Eubacteria are unicellular prokaryotic microorganisms that have rigid cell walls, flagella, DNA in the form of a single circular chromosome, and lack a nucleus. They include all types of bacteria except archaebacteria. Eubacteria are classified into gram-positive and gram-negative types based on their cell wall composition and stain type. They reproduce through binary fission or budding and range in size from 0.2 to over 5 micrometers.