STEPTRACKERMICROBIOLOGY | ATYPICAL BACTERIA | HIGH-YIELD REVIEW

Mycoplasma, Chlamydiae & Rickettsiae

Bacteria that don't play by Gram-stain rules — no wall, or living inside our cells — treated with tetracyclines and macrolides, not β-lactams.

FOCUS ON
Mycoplasma pneumoniae: no wall (cholesterol membrane), "walking pneumonia" in young adults, cold agglutinins (IgM anti-I → hemolysis), erythema multiforme → macrolide/doxycycline. Chlamydia: obligate intracellular, elementary body (infects) → reticulate body (replicates) in inclusions; serotypes A–C trachoma, D–K genital/neonatal, L1–L3 LGV. Rickettsiae (vector-borne): RMSF (wrists/ankles → trunk, palms/soles), typhus (trunk → out, spares palms/soles), Ehrlichia/Anaplasma (no rash, morulae). Coxiella: Q fever, no vector, no rash.
ORGANISM · DISEASE
BIOLOGY · TRANSMISSION
CLINICAL PICTURE
CLUE · TREATMENT
Mycoplasma pneumoniae
No cell wall; sterols; Eaton agar; respiratory droplets, closed groups (military, dorms)
Gradual fever, dry cough, headache; patchy interstitial infiltrates worse than exam
Cold agglutinins, erythema multiforme, SJS · azithromycin or doxycycline
Ureaplasma / M genitalium
Wall-less, sexually transmitted
Nongonococcal urethritis, cervicitis
Doxycycline, moxifloxacin (M genitalium)
C trachomatis D–K
Sexual or vertical; EB ↔ RB cycle
Urethritis, cervicitis, PID, ectopic pregnancy, reactive arthritis; neonatal conjunctivitis (day 5–14) and staccato-cough pneumonia
NAAT · azithromycin or doxycycline (treat partner)
C trachomatis L1–L3 (LGV)
Invades lymphatics
Painless genital ulcer → painful inguinal buboes ("groove sign") → proctocolitis
Doxycycline 3 weeks
C trachomatis A–C
Fomites, flies, hands
Trachoma: follicular conjunctivitis → scarring, trichiasis, blindness
Azithromycin
Chlamydophila pneumoniae · psittaci
Droplets · birds (parrots)
Atypical pneumonia · psittacosis (pneumonia + splenomegaly, hepatitis)
Doxycycline
Rickettsia rickettsii — RMSF
Dermacentor tick; endothelial infection → vasculitis
Fever, headache, rash starting at wrists/ankles spreading centrally, includes palms and soles
Southeast/central US · doxycycline (even in children) — don't wait for tests
Typhus
Endemic: R typhi (fleas) · epidemic: R prowazekii (lice)
Rash starts on trunk and spreads outward, spares palms/soles
Doxycycline
Ehrlichia · Anaplasma
Ticks (Amblyomma · Ixodes); infect monocytes · granulocytes
Fever, headache, leukopenia, thrombocytopenia, ↑ LFTs; usually no rash; morulae in cytoplasm
Doxycycline
Coxiella burnetii — Q fever
Spore-like form inhaled from cattle/sheep amniotic fluid; no vector
Pneumonia, hepatitis, fever; chronic → culture-negative endocarditis
No rash, no vector · doxycycline
Weil-Felix test
Antirickettsial antibodies cross-react with Proteus antigens
+ in typhus and RMSF; − in Q fever
Historical test

Mechanism — the Chlamydia life cycle

Elementary bodySmall, infectious, enters cell
→
Reticulate bodyReplicates by fission in an inclusion
→
Reorganizes into EBs
→
Cell lysesEBs spread

Elementary body = infectious form; reticulate body = replicating form. No peptidoglycan muramic acid → β-lactams don't work.

Rash direction

DISEASE
RASH STARTS
PALMS/SOLES
RMSF
Wrists, ankles → trunk
Involved
Typhus
Trunk → limbs
Spared
Ehrlichiosis
Usually none
—
Q fever
None
—

How it's tested

COMMON CONFUSIONPalms-and-soles rash: RMSF, secondary syphilis, coxsackie A (hand-foot-mouth).
THE TRAPDoxycycline is the drug for RMSF in children too — the benefit outweighs tooth staining; delay kills.
YOUR APPROACHNo Gram stain organisms → wall-less (Mycoplasma) or intracellular → vector and rash pattern → doxycycline or macrolide.

If you see… think…

College student, walking pneumonia, cold agglutinins → Mycoplasma pneumoniae
Hemolysis that worsens in the cold after pneumonia → Mycoplasma (IgM anti-I)
Neonate day 10 with conjunctivitis then staccato cough → C trachomatis
Painless ulcer then tender inguinal buboes → LGV (L1–L3)
Pneumonia in a parrot owner → C psittaci
Rash from wrists inward to palms after a tick bite in North Carolina → RMSF
Rash starting on trunk, sparing palms, after lice → epidemic typhus
Tick bite, fever, leukopenia, morulae in monocytes → Ehrlichia
Farm worker, pneumonia, culture-negative endocarditis → Coxiella (Q fever)
Treatment for nearly all of these → doxycycline (or macrolide)