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 (blennorrhoea), 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. Nucleic acid amplification tests (NAATs) are now the choice for genital Chlamydia trachomatis infections.
Antibiotic sensitivity
Tetracycline is effective for all chlamydial infections.
Rickettsiae
Rickettsiae are pleomorphic coccobacilli smaller but similar to Gram-negative bacteria resembling them structurally and metabolically; they do not stain with Gram stain. 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 the Rickettsiae: Rickettsia and
Coxiella.
Rickettsia
Rickettsial diseases include:
■ typhus, an acute febrile illness, now rare, with a maculopapular rash transmitted by the rat flea; the fatality rate is frequently high as a result of haemorrhagic complications
■ spotted fevers: Rocky Mountain spotted fever and other tick-borne fevers.
Coxiella
Coxiella burnetii, an organism closely resembling rickettsiae, causes Q fever, a typhus-like illness. Usually Q fever presents as a 'non-bacterial' pneumonia, but lesions may be seen in the brain and other organs, including the heart, with resultant infective endocarditis.
Culture and diagnosis
■ grows well in all culture lines and yolk sac
■ serology: rising titre of antibody in paired sera
■ molecular identification methods are now common.
Antibiotic sensitivity
Tetracycline or doxycycline.
Mycoplasmas
Mycoplasmas are the smallest prokaryotes capable of binary fission, and they grow, albeit slowly, on inanimate media. There are more than 200 species of these cell wall-free bacteria considered to be parasite living within eukaryotic cells. They do not possess a peptidoglycan cell wall and are bound by a plasma membrane consisting of lipids and sterols (including cholesterol derived from the host cell). Hence, they are highly pleomorphic. They cause both human and animal diseases and are normal commensals of the human mucous membranes, including the oral cavity.
The most important species of the genus Mycoplasma is Mycoplasma pneumoniae, which causes:
■ a common pneumonia, atypical pneumonia
■ mucocutaneous eruptions, including the oral mucosa
■ haemolytic anaemia.
Mycoplasma pneumoniae
Primary atypical pneumonia
Primary atypical pneumonia takes the form of fever, nonproductive cough, severe headache, weakness and tiredness; it is an important cause of community-acquired pneumonia. The acute illness lasts for about 2 weeks, but in a majority, the symptoms last longer.
Mucocutaneous eruptions
M. pneumoniae may cause skin rashes and ulcerations of both the oral and vaginal mucosa. These appear as maculopapular, vesicular or erythematous eruptions. The skin lesions, which often affect the extremities, have a target or iris appearance (target lesions). In the oral mucosa, erythematous patches may appear first, quickly becoming bullous and erosive. This leads to extensive blood encrustations, especially the labial lesions. When the oral ulceration is associated with the skin rash and conjunctivitis, it is called Stevens-Johnson syndrome.
Culture and diagnosis
Mycoplasma can be cultured in special media but is a slow- grower (about 10 days); the colonies have a characteristic 'fried- egg' appearance. Immunofluorescence of colonies transferred to glass slides is useful (as they do not take up the Gram stain well).
Serology is useful as the culture results are delayed. Complement fixation testing for M. pneumoniae antibodies is diagnostic. NAATs are increasingly being employed for laboratory diagnosis.
Antibiotic sensitivity
Tetracycline for adults and erythromycin for children.
Oral mycoplasmas
Mycoplasmas have been isolated from saliva, oral mucosa and dental plaque of healthy humans, but their significance is not clear. Estimates of the oral carriage of mycoplasma vary from 6% to 32% and they appear to exist as oral commensals in some adults. The oral species are poorly characterized and include Mycoplasma buccale, Mycoplasma orale and Mycoplasma salivarium. The latter two species have been isolated from salivary glands and are thought to play a role in salivary gland hypofunction. Isolation of M. pneumoniae from the oral cavity is a likely sign of disease.
Key facts
• Chlamydiae are obligatory intracellular parasites related to Gram-negative bacteria.
• Chlamydia trachomatis causes ocular (neonatal conjunctivitis, keratoconjunctivitis, blindness: trachoma), genital (non-specific urethritis, lymphogranuloma venereum) and respiratory tract (pneumonia) infections.
Rickettsiae are tiny coccobacilli resembling Gram-negative bacteria and, like chlamydiae, are obligatory intracellular parasites.
All members of the genus Rickettsia are transmitted to humans by bites of infected arthropods.
• Rickettsial diseases include typhus, an acute febrile illness (frequently fatal) with a maculopapular rash.
• Mycoplasmas are the smallest prokaryotes capable of binary fission and exist as pleomorphic morphological forms (as they lack peptidoglycan cell wall).
• Mycoplasma pneumoniae is an important human pathogen and causes atypical pneumonia, haemolytic anaemia and mucocutaneous eruptions.
• Mucocutaneous eruptions often affect the extremities and have a target or iris appearance (target lesions).
• The oral mucosal lesions of M. pneumoniae appear erythematous at first and quickly become bullous and erosive, leading to extensive blood encrustations.
• Oral mycoplasmas (Mycoplasma buccale, Mycoplasma orale, Mycoplasma salivarium) have been isolated from saliva, oral mucosa and dental plaque, but their significance in either health or disease is unclear.
Review questions (answers on p. 365)
Please indicate which answers are true, and which are false.
20.1 Chlamydial infections:
A. may cause primary atypical pneumonia
B. can lead to blindness
C. are the commonest cause of non-gonococcal urethritis
D. are diagnosed by culturing the organism on selective agar media
E. are treated by tetracycline
20.2 Rickettsiae:
A. are obligatory intracellular parasites
B. commonly have an arthropod vector
C. cause spotted fevers
D. infections are often diagnosed by serological tests
E. infections are best treated with cephalosporins
20.3 Mycoplasma:
A. are highly pleomorphic obligatory intracellular parasites
B. cause oral mucosal ulcerations
C. skin lesions have characteristic target appearance
D. cannot be grown in vitro
E. infections in children are treated by erythromycin
Further reading
Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.). (2013). Chlamydia spp. In Jawetz, Melnick & Adelberg's medical microbiology (26th ed., pp. 359-368). New York: McGraw Hill. Chapter 27. [e-Book] Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.). (2013). Mycoplasma and cell wall-defective bacteria. In Jawetz, Melnick & Adelberg's
medical microbiology (26th ed., pp. 341-345). New York: McGraw Hill. Chapter 25. [e-Book]
Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.). (2013). Rickettsia and related genera. In Jawetz, Melnick & Adelberg's medical microbiology (26th ed., pp. 349-356). New York: McGraw Hill. Chapter 26. [e-Book]