Obligate Intracellular Organisms

Bacterial Intracellular Organisms

Intracellular organism

Lives in a phagosome &prevents phagolysosomal

fusion

Escapes from phagosome& lives in cytosol

Lives in phagolysosome

Pathogenesis not wellunderstood. Adheresto endothelial cells

and is engulfed

•Ricketssiae•Listeria•shigella

•Chlamydiae spp•M. Tuberculosis•Legionella spp

•Toxoplasma gondii

•Leishmenia•Coxiella

•Bartonellaspp

Chlamydia Species

• Obligate intracellular organisms• Small round to ovoid cells, 0.3µm• Cell has peptidoglycan and an outer lipid layer

resembling that of a Gram negative bacteria• Genome much smaller than that of other bacteria• They cannot make ATP (adinosine triphosphate) –

dependent on host cell for energy production • They import nutrients from the cytosol into the

endosome with the help of tubular projections on the surface of the reticulate body

Chlamydia Species

• Growth characteristics: cannot be grown on artificial bacteriologic medium, only grows within living tissue or tissue culture cells

• Replication is by binary fission but they undergo morphologic variation during replication cycle– have distinct elementary bodies (EB) adapted for

extracellular survival and initiation of infection and

– reproductive reticulate body (RB) forms for intracellular replication

Chlamydia Species

• Antigenic composition:

• Serotyping is based on specific cell wall proteins

• Each serotype is associated with a specific disease

Epidemiology

• Chlamydiae are susceptible to environmental conditions, survive only a short time outside the host

• Transmission is by direct contact among humans

• C. psittaci is pathogenic for birds and domestic fowl and is transmitted to humans by inhalation of bacteria in droplets or dust

Pathogenicity• Highly infectious• Pathogenesis of disease caused by these bacteria is

not well understood• Different strains of both C. trachomatis and C.

psittaci show different degrees of virulence– Phagocytosed chlamydiae prevent fusion of lysosome

to the phagosome thus escape intracellular destruction by lysosomal enzymes

– Produce heat-labile toxin – Competition for nutrients – C. trachomatis may exist in a latent state and may

reactivate if host becomes immunosuppressed

Diseases caused by Chlamydia species

• Chlamydia cause persistent and recurrent infections

• Three species cause human disease: 

• C. trachomatis: trachoma a chronic infection which causes blindness , inclusion conjunctivitis an acute infection, urethritis, cervicitis, salpingitis and lymphogranuloma venerium (LGV)

• C psittaci: which causes pneumonia,

• C. pneumoniae: which causes pneumonia, pharyngitis, bronchitis

Laboratory Diagnosis and Treatment

• C. trachomatis is detected using– Direct fluorescent antibody staining of genital exudates

with fluorescein-labelled monoclonal antibodies against MOMP or LPS. 

– Serologic techniques are not used for C. trachomatis and culture is rarely done. 

– Nucleic acid hybridization techniques are widely used to detect asymptomatic genital infection in women.

• C. psittaci pneumonia: a 4 fold rise in CF antibody to chlamydial group antigen

• Treatment of Chlamydia includes macrolides and tetracyclines

Rickettsia

• Obligate intracellular pathogens

• Small GNB 0.3-0.5µm

• Stain poorly with bacteriologic stains

• Visualized easily with Giemsa

• Important members include: R. prowazekii, R. typhi, and R rickettsii

Growth characteristics

• Only grows within living tissue or tissue culture cells

• Intracellular growth: enter host by endocytosis, produces a phospholipase which destroys the phagosome and it begins to grow in the cytoplasm of host cell.– require coenzyme A, nicotinamide-adenine dinucleotide

(NAD), + energy from host cell

• Free rickettsiae: cease metabolic activity and begin to leak intracellular constituents resulting in lack of infectivity within a short period

Life Cycle

Pathogenicity

• Generally have animal reservoirs

• Transmitted by the bite of an arthropod vector

Pathogenesis

• Bacteria invade the vascular epithelial cells and become widely disseminated

• Damage to the endothelial cells in small vessels causes vasculitis, obstruction, capillary leaks and thrombosis. 

• This causes a rash, organ damage and potentially shock.

Clinical Disease

• Spotted fever– Fever, headache

and rash– Eschar = black scar

at site of bite

Diagnosis and Treatment

• By necessity, the diagnosis must be made clinically

• If a rickettsial illness is suspected, treat first with an antibiotic that can reach a therapeutic concentration inside infected cells (tetracyclines, long acting macrolides, fluoroquinolones etc.), and then confirm the diagnosis with serology.