SN7 SHORT NOTE 5M

Describe Legionella pneumophila. Add a note on the other atypical organisms.

Microbiology · 2nd Professional — Para-Clinical

Legionella Pneumophila

A thin, faintly staining gram-negative bacillus that is poorly seen on Gram stain of sputum — a characteristic and diagnostically important negative
Requires L-cysteine and iron, and grows only on buffered charcoal yeast extract (BCYE) agar, not on ordinary media; growth takes 3 to 5 days
An intracellular pathogen of amoebae in the environment, which is how it survives in water systems and why it also survives inside human macrophages — the amoeba is effectively a training ground
Habitat — warm water systems: cooling towers, air conditioning, hot water tanks, showers, spa pools, humidifiers and decorative fountains; it is favoured by temperatures of 25 to 45°C, stagnation and biofilm
Transmission is by inhalation of aerosol; there is NO person-to-person spread, which is why outbreaks are traced to a source rather than to contacts

Form	Features
Legionnaires disease	A severe atypical pneumonia with high fever, relative bradycardia, confusion, diarrhoea, hyponatraemia and deranged liver enzymes — the extrapulmonary features are what suggest it. Radiographic changes exceed the auscultatory findings. Mortality is appreciable, particularly in the elderly, smokers and the immunosuppressed
Pontiac fever	A self-limiting flu-like illness without pneumonia, with a high attack rate and full recovery

CLINICAL PEARL

Hyponatraemia, diarrhoea and confusion in a patient with pneumonia should suggest Legionella. No single feature is specific, but the combination of extrapulmonary findings with a severe pneumonia is characteristic, and it matters because beta-lactams do not work — the organism is intracellular. A patient deteriorating on ceftriaxone with these features needs a macrolide or fluoroquinolone and a urinary antigen test, not a broader beta-lactam.

Diagnosis and Treatment

Urinary antigen test — the mainstay: rapid, unaffected by prior antibiotics, and positive for weeks. Its limitation is that it detects only serogroup 1, which nonetheless causes most disease
Culture on BCYE agar — the reference method, needed for outbreak typing and to detect other serogroups
Direct fluorescent antibody, PCR on respiratory samples; serology with a fourfold rise, which is retrospective
Treatment — an agent that penetrates cells: a macrolide (azithromycin) or a fluoroquinolone (levofloxacin). beta-lactams are ineffective because the organism is intracellular and also produces beta-lactamase

Other Atypical and Miscellaneous Organisms

Organism	Key features
Mycoplasma pneumoniae	The smallest free-living organism; NO cell wall, so it is pleomorphic, unaffected by beta-lactams, and its membrane contains sterols. "Fried egg" colonies on Hayflick medium. Causes atypical pneumonia in young people ("walking pneumonia"), with cold agglutinins, erythema multiforme and haemolysis. Treated with macrolides or doxycycline
Chlamydia trachomatis	An obligate intracellular bacterium with a unique biphasic cycle — the infectious elementary body and the replicating reticulate body. Serovars A–C cause trachoma (a leading preventable blindness, targeted by the safe strategy); D–K cause non-gonococcal urethritis, cervicitis, pelvic inflammatory disease and inclusion conjunctivitis of the newborn; L1–L3 cause lymphogranuloma venereum. Treated with doxycycline or azithromycin
Rickettsiae	Obligate intracellular; arthropod-borne. Scrub typhus (Orientia tsutsugamushi, from mite larvae, with an eschar) is common and important in India; also typhus and spotted fevers. The WEIL-FELIX test uses Proteus antigens. Treated with doxycycline
Coxiella burnetii	Causes Q fever by inhalation rather than by a vector; from parturient animals; causes pneumonia, hepatitis and culture-negative endocarditis
Bartonella	Cat scratch disease; bacillary angiomatosis in HIV

Applied Aspects

Cover atypical organisms in severe community-acquired pneumonia with a macrolide or fluoroquinolone alongside a beta-lactam; a beta-lactam alone will miss Legionella, Mycoplasma and Chlamydia entirely
Investigate the water system, not the contacts, in a Legionella cluster; control means chlorination, raising hot water temperature above 60°C, avoiding stagnation and maintaining cooling towers
Consider scrub typhus in any acute undifferentiated fever in India, particularly after rural or agricultural exposure; search carefully for an eschar in the axilla, groin and skin folds, and treat empirically with doxycycline, which is dramatically effective and cheap
Chlamydial infection is often asymptomatic in women and is a leading preventable cause of tubal infertility, which is the argument for screening and for partner treatment
Trachoma is prevented by facial cleanliness and environmental improvement as much as by azithromycin, and India was validated as having eliminated it as a public health problem — a substantial achievement
Remember that organisms without a cell wall or living inside cells will not respond to beta-lactams; this single principle covers Mycoplasma, Chlamydia, Rickettsia, Legionella and Coxiella together

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