Chlamydia and Rickettsia (Gram-Negative)

These bacteria are obligate intracellular parasites, meaning they can only survive inside animal cells. Chlamydia and rickettsia resemble viruses because of their size and parasitism. Unlike viruses, they can synthesize their own proteins, posses both RNA and DNA, and are sensitive to antibiotics. Chlamydia spreads from person to person, and rickettsia spreads from arthropod to person.

Chlamydia life cycle 
The bacterium has a dense, small, inert form called the elementary body (EB). It is spread from cell to cell in the host, and becomes an initial body (IB), or reticulate body. IBs replicate before some are transformed back to EBs, which infect more cells.

Chlamydia trachomatis 
Primarily infects the eyes and genitals, and causes STD.

• Diseases: 
• Trachoma, a type of chronic pink eye (conjunctivitis) and causes preventable blindness in the course of 10-15 years.
• Inclusion conjunctivitis of newborns by infection from the birth canal. Causes swelling, inflammation and pus.
• Infant pneumonia. 
• Urethritis. As Neisseria gonorrhoeae most commonly causes urethritis, the disease caused by other bacteria is labeled nongonococcal urethritis (NGU). 
• Cervicitis. 
• Pelvic inflammatory disease (PID).
• Epididymitis, which can develop in men with urethritis. 
• Lymphogranuloma venereum, an STD that exhibits painless bumps or ulceration on the genitals. Once bacteria migrate to the lymph nodes, the nodes swell and may break open.

Chlamydia psittaci
Infection is transferred from birds to humans. This bacteria common causes an atypical pneumonia called psittacosis.

Rickettsia rickettsii
Transmitted to humans by ticks. Bacteria proliferate in the blood vessel and capillary endothelium, and cause Rocky Mountain spotted fever. The skin rash of the disease is caused by small hemorrhages and inflammation.

Rickettsia akari
Transmitted to humans by mites. Causes rickettsialpox, which starts with a localized red bump on the bitten skin. The bump turns into a blister. Later, fever and headache develop and pox covers the body.

Rickettsia prowazekii 
Causes epidemic typhus, especially in places of overcrowding, poverty and unsanitary conditions. (Epidemic is widespread and sudden. Endemic affects a population for a consistent time). Disease is transmitted by lice. Symptoms include fever, headache and small pustules on the skin. Unlike Rocky Mountain spotted fever, there is no rash on the palms, soles or face.

Rickettsia typhi
The endemic form of typhus, murine typhus, but symptoms are not as severe. Transmitted by fleas.

Rickettsia tsutsugamushi
Transmitted to humans by mite larvae. Causes scrub typhus.

Rickettsia parkeri
Infection started in the southeast coast of the US. Symptoms include fever, headache, eschar (scab) and regional lymphadenopathy (abnormal lymph node size). 

Rickettsia africae
Causes African tick-bite fever (ATBF).