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Chapter 21

Miscellaneous Bacterial Pathogens: Mycoplasmas, Rickettsias, Chlamydias, Spirochetes, and Slightly Curved Bacilli

Miscellanous Bacterial Pathogens

• Stain pink in a Gram stain but differ from typical Gram-negative organisms

• Have different morphology, growth habits, or reproductive strategies

• Traditionally discussed separately due to their unique features

Mycoplasmas

• Smallest free-living microbes

• Lack cytochromes, enzymes of the Krebs cycle, and cell walls

• Can colonize the mucous membranes of the respiratory and urinary tracts

Mycoplasma pneumoniae

• Attaches specifically to receptors located at the bases of cilia on epithelial cells lining the respiratory tracts of humans

• Causes primary atypical pneumonia, or walking pneumonia

• Symptoms such as fever, headache, and sore throat are not typical of other types of pneumonia

• Not usually severe enough to require hospitalization or to cause death

• Spread by nasal secretions among people in close contact

Mycoplasma pneumoniae

• Diagnosis is difficult because mycoplasmas are small and grow slowly

• Prevention can be difficult because patients can be infective for long periods of time without signs or symptoms

Rickettsias

• Extremely small (not much bigger than a smallpox virus)

• Appear almost wall-less due to the small amount of peptidoglycan present

• Obligate intracellular parasites

• Unusual because they have functional genes for protein synthesis, ATP production, and reproduction

• Three genera cause disease in humans

• Rickettsia, Orienta, and Ehrlichia

Chlamydias

• Do not have cell walls

• Have two membranes but without any peptidoglycan between them

• Grow and multiply only within the vesicles of host cells

• Have a unique developmental cycle involving two forms

• Both forms can occur within the phagosome of a host cell

Chlamydia trachomatis

• Has a limited host range

• One strain infects mice, all others infect humans

• Infect the conjunctiva, lungs, urinary tract, or genital tract

• Enters the body through abrasions and lacerations

• Clinical manifestations result from the destruction of infected cells at the infection site, and from the resulting inflammatory response

Chlamydia trachomatis

• Causes two main types of disease

• Sexually transmitted diseases

• Causes the most common sexually transmitted disease in the United States

• Ocular disease called trachoma

• Occur particularly in children

• Endemic in crowded, poor communities with poor hygiene, inadequate sanitation, and inferior medical care

Sexually Transmitted Diseases

• Lymphogranuloma venereum

• Characterized by a transient genital lesion and swollen, painfully inflamed, inguinal lymph nodes

• Occurs in three stages

• Initial stage

• Produces a lesion at the infection site that is small painless, and heals rapidly

• Second stage

• Buboes develop at the infection site

Sexually Transmitted Diseases

• Third stage

• Only some cases progress to this stage

• Characterized by genital sores, constriction of the urethra, and genital elephantiasis

• Most infections in women are asymptomatic but men often have symptoms

• Women can develop pelvic inflammatory disease if reinfected with C. trachomatis

Trachoma

• Disease of the eye

• Leading cause of nontraumatic blindness in humans

• Bacteria multiply in the conjunctival cells resulting in scarring

• The scarring causes the eyelashes to turn inwards and abrade the eye that can eventually result in blindness

• Typically a disease of children who have been infected during birth

• Infection of the eye with bacteria from the genitalia can also result in disease

Diagnosis, Treatment, and Prevention

• Diagnosis

• Demonstration of the bacteria inside cells from the site of infection

• Treatment

• Antibiotics can be administered for genital and ocular infections

• Surgical correction of eyelid deformities from Trachoma may prevent blindness

Diagnosis, Treatment, and Prevention

• Prevention

• Abstinence and safe sex can prevent sexually transmitted chlamydial infection

• Blindness can only be prevented by prompt treatment with antibacterial agents and preventing reinfections

Chlamydia pneumoniae

• Causes bronchitis, pneumonia, and sinusitis

• Has been implicated in some cases of atherosclerosis

• Most infections are mild and don’t require hospitalization

• Some more severe cases can resemble primary atypical pneumonia caused by Mycoplasma pneumoniae

• Prevention is difficult because C. pneumoniae is ubiquitous and spreads via respiratory droplets

Chlamydia psittaci

• Causes ornithosis (psitticosis), a disease of birds, that can be transmitted to humans

• Usually causes flulike symptoms

• Rarely nonrespiratory conditions are observed

• Individuals that handle animals are at greatest risk of infection

• Transmission occurs via inhalation of aerosols or through contact with infected material or a pet bird

• Diagnosis is difficult because symptoms are similar to those of many other respiratory infections

Spirochetes

• Thin, tightly coiled, helically shaped bacteria

• Moves in a corkscrew fashion through its environment

• This movement is thought to enable pathogenic spirochetes to burrow through their hosts’ tissues

• 3 genera cause human disease

• Treponema, Borrelia, and Leptospira

Treponema pallidum

• Cannot survive in the environment

• Lives naturally only in humans as an obligate parasite

• Causative agent of syphilis

• Syphilis occurs worldwide

• Transmission is almost solely via sexual contact

• Endemic among sex workers, men who have sex with men, and users of illegal drugs

• Can also be spread from an infected mother to her fetus

• Often results in the death of the fetus or in mental retardation and malformation

Treponema pallidum

• Syphilis can proceed through four stages

• Primary, secondary, latent, and tertiary syphilis

Diagnosis, Treatment, and Prevention

• Diagnosis

• Primary, secondary, and congenital can be readily diagnosed with antibody tests against bacterial antigens

