Rickettsia Biology of Obligate Intracellular Bacteria

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:

1. The cell wall of this bacteria consists of an acid called diaminopimelic acid.

2. There is no presence of teichoic acid in Rickettsia because they are gram-negative bacteria.

3. 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?

1. 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.  

2. Infection occurs in mammals that are nonhumans. For instance, Rickettsia species have been said to afflict Lama guanacoe.

3. 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.  

4. Rickettsia bacteria cannot survive outside the host’s body.

5. They are nonmotile bacteria.