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The Family SimkaniaceaeDescription of Simkania gen. novSimkania (Sim.ka«ni.a. M.L. fem. n. Simkania arbitrary name formed from the personal name Simona Kahane). Currently the description of Simkania corresponds to the description of the family Simkaniaceae . The natural host of Simkania is not known, but serological evidence and PCR indicate that it is widespread among humans.New members of the genus Simkania should have 16S or 23S rRNA genes that are"95% identical to those of Simkania negevensis, the type species [Everett et al., 1999].A multi locus typing scheme based on seven housekeeping genes of members of the Chlamydiales supported the current distinction of the Family Simkaniaceae among the order Chlamydiales [Pannekoek et al., 2008]. Simkania negevensisDescription of Simkania negevensis sp. nov.Simkania negevensis (ne.ge.ven«sis. M.L. adj. negevensis of or pertaining to the Negev, a desert in southern Israel).The species Simkania negevensis currently includes only the type strain, Simkania negevensis strain ZT =ATCCVR1471T. Simkania negevensis was discovered as a bacterial contaminant in cell cultures. The description of this species is derived from that of the `micro-organism Z' (Kahane et al., 1993, 1995, 1999). Strain ZT does not contain an extrachromosomal plasmid and has an unusually slow developmental cycle in cultured Vero cells (up to 14 days), compared to other chlamydiae. Unlike other chlamydial rRNA genes that have been characterized, Simkania negevensis has a group I intron in 23S rRNA position 1931 (E. >coli numbering) and this intron is not spliced out of the rRNA. The intron is closely related to group I introns in the 23S rRNA of chloroplasts and mitochondria in algae and amoebae [Everett et al., 1999]. Environmental and other studies indicate that there are many more Simkania-like agents with a close relationship to the family (Horn & Wagner, 2001; Ossewaarde & Meijer, 2000). The organism is capable of growing and surviving within acanthamoebal cysts (Kahane et al., 2001) and the DNA of organisms related to Simkania has been identified in environmental samples from sewage and waste water treatment plants (Horn & Wagner, 2001). Developmental cycle.S. negevensis has a slow developmental cycle in cultured Vero cells (12–14 days) compared with other Chlamydiales. By light microscopy, changes in the appearance of the infected cell as development progresses are dramatic. Tarry masses of tiny vacuoles appear in the cytoplasm. They enlarge and become angular, but appear empty until about 7 days after infection. However, in the final 7–14 days of the developmental cycle the vacuoles fill with small, flickering particles. Transmission electron microscopy revealed that reticulate bodies (RBs) of S. negevensis in Vero cells were similar to the RBs of other chlamydial species. Electron-dense forms of S. negevensis analogous to elementary bodies (EBs) began to appear on the third day post-infection, but quantitatively did not account for the high titre of infectivity in extracts from these host cells. These EBs had both electron-dense and electron-lucent areas, a characteristic seen only in a few chlamydial species. S. negevensis infectivity did not appreciably change during the ensuing 12 days required for host cell lysis, despite an eightfold increase in the proportion of electron-dense bacteria over this time. The emergence of electron-dense bodies, increase in infectivity and host-cell lysis were not synchronized developmental events, suggesting that the RBs of S. negevensis, unlike the Chlamydiaceae, may also be infectious [Kahane et al., 2002]. This remains to be confirmed. Genome and taxonomyS. negevensis has a genome length of 1.7 Mb compared to 1.0 - 1.2 Mb for the Chlamydiaceae. Its full length 16S and 23S rDNA sequences are 80 - 87% identical to the Chlamydiaceae. Within the Chlamydiaceae identity for these gene sequences are >90% [Everett et al., 1999a; Kahane et al., 1999]. Strong evidence that S. negevensis is a member of the Order Chlamydiales comes from the fact that, like other members of the order, it has characteristic insertions and deletions in the MurA gene associated with peptidoglycan synthesis [Griffiths & Gupta, 2002]. Apart from its prolonged developmental cycle, S. negevensis differs from the other members of the order Chlamydiales in two further respects:
Clinical significanceS. negevensis has been associated with community acquired pneumonia in adults (Lieberman et al., 1997), with bronchiolitis in infants (Kahane et al., 1997) and with exacerbations of chronic obstructive pulmonary disease (Lieberman et al., 2002). Eight new isolates of Simkania negevensis were isolated from 200 nasopharyngeal washes from cases of respiratory infection in the Negev Desert. Seven of these 8 isolates had identical nucleotide sequences in a small amplified segment of the 16S ribosomal DNA. The discrepant isolate differed in one nucleotide base only. Antibodies raised against these strains showed very little cross reactivity with members of other families in the Order Chlamydiales. (Friedman et al., 1998; Kahane et al., 2000). Antibody to S. negevensis was found in the serum of UK persons with acute respiratory tract infections (n=120), rising from an antibody prevalence for IgG of 15% in British children age 1 to 4 years up to 74% in adults age greater than 19 years. For C. pneumoniae in the same group, the comparative figure was 0% prevalence before age 5 and 49% over age 19. These data suggest the cumulative acquisition of antibody to S. negevensis with age in the UK population and indicate that the occurrence of antibody to S. negevensis was independent of antibody to C. pneumoniae (Friedman et al., 2000). Substantial prevalences of antibody to S. negevensis were also found in the sera of pregnant women from Vancouver, Canada (68% versus 35% for C. pneumoniae; n=100); in sera from Brooklyn, New York as well as the Negev (Friedman, Dvoskin & Hartley , 2000). Further evidence of prior S. negevensis infection was found in paired serum samples from Danish patients with persistent cough (Friedman, Birkebaek & Dvoskin, 2000). Collectively, these data suggest that S. negevensis infection and organisms related to Simkania may have a wide geographical and environmental distribution. Simkania-like nucleotide sequences have been detected from fresh-water sources, suggesting that amoebae may be a natural host for this organism. Most of the studies of possible S. negevensis infection are based on serology rather than direct detection of the organisms or its DNA. So far there is little evidence that S. negevensis is a significant cause of disease in humans but, interestingly, a Simkania-related nucleotide sequence was detected by PCR from an aortic aneurysm [Ossewaarde & Meijer, 1999] indicating the need for further research using direct detection methodology. For a more detailed review of possible infections caused by these organisms see Friedman et al., 2003. 'Candidatus Fritschea bemisiae' and 'Candidatus Fritschea eriococci'An electron microscope study of bacteriocytes in Bemisia tabaci (whiteflies) revealed two morphological types: electron dense globular bodies (0.2-0.25 nm) and a less dense morphological form (0.7-0.8 nm wide, 0.7-2.5 nm long [Costa et al., 1995]. A 16.6 kb gene segment that included the 16S rDNA and 23S rDNA operon was cloned and sequenced and found to contain six genes encoding proteins similar to those found in the Chlamydiales. The full length sequences of 16S rDNA and 23S rDNA of another strain were obtained from the whitefly Eriococcus spurius. Neither lineage of bacteria has been cultured outside insects, so these chlamydiae are being proposed as belonging to a new genus within the family Simkaniaceae with two new species, 'Candidatus Fritschea bemisiae' and 'Candidatus Fritschea eriococci' [Thao et al., 2003]. F. bemisiae but not F. eriococci contains the group 1 intron in the locus of the 23S DNA that was reported by Everett et al., 1999 in S. negevensis [Everett, personal communication]. For a review of prokaryotes associated with Bemisia tabaci see Zchori-Fein et al., 2002. Xenoturbella endosymbiontsXenoturbella is an enigmatic marine animal that is merely a ciliated bag with epithelial epidermis and gastrodermis and a ventral opening (“mouth”), but without any distinct organs except a “statocyst” containing motile flagellated cells. Ultrastructural observations of the gastrodermal cells in Xenoturbella revealed numerous chlamydiae which were shown to be related to "Candidatus Fritschea" and Simkania (Simkaniaceae) by 16S and 23S rRNA. Their 23S rRNA gene contain a characteristic intron encoding a putative homing endonuclease. The chlamydiae were pleomorphic and formed intravacuolar colonies. They had flattened disk-shaped elementary bodies, either oval or bow tie-shaped in cross-section, and reticulate bodies that were spherical, polygonal or irregularly shaped. All stages had a five-layered cell wall with a rippled appearance. The association between these chlamydiae and Xenoturbella was characterized by an absence of cytopathological effects; limited host cell response against the chlamydiae; the confinement of the chlamydiae to inclusions in some part of the host cell; and complete and uniform infection of all examined hosts [Israelsson, 2007].
[MEW] May 2004 NEXT: 'chlamydia-like' organisms Web resourceNCBI protein sequences of F. bemisiae NCBI nucleotide sequence of 16 and 23S rRNA genes F. eriococci NCBI nucleotide sequences S. negevensis NCBI protein sequences S. negevensis References
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