User:JMiller25/Rickettsia rickettsii

History

The first documented case of Rocky Mountain Spotted Fever (RMSF) was in the Boise, Idaho in 1896 after being recognized by Major Marshall H. Wood. At the time of discovery, not much information was known about the disease. It was originally called "Black Measles" due to the infected area turning black during the late stages of the disease. The first clinical description of Rocky Mountain Spotted Fever was reported in Snake River Valley in 1899 by Edward E. Maxey. At the time, 69% of individuals diagnosed with RMSF died. It was theorized that the wood tick played a critical role as a vector of disease, based on apparent correlation between tick exposure and disease, as well as the geographic and seasonal patterns.

Howard Ricketts (1871–1910), an associate professor of pathology at the University of Chicago, in 1902 was the first to identify and study R. rickettsii on a microbial level. His research consisted of interviewing victims of the disease, as well as collecting infected animals to study. He was also known to inject himself with pathogens to more accurately document their effects. Ricketts was the first to identify the pathogen responsible for RMSF as a gram negative bacillus bacteria, and confirm a route of transmission from infected ticks in a guinea pig model. His research provided valuable information on the organism's vector and route of transmission.

Simeon Burt Wolbach is credited for the first detailed, published description of the pathogenic agent that causes R. rickettsii in 1919. Wolbach described RMSF using the process of Giemsa staining, and positively identified the bacterium as frequently residing within endothelial cells.

Today, the once lethal infection RMSF has become curable due to the modern availability of antibiotics. Broad spectrum antibiotics chloramphenicol and tetracycline-class drugs, like doxycycline, were first harnessed as treatment for RMSF in the late 1940s. Before their discovery, 1 in 5 infected patients died. Treatment recommendations changed in the 1990s to support primary therapeutic use of tetracycline-class drugs, and the current recommended treatment reflects this, as doxycycline is most commonly prescribed. {{41}} This change in treatment recommendation coincided with a decrease in annual case-fatality rates (CFRs) from the 1980s on to the early 1990s. Since then, the fatality rate has dropped to between 5 and 10%.

Physiology

Metabolic Pathways

R. rickettsii are obligate intracellular bacteria, meaning they need a host cell in order to replicate and survive. In fact, no glycolytic enzymes remain in R. rickettsii's genome. It is theorized that while R. rickettsii once possessed complete, complex metabolic pathways that allowed it to survive outside a host, evolutionary pressures caused progressive genomic reduction that now limits metabolism to the tricarboxylic acid cycle (TCA). Remnants of these lost metabolic pathways can be seen in analyses of R. rickettsii's genome, which contain some identified remnant enzymes of pathways that remain unfunctional in vivo. This decrease in available metabolic pathways has left R. rickettsii largely dependant on a range of transport systems to harvest essential amino acids, nucleic acids, and other metabolites from its host.

Carbon, Lipids, Peptidoglycan & Small Molecules

The primary carbon source of R. rickettsii is pyruvate, though many other amino acids and TCA cycle intermediates such as glutamine, glutamate, and malate can be used. For lipid metabolism, a complete map of fatty acid synthesis enzymes have been found, allowing R. rickettsii to construct a viable cell membrane with lipopolysaccharides (LPS). Regarding synthesis of a peptidoglycan layer, there is speculation as to whether key components are synthesized internally or imported and modified for use. Pathways for producing critical small molecules such as riboflavin (B2), nicotinamide (B3), pantothenate (B5), pyridoxine (B6), and biotin (B7) are all missing key enzymes, forcing R. rickettsii to rely solely on transmembrane transport proteins.

Overall, R. rickettsii has a genome that does not encode many of the enzymes and proteins that are required for several pathways besides the TCA cycle. These bacteria import many of the intermediates, cofactors, and byproducts from the host cells' metabolic pathways to use for their own benefit and synthesis of necessary structures and energy for survival.

^^= My own writings

{ Outside a host, R. rickettsii is unable to utilize glycolysis and the pentose phosphate pathway due to a reduced genome, and since these pathways are unusable, the bacterium must use the tricarboxylic acid cycle (TCA) as an alternative pathway. However, this can only be done through the use of the host cell's metabolites. }

The primary carbon source of R. rickettsii is pyruvate, though many other amino acids and TCA cycle intermediates such as glutamine, glutamate, and malate can be used. For lipid metabolism, a complete map of fatty acid synthesis enzymes have been found, allowing R. rickettsii to construct a viable cell membrane with lipopolysaccharides (LPS). Regarding synthesis of a peptidoglycan layer, there is speculation as to whether key components are synthesized internally or imported and modified for use.

Overall, R. rickettsii has a genome that does not encode many of the enzymes and proteins that are required for several pathways besides the TCA cycle. These bacteria import many of the intermediates, cofactors, and byproducts from the host cells' metabolic pathways to use for their own benefit and synthesis of necessary structures and energy for survival.

One of the metabolites that R. rickettsii uses from its host cell in order to synthesize peptidoglycan and lipopolysaccharides is a sugar called UDP-N-acetyl-α-d-glucosamine.

https://journals.asm.org/doi/full/10.1128/mbio.00859-17

Morphology

[edit] R. rickettsii has many vital proteins within its cellular membranes. One of these proteins is YbgF, which maintains the structure of the cellular membrane. YbgF is found within both the inner and outer membranes along with another protein called TolC. TolC is a transport protein that connects to other transport proteins within the periplasmic space and inner membrane. These two proteins are believed to be associated with pathogenicity of this microbe and serve as specific points that antibodies can bind to in order to prevent the bacteria from interacting with host cells.[3]

R. rickettsii also has an outer layer or a "microcapsule", which acts similarly to the S-layer or slime layer of other bacteria. This slime layer consists mostly of polysaccharides, and the "microcapsule" contributes to mechanisms involving anti-phagocytosis and attachment to host cells. [25]

new edits to above text: R. rickettsii has an outer layer or a "microcapsule", in addition to the traditional peptidoglycan cell wall. The function of the microcapsule resembles that of slime layers, or S-layers, of other bacteria. This slime layer consists mostly of polysaccharides and is constantly undergoing changes in reaction to chemical or physiological events. Research to precisely determine the function of the slime layer is currently limited due to high risk of infection while working with this bacterium; however, scientists can infer based on conclusions from other studies that it is likely that this slime layer is used for antiphagocytic properties, which prevent phagocytes from engulfing and killing the R. rickettsii bacteria, and attachment to host cells in preparation to penetrate and infect those hose cells.

→ Silverman DJ, Wisseman CL Jr, Waddell AD, Jones M. External layers of Rickettsia prowazekii and Rickettsia rickettsii: occurrence of a slime layer. Infect Immun. 1978 Oct;22(1):233-46. doi: 10.1128/iai.22.1.233-246.1978. PMID: 83297; PMCID: PMC422141.

Preventative Measures

Prevention of Rocky Mountain Spotted Fever begins with identifying and avoiding vectors that cause exposure including ticks, lice, mites, and fleas. Being cognizant of where endemic areas are and taking precautions when inhabiting or traveling may also decrease the likelihood of contracting the disease. As of now, there are no vaccines that allow a host's body to anticipate RMSF. Due to increased antibiotic resistance preemptive antibiotic prophylaxis is strongly discouraged in the United States.

"This photograph depicts, Field EIS officer, Heather Walker, DVM, MPH (EISO Class of '23), as she was placing a flea and tick collar on a community owned dog, for a Rocky Mountain Spotted Fever campaign in Arizona."-CDC

When coming into contact with vectors, specifically ticks, there are additional preventative measures that can be taken. Many ticks are present in dense bush, grass, and wooded areas. Ticks also can travel on animals, so caution should be taken with a pet as well. First examine them and yourself by looking: under the arms, within and around the ears, inside the belly button, back of the knees, within and around the hair, in between the legs, and around the waist. It is then recommended to remove the clothing and place it in the dryer on high for at least 10 minutes. If items need to be washed before drying ensure that hot water is used as cold or medium heat will not remove the threat. Finally, it has been proved that showering two hours after leaving the outside environment lowers the risk of obtaining Lyme disease, a disease whose vector is ticks, therefore it may assist in reducing the risk of other tick-borne diseases.

An alternative or additive measure may be the use of Environmental Protection Agency (EPA)-registered insect repellents while in areas prone to ticks. These repellents contain at least one of the following active ingredients; picaridin, IR3535, DEET, Oil of Lemon Eucalyptus (OLE), para-menthane-doil (PMD), or 2-undecanone. Avoid application of repellent that contains OLE or PMD in infants and toddlers, or until they reach 3 years of age.

Pathophysiology

[edit] While humans are hosts for R. rickettsii, they do not contribute to rickettsial transmission. Rather, the pathogen is maintained through its vector: ticks. R. rickettsii invades vascular endothelial cells that line both small and medium sized the blood vessels in the host's body. There is an extensive immune response to this pathogen that triggers different pathways in the macrovascular and microvascular systems. As the cells are damaged there is an increased permeability of the vessels (extravasation), microvascular hemorrhages, and necrosis. Additionally, there is evidence that suggests the ability to cross the blood-brain barrier and into the hosts central nervous system, allowing the pathogen to target the brain's vasculature.The pathogen causes changes in the host cell's cytoskeleton that induces phagocytosis. Consequently, R. rickettsii replicates further and infects other cells in the host's body. R. rickettsii's survival in the immune system cells increases the pathogen's virulence in mammalian hosts.

Nitric oxide has the ability to inhibit the pathogen by negatively effecting attachment, intracellular growth, and subversion of the host cell. It does so by depleting R. rickettsii of ATP as nitric oxide targets cytochrome bo oxidase and cytochrome bd oxidase complexes which are necessary for ATP-synthase. The impact of nitric oxide on the metabolism however is significant enough to limit the pathogens ability to attach to host cells. As translation is also majorly ATP-dependent, the introduction of nitric oxide can greatly reduce the process of translation and therefore protein synthesis, making R. rickettsii incapable of subverting the host cell. Understanding of these inhibitory processes may allow for more advance care and possible antimicrobial treatments to be developed.

Actin-Based Motility (ABM) is a virulence factor that allows for the pathogen to evade the host's immune cells and spread to neighboring cells. It is suggested that the Sca2 gene, which is an actin-polymerizing determinant, is a distinguishing factor for the Rickettsia family, as R. rickettsii mutants with a Sca2 transposon the bacteria can avoid autophagic processes by host phagocytic cells. This leads to an increase in disease manifestation for the host.

R. rickettsii is also able to suppress immune responses while dwelling in infected cells by creating inhibitory proteins such as Rickettsial ankyrin repeat protein 2 (RARP2). RARP2 mediates the fragmentation of TGN, or the trans-Golgi network, causing attenuation of vesicular transport and glycosylation defects in infected host cells. There are two important proteins within the host cell that are affected by these glycosylation defects: TGN46 and major histocompatibility complex class 1 (MHC-I). MHC-I is an important protein for defending against pathogens as it functions as an antigen presenting complex signaling its infection status to lyphocytes. However, since RARP2 causes attenuation of vesicular transport, MHC-I is unable to be transported to the plasma membrane and the infected cell will not be able to alert host immune cells. Thus, the bacterial cells are able to avoid certain immune responses and allow for proliferation within a host cell.

This pathogen also has the unique ability, as seen in studies on ticks, to prevent apoptosis. It does so by affecting apoptosis regulators such as caspases, Bcl-2 proteins, or possibly the p53 tumor suppressor pathway. This provides an advantage, allowing R. rickettsii to proliferate further due to prolonged survival and increased time for replication. By doing so, R. rickettsii also increases its transmission abilities. Without this process in place, the pathogen may not survive long enough in the vector to properly replicate and infect other hosts.

R. rickettsii is an obligate intracellular alpha proteobacterium that belongs to the Rickettsiacaea family. Within the Rickettsia species, these bacteria are divided into four clades. The clades include the ancestral group, spotted fever group (SFG), typhus group, and transitional group, and the determining factors for classifying into each group depends on phenotypic characteristics, phylogenetic organization, or the type of vector host they inhabit. R. rickettsii falls into the largest group of them all, the SFG group. R. rickettsii has a genome that consists of about 1.27 Mbp with ~1,350 predicted genes, which is smaller compared to most other bacteria. This small genome size allows the bacteria to maintain an intracellular lifestyle with increased pathogenicity from gene reduction. It is maintained in its tick host by transovarial transmission. The multiplication of R. rickettsii is by binary fission inside the cytosol.

Transmission in mammals

[edit] Due to its confinement in the midgut and small intestine, Rickettsia rickettsii can be transmitted to mammals, including humans.

Transmission can occur in multiple ways. The most common way of contraction is by the bite of an infected tick. After getting bitten by an infected tick, R. rickettsiae is transmitted into the bloodstream by tick salivary secretions. The saliva of ticks contain immunomodulatory agents which effect immune defenses in the host. This allows for R.rickettsiae to be transmitted with little to no resistance from the host's immune system.Another way to contract the infection is through contact with an infected host's feces. If an infected host's feces come into contact with an open skin barrier, it is possible for the disease to be transmitted. An uninfected host can become infected when eating food that contains the feces of the infected vector.

Marshall --> look for new citations for [27] and [28] due to them being very old

Ticks can contract R. rickettsii by many means. An uninfected tick can become infected through feeding on the blood of an infected vertebrate host during the larval or nymph stages. This mode of transmission is called transstadial transmission.[17] Once a tick becomes infected with this pathogen, they are infected for life. This pathogen, however, will not harm the tick itself and will only cause symptoms in mammals the tick infects. Both the American Dog Tick and the Rocky Mountain Wood Tick serve as long-term reservoirs for Rickettsia rickettsii, infecting the posterior diverticula of the midgut, the small intestine, and the ovaries.[17] In addition, an infected male tick can transmit the organism to an uninfected female during mating,[18] and infected female ticks can transmit the infection to their offspring in a process known as transovarian passage.[19][20] This process, however, is unlikely to play a major role in the maintenance of R. rickettsii within a population.[21] Notably, R. rickettsii is inefficient at infecting the ovaries of adult female ticks, resulting in a lowered rate of vertical transmission.[21] Rickettsial colonization of the ovaries sees higher success when ticks obtain the pathogen as a larva or nymph.[22] Reduced fecundity is also observed in ticks infected with R. rickettsii.[18] As a result of these limitations, long-term maintenance of R. rickettsii in populations of ticks relies mainly on horizontal transmission through the exchange of bacteria during feedings of infected hosts.[18][23]