Atypical Bacteria - Part 1 Quiz

Atypical Bacteria - Part 1

• Atypical bacteria are categorized as intracellular or extracellular
• Intracellular bacteria include Chlamydia, Legionella, Mycobacterium, and Rickettsiales, among others.
• Extracellular bacteria include Mycoplasma, Spirochetes (Leptospira, Borellia, Treponema), etc.
• Key bacteria covered in this lecture include Chlamydia and Chlamydophila, Legionella, and Mycobacterium.

Recommended Reading and Learning Objectives

• Review appropriate chapters from medical microbiology
• Describe each pathogen's cell wall properties (intracellular or extracellular)
• Detail the pathogen's natural habitat (human, animal, soil, etc.)
• Explain transmission mechanisms
• Identify clinical manifestations of diseases
• Describe pathogenicity, linking virulence factors
• Explain diagnostic features

Case Study

• A 19-year-old male presented to an STD clinic, with a purulent urethral discharge
• A gram stain revealed no gram-negative diplococci
• The most likely clinical diagnosis is non-gonococcal urethritis
• Possible etiological diagnoses are: Chlamydia trachomatis, Ureaplasma urealyticum, Mycoplasma hominis, Mycoplasma genitalium, and Trichomonas vaginalis

Chlamydia and Chlamydophila

• Worldwide human pathogens
• They do not gram stain, but have characteristics similar to gram-negative bacteria
• Outer membrane, LPS
• Obligate intracellular pathogens; require cell culture for isolation
• Once thought to be viruses, but have DNA, RNA, and ribosomes, and make their own macromolecules.
• Obtain energy from host ATP
• Unique life cycle in two forms: elementary bodies (EB) and reticulate bodies (RB)
• EB are metabolically inactive, infectious
• RB are metabolically active, non-infectious
• Chlamydia's life cycle includes attachment, transcription of DNA, RNA and protein synthesis, binary fission, and continued multiplication
• Persistent infection possible with presence of gamma-IFN
• Chlamydia trachomatis is the species of importance, with types concerning ocular, genital, and invasive infections
• Different subtypes (serotypes) of C. trachomatis cause different diseases (trachoma, genital, lymphogranuloma)

Virulence Factors of Chlamydia

• Intracellular replication in numerous cell types
• Safe from complement, antibodies, and phagocytosis
• Prevent fusion of endosome with lysosome
• Endosome becomes inclusion body for replicating RBs
• Transport proteins (IncA, etc.) facilitate nutrient uptake from cytoplasm into inclusion body
• Secretion systems function as "molecular straws" transporting nutrients, and inhibit apoptosis
• Proteins also inhibit MHC Class I expression, decreasing antigen presentation to CD8 cells

Pathogenesis of Chlamydia trachomatis

• Elementary bodies enter epithelial cells of conjunctiva and urogenital tracts
• Organism replicates as previously described.
• Infected cells release inflammatory mediators (IL-1, IL-8, others) leading to acute, then chronic inflammation.
• Disease manifestations result from chronic inflammation, tissue reorganization, and scarring, especially if left untreated
• Infected cells stimulated with gamma-IFN can temporarily halt replication but not kill RBs
• The lack of this inhibition may lead to chronic infection
• Gamma -IFN induces the expression of indoleamine-2,3-dioxygenase (IDO) which degrades tryptophan. This reduced tryptophan leads to the death of Chlamydia spp. through tryptophan starvation. However, some reticulate bodies acquire a non-replicating, but viable, persistent form. Removing IFN-gamma and replenishing tryptophan causes re-differentiation into infectious elementary bodies.

Important Members of the Chlamydia family

• C. trachomatis causes ocular infections, genital infections, and neonatal infections. Many serotypes make vaccine creation difficult and cause repeat infections.
• C. pneumoniae causes respiratory infections
• C. psittaci, a bird pathogen, causes psittacosis in humans
• Flu-like illness with lower respiratory symptoms associated with bird excretion

Legionella pneumophila

• Gram-negative bacteria that do not stain well with Gram stain.
• Pleomorphic in culture (have several forms in different settings)
• Legionella pneumophila serotypes 1 and 6 cause over 90% of human infections

Transmission and Epidemiology of Legionella sp.

• Aquatic saprophytes that live in biofilms and amoebae in water
• Found in wide variety of natural and man-made water sources (lakes, rivers, ponds, showers, fountains, spas, etc.)
• Transmitted through water mist inhalation.
• Survive high temperatures and treatment with chlorine.

