Lecture 17: Anthrax and Bacterial Pneumonia

Legionella pneumophila

15 serogroups, serogroup 1 causes >80% of diseases

Aerobic, gram-negative, unencapsulated bacillus

Has a single polar flagellum

Outer membrane contains LPS

Grows aerobically but requires specialized growth medium (fastidious)

Requires increased iron and cysteine for growth - Buffered charcoal yeast extract (BCYE) agar is commonly used

Catalase and Oxidase positive

Found naturally in freshwater environments

Legionella pneumophila Pathogenesis

Inhalation of aerosols

Bacteria are internalized by host cells through endocytosis

Inhibits phagosome – lysosome fusion

Host cell is killed when the phagosome lyses, releasing toxic enzymes

Bacteria are released upon cell lysis

Legionella pneumophila Risk Factors

Smoking

Chronic lung disease

Immunosuppression

Elderly

Alcohol abuse

Pontiac fever

Fever, chills, malaise, myalgia, headache

No sign of pneumonia

Self-limited illness; symptoms last 2-5 days and resolve spontaneously without treatment

Legionnaires’ Disease

A severe, acute atypical pneumonia with a high mortality rate

2-10-day incubation period, abrupt onset of symptoms:

Fever, chills, dry/nonproductive cough, Headache

Radiographic patchy unilateral opacities, which can progress to consolidations

GI and neurologic symptoms common

Nausea, vomiting, diarrhea

Lethargy, altered mental status

Death is due to shock or respiratory failure

Legionella pneumophila Diagnosis

Culture on BCYE agar is considered the gold standard

Samples – sputum, bronchoalveolar lavage

PCR – sputum or bronchoalveolar lavage specimen

Diagnostic accuracy is high

Urine antigen test for serogroup 1

Legionella pneumophila Treatment

Antibiotics:

Azithromycin – Macrolide – binds to 50S ribosomal subunit and blocks protein synthesis

Levofloxacin – Quinolone – Inhibits DNA gyrase – blocks replication

Tetracyclines (e.g. doxycycline) – Inhibits 30S ribosomal subunit

Chlamydias

Gram-negative obligate intracellular bacteria

Chlamydia trachomatis

Chlamydophila pneumoniae

Non-motile, coccoid-shaped

Elementary bodies (EB): Infectious stage of life cycle

Reticulate bodies (RB): Intracellular stage

“Energy parasite” - relies entirely on host cells for ATP

Outer membrane contains:

Lipopolysaccharide (LPS)

Cysteine-rich proteins – extensively cross-linked by disulfide bonds to provide structural rigidity

Major outer membrane proteins (MOMPs) – Elicit immune response

Pathogenesis:

Has a high affinity for epithelial cells

Lyse host cells

Induce inflammatory response

Chlamydia trachomatis

Neonatal infection : Infant pneumonia

Spreads from infected mother’s birth canal to child

Symptoms appear 2-12 weeks after infection

Symptoms:

Most are modestly ill and afebrile

Staccato cough (Partussis-like)

Severe conjunctivitis

Nasal obstruction

Tachypnea, rales

Eosinophilia

Chest radiography: nonspecific

Chlamydophila pneumoniae

Adult atypical pneumonia

Transmitted from person to person, likely via respiratory droplets

Outbreaks in closed communities

Symptoms:

Typically mild: fever, cough, shortness of breath

Chest radiograph usually nonspecific: unilateral alveolar opacities are common

Chlamydias Diagnosis

PCR-based assays (NAAT) – Most sensitive

Multiplex (can test for multiple pathogens)

Samples from nasopharyngeal swabs, sputum, and bronchoalveolar lavage fluid

Cell culture: microscopy to see inclusions

Requires specialized equipment

Chlamydias Treatment

Macrolides (e.g., Erythromycin or azithromycin): children and

adults

Oral erythromycin is best for infants with conjunctivitis or pneumonia

Treatment of conjunctivitis with oral antibiotics ensures any nasopharynx infection is also cleared

