Sexually transmitted infections or STIs — such as chlamydia, gonorrhea and herpes — carry a long history of social stigma and shame. Their persistence as a taboo topic, however, is interesting because their incidence is quite common. Take Chlamydia trachomatis, for which over 1.7 million new infections of the genital tract are reported to the CDC each year. Clinically speaking, Chlamydia trachomatis serves as both the most common causative agent of bacterial sexually transmitted infection and of infectious blindness worldwide. While infections are treatable, strains of C. trachomatis can cause various diseases if left untreated. Common diseases which may result from untreated C. trachomatis infection include pelvic inflammatory disease (PID) and ectopic pregnancy, and many of these diagnoses occur among young women. Therefore, it is important that sexually active young people are tested for STIs including chlamydia, and that testing becomes more widely accessible in order to reduce disease incidence. Research on Chlamydia species can help identify how to best test for and treat infections (i.e. when to treat and with which antibiotics).
Chlamydia trachomatis, biologically speaking, is a gram-negative bacterium which can only replicate in host cells. C. trachomatis makes use of a two-phase developmental (or life) cycle characterized by two forms of the bacteria: elementary bodies (EBs) and reticulate bodies (RBs). A diagram depicting the life cycle and conversions between these bodies can be seen below. Characteristically, EBs are infectious, extracellular and smaller in size. RBs replicate, are intracellular (in host cells) and larger. If development of either of these forms is prevented (naturally or through scientific manipulation), the infection cycle of C. trachomatis is effectively stopped. EBs attach to and enter host cells – they are also surrounded by a vacuole called an inclusion. After invading a host cell, infectious EBs begin to differentiate into RBs, then replicate and divide via a process which depends on peptidoglycan synthesis. Peptidoglycan (PG) is a mesh-like structure composed of amino acids and sugars which forms the cell wall of many bacteria – its synthesis and turnover at the septum (between daughter cells) is crucial for the conversion from EBs to RBs (and back to EBs). C. trachomatis EBs eventually leave their host cell through one of two mechanisms. In one method, the inclusion (C. trachomatis-containing vacuole) membrane may expand enough that it kills the host cell, freeing the newly-developed EBs. Alternatively, sections of the inclusion may start to bleb off into the environment surrounding the host cell – this occurs through a process called “extrusion.”
Figure 1. Diagram of Chlamydia trachomatis developmental cycle
As scientists know from early research on Chlamydia, a third form of the bacteria exists which does not replicate: aberrant bodies (ABs). These bodies are able to remain living inside the inclusion vacuole for long periods of time. ABs tend to be much larger than the other forms in size and develop out of instances of cell division inhibition while cell growth persists. The pathways which result in AB formation are not yet fully understood, though they may develop due to an underlying stress response pathway which affords C. trachomatis a degree of protection during infection. Persistence of the bacteria, which refers here to both the reversible inhibition of cell division (causing developmental interruptions) and to the increase in bacterial cell size, requires further study. In order to uncover how C. trachomatis ABs contribute to disease production and persistence of the bacteria in their hosts, study authors Mary Brockett and George Liechti carried out the research described below.
In “Persistence alters the Interaction between Chlamydia trachomatis and its Host Cell,” researchers Brockett and Liechti examine the foundations of aberrant forms of Chlamydia and how their structural and functional differences impact how the microbe interacts with its host cell. They found that aberrant bodies are different in terms of how they present effector proteins on the inclusion surface in the mid and late stages of their development cycle.
They utilize C. trachomatis cell lines in their experiment to determine the physiological characteristics that define aberrant forms. Focusing on how persistence affects interaction with the host cells, they induce persistence through the use of antibiotics that either target the peptidoglycan or induce osmotic stress or they used IFN-𝛾, a protein that depletes tryptophan and alters protein expression.
Figure 2 (Fig. 8 in the text) above shows in depth the various stages that Chlamydia undergoes throughout its developmental cycle. The EB form utilizes a type III secretion system to inject different effector proteins. These effectors perform diverse activities as indicated in the figure, which help the pathogenic bacteria invade host tissue, suppress the host immune system, and survive. The functions of these effector proteins are crucial for the time-dependent model that the authors use to illustrate how AB alters the interaction between the microbe and the host cell. The authors propose that inducing persistence prior to the mid stage may be advantageous since C. trachomatis appears to exhibit the highest tolerance to aberrance inducing conditions at this midway point.
