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Growth, Cell Division, and Fragmentation in a Species of Flexibacter

¹ Department of Microbiology, and Mathematics Department, Indiana University, Bloomington, Indiana 47401

ABSTRACT

Flexibacter FS-1, a gram-negative gliding bacterium was grown in liquid culture as long (over 100-µm) filaments. The filaments possessed a triple-track wall which resembled that found in other gram-negative bacteria. Although phase-contrast microscopy indicated that the long filaments were nonseptate, electron microscopy revealed three or four septa along the length of each filament. The septa contained lysozyme-sensitive, electron-opaque material, presumed to be peptidoglycan, sandwiched between cell membranes. The outer triple track wall was not part of the septum. Mesosomes were seen in various areas of the cell and frequently were observed attached to septa in different stages of completion. Studies of the organism in slide culture revealed that individual filaments grew in an exponential fashion and divided in the middle despite the long length and multiseptate condition. When the temperature of a liquid culture growing exponentially with a generation time of 90 minutes was shifted from 30 to 35 C, the filaments fragmented into three or four shorter cells within 120 min. The short cells continued to grow exponentially at 35 C at approximately the same rate as at 30 C. When the culture was shifted back to 30 C, the cells immediately stopped dividing and began to elongate. After a period of 2 to 3 hr, cell division resumed. It is suggested that the shift-up in temperature induced the completion of the cross wall (centripetal growth of the triple-track wall) and cell separation at the sites of previously formed septa, whereas the shift-down in temperature caused a transient inhibition of cross-wall formation but not of growth. Fragmentation was inhibited by sodium azide but took place despite the inhibition of protein synthesis by chloramphenicol or the inhibition of deoxyribonucleic acid synthesis by mitomycin C.