How Does Rickettsia Affect Humans? Symptoms & Prevention
Rickettsia bacteria is a genus of nonmotile, gram-negative, and non-spore-forming bacteria. It is a greatly pleomorphic bacteria that may occur in three forms. These three forms are cocci (which have a diameter of 0.1μm), bacilli (which have a length of 1-4 μm), and threads (which are up to about 10 μm long). The term rickettsia has nothing to do with rickets, the deficiency disease that occurs due to lack of vitamin D. Rickettsia was named after Howard Taylor Ricketts to credit his work on tick-borne spotted fever. Rickettsia commonly applies to any members belonging to the order Rickettsiales. They belong to the phylum Proteobacteria. Let us take a look at the diagram of rickettsia.
Classification of Rickettsia
The classification of the rickettsia bacteria was made into three groups by a method called serology which is the scientific study of body fluids and serum. These three groups were namely ‘spotted fever group’, ‘typhus group’ and ‘scrub typhus group’. The grouping was then confirmed by the DNA sequencing method. Human pathogens are included in all three of these groups.
The scrub typhus group was later reclassified as a related new genus called Orientia, but they still belonged in the order Rickettsiales and are therefore grouped with the other rickettsial diseases.
Rickettsias are noticed to be more widespread than they were previously known to be. They are said to be associated with arthropods, leeches, and protists.
Rickettsia Characteristics
Rickettsia is a small bacteria that approximately range from 0.3 to 0.5 μm in their diameter and from 0.8 to 2.0 μm in their length. They are notably smaller than eukaryotic cells which roughly range from 10 to 100 μm in diameter. The variation in the size of these species can be because of their type or the general shape of the bacteria.
Some bacteria can be spherical or coccoid in shape like Rickettsia rickettsii. Some can be rod-shaped like Rickettsia Typhi.
Several species found in this genus are pleomorphic and hence have the capability of changing their shape depending on environmental conditions.
When they are observed under a microscope, these species can appear singly or in pairs. They may also form strands.
Rickettsia is a gram-negative bacteria and hence, has the presence of a thin peptidoglycan layer in their cell wall. Just like gram-negative bacteria, their cell envelope contains three main layers. This includes the inner cell membrane, the thin cell wall which surrounds the inner membrane, and the outermost membrane surrounding the cell wall.
Although Rickettsia is said to be gram-negative, the observations may differ when they are stained in Gram stain. Due to this reason, other stains such as Giemsa are used often for their identification.
Other structural characteristics of the Rickettsia bacteria involve:
The cell wall of this bacteria consists of an acid called diaminopimelic acid.
There is no presence of teichoic acid in Rickettsia because they are gram-negative bacteria.
Rickettsia species have the presence of lipopolysaccharide (LPS) that further consists of lipid and polysaccharide.
Genomics
Some parts of the rickettsial genomes resemble the ones in mitochondria. The deciphered genome of the R. prowazekii species is 1,111,523 bp in length, and it is said to contain 834 genes. It consists of no genes that are required for anaerobic glycolysis, or genes that are involved in the biosynthesis and regulation of amino acids and nucleosides. Due to this reason, they are said to be similar to mitochondrial genomes.
The production of ATP in Rickettsia is identical to the one we find in mitochondria. Hence, it is said that Rickettsia in all probabilities is the closest relative to the mitochondria because of their phylogenetic features. However, the genome of R. prowazekii consists of a complete set of genes that encodes for the tricarboxylic acid cycle and the respiratory chain complex.
There has been a recent discovery of another parallel between Rickettsia meaning and viruses. This development may become a basis to fight HIV infection. It has appeared that the human immune response to the scrub typhus pathogen, Orientia tsutsugamushi, provides an effect that is beneficial against the progress of the HIV infection. This negatively influences the virus replication process.
Did You Know?
In March 2010, a case of bacterial meningitis was reported by Swedish researchers. Here, a woman was affected, and this was caused by Rickettsia Helvetica which was previously thought to be a harmless bacteria.
Infection occurs in mammals that are nonhumans. For instance, Rickettsia species have been said to afflict Lama guanacoe.
Rickettsial organisms are parasites that are obligate intracellular by nature. They invade the vascular endothelial cells in target organs and damage them. As a result, this causes an increase in vascular permeability with consequent oedema, hypertension, and hypoalbuminemia.
Rickettsia bacteria cannot survive outside the host’s body.
They are nonmotile bacteria.
FAQs on Rickettsia Explained: Types, Characteristics & Genomics
1. What is Rickettsia and what are its main characteristics?
Rickettsia is a genus of bacteria that are known for being obligate intracellular parasites. This means they can only survive and reproduce inside the living cells of another organism (a eukaryotic host). Key characteristics of Rickettsia include: they are Gram-negative, non-motile, non-spore-forming, and typically have a rod-like or coccobacilli shape. They are much smaller than typical bacteria and cannot be grown on standard artificial laboratory media.
2. What are the major diseases caused by Rickettsia bacteria?
Rickettsia bacteria cause a group of diseases known as rickettsioses, which are often transmitted by arthropods. Some of the most significant diseases include:
Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, it is one of the most severe rickettsial diseases.
Typhus: This includes Epidemic Typhus (caused by R. prowazekii) and Murine Typhus (caused by R. typhi).
Rickettsialpox: Caused by Rickettsia akari, transmitted by mites.
African Tick-Bite Fever: Caused by Rickettsia africae.
Scrub Typhus: Although closely related, this is caused by the genus Orientia (formerly considered Rickettsia).
3. How is Rickettsia transmitted to humans?
Rickettsia bacteria are primarily transmitted to humans through the bites of infected arthropod vectors. Humans are typically accidental hosts. The specific vector depends on the Rickettsia species:
Ticks are common vectors for diseases like Rocky Mountain Spotted Fever and African Tick-Bite Fever.
Lice transmit Epidemic Typhus.
Fleas (often from rodents) transmit Murine Typhus.
Mites (chiggers) are responsible for transmitting Rickettsialpox and Scrub Typhus.
4. How are Rickettsia species scientifically classified?
The genus Rickettsia is primarily classified into two major groups based on their antigenic properties and genetic characteristics:
Spotted Fever Group (SFG): This is the largest group, containing species like Rickettsia rickettsii (causes RMSF) and Rickettsia conorii (causes Mediterranean spotted fever). These are primarily transmitted by ticks.
Typhus Group (TG): This group includes species like Rickettsia prowazekii (causes epidemic typhus, louse-borne) and Rickettsia typhi (causes murine typhus, flea-borne).
A third, transitional group is also sometimes recognised, containing species like Rickettsia akari.
5. Why is Rickettsia considered an obligate intracellular parasite?
Rickettsia is considered an obligate intracellular parasite because it lacks the complete metabolic machinery to create its own energy and essential nutrients for survival. Specifically, it has a 'leaky' cell membrane and cannot synthesise its own adenosine triphosphate (ATP), the main energy currency of the cell. It must steal ATP and other vital molecules like amino acids directly from the cytoplasm of its host's cells. This complete dependency on the host cell's resources makes it impossible for Rickettsia to survive or replicate in a non-living environment.
6. How does a Rickettsia bacterium differ from a typical bacterium or a virus?
Rickettsia occupies a unique biological space between typical bacteria and viruses. Here’s how they compare:
Versus Typical Bacteria: Unlike most bacteria that are free-living, Rickettsia is an obligate intracellular parasite. It is also significantly smaller and cannot be cultured on standard artificial media, requiring living cells (like tissue cultures) for growth.
Versus Viruses: While both are obligate intracellular parasites, Rickettsia is a true bacterium. It has a cellular structure (with a cell wall and membrane), contains both DNA and RNA, possesses its own ribosomes to make proteins, and is susceptible to antibiotics like doxycycline. Viruses lack cellular structure, have either DNA or RNA (but not both), and do not respond to antibiotics.
7. How does an infection with Rickettsia lead to symptoms like fever and rash?
The primary target of Rickettsia in the human body is the endothelial cells, which form the inner lining of blood vessels. After entering the bloodstream via a vector's bite, the bacteria invade these cells. Their replication causes direct injury and inflammation of the blood vessels, a condition called vasculitis. This widespread damage increases the permeability of small blood vessels, causing fluid to leak into surrounding tissues. This process explains the classic symptoms:
Fever and headache are caused by the body's systemic inflammatory response to the infection.
Rash (often petechial - small red or purple spots) appears as blood leaks from the damaged capillaries into the skin.
8. What does the genomics of Rickettsia reveal about its evolution?
The study of Rickettsia's genomics provides strong evidence for a process called reductive evolution. As these bacteria adapted to a stable life inside a host cell, they shed many genes that were essential for a free-living existence but became redundant in the nutrient-rich environment of the host's cytoplasm. This has resulted in one of the smallest known bacterial genomes. Interestingly, the Rickettsia genome shares remarkable similarities with that of mitochondria, the powerhouses of our own cells. This supports the widely accepted endosymbiotic theory, which proposes that mitochondria evolved from an ancient, Rickettsia-like bacterium that was engulfed by an early eukaryotic cell.
