15.1 Intracellular Bacteria

Obligate Intracellular Pathogens

Intracellular bacteria are a functional category — not a taxonomic one — defined by their inability to replicate outside a living host cell. The PA-CAT Bulletin of Information, rev. 20240815, groups these organisms under the Bacteria heading of the Microbiology blueprint (Table 7). Two genera are classical obligate intracellular pathogens: Chlamydia and Rickettsia. Both are small, Gram-negative–like organisms that are structurally deficient in ATP generation, which is why they were once called 'energy parasites.'

Why Intracellular Survival Matters

These bacteria survive inside host cells by commandeering host ATP, amino acids, and nucleotides. Their small genomes (~1 Mb) reflect loss of genes they no longer need because the host cell supplies the products. This lifestyle has two clinical consequences:

1. They cannot be cultured on artificial media — diagnosis relies on serology, nucleic acid amplification (NAAT), or cell culture.
2. Antibiotic choice is limited to agents that penetrate host cells — tetracyclines (doxycycline) and macrolides (azithromycin) are mainstays; beta-lactams are ineffective.

Chlamydia: A Biphasic Lifecycle

Chlamydia species are unique among bacteria in having two morphologic forms. The elementary body (EB) is the infectious, extracellular form: small, dense, and metabolically inert, it survives outside cells and attaches to host epithelium. After endocytosis, it reorganizes into the reticulate body (RB), a larger, metabolically active form that divides by binary fission within a membrane-bound inclusion. RBs recondense back into EBs, which are released by cell lysis or extrusion to infect new cells.

A distinctive feature of chlamydial infections is that cell-mediated immunity (Th1 response, IFN-γ) is central to control, but it also drives the scarring pathology of trachoma and PID. Latent or persistent infection occurs when EBs arrest in a nonreplicating state.

Rickettsia and Related Genera

Rickettsia are transmitted by arthropod vectors — ticks, lice, fleas, and mites — and target endothelial cells, producing a vasculitic rash that is the clinical hallmark. Rickettsia rickettsii causes Rocky Mountain spotted fever (RMSF), the most severe rickettsiosis in the United States. The Dermacentor tick transmits it; despite the name, most cases occur in the southeastern and south-central U.S. The rash classically begins on the wrists and ankles and spreads centrally, often involving palms and soles — a pattern that helps distinguish RMSF from viral exanthems.

Comparison of Key Rickettsial Diseases

Coxiella burnetii, formerly classified with rickettsiae, is distinct: it forms endospore-like structures that resist heat and desiccation, is transmitted primarily by inhalation of aerosols from infected livestock birth products (not arthropod bite), and causes Q fever — typically pneumonitis without rash, plus granulomatous hepatitis. Its acute form responds to doxycycline; chronic forms (endocarditis) require prolonged therapy with doxycycline plus hydroxychloroquine.

Mycoplasma: The Cell-Wall–Deficient Bacterium

Mycoplasma pneumoniae is the smallest free-living bacterium and lacks a peptidoglycan cell wall entirely. This has three exam-relevant consequences:

• It is not seen on Gram stain (no wall to retain crystal violet) and is invisible to beta-lactam antibiotics.
• It grows slowly on special media (Eaton's PPLO agar), requiring cholesterol.
• Cold agglutinins — IgM autoantibodies that agglutinate red cells at 4 °C — appear in about half of cases and are a useful (if nonspecific) clue.

Clinically, M. pneumoniae produces atypical ('walking') pneumonia: gradual onset, dry cough, low-grade fever, patchy interstitial infiltrates on chest imaging that look worse than the patient appears. Extrapulmonary manifestations include erythema multiforme and, less often, Guillain-Barré–like neuropathy.

Antibiotic Cross-Reference

A critical PA-CAT point: doxycycline is first-line for suspected RMSF and should not be withheld pending serologic confirmation, because untreated RMSF has a case-fatality rate exceeding 20%.