Legionella in Man-Made Water Sources

• Pathogenic Legionella can produce aerosols in engineered and natural water-systems, leading to lung infections, resulting in Legionellosis

Legionnaires' Disease/Legionellosis

• Community-acquired and hospital-acquired pneumonia
• ~10,000 reported cases per year in the US.
• Risk factors include elderly, patients with smoking, renal, or liver disease, diabetes, immune suppression (e.g., from cancer or AIDS), transplantation, and corticosteroid use.
• Pontiac Fever, a milder disease, is also associated with Legionella

Pathogenesis of Legionnaires' disease

• Infection through inhalation, resulting in growth in alveolar macrophages
• Inhibit formation of phagolysosome, surrounding phagosome with materials from endoplasmic reticulum
• Incubation period is 2-10 days
• Symptoms are abrupt onset of fever, chills, nonproductive cough, and headache

Diagnosis of Legionellosis

• Clues include pneumonia with both pulmonary and nonpulmonary symptoms, negative sputum gram stain
• PCR, urine antigen test, and culture using buffered charcoal yeast extract (BCYE)

Prevention of Legionella Infection

• Superheating of water to 70-80°C
• Installing copper-silver ionization units

Tuberculosis

• A contagious disease caused by Mycobacterium tuberculosis
• Person-to-person transmission occurs through inhaling droplets from a cough or sneeze.
• Global health concern

Global Impact of Tuberculosis

• One of the leading causes of death worldwide
• Estimated 1/3 of world population is infected.
• Estimated 8 million new cases annually.
• 1.5-3.0 million people die annually from tuberculosis
• Antibiotic resistance is a major problem.

Mycobacterium

• Acid-fast, aerobic rods.
• Cell wall contains lipids, making them resistant to chemicals and drying
• Important species include M. tuberculosis, M. leprae, and M. intracellulare
• M. tuberculosis (TB)
• M. leprae (leprosy), with pockets of high endemicity
• M. intracellulare, a disseminated disease in immune-compromised individuals

TB Incidence & Risk Factors

• TB incidence varies geographically
• Foreign-born people from high-TB countries, close contacts, people in high-risk congregate settings (prisons/shelters), HIV/AIDS patients, injection drug users, medically underserved and low-income populations have elevated risk.

Pathogenesis of Mycobacterium tuberculosis

• Inhalation of droplets containing bacteria
• Bacteria access lungs and replicate within alveolar macrophages
• Replication progresses very slowly
• Outcome depends on host immune response; Latent or active infection

Latent Infection

• Bacteria are contained by granulomas, preventing replication, but staying viable for years/decades
• Over time, granulomas can turn into calcified nodules (Gohn complex).

Active Infection (Pulmonary)

• Majority of people develop miliary TB (usually in immune compromised people/children).
• Pulmonary TB; TB primarily in the lung
• Secondary (reactivated) TB
• Immune suppression allows latent TB to reactivate, resulting in granuloma breakdown and releasing organisms into the body

Primary vs Reactivated Pulmonary TB

• Primary infection and ensuing pulmonary disease is more common in children and less in adults.
• Reactivated TB infection and disease is more common in adults whose immune system is compromised.

Manifestations of Reactivated Pulmonary TB

• Insidious onset of fever, night sweats, weight loss, anorexia, malaise, and weakness.
• Productive cough, often with blood-streaked sputum
• Apical and posterior segments of upper lobes are most often affected
• Cavitary lesions in CXR are indicative of reactivated pulmonary TB

Miliary (Non-Pulmonary) TB

• Miliary TB is a disseminated form of TB
• Can affect lymph nodes, meninges (meningitis).
• Back pain associated with Potts disease/involvement of the spine is common
• Chronic arthritis
• Gastrointestinal system involvement, including ulcers, malabsorption, and diarrhea

Diagnosis of Active TB

• TB skin test or interferon gamma release assay
• Acid-fast stain of sputum
• Sputum sample for nucleic acid detection, culture and sensitivity testing.
• NA detection using RNA probes
• PCR can detect M. tuberculosis.

Treatment of TB

• Directly observed therapy is a globally used method to encourage patient compliance
• Latent TB is treated with a 2-drug regimen for 9 months
• Active TB is treated with 4-drug regimen for both two and longer periods of time, depending on resistance and severity

Case Study - Mycobacterium avium complex (MAC) infection

• A 23-year-old AIDS patient with a 3-month history of fever, night sweats, fatigue, weight loss (22 pounds), and diarrhea.
• Low CD4 count (below 50/mm3) and high viral load (55,000/mm3) for the preceding several months.
• Routine blood cultures negative after 48 hrs but grew after ten days
• DNA probes identified Mycobacterium avium complex organisms.

Pathogenesis of Disseminated MAC in AIDS Patients

• Disseminated infection in severely immunocompromised patients with CD4 counts below 50/uL
• Organism enters body through ingestion or inhalation, crosses mucosal epithelium and infects macrophages
• Macrophages carry the organism throughout the body to other organs.
• Macrophages replicate the organisms and secrete a variety of cytokines.
• High burden of microbes in macrophages causes a "cytokine storm" that leads to widespread inflammation and tissue harm.

Questions regarding the case study and organisms.

• Diagnostic features of organisms
• Sources of organisms
• Other diseases caused by organism presented in case study
• Treatment options