Tetracyclines (e.g., doxycycline): Older children and adults (do not use doxycycline for children or pregnant women)

Fluoroquinolones can also be used

Coxiella burnetii

Q Fever

Gram-negative coccobacilli - zoonotic transmission: comes from cattle, goats, sheep

Diagnosis:

Symptoms and history (contact with pregnant animals or animal birth products)

PCR

Serology

Culture is difficult and not practical

Most acute infections resolve spontaneously

Antibiotic therapy reduces the length of the fever and the likelihood of chronic infection

For acute Q fever: 14-day course of doxycycline, trimethoprim-sulfamethoxazole for pregnant women

For chronic Q fever: 18-month course of doxycycline

Bacillus sp.

Gram positive rods

Forms chains

Aerobic and facultative anaerobic organisms

Spore forming

Endospores are stable in extreme conditions

Bacillus anthracis

Spores are the infectious agent

Unique cell wall

Thick peptidoglycan

Secondary cell wall polysaccharides (SCWP)

Covalently bound to peptidoglycan

Anchors S-layer to peptodoglycan

S-layer:

Crystalline

Sap – primary protein during exponential growth

EA1 – replaces Sap as cell enters stationary growth phase

Responsible for mechanical stability and virulence

Virulent strains carry two extrachromosomal plasmids

pXO1:

Lethal factor (LF)

Edema factor (EF)

Protective antigen (PA)

LF+PA → Lethal toxin

EF+PA → Edema toxin

pXO2:

Encodes for capsule production

capBCDAE operon encodes genes necessary for poly-γ-D-glutamic acid capsule

Inhibits phagocytosis

PA receptor on host cells: anthrax toxin receptor/tumor endothelial marker 8 (ATR/TEM8)

Lethal toxin

Zinc dependent protease

Cleaves host cell MAP kinase proteins

Leads to host cell death and release of TNFα

Edema toxin

Adenylate cyclase

Increases cAMP levels in host cell

Leads to swelling and edema

Cutaneous anthrax

Most common

Spores get into body via cut or scrape

Starts 1 to 7 days post exposure

Edema leads to formation of papule at site of infection

Painless ulcer (eschar) forms and leads to necrotic area – fully developed by 7 days

Lymph nodes also become swollen and painful

Inhalation anthrax

Incubation period as long as 6 weeks

Spores remain ungerminated in lower airways

Alveolar macrophages ingest spores and carry them to mediastinal lymph nodes

Enlargement of mediastinal lymph nodes – pronounced mediastinal widening

Initial symptoms are general

Fever, malaise, cough

Secondary symptoms show rapid progression

Severe fever and edema

Meningitis

Respiratory failure and sepsis precedes death

Pneumonia very rarely develops despite inhalation route of infection

Systemic disease that involves multiple systems

All patients progress to the point of shock and death within 3 days of presenting symptoms

Gastrointestinal anthrax

Rare

Spores ingested via contaminated meat

Starts 1 to 7 days post exposure

Invasion of intestinal tract:

Phase I: low grade fever, syncope and malaise

Phase II (24 hours after Phase I): mild to severe abdominal pain, nausea, and vomiting sometimes with blood

Ulcers can form in mouth and espohagus

Sepsis rapidly occurs as the bacterium spreads and continues making toxin

Bacillus anthracis Diagnosis

Gram stain from: Pus, blood, pleural fluid, CSF – Gram-positive rods

Culturing can be dangerous: Non-hemolytic colonies on blood agar

Rapid ELISA that measures total antibody to PA

PCR of toxin genes: Genetic similarities between strains in the Bacillus group strains prevent efficient PCR analysis

Bacillus anthracis Treatment

Antibiotics:

If inhalation anthrax is suspected, prophylaxis with doxycycline and ciprofloxacin can be used for 60 days

Latency in spore germination requires lengthy abx course

Immediate vaccination (purified, inactivated toxins)

Cutaneous anthrax treated with penicillins

Obiltoxaximab and Raxibacumab are monoclonal antibodies to PA that are used to treat infected or exposed individuals