20: Chlamydiae, rickettsiae and mycoplasmas

Chapter 20 Chlamydiae, rickettsiae and mycoplasmas

Chlamydiae, rickettsiae and mycoplasmas are a miscellaneous group of organisms with properties common to both bacteria and viruses. Although they are categorized together in this chapter for the sake of convenience, they differ markedly from each other and cause divergent human diseases. A comparison of bacteria, chlamydiae, rickettsiae, mycoplasmas and viruses is given in Chapter 2, Table 2.1.

Chlamydiae

The chlamydiae are a group of microorganisms related to Gram-negative bacteria. However, unlike bacteria, they are unable to grow on inanimate culture media. They are therefore obligatory intracellular parasites. Their main characteristics include the following:

• larger than most viruses and hence visible by light microscopy
• both DNA and RNA are present
• obligate intracellular parasites with a complex growth cycle
• sensitive to tetracycline, erythromycin, sulphonamides.

There are three species in the genus Chlamydia:

1. Chlamydia trachomatis is an agent of many diseases (see below).
2. Chlamydia pneumoniae causes acute respiratory tract infection, including sore throat, mild pneumonia and fever in humans.
3. Chlamydia psittaci primarily causes disease (psittacosis) in birds such as pet parrots and budgerigars, from which humans contract the infection. The human infection, also known as psittacosis, takes the form of a primary atypical pneumonia.

Chlamydia trachomatis

Causes a spectrum of diseases:

• ocular infections – neonatal conjunctivitis (blenorrhoea), keratoconjunctivitis, blindness (trachoma). Trachoma is a major cause of blindness in the developing world
• genital infections – non-specific urethritis, the most common sexually transmitted disease in the UK. In the tropics, it causes lymphogranuloma venereum
• pneumonia – in neonates.

Culture and diagnosis

Identified by tissue culture (e.g. HeLa cells), serology (complement fixation test) and fluorescent antibody staining of smears from the lesion.

Antibiotic sensitivity

Tetracycline is effective for all chlamydial infections.

Rickettsiae

Rickettsiae are pleomorphic organisms, smaller than bacteria but resembling them structurally and metabolically, including cell wall formation. They, like Chlamydia and viruses, are obligate intracellular parasites. The best-known human rickettsial disease is typhus, which spreads wildly in conditions of malnutrition and poverty. Rickettsiae are:

• coccobacilli, with a multilayered outer cell wall resembling that of Gram-negative bacteria
• obligate intracellular parasites that replicate by binary fission
• visible by light microscope when special stains are used (e.g. Giemsa)
• able to infect many species, including arthropods, birds and mammals; members of the genus are transmitted to humans via bites of infected arthropods
• sensitive to tetracycline and chloramphenicol.

There are two genera within/>

Related posts:

  1. 16: Vibrios, campylobacters and Wolinella
  2. 27: Infections of the genitourinary tract
  3. 11: Streptococci, staphylococci and micrococci
  4. 32: Microbiology of dental caries
  5. 26: Infections of the gastrointestinal tract
  6. 4: Viruses and prions

Stay updated, free dental videos. Join our Telegram channel
Tags:
Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 20: Chlamydiae, rickettsiae and mycoplasmas

VIDEdental - Online dental courses
Get VIDEdental app for watching clinical videos