# Atypical Bacteria Overview > Atypical bacteria overview for NEET-PG Microbiology: Mycoplasma, chlamydiae, legionellae and Coxiella, why beta-lactams fail, and how diagnosis is made. - Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/atypical-bacteria-overview - Exam / course: NEET-PG · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Atypical Bacteria Overview", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/atypical-bacteria-overview ## Direct answer The chest radiograph looks worse than the auscultation, the cough is dry, the patient is walking about — that is the clinical signature of the atypical bacteria: Mycoplasma pneumoniae, Chlamydophila pneumoniae and psittaci, Legionella pneumophila and Coxiella burnetii. They are called atypical because they defy the standard bacterial workup: they are either cell-wall-less (Mycoplasma) or obligate intracellular (chlamydiae, Coxiella) or fastidious (Legionella), so Gram stain of sputum and ordinary blood agar culture are unhelpful. Diagnosis rests on serology, urinary antigen or polymerase chain reaction, and treatment must be a macrolide, tetracycline or fluoroquinolone because beta-lactams have nothing to attack. ## What you must remember - **Definition:** organisms not visualised on routine Gram stain and not grown on ordinary media, producing pneumonia with gradual onset, dry cough, headache and patchy lower-zone infiltrates. - **Mycoplasma pneumoniae:** smallest free-living organism, no cell wall, fried-egg colonies on Hayflick's medium after 5-10 days; cold agglutinins (IgM anti-I) appear in about half of cases. - **Chlamydophila pneumoniae:** TWAR organism causing pharyngitis with prolonged subacute cough; micro-immunofluorescence is the reference serology, and it has no animal reservoir. - **Chlamydophila psittaci:** psittacosis from birds (parrots, poultry), splenomegaly with pneumonia is a clue, and tetracycline is preferred. - **Legionella pneumophila:** grows only on buffered charcoal yeast extract agar, serogroup 1 urinary antigen detects most clinical disease, hyponatraemia and diarrhoea accompany the pneumonia. - **Coxiella burnetii:** Q fever has no rash and a negative Weil-Felix because the organism lacks the alkali-stable antigen; phase II antibody dominates acute infection and phase I chronic infection such as endocarditis. - **Therapeutic principle:** erythromycin, azithromycin, doxycycline or a respiratory fluoroquinolone; penicillins and cephalosporins fail categorically in every member of this group. ## How to work through a non-resolving pneumonia Consider a 21-year-old hostel resident with ten days of dry cough, fever 38.5 degrees and inspiratory crackles at the left base; the radiograph shows a patchy lower-lobe infiltrate far more impressive than the chest signs, the white count is normal and sputum Gram stain shows no predominant organism. Within this framework, cold agglutinin positivity or a raised Mycoplasma IgM supports mycoplasma pneumonia, remembering that the textbook test — agglutination of group O red cells at 4 degrees Celsius that reverses at 37 degrees — appears in only about half of cases and also occurs in some viral and lymphoproliferative disease. Shift the patient to a 60-year-old smoker who returned from a hotel conference with high fever, diarrhoea, confusion and a serum sodium of 126: that is the Legionella gestalt. Ask specifically about cooling towers and hot-water systems, send the serogroup 1 urinary antigen (result within an hour, positive in roughly 80% of clinical legionellosis because serogroup 1 causes most disease), and treat with azithromycin or levofloxacin. If instead the exposure history is to parturient sheep or cattle (Coxiella is aerosolised from placental products and survives years in dust), think Q fever: a granulomatous hepatitis with doughnut granulomas, no rash, no Weil-Felix reaction, and a doxycycline-plus-hydroxychloroquine regimen if endocarditis supervenes. ## Viva angles examiners love Why does penicillin not work on Mycoplasma? Because beta-lactams act on the peptidoglycan cell wall that Mycoplasma never built; the same answer explains why it is naturally resistant to vancomycin and lysozyme, and why its colonies look like fried eggs on medium enriched with serum and yeast extract supplying the sterols it cannot synthesise. The reverse viva question is why atypical coverage is added to beta-lactam therapy in hospitalised community-acquired pneumonia — because sputum Gram stain cannot exclude these organisms, guidelines cover both typical and atypical organisms empirically. ## Frequently asked questions ### Which organisms constitute the atypical pneumonia group? Mycoplasma pneumoniae, Chlamydophila pneumoniae and psittaci, Legionella pneumophila and Coxiella burnetii; some lists add Chlamydia trachomatis in neonates. ### What are cold agglutinins? IgM antibodies against the I antigen of human group O red cells, detectable at 4 degrees Celsius and reversible at 37 degrees, present in roughly half of Mycoplasma pneumoniae cases; they explain the associated autoimmune haemolytic anaemia. ### Which medium grows Legionella? Buffered charcoal yeast extract agar with alpha-ketoglutarate, supplying L-cysteine and iron that the organism cannot obtain from blood agar — failure to grow on blood agar is itself a diagnostic clue. ### Why is Coxiella burnetii negative in Weil-Felix? Weil-Felix uses cross-reacting Proteus antigens shared by rickettsiae with alkali-stable polysaccharide; Coxiella lacks this antigen, so Q fever is Weil-Felix negative, as is trench fever. ### What are phase I and phase II antigens of Coxiella? Acute Q fever produces antibody mainly to phase II (rough, avirulent laboratory phase) antigens, while chronic disease such as endocarditis produces high phase I (smooth, virulent) antibody titres.