Simkania negevensis

Classification

Domain: Bacteria Kingdom: Bacteria Phylum: Chlamydiae Class: Chlamydiae Order: Chlamydiales Family: Simkaniaceae Genus: Simkania Species: negevensis

Discovery

S. negevensis was first discovered in 1993 as a contaminant of human and simian cell cultures of unknown origin in an Israeli laboratory by Kahane and colleagues, who originally characterized it as a chlamydia-like microorganism called ‘Z’ (1993).

Characteristics & Growth

Simkania negevensis is an obligate intracellular gram-negative bacterium that has been implicated in various respiratory diseases. Its growth cycle includes two distinct phases, the elementary and reticulate bodies, which are typical of the Chlamydiales. The main function of reticulate bodies (RBs) is to replicate through binary fission and are usually not infectious. Unlike C. trachomatis, the RBs of S. negevensis may also be infectious. The elementary bodies (EBs) do not replicate and are specialized for transmitting infection to other cells upon host cell lysis, so their electron-dense forms appear later during infection, after about 3 days (Kahane, Kimmel, & Friedman, 2002). Mature EBs of S. negevensis are distinguishable from other Chlamydiales by their quadrangular or dumbbell shapes (Kahane et al., 2002; Michel et al., 2005). After infection, S. negevensis undergoes rapid growth for 2-3 days, during which time reticulate bodies become abundant, and then enters a stationary phase for about 8 days . This cycle is much longer than most Chlamydiales, which typically lyse their host cells 2 to 3 days after infection.

The intracellular lifestyle of S. negevensis is supported by four nucleotide transport proteins that allow it to exploit the host cell’s energy and nucleotide pools by importing ATP, GDP, GTP, and RNA nucleotides. Since S. negevensis seems unable to make its own nucleotides from scratch, these proteins are vital to the biosynthesis of RNA and DNA.

Genome

The full-length 16S and 23S rRNA sequences are 80-87% identical with Chlamydiaceae, and the genome length is 1.7 Mbp, which is 2-3 times larger than the genome of Chlamydiaceae (Kahane, Everett, Kimmel, & Friedman, 1999). The G + C content of S. negevensis and other Chlamydiae is about 40% despite the smaller genome size of Chlamydia (Collingro et al., 2011; Kahane et al., 1999). Simkania contains a 132-kb plasmid that is ten times larger than that found in Chlamydiaceae and encodes for proteins involved in metabolic processes, the maintenance and propagation of the plasmid, and virulence (Collingro et al., 2011).

Pathogenicity

Infants with bronchiolitis and adults with community-acquired pneumonia (CAP) or chronic obstructive pulmonary disease (COPD) have been linked to infection with S. negevensis (Karunakaran, Mehlitz, & Rudel, 2011; Lieberman, Kahane, Lieberman, & Friedman, 1997). Evidence of S. negevensis infection has been supported with culture, serological tests, and PCR. Despite a lack of reports concerning widespread S. negevensis infection in humans, a high seroprevalence (46-80%) of S. negevensis antibodies has been identified in healthy adults from all over the world (Lieberman et al., 1997). This may be due to the bacteria’s ability to survive and replicate within amoeba, thus supporting a broad distribution of S. negevensis, particularly through aquatic environments (Michel et al., 2005).

Similar to Chlamydia-infected cells, S. negevensis-infected cells display anti-apoptotic mechanisms to ensure host cell survival, allowing the bacteria to continue replication. The apoptotic resistance of cells infected with S. negevensis reaches a peak during the rapid growth phase (2-3 days), and the cells become slightly more susceptible to apoptosis after this phase (Collingro et al., 2011).

Patients with CAP made quick recoveries after treatment with erythromycin (Lieberman et al., 1997). S. negevensis is sensitive to tetracyclines and macrolides, but its growth is unaffected by sulfadiazine (Donati et al., 2013; Kahane et al., 1999, 1993). S. negevensis is distinguished from Chlamydiaceae by its resistance to ampicillin, penicillin G, and cyclosporine (Kahane et al., 1993).

Because S. negevensis is able to thrive in both amoeba and human host cells and contains a conjugative plasmid, it has considerable potential as a model organism (Collingro et al., 2011; Michel et al., 2005).