• Tertiary syphilis is difficult to diagnose

• Treatment

• Penicillin is the drug of choice except with tertiary syphilis which is a hyperimmune response and not an active infection

• Prevention

• Abstinence and safe sex are the primary ways to avoid contracting syphilis

Nonvenereal Treponemal Diseases

• Treponema species cause three nonvenereal diseases that occur primarily in impoverished children who live in unsanitary conditions

• Bejel

• T. pallidum endemicum is the causative agent

• Results in the formation of lesions around the lips and inside the mouth

• The bacteria are spread by contaminated eating utensils

Nonvenereal Treponemal Diseases

• Pinta

• T. carateum is the causative agent

• Causes a skin disease that can result in scarring and disfigurement

• Spread by skin-to-skin contact

• Yaws

• T. pallidum pertenue is the causative agent

• Characterized initially by skin lesions that can develop into large draining lesions

• Spread via contact with the bacteria in the fluid draining from the lesions

Borrelia

• Lightly staining, Gram-negative spirochetes

• Cause two diseases in humans

• Lyme disease

• Relapsing fever

Lyme Disease

• Borrelia burgdorferi is the causative agent

• Bacteria are transmitted to humans via a tick bite

• Hard ticks of the genus Ixodes are the vectors of Lyme disease

• The tick life cycle is important in understanding the spread of Lyme disease

Lyme Disease Pathology

• Shows a broad range of signs and symptoms

• 3 phases of disease in untreated patients

• An expanding red “bull’s eye” rash occurs at the site of infection

• Neurological symptoms and cardiac dysfunction

• Severe arthritis that can last for years

• Pathology of this stage is largely a result of the body’s immune response

Lyme Disease Pathology

• The increase of cases is a result of humans coming in closer association with ticks infected with Borrelia

• Antimicrobial drugs can effectively treat the first stage of Lyme disease

• Treatment of later stages is difficult because symptoms result from the immune response rather than the presence of bacteria

• Prevention is best achieved by taking precautions to avoid ticks

Relapsing Fever

• 2 types of relapsing fever

• Epidemic relapsing fever

• Borrelia recurrentis is the causative agent

• Transmitted to humans by the human body louse

• Endemic relapsing fever

• Several Borrelia species can cause this disease

• Transmitted to humans by soft ticks of the genus Ornithodoros

Relapsing Fever

• Both types of relapsing fever are characterized by recurring episodes of fever and septicemia separated by symptom free intervals

• Pattern results from the body’s repeated efforts to remove the spirochetes,which continually change their antigenic surface components

• Observation of the spirochetes is the primary method of diagnosis

• Successful treatment is with antimicrobial drugs

• Prevention involves avoidance of ticks and lice, good personal hygiene, and use of repellent chemicals

Leptospira interrogans

• Motile, obligately aerobic bacteria

• Normally found in numerous wild and domestic animals

• Causes the zoonotic disease, leptospirosis

• Humans become infected through direct contact with the urine of infected animals or indirectly via contact of contaminated streams, lakes, or moist soil

• Leptospira gain initial access via invisible cuts and abrasions in the skin and mucous membranes

• Travels via the bloodstream throughout the body

Vibrio

• Members of this genus share many characteristics with enteric bacteria such as Escherichia and Salmonella

• Found in water environments worldwide

• Vibrio cholerae is the most common species to infect humans

• Causes cholera

• Humans become infected with V. cholerae by ingesting contaminated food and water

• Found most often in communities with poor sewage and water treatment

Vibrio

• A large inoculum is required to cause disease because the bacteria are susceptible to the acidic stomach environment

• Cholera toxin is the most important virulence factor of V. cholerae

Cholera Pathology

• Some infections are asymptomatic or cause mild diarrhea

• Can cause severe disease resulting in abrupt watery diarrhea and vomiting

• “Rice-water stool” is characteristic

• Results in severe fluid and electrolyte loss

• Can progress to coma and death

Diagnosis, Treatment, and Prevention

• Diagnosis

• Usually based on the characteristic diarrhea

• Treatment

• Fluid and electrolyte replacement

• Antimicrobial drugs are not as important because they are lost in the watery stool

• Prevention

• Adequate sewage and water treatment can limit the spread of V. cholerae

Other Diseases of Vibrio

• Vibrio parahaemolyticus

• Results from ingestion of shellfish

• Causes cholera-like gastroenteritis

• Vibrio vulnificus

• Causes septicemia following consumption of contaminated shellfish

• Infections can results from washing wounds with contaminated seawater

Campylobacter jejuni

• Likely the most common cause of gastroenteritis in the United States

• Many animals serve as reservoirs for the bacteria

• Humans become infected by consuming contaminated food, milk, or water

• Poultry is the most common source of infection

• Infections produce bloody and frequent diarrhea that is self-limiting

• Spread of the bacteria can be reduced by proper food handling and preparation

Helicobacter pylori

• Slightly helical, highly motile bacterium that colonizes the stomach of its hosts

• Causes most (if not all) peptic ulcers

• H.pylori produces numerous virulence factors that enable it to colonize the stomach

Diagnosis, Treatment, and Prevention

• Diagnosis

• Presence of H. pylori can be demonstrated by a positive urease test

• Biochemical tests provide a definitive identification

• Treatment

• Antimicrobial drugs are used in combination with drugs that inhibit acid production

• Prevention

• Prevention is difficult because the exact mode of transmission is unknown

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