The outer layer of Bacterian is made of rock. It has craters all around the planet with hatches inside the craters that send out Bacterian ships. The middle layer of Bacterian is completely mechanical; It has smaller hatches, Ducker robots, moving ceilings and floors, mini spider-walkers, a gunwall, and a normal sized spider walker. The core of Bacterian is organic with regenerating walls, floating brains, and organic turrets as the planet's final defense. The inner core contains the nucleus, surrounded by faces, and an mechanical escape route should the nucleus be destroyed.
Anime
In Salamander 3 OVA, Bacterian appears as a planet located in an unknown star system known as the Lazaford System. In this Anime, Bacterian appears to be a planet of crystal surrounding by rings. The crystals are the Bacterians in origin. There is a Bacterian sun that contains Bacterians in their crystal form on the inside just like planet Bacterian; this implies the Bacterian sun and Bacterian are the same.
Each time Bacterian is destroyed, a piece of it either regenerates into a new planet Bacterian or infects a planet into a fortress.
The strange part about Bacterian though, is the when the player entered it in Gradius III AC, some part of the planet was mechanical.
You may or may not notice it, but like most Fortresses, when you enter Bacterian, it may look mechanical, but when you get to the inner depths of it, it looks completely or partly organic.
