Abstract
A recent study proposed the novel classification of the family Mycobacteriaceae based on the genome analysis of core proteins in 150 Mycobacterium species. The results from these analyses supported the existence of five distinct monophyletic groups within the genus Mycobacterium. That is, Mycobacterium has been divided into two novel genera for rapid grower Mycobacteroides and Mycolicibacterium, and into three genera for slow grower Mycolicibacter, Mycolicibacillus, and an emended genus Mycobacterium, which include all the major human pathogens. Here, cryo-TEM examinations of 1,816 cells of 31 species (34 strains) belonging to the five novel genera were performed. The fundamental morphological properties of every single cell, such as cell diameter, cell length, cell perimeter, cell circularity, and aspect ratio were measured and compared between these genera. In 50 comparisons on the five parameters between any two genera, only five comparisons showed “non-significant” differences. That is, there are non-significant differences between slow grower genus Mycolicibacillus and genus Mycobacterium in average cell diameter (p = 0.15), between rapid grower genus Mycobacteroides and slow grower genus Mycobacterium in average cell length (p > 0.24), between genus Mycobacteroides and genus Mycobacterium (p > 0.68) and between genus Mycolicibacter and genus Mycolicibacillus (p > 0.11) in average cell perimeter, and between genus Mycolicibacterium and genus Mycobacterium in circularity (p > 0.73). The other 45 comparisons showed significant differences between the genera. Genus Mycobacteroides showed the longest average cell diameter, whereas the genus Mycolicibacter showed the shortest average diameter. Genus Mycolicibacterium showed the most extended average cell length, perimeter, and aspect ratio, whereas the genus Mycolicibacillus showed the shortest average cell length, perimeter, and aspect ratio. Genus Mycolicibacillus showed the highest average cell circularity, whereas genus Mycobacterium showed the lowest average cell circularity. These fundamental morphological data strongly support the new classification in the family Mycobacteriaceae, and this classification is rational and effective in the study of the members of the family Mycobacteriaceae. Because both the genus Mycolicibacterium and the genus Mycobacterium contain many species and showed larger significant standard deviations in every parameter, these genera may be divided into novel genera which show common genotype and phenotypes in morphology and pathogenicity.
Highlights
Mycobacterium is often described as “0.2–0.6 μm by 1.0– 10.0 μm, straight or slightly curved rod-shaped bacteria, sometimes branched, acid-fast.” Methods of measuring cell morphology include light microscopic examination of stained bacilli or electron microscopy of transmission or scanning
Rapid and slow grower species belonging to 5 genera in the family Mycobacteriaceae were examined cryo-transmission electron microscopy (TEM). 1,816 cells of 31 species (34 strains) were examined (Supplementary Figures S4–S8)
Representative cryo-TEM images of rapid grower species belonging to genus Mycobacteroides (6 species) and genus Mycolicibacterium (6 species, 7 strains) were shown in Figures 1, 2, respectively
Summary
Mycobacterium is often described as “0.2–0.6 μm by 1.0– 10.0 μm, straight or slightly curved rod-shaped bacteria, sometimes branched, acid-fast.” Methods of measuring cell morphology include light microscopic examination of stained bacilli or electron microscopy of transmission or scanning. Methods of measuring cell morphology include light microscopic examination of stained bacilli or electron microscopy of transmission or scanning. This study conducted whole-mount ice-embedded cryo-TEM examinations of more than 1,800 cells of species belonging to the family Mycobacteriaceae, obtaining precise cell morphology data on the cell length, cell diameter, cell perimeter, cell circularity, and aspect ratio. The present study was based on the novel classification and comparison of fundamental cell morphology data obtained from cryo-TEM examination between the five genera. Most comparisons of the cell length, the cell diameter, the cell perimeter, the cell circularity, and the aspect ratio revealed significant differences between the five genera These data suggest and support the idea that the novel classification of the family Mycobacteriaceae is proper and reasonable from the viewpoint of cell morphology and the ultimate phenotype
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