This cytological study examines structural differences between smooth and rough bacterial variants in genera Bacterium and Bacillus. Smooth cultures contain unicellular bacteria with two chromatinic bodies that divide by constriction after forming a membranous septum. Rough cultures comprise multicellular bacteria, typically containing four cellular units separated by membranous septa, each unit containing a single chromatinic body. Using acid-Giemsa and tannic-acid-violet staining techniques, Bisset demonstrates that smooth bacteria divide via inward growth of the existing cell wall, while rough bacteria form complete new cell walls secreted internally by membranous septa. The study involved Gram-negative intestinal bacteria including Bacterium coli and Gram-positive sporing bacilli isolated from laboratory dust. Chromatinic bodies divide before cell division in both forms, though rough bacteria maintain a four-celled structure with eight chromatinic bodies before dividing. The parallel between smooth and rough colony morphologies in different bacterial groups reflects fundamental differences in cellular structure and division mechanisms, though the biological significance of this variation remains unclear.

Key findings

• Smooth bacterial variants are unicellular with two chromatinic bodies dividing by constriction; rough variants are multicellular with four units, each containing one chromatinic body
• Rough bacteria form transverse septa continuous with the cell wall, while smooth bacteria divide by inward constriction of existing cell wall material
• Cell wall septa in rough bacteria are secreted internally within membranous septa, similar to septation in higher plant cells
• Rough and smooth morphologies in Bacterium and Bacillus genera show consistent cellular and divisional characteristics distinct from their respective colony structures