The theoretical knowledge must be reinforced by information obtained by its application to classical microbiological practice. This approach for designing instruments, which exploits theoretical knowledge substantiated by actual practice, is not new, but it has not been used in microbiology. Many of the microbiological analytical systems on or near the market have mainly depended on instrumentation of traditional microbiological analysis, considering only an end-point reaction, with little regard to the theoretical aspects of microbiology. An ideal clinical microbiological analysis system would give the results of detection, enumeration, identification, and graded antibiotic susceptibility within a few seconds, or minutes at most. To cover the scope of needs for developing an automated clinical microbiology analyzer, five rules were formulated based on the properties of microorganisms growing in a fluid medium. The rules are the concentration rule, the dilution rule, the equilibrium rule, the reaction rule, and the dimension rule.