This 1975 study examined the ultrastructure of extremely thermophilic acidophilic microorganisms (MT strains) isolated from volcanic hot springs near Naples using electron microscopy. The cells were nearly spherical, 0.7-1.0 μm in diameter, with primitive morphology consisting of ground cytoplasm, ribosomes, and randomly distributed DNA strands. Cells were enclosed by a plasma membrane and an approximately 20 nm thick extracellular coat displaying a regular hexagonal pattern. DNA strands remained dispersed throughout the cytoplasm during interphase but condensed into distinct nucleoid areas during cell division via binary fission with median constriction. The organisms possessed pili approximately 10 nm in diameter and lacked typical bacterial peptidoglycan walls and mesosomes. The authors encountered significant technical challenges during specimen preparation, finding that fixation in acidic culture medium caused cytoplasmic precipitation; optimal preservation required either separating cells by low-speed centrifugation or neutralizing the medium before fixation. The ultrastructural features were compared with other thermophilic acidophiles including Sulfolobus acidocaldarius and Thermoplasma acidophila, suggesting these organisms represent a distinct group with features intermediate between typical bacteria and mycoplasmas.

Key findings

• MT thermophilic acidophilic cells are nearly spherical (0.7-1.0 μm diameter) with primitive morphology lacking peptidoglycan walls and mesosomes, containing dispersed ribosomes and DNA strands
• Cells possess a hexagonally-patterned extracellular coat (~20 nm thick) with regular substructure and pili (~10 nm diameter), but lack typical bacterial cell wall components
• DNA strands remain uniformly distributed during interphase but condense into distinct nucleoids only during binary fission via cell constriction
• Technical optimization was critical: low-speed centrifugation and pH-buffered fixatives preserved ultrastructure better than direct fixation in acidic medium
• Ultrastructural features suggest these organisms represent a distinct group with characteristics intermediate between conventional bacteria and mycoplasmas