Figure 3 (Fig. 9 in the text) above shows a model of Chlamydial persistence in the context of infection. The timing of persistence induction is critical and determines the fate of the bacterium. As shown above, if aberrance formation is induced less than 4 hours post infection, this prevents the secretion of early effector proteins that are important for evading the host cell and may result in apoptosis. Whereas if aberrance is induced more than 12 hours post infection, RB replication and shedding of peptidoglycan would have already occurred and the infected cells would be targeted by the cell mediated immune response. The researchers found that when aberrance is induced during mid stage 8-12 hours post infection, the inclusion is stable within the host cell and the cell is removed from the endocytic pathway. Also, the cell is able to avoid the subsequent targeting of the host cell for cell mediated killing. These findings are consistent with their hypothesis that inducing persistence prior to the mid stage allows C. trachomatis to remain intact and avoid the cell-immune response.
In the discussion portion of the article, the researchers explore in depth how antibiotics that target peptidoglycans trigger C. trachomatis persistence through transformation into the AB state. Particularly, they focus on when and why this happens. Through previous research, it has been theorized that entering an aberrant state can provide some protection from antibiotics in charge of disrupting bacterial peptidoglycan formation and when researchers put this to the test, they identified some significant activity. In these antibiotic environments, peptidoglycan degradation remains unaffected, while synthesis was inhibited. Luckily, those involved identified an explanation for this; entering into a persistent state “turns off” genes for protein synthesis which is hypothesized to then “turn on” PG degradation. When peptidoglycan production is “normal” in the RB state, the authors identify that these molecules have the ability to signal an increase in levels of IL-8, a peptide that serves a large role in inflammation. During persistence, without PG synthesis, IL-8 also remains unsynthesized, which lowers the bacteria’s immunogenic profile, or the bacteria’s ability to stay “incognito” in the host cell. In collaboration with a direct effect on PG activity, these AB state bacteria lacking PG also tend to lack mid and late-stage inclusion proteins compared to their controls. This lack of proteins also extends to the T3S effector protein, whose levels significantly decrease, leading to the conclusion that entering into this state prevents the maturation of the bacteria’s inclusion.
Time becomes a significant factor in bacterial persistence when entering into the AB state. Prematurely entering into this state would cause a lack of the early effector proteins leading to host cell apoptosis, however entering too late would allow already synthesized proteins to signal IL-8 release. Therefore, researchers identified that 8 -12 hours past infection was the target time for entering into the AB state. This is significant when considering antibiotic resistance and treatment; when treated with the antibiotic azithromycin, if the bacteria are given the time to enter into the mid-stage, they are more resistant to the antibiotic’s effects as they can enter persistence at the most stable time.
Before this study, ABs were previously understood to be “dead” with little use to the bacterial cell, however this article’s evidence discredits this idea, instead identifying the following about the importance of ABs: their smaller inclusion lacking mid and late stage proteins actually make them harder to identify by their host and by other immune cells. However, due to possessing early stage proteins, the cell is able to avoid host cell apoptosis. In conclusion, these mechanisms – when coupled with the lack of IL-8 peptides – allows the bacteria to more successfully evade identification and continue infection.
In terms of future research directions, the article authors propose several possible avenues of further experimentation. The most general is the need for further analysis of both PG and inclusion protein levels in host cells to provide even more definitive evidence towards the authors’ conclusions. Further research should also continue to analyze PG synthesis and degradation when C. trachomatis is in other states/forms. Microbiological research on Chlamydia species is crucial to help identify methods to test for and treat infections in patients. The more we learn about C. trachomatis in particular, the more readily we can prevent the range of diseases including pelvic inflammatory disease and infectious blindness which may result from this pathogen. We as the authors were thrilled to uncover more about this bacteria, especially given the misinformation and stigmatization around STIs.
About the Authors:
Shoshi Daniels ‘23 is a Biology major and Anthropology minor. She is also completing a 5C certificate in Reproductive Health, Rights and Justice. Shoshi is an EMT and currently works at a homeless shelter in Amherst. After she graduates next Fall, she plans on attending nursing school. In her free time, she enjoys going to concerts, traveling and being in nature.
Gabi Davis ‘22 is a Biochemistry and Film, Media, Theatre double major. With their fields of study, they are very interested in studying the intersection between the two through documentarial work. They are currently interning at a film equipment rental house with clientele across all of New England. In their free time, they enjoy painting and watching movies with friends.
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