medical microbiology ii lecture 11
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MEDICAL MICROBIOLOGY II
Lesson 11
Rickettsia and Legionella
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Rickettsiaceae
The family of microorganisms that cause typhus fever and related diseases mostly transmitted by arthropods
Closely related to viruses in their inability to grow on artificial culture medium and their being obligate intracellular parasites
True bacteria, have DNA and RNA, cell wall, are susceptible to antibiotics, and divide by binary fission. They are smaller than bacteria but not filterable
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Rickettsiaceae
Contain 3 genera: Rickettsia, Coxiella and Rochalimaea
Rickettsia contains agents of typhus fever, spotted fever and scrub typhus
Rochalimaea contains the agent of trench fever (the microorganism differs from rickettsia agents: usually extracellular in the arthropod host and able to grow on blood agar
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Rickettsiaceae
Coxiella contains agent of Q fever which differs from Rickettsia in being smaller, more resistant to physical and chemical agents and transmissible without an arthropod vector
Rickettsiaceae can also be subdivided according to the type of diseases they produce, the vectors that transmit them and their geographical distribution
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Rickettsia
Small Gram negative pleomorphic rods, non-motile, non-capsulated (but R. prowazeki has an amorphous capsule)
Do not grow on cell-free media but R. quintana is able to grow on blood agar (32 -35 C)
Most are found in normally in the alimentary canal of insects - lice and mites
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Rickettsia
R. prowazeki has man as its only natural maintenance host and the human body louse is the vector
The rickettsiae multiply in the gut of the louse and are found in the faeces of the louse
Infection occurs through contaminated louse faeces being rubbed in by scratching damaged skin
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Rickettsia
Infection can also occur through inhalation of dried louse faeces, or through the conjunctiva or through blood transfusion
The vector for R. tsutsugamushi is the thrombiculid mite and its natural reservoirs are rats, mice, voles and quail
Infection occurs through the bite of an infected larval mite as the rickettsiae are contained in the saliva of the mite
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Rickettsia
Rats are natural reservoirs of R. typhi (R. mooseri) and infection is transmitted by the rat flea, Xenopsylla cheopis
The rickettsiae multiply in the gut of the flea
Human infection is through inhalation of rickettsial particles in dried flea faeces
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Rickettsia
The natural reservoirs for R. conori and R. siberica are dogs, rats, mice and probably rabbits
The rickettsiae are transmitted by hard ticks
Human infection is through the bite of infected tick
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Pathogenicity
Pathogenesis begins with the invasion and multiplication of the rickettsiae in the endothelial cells of small blood vessels, smooth muscle cells and histiocytes
Tissues most involved are those of the skin, heart, brain and lungs
The invading organisms cause damage of tissues with a local cellular response in the infected cells become severely inflamed
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Pathogenicity
Capillaries become blocked and there may be bleeding in severe cases
Most infections are characterised by high continuous fever, severe headache, general body pains and weakness
There is a characteristic macular or maculopapular rash appearing toward the end of the first week
Other signs may include oedema, nose bleeds and involvement of the eyes
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Pathogenicity
Towards the second week the patient may become delirious, confused and eventually comatose
Fatality ranges from 15 - 70% and increases with age
Death usually occurs from general toxaemia, encephalitis, pneumonia or myocarditis
Typhus and spotted fever groups give the most severe symptoms
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Pathogenicity
Following rickettsial infection, gangrene of the fingers, toes and other parts of the body may occur
In patients who recover, the organisms may remain latent in the lymphoid tissue and can be triggered off again in years to come
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Laboratory Diagnosis
Most lab are restricted to performing only serological testing because of the difficulties in working with and the risk of infection with rickettsiae organism
Specimens: blood, tissues and sputum (should be labeled as biohazards)
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Laboratory Diagnosis
1. Microscopy
Smears may be examined directly after staining by Giemsa, Castaneda or Macchiavellomethods
Also by IF
Organisms are pleomorphic, forming bacillary and coccal forms, small, measuring less than 0.5 m in diameter
Usually seen in tissue smears near cells or free within the cytoplasm
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Laboratory Diagnosis
2. Culture
Can be cultivated in animals, egg yolk or cell culture (only to be performed in well equipped lab)
3. Serology (ELISA of IF using specific rickettsial antigens
The immunological response to rickettsial infections is firstly the production of IgM antibodies
Followed in the later stages of the disease by IgG antibodies (may remain in circulation for years)
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Laboratory Diagnosis
4. Weil-Felix reaction
A non-specific agglutination test in which rickettsial sera are tested for agglutinins to the O antigens of certain non-motile strains of Proteus vulgaris (OX19 and OX2) and Proteus mirabilis (OXK)
It is based on cross-reaction between rickettsial antibodies and the antigens of the Proteusstrains
Simple and useful test to diagnose acute rickettsial infections
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Laboratory Diagnosis
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Laboratory Diagnosis
5. Antibiotic sensitivity
Usually sensitive to erythromycin, tetracycline, chloramphenicol
Usually resistant to penicillin
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Legionella
Gram negative, aerobic, non-sporing, non acid-fast and encapsulated bacteria, 0.3 - 0.7 m
Catalase and oxidase variable
Primary isolation may take up to 10 days incubation at 35 C
Growth is enhanced in an atmosphere containing 5% carbon dioxide
Most members are motile and produce beta-lactamase
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Legionella
Type species: L. pneumophilia, L. micdadei, L. bozemanii, L. longbeachae, L. dumoffii, and L. gormanii
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Pathogenicity
L. pneumophilia causes Legionnaires disease or Legionellosis - an atypical pneumonia with high fatality
It was first known as Legionnaires disease bacterium (LDB)
It derived its name from the epidemic at the American Legion convention in Philadelphia in 1976 (29 death out of 183 cases)
First named L. legionella, then later changed to L. pneumophila
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Pathogenicity
L. pneumophila is an environmental microorganism
Human infection usually occurs from transmission through aerosols of organisms that have colonised water supplies and air conditioning systems
Person to person transmission is rare Legionellosis usually presents as non-
productive cough, chest pain and high fever Other symptoms include mental confusion,
diarrhoea, nausea and vomiting
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Laboratory Diagnosis
Specimens: sputum, bronco-alveolar lavage, respiratory tissue and rarely pleural effusion and blood for culture. It may be necessary to culture samples from hospital water supplies and samples from cooling systems of air conditioners. Specimens must be sent ASAP but if not, specimen should be kept in refrigerator
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Laboratory Diagnosis
1. Microscopy
Gram stained sputum - faintly stained Gram negative bacilli
Direct fluorescent antibody method are also valuable though they may not be totally specific as occasionally cross-reactions occur with non-legionella bacteria
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Laboratory Diagnosis
2. Culture
Fastidious and slow growing, require growth supplements and cysteine for growth
Primary isolation medium is the buffered charcoal yeast extract (CYE) agar, medium is made selective by addition of vancomycin, polymixin B and anisomycin
Medium is inoculated with 1:10 dilution of sputum or lavage, and incubated at 35 C
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Laboratory Diagnosis
Colonies of about 1 - 2 mm may appear after 3 days incubation
The colonies are round, greyish-pink and opalescent with entire edges
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Laboratory Diagnosis
3. Identification test
Presumptive identification of Legionella is made when the suspected isolate is found to be cysteine dependent and faintly Gram negative bacillus
When sub-cultured on a cysteine deficient medium, the organisms fail to grow
4. Serology
Indirect immunoflourescent method is usually used to detect L. pneumophilia antibodies
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Laboratory Diagnosis
A rapid microagglutination test is commercially available
5. Antibiotic sensitivity Sensitive to erythromycin, rifampicin,
cotrimoxazole and 4-fluoroquinolones
Most strains are resistant to penicillin due to their beta-lactamase production
Aminoglycosides are also effective therapy
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