, visible only through the microscope, consisting of gonidia or regenerative rods. By repeated exposure ofAzotobacter to penicillin populations could be obtained that were adapted to high concentrations of this antibiotic.