14.3 Identification Methods, Direct Molecular Detection, and TB Therapy

14.3 Identification Methods, Direct Molecular Detection, and TB Therapy

Quick Answer: Identify cultured mycobacteria with sequencing or MALDI-TOF MS after inactivation—never live AFB on an open target. Direct NAAT on smear-positive sputum detects M. tuberculosis complex and can flag rpoB rifampin resistance; it does not replace culture and does not identify NTM. First-line TB therapy is RIPE (rifampin, isoniazid, pyrazinamide, ethambutol). MDR-TB is resistance to at least isoniazid and rifampin; XDR is more extensive. Broth-microdilution and critical-concentration AST tables are SM-only, not M-tested detail.

Outline III.A.6.a is on the M examination: M. tuberculosis antimicrobial therapy at a conceptual level. Identification methods (sequencing, MALDI) and direct molecular detection sit beside that therapy item because a rifampin-resistant NAAT result changes both isolation and treatment before the 21-day slant is mature. Detailed AST methods (III.A.6.b–c and III.A.7: critical concentrations, broth microdilution panels for NTM) are SM-only. Know that susceptibility testing exists and is specialized; do not memorize MIC tables for the M exam.

Identification from culture: sequencing and MALDI-TOF MS

Once an AFB isolate exists, the name must come from a method that can separate M. tuberculosis complex from NTM and NTM from each other. Colonial pigment and cords (14.2) are presumptive. They are not a final identification.

Sequencing targets conserved genes with species-level discriminatory regions: 16S rRNA, rpoB, and hsp65 (and sometimes internal transcribed spacer). A pure isolate is required. Mixed MGIT broths produce unreadable or composite sequences—subculture to 7H11 first. Sequencing resolves many M. abscessus group and MAC-related names that older biochemicals could not, but some complexes still need more than one gene. Use sequencing when MALDI is inconclusive, when the morphology disagrees with the score, or when a rare NTM would change therapy.

MALDI-TOF MS ionizes abundant proteins and matches the spectrum to a mycobacterial library. Mycobacteria do not go on the target as a live smear from LJ. Inactivate first: heat (often ~95 °C), ethanol kill, and/or tube extraction with formic acid and acetonitrile per the validated SOP. Inactivation is a biosafety step, not a convenience. Live M. tuberculosis on an open MALDI plate is a BSL-3 breach. Even NTM should be extracted so the library sees a reproducible protein set through the waxy wall.

MALDI is fast after inactivation but is only as good as the library and the extraction. Closely related species may not separate. A low score or a result that contradicts cords, pigment, or growth rate is a reflex to sequencing (or a validated DNA probe), not a report. Historical culture-confirmation probes (for example MTB complex, MAC, M. kansasii, M. gordonae) still appear in some laboratories; treat them as culture identification methods, not as sputum screens.

Nocardia is identified from aerobic actinomycete workflows: modified acid-fast morphology, MALDI with an appropriate library, or sequencing. Do not force Nocardia through a mycobacterial biochemical panel.

Direct molecular detection on specimens

NAAT (nucleic acid amplification) can detect M. tuberculosis complex directly in sputum, especially smear-positive specimens where the organism load is high. Many current assays also interrogate rpoB (rifampin-resistance determining region). An rpoB mutation associated with rifampin resistance is a surrogate marker that the isolate may be MDR, because rifampin resistance rarely travels alone in clinical TB. That result is a critical, same-day public-health event: airborne isolation, contact investigation, and a regimen that does not assume rifampin will work.

Limits the exam expects:

NAAT does not replace culture. Culture is still required to recover an isolate, to detect NTM, to identify mixed infections, and to perform whatever susceptibility testing the laboratory (or a reference laboratory) offers. NAAT does not diagnose MAC, M. kansasii, M. abscessus, or M. gordonae. A CF patient with a smear-positive sputum and a negative MTB NAAT is an NTM workup, not “NAAT-negative TB that can go home without isolation” until the clinical picture agrees.

RIPE, MDR, and XDR—conceptual therapy, not AST tables

Drug-susceptible active TB is treated with combination therapy so resistant mutants are not selected. The first-line intensive combination is remembered as RIPE:

A typical drug-susceptible pulmonary course is an intensive RIPE phase followed by a continuation phase of isoniazid plus rifampin (often thought of as about 2 months then 4 months, ~6 months total for many pulmonary cases). Exact duration is a clinical decision; the M point is combination RIPE, not monotherapy. Never treat active TB with a single drug.

MDR-TB means resistance to at least isoniazid and rifampin—the two most important first-line agents. An rpoB-resistant NAAT is therefore treated as an MDR alarm until proven otherwise. XDR-TB denotes more extensive resistance beyond MDR (historically including a fluoroquinolone plus an additional core second-line agent; WHO language has evolved). For M, memorize the hierarchy: susceptible → rifampin-resistant / MDR → XDR as successively harder disease. Do not memorize broth-microdilution MIC breakpoints, critical-concentration tables, or NTM drug panels; those AST details are SM-only (III.A.6.b–c, III.A.7).

NTM and Nocardia are not treated with a reflex RIPE course. MAC, M. kansasii, M. abscessus, and Nocardia each have different clinical regimens. The laboratory contribution at M level is correct