Three species of genus Chlamydia in family Chlamydiaceae, namely C. pneumoniae, C. psittaci, and C. trachomatis are known to cause respiratory infections in humans. C. pneumoniae was previously known as Taiwan acute respiratory agent (TWAR) named from the two original isolates: TW-183 of Taiwan and an acute respiratory isolate designated AR-39. Most respiratory infections caused (about 70%) by C. pneumoniae are asymptomatic or only mild, but a few (30%) of them lead to more serious respiratory diseases, such as community-acquired pneumonia (CAP), bronchitis and upper respiratory tract infections (URTIs). It is transmitted from human to human through respiratory droplets without any other known animal hosts.
Fig. 1 C. pneumoniae developmental cycle (Di Pietro M, et al. 2013)
C. pneumoniae is a gram-negative obligate intracellular bacterium with the size of 0.2-1um. It exists in two different forms; infectious elementary body (EB) and the reticulate body (RB) with metabolic activity and replication after causing cell lysis. EB is an extracellular infection form of Chlamydia, with inactive metabolism. It enters the respiratory tract through inhalation, and then attaches to the mucosal surface. The EB enters cells through receptor-mediated endocytosis and differentiates into RB in inclusion bodies. RBs are able to change the host cell pathway and replicate within 24 hours after infection. After about 48 to 72 hours, the new born RB will turn to EB and are released back to the lung. And these new EB will continue to infect other host cells, either in the same organism or in a new host.
Serological analysis is a routine method for the diagnosis of C. pneumoniae infection. The increase of C. pneumoniae IgM titer in CAP patients can be regarded as a sign of acute infection, while the increase of IgG (by four times) indicates previous infection. Chlamydia culture can be conducted in a special medium using respiratory secretions, but it is very difficult and requires professional equipment. Polymerase chain reaction (PCR) based detection can be carried out on nasopharyngeal swabs, sputum or bronchoalveolar lavage, but it is difficult to implement due to its complexity.
The Centers for Disease Control and Prevention (CDC) recommended azithromycin as the first-choice drug for the treatment of C. pneumoniae infection. Tetracycline, clarithromycin, doxycycline or fluoroquinolone are treated as alternatives. It is reported that AZD0914, a new research drug that inhibits DNA cyclooxygenase and topoisomerase IV, has the same effect on chlamydia infection as other proven antibiotics. Effective and safe C. pneumoniae vaccines is under development. A new chlamydia vaccine based on recombinant protein subunit (CTH522) has passed the first human clinical trial. This vaccine targets C. trachomatis, which seems to have many similarities and weaknesses with C. pneumoniae. It may be contributed to the research and development of C. pneumoniae vaccine.
