**3.** *C. pneumoniae***: a pathogen causing more than pneumoniae**

*C. pneumoniae* is a widespread cause of infection, with an estimated seroprevalence of over 50% among adults in many countries [35–37]. However, the prevalence is relatively low in children under 5 to 10 years old (7–8%), but it sharply increases to 40–55% in those aged 20 and continues to increase gradually, reaching 70–80% in the elderly [38, 39].

In 1989, Grayston identified *C. pneumoniae* as a novel species based on its distinct morphology, DNA sequence, and associated clinical disease spectrum within the Chlamydiae family [40]. Subsequently, *C. pneumoniae* has been linked to 6–22% of upper and lower respiratory tract infections, including pharyngitis, laryngitis, sinusitis, bronchitis, and pneumonia, in both children and adults [41–43]. This obligatory intracellular pathogen has been associated with an extensive range of conditions, such as cardiovascular disease, Alzheimer's disease, arthritis, lung cancer, diabetes, and asthma [44].

### **3.1 Biology and developmental cycle**

*C. pneumoniae* is a human pathogen belonging to the Chlamydiae family. Its developmental cycle is complex and involves alternating between an infectious, extracellular elementary body, and a noninfectious, intracellular reticulate body. These two forms exist in a membrane-bound compartment called an inclusion, located inside a mucosal cell. After multiple replications, the reticulate body returns to the elementary body form and is released from the host cell, enabling it to infect nearby cells. This life cycle plays a crucial role in the molecular pathogenesis of chronic chlamydial infections.

To inject effector molecules into host cells, Chlamydia spp. utilizes a type III secretion system (T3SS). This T3SS produces a unique family of proteins known as inclusion membrane proteins (Incs). Incs are essential for the intracellular survival of Chlamydia spp. as they recruit host proteins to the inclusion, hijack the endocytic-lysosomal pathway, and help maintain the structural integrity of the inclusion. Additionally, Incs can enhance virulence by interfering with host antimicrobial pathways, promoting resistance to apoptosis, or constructing novel complexes with unique functions [45, 46].

Although studies of *C. trachomatis* have contributed significantly to our understanding of chlamydial infection and metabolism in humans, not all of these findings apply to *C. pneumoniae*. Significant differences in transcription, metabolism, and morphology exist between *C. pneumoniae* and other chlamydial species.

Research has shown that *C. pneumoniae* can modulate host cell apoptosis to evade detection by the host's immune system by interfering with tumor necrosis factoralpha (TNF-alpha) and various signaling pathways [47]. This trait of the microorganism suggests that if the host cell can survive after the expulsion of extracellular vesicles, it could enable further reinfection and the maintenance of chronic, asymptomatic disease. The ability of *C. pneumoniae* to spread from the lungs to distant body parts and persist in those tissues for an extended period is essential to the development of the infection [48].

Persistence of Chlamydia Infection: Chlamydia infection is caused by the direct effects of chlamydial proteins, as well as mechanisms that utilize the host cell's machinery. When exposed to stressful conditions, Chlamydiae cease production of infectious extracellular bodies (EBs) and instead form viable but noninfectious forms characterized by a continued synthesis of unprocessed 16S rRNA and genomic replication [47]. These persistent forms can remain in the host for a prolonged period and are often associated with enlarged and malformed RBs, which can return to the normal developmental cycle when the inducing factor is removed [44]. *In vitro*, experiments have shown that several factors, including exposure to interferon-gamma (IFN) or antibiotics (such as penicillin and amoxicillin) and nutrient deprivation, can trigger the formation of persistent forms.
