Rickettsiaceae, rickettsia-like endosymbionts, and the origin of mitochondria.

Accumulating evolutionary data point to a monophyletic origin of mitochondria from the order Rickettsiales. This large group of obligate intracellular alpha-Proteobacteria includes the family Rickettsiaceae and several rickettsia-like endosymbionts (RLEs). Detailed phylogenetic analysis of small subunit (SSU) rRNA and chaperonin 60 (Cpn60) sequences testify to polyphyly of the Rickettsiales, and consistently indicate a sisterhood of Rickettsiaceae and mitochondria that excludes RLEs. Thus RLEs are considered as the nearest extant relatives of an extinct last common ancestor of mitochondria and rickettsiae. Phylogenetic inferences prompt the following assumptions. (1) Mitochondrial origin has been predisposed by the long-term endosymbiotic relationship between rickettsia-like bacteria and proto-eukaryotes, in which many endosymbiont genes have been lost while some indispensable genes have been transferred to the host genome. (2) The obligate dependence of rickettsiae upon a eukaryotic host rests on the import of proteins encoded by these transferred genes. The nature of a proto-eukaryotic cell still remains elusive. The divergence of Rickettsiaceae and mitochondria based on Cpn60, and the evolutionary history of two aminoacyl-tRNA synthetases favor the hypothesis that it was a chimera created by fusion of an archaebacterium and a eubacterium not long before an endosymbiotic event. These and other, mostly biochemical data suggest that all the mitochondrion-related organelles, i.e., both aerobically and anaerobically respiring mitochondria and hydrogenosomes, have originated from the same RLE, while hydrogenosomal energy metabolism may have a separate origin resulting from a eubacterial fusion partner.

Morphologically variable bacilli of cat scratch disease are identified by immunocytochemical labeling with antibodies to Rochalimaea henselae.

The identification of the causative organisms of cat scratch disease (CSD) has been elusive. The demonstration of Warthin-Starry stain-positive pleomorphic bacilli in lymph nodes of patients with CSD and recent serologic and epidemiologic data suggest an etiologic role of Rochalimaea henselae in CSD. The authors studied lymph node biopsy specimens of 46 patients with illnesses clinically consistent with CSD and found pleomorphic bacilli in 15 (33%). The organisms were labeled by polyclonal rabbit antibodies induced by outer surface proteins of R henselae. This finding further supports the possibility of an important role of R henselae in the pathogenesis of CSD.

Action of penicillin G on endosymbiote lambda particles of Paramecium aurelia.

The kinetics of loss from the cytoplasm and changes in ultrastructure of symbiont lambda particles after treatment of axenically cultivated lambda-bearing Paramecium aurelia with penicillin G was investigated. Low concentrations (1 to 2 unit/ml) of the antibiotic caused many particles within the cell to become filamentous; high concentrations (2,000 unit/ml) caused lysis of the particles without noticeably affecting the protozoan. The ED(50) value (2 to 3 unit/ml) was within the range of values found to cause lysis of many gram-negative bacteria. Rapidly dividing lambda were more vulnerable to the action of the antibiotic than slowly dividing particles. Nondividing particles were not affected by exposure to the antibiotic. Ultrastructural changes observed in lambda during lysis by penicillin G were consistent with the view that penicillin interferes with the synthesis of a vital component of the cell envelope of the particle, possibly a peptidoglycan similar to that found in the cell walls of bacteria. The deoxyribonucleic acid of lambda was dispersed throughout the particle as electron dense fibers enclosed within electron transparent areas. The cell envelope appeared to consist of at least two morphologically distinguishable layers, an inner layer homologous to the plasma membrane of bacteria and an outer layer homologous to the bacterial cell wall. Lambda may be regarded as a randomly distributed population of bacteria growing and dividing synchronously within the collective cytoplasm of its protozoan host.