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• Rickettsiae


  Diverse collection of obligate intracellular (resemble viruses), Gram negative bacteria found in ticks, lice, fleas, mites, and mammals
• Rickettsiae


  • Small G-ve bacteria
  • Classified as bacteria, but they resemble viruses in that they are obligate intracellular parasites and are unable to survive as free living organism
  • Best isolated either in yolk sac of embryonated egg, in guinea pig or in mice
  • Can be seen by light microscope if stained blue with Giemsa stain
  • Contains DNA and RNA
  • Replicate intracellularly by binary fission
  • Sensitive to chloramphenicol and tetracycline
  • Cell wall contains muramic acid
• Rickettsiae classification groups


  • Typhus (R. Prowazaki, R. Typhi)
  • Spotted fever (R. Conorii, R. Rickettsii)
  • Rickettsial pox (R. Akari)
  • Scrub typhus (Orientia tsutsugamushi)
• Weil Felix reaction


  Tube agglutination test based on the cross-reaction which occurs between antibodies produced in acute R infection and the OX19, OX2, OXK strains of proteus
• Rickettsiae species are transmitted by the bite of infected ticks or mites or by the feces of infected lice or fleas
• From the portal of entry in the skin, rickettsiae spread via the bloodstream. They invade capillary endothelium causing vasculitis in brain, heart and other organs. DIC and vascular occlusion may occur
• Disease may be fatal and survivors usually remain with long lasting immunity
• Clinical manifestations of Rickettsiae infections

  • Rocky Mountain spotted fever
  • Rickettsial pox
  • Other spotted fevers
  • Epidemic typhus
  • Murine typhus
• Diagnostic methods for Rickettsiae

  • Isolation of the organism in the first week of infection
  • Serology usually in the second week
  • Weil Felix reaction
  • Complement fixation test
  • Rickettsial agglutination test
  • Immunofluorescence
  • Passive hemagglutination test
  • PCR
• Rickettsiae species are susceptible to the broad-spectrum antibiotics, doxycycline, tetracycline, and chloramphenicol
• Prevention of exposure to infected arthropods offers some protection
• Chlamydiae


  A large group of obligate intracellular parasites closely related to Gram negative bacteria
• Chlamydiae


  • Contain both DNA & RNA
  • Replicate only in living cells (like virus) as it can't synthesize ATP
  • Unable to grow on inanimate media (unlike bacteria)
  • Multiply by binary fission and possess cell wall, ribosomes and synthesis protein
  • Stain with Giemsa stain
  • Produce intracytoplasmic basophilic inclusions
  • Their growth can be inhibited by many antimicrobial drugs
• Chlamydiae reproduction


  1. Infective chlamydial particle called elementary body is ingested by a host cell

  2. Host cell surrounds the elementary body with a membrane to form an inclusion body

  3. Elementary body enlarges to form an initial or reticulate body

  4. Reticulate body multiplies by binary fission to form new reticulate bodies

  5. Within 24-48 hrs, the reticulate bodies form themselves to infective elementary bodies which are released when the host cell rupture
• Chlamydiae antigenic structure


  • Group antigens shared by all chlamydiae
  • Specific antigens (species-specific or immunotypes-specific)
• Chlamydiae species


  • C. trachomatis
  • C. Psittaci
  • C. Pneumoniae
• Infections due to C. trachomatis

  • Trachoma
  • Genital chlamydial infections and inclusion conjunctivitis
  • Lymphogranuloma venereum
• Chlamydiae trachomatis immunotypes A, B, Ba & C cause trachoma
• Chlamydiae trachomatis immunotypes D-K cause genital infections and inclusion conjunctivitis
• Chlamydiae trachomatis immunotypes L1-L3 cause lymphogranuloma venereum
• Antibiotics with activity against C. trachomatis are sulphonamides, tetracycline, chloramphenicol and rifampicins
• Psittacosis is a disease of birds, especially psittacine birds, caused by C. Psittaci
• C. Pneumoniae causes atypical pneumonia in humans and has been associated with atherosclerosis, heart disease, adult asthma and Alzheimer's disease
• Laboratory diagnosis of chlamydial infections

  • Microscopy & staining
  • Culture
  • Antigenic detection
  • Serology
  • Molecular techniques
• Mycoplasmas


  Smallest organism that can be free living in nature and can also grow on laboratory media. Few can cause human diseases particularly in respiratory tract & urogenital tract
• Mycoplasmas


  • Very small in size (125-250 nm)
  • Lack a rigid cell wall and thus they are highly pleomorphic, poorly stained gram negative and completely resistant to penicillins and cephalosporins
  • They require cholesterol for growth
• Mycoplasma species of medical importance

  • Mycoplasma pneumoniae
  • Mycoplasma hominis
  • Mycoplasma genitalium
  • Ureaplasma urealyticum
• Mycoplasma pneumoniae causes atypical pneumonia, brochopneumonia, acute pharyngitis, anemia (cross-reaction) and septic arthritis
• Mycoplasma hominis causes post-partum fever and salpingitis
• Mycoplasma genitalium causes genital infections
• Ureaplasma urealyticum causes non-gonococcal urethritis, which may play a role in male infertility
• Diagnostic methods for Mycoplasma infections

  • Direct smear
  • Culture
  • Antigenic detection
  • Serology
  • PCR
• New quinolones such as trovafloxacin and sparfloxacin are the drug of choice for treating Mycoplasma infections