The requirement of high salt (NaCl) concentrations for growth of microorganisms. Microorganisms (mainly bacteria) can be classified by their physiological tolerance to salt. Most normal eubacteria, such as Escherichia coli and Pseudomonas fluorescens, and most fresh-water microorganisms, are nonhalophiles (best growth at less than 1.2% NaCl). Slight halophiles (1.2–3% NaCl) include many marine microorganisms. Moderate halophiles (3–15% NaCl) include Vibrio costicola, Paracoccus halodenitrificans, and many others. Borderline extreme halophiles (9–25% NaCl) include the photosynthetic bacterium Ectothiorhodospira halophila, the actinomycete Actinopolyspora halophila, and the halophylic archaebacteria Halobacterium volcanii and H. mediterranei. Extreme halophiles (require at least 10% NaCl; optima 15–30% NaCl) are Halobacterium salinarium and Halococcus morrhuae.