Rickettsiae and Spirochetes

Rickettsiae and Spirochetes

Contents

Ricketssiae

• Morphology

• Classification

• Diseases

Spirochetes

• Morphology

• Classification

• Diseases

Intended learning objectives

At the end of this lecture, the student will be able to

• Explain the characteristic features of ricketssiae

• List certain ricketssial diseases and their symptoms

• Explain the special features in the morphology of spirochetes

• Identify clinically important spirochetes

Study of Rickettsiae

• They are named after Howard Taylor Ricketts

• Non-motile

• Gram-negative

• Non-sporeforming

• Highly pleomorphic bacteria

• Can present as cocci(0.1 μm in diameter)

• Rods (1–4 μm long)

• Thread-like (10 μm long).

• Obligate intracellular parasites

• Rickettsia survival depends on entry, growth, and replication within the cytoplasm

• Cannot live in artificial nutrient environments

• Grown either in tissue or embryo cultures (typically, chicken embryos are used) of eukaryotic host cells (typically endothelial cells)

• In the past it was positioned somewhere between viruses and true bacteria

Rickettsial diseases

• Rickettsia species are carried by many chiggers, ticks, fleas, and lice,

• Typical rickettsia is very similar to that of Gram-negative bacteria.

• Outer structures - innermost cytoplasmic membrane, a thin electron dense rigid cell wall and an outer layer.

• The outer layer resembles typical membranes in its chemical composition and its trilaminar appearance.

Structure

• The cell wall is chemically similar to that of Gram-negative bacteria but it contains diaminopimelic acid and lacks teichoic acid.

• Intracytoplasmic invaginations of the plasma membrane (mesosomes) and ribosomes are also seen.

• There are no discrete nuclear structures

• Cause diseases in humans such as

– Typhus

– Rickettsial pox

– Boutonneuse fever

– African tick bite fever

– Rocky Mountain spotted fever

– Flinders Island spotted fever

– Queensland tick typhus

Transmission

• In their arthropod vectors, the rickettsia multiply in the epithelium of the intestinal tract

• They are excreted in the feces, but occasionally gain access to the arthropods salivary glands.

• They are transmitted to man, via the arthropod saliva, through a bite.

• In their mammalian host, they are found principally in the endothelium of the small blood vessels, particularly in those of the brain, skin and heart.

• Hyperplasia of endothelial cells and localized thrombus formation lead to obstruction of blood flow, with escape of RBC's into the surrounding tissue.

Pathogenicity

• Inflammatory cells also accumulate around affected segments of blood vessels.

• This angiitis appears to account for some of the more prominent clinical manifestations, such as petechial rash, stupor and terminal shock.

• Death is ascribed to damage of endothelial cells, resulting in leakage of plasma, decrease in blood volume, and shock.

Symptoms of rickettsioses

• Rickettsioses are difficult to diagnose

• Common symptoms that typically develop within 1–2 weeks of infection

• Fever, headache, malaise, rash, nausea, and vomiting

• Rickettsia infections damage the permeability of blood capillaries, which results in a characteristic spotted rash

Diagnosis and treatment

• Diagnosis is usually based on clinical recognition and serology

• Treatment of patients with possible rickettsioses should be started early and should never await confirmatory testing

• Immediate empiric treatment with a tetracycline is recommended, most commonly doxycycline

• Broad-spectrum antibiotics are not usually helpful

• Chloramphenicol may be an alternative

• No vaccine is available for preventing rickettsial infections

Study of spirochaetes

• The phylum Spirochaetes contains gram-negative, chemoheterotrophic bacteria

• Spirochetes can be anaerobic, facultatively anaerobic, or aerobic.

• Distinguished by their structure and mechanism of motility

• They are slender, long bacteria (0.1 to 3.0 m by 5 to 250 m) with a flexible, helical shape

Distinguishing spirochetes from other bacteria

• Spirochetes differ greatly from other bacteria with respect to motility

• Can move through very viscous solutions though they lack external rotating flagella

• When in contact with a solid surface, they exhibit creeping

• Or crawling movements

• Their unique pattern of motility is due to an unusual morphological structure called the axial filament

Spirochaete morphology

• The central protoplasmic cylinder contains cytoplasm and the nucleoid

• Bounded by a plasma membrane and gram-negative type cell wall

• Two to more than a hundred procaryotic flagella, called axial fibrils, periplasmic flagella or endoflagella, extend from both ends of the cylinder and often overlap one another in the center third of the cell

• The whole complex of periplasmic flagella, the axial filament, lies inside a flexible outer sheath or outer membrane.

• The outer sheath contains lipid, protein, and carbohydrate and varies in structure between different genera

• The outer sheath of Treponema pallidum has few proteins exposed on its surface.

• This allows the syphilis spirochete to avoid attack by host antibodies

Spirochete morphology

• One end of each axial filament is attached near a pole of the cell

• By rotating its axial filament, the cell rotates in the opposite direction, like a corkscrew

• The movement is very efficient in moving the organism through liquids

• Many spirochetes are found in the human oral cavity

Spirochete - classes

Treponema

• The spirochetes include a number of important pathogenic bacteria

• The best known is the genus Treponema, which includes Treponema pallidum, the cause of syphilis

Borrelia

• Members of the genus Borrelia cause relapsing fever and Lyme disease, serious diseases that are usually transmitted by ticks or lice

Leptospira

• Leptospirosis is a disease usually spread to humans by water contaminated by Leptospira species.

• The bacteria are excreted in the urine of such animals as dogs, rats and swine. so domestic dogs and cats are routinely immunized against leptospirosis.

Symptoms

• High fever

• Headache

• Chills

• Muscle aches

• Vomiting

• Yellow skin and eyes

• Red eyes

• Abdominal Pain

• Diarrhoea

• Rash

Leptospira interrogans, the cause of leptospirosis