Rickettsias and Chlamydias

1. Rickettsias and Chlamydias

2. Rickettsia
 Classification – Family Rickettsiaceae with 3 medically
important genera
 Rickettsia
 Coxiella
 Rochlimaea
 Morphology and cultural characteristics
 All are obligate intracellular parasites (except Rochlimaea)
 Rochlimaea can be cultivated on artificial media containing blood

3. Rickettsia
 Others are grown in embryonated eggs or tissue culture
 Cultivation is costly and hazardous because aerosol
transmission can easily occur
 Small, pleomorphic coccobacilli
 Gram stain poorly, but appear to be Gram negative
 Stain readily with Giemsa
 All, except Coxiella, are transmitted by arthropod vectors.

4. Rickettsia
 Transmission to humans occurs via an arthropod vector
(lice, fleas, ticks or mites)
 The organisms multiply within the cytoplasm, and
sometime within the nucleus of the host cells
 In the lab they are cultured in
 A) host animals such as guinea pigs or mice
 B) embroynated chicken eggs
 C) tissue cell cultures

6. Diseases caused
 Rocky mountain spotted fever (ticks)- Rickettsia rickettsia
 Classical typhus fever (lice)- Rickettsia typhi
 Murine typhus fever (fleas)- Rickettsia typhi
 Rickettsial pox (mites)- Rickettsia akari
 Scrub typhus (mites)- Orientia tsutsugamushi

8. Chlamydia

9. Chlamydia
 Classification – order Chlamydiales – contains one medically
important genus – Chlamydia
 Are Gram negative obligate intracellular parasites
 Differences with Rickettsias
 Inability (cannot) to make ATP
 Have a complex developmental cycle

10.  Chlamydial reproduction begins with the attachment of
an elementary body (EB) to the cell surface.
 #Elementary bodies are 0.2 to 0.6 µm in diameter,
contain electron-dense nuclear material and a rigid cell
wall, and are infectious.
 #The host cell phagocytoses the EB, which are held in
inclusion bodies where the EB reorganizes itself to form
a reticulate body (RB) or initial body.

11.  The RB is specialized for reproduction rather than
infection.
 Reticulate bodies are 0.6 to 1.5 µm in diameter and
have less dense nuclear material and more ribosomes
than EBs; their walls are also more flexible.
 About 8 to 10 hours after infection, the reticulate body
undergoes binary fission and RB reproduction
continues until the host cell dies

14. A Cell Infected with, Chlamydia trachomatis
Elementary Body
Initial Body/
Reticulate Body

15. Pathogenicity
 C. trachomatis infects humans and mice. In humans it
causes trachoma, non-gonococcal urethritis, and other
diseases.
 Chlamydiophila psittaci causes psittacosis in
humans. However, unlike C. trachomatis, it also infects
many other animals (e.g., parrots, turkeys, sheep,
cattle, and cats)
 Chlamydiophila pneumoniae is a common cause of
human pneumonia.

16. Joint and central nervous
system infections