thought to be viruses that invaded cells. They are classified as bacteria because
they have bacterial cell walls and contain DNA and RNA, which is not the case
with a virus. Rickettsias and Chlamydias have no means of mobility because
they lack flagella. They are also gram-negative.
Rickettsias are small rod-shaped or spherical bacteria that live in the cells of
ticks, lice, fleas, and mites (arthropods) and can be transmitted to humans when
bitten by arthropods, causing rickettsial disease. Rickettsial diseasecauses cap-
illaries to become permeable resulting in a rash. Rickettsias reproduce by binary
fission where a cell wall forms across the cell and the two halves separate to
become individual cells.
Common Rickettsias are:
- Rickettsia prowazekii. Rickettsia prowazekiiis transmitted by lice and
causes endemic typhus. - R. rickettsii. R. rickettsiiis transmitted by ticks and causes Rocky Mountain
spotted fever. - R. tsutsugamushi. R. tsutsugamushiis transmitted by arthropods and cause
scrub typhusthat presents with fewer, rash and inflammation of the lymph
nodes. - Coxiella burnetti. Coxiella burnettiis transmitted by aerosols or contami-
nated milk and causes Q fever, which is similar to pneumonia. - Bartonella bacilliformis. Bartonella bacilliformisis transmitted by arthro-
pods and causes a wart-like rash called Oroya fever.
Chlamydiasare very small spherical or coccoid bacteria that are non-motile
and can be transmitted from person-to-person contact or by airborne respiration.
Chlamydias are not transmitted by arthropods. They are so small that they mul-
tiply in host cells. There are three species of chlamydia: - Chlamydia trachomatis. C.trachomatis causes trachoma, which is a com-
mon cause of blindness and the non-gonorrhea sexually transmitted dis-
ease urethritis (inflammation of the urethra). - C. penemoniaecauses a mild form of pneumonia in adolescence.
- C. psittacicauses psittacosis.
Mycoplasmas
Mycoplasmasare very small facultatively anaerobic bacteria (some are oblig-
ately anaerobic) that have taken on many shapes (pleomorphic) and were once
CHAPTER 10 The Prokaryotes^159