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The presence of a periplasmic space within the cell envelope of Thermus thermophilus was analyzed in a mutant (HB8ΔUTR1) defective in the regulation of its S-layer (surface crystalline layer). This mutant forms round multicellular bodies (MBs) surrounded by a common envelope as the culture approaches the stationary phase. Confocal microscopy revealed that the origin of the MBs is the progressive detachment of the external layers coupled with the accumulation of NH₂-containing material between the external envelopes and the peptidoglycan. A specific pattern of proteins was found as soluble components of the intercellular space of the MBs by a single fractionation procedure, suggesting that they are periplasmic-like components. To demonstrate this, we cloned a gene (phoA) from T. thermophilus HB8 encoding a signal peptide-wearing alkaline phosphatase (AP), and engineered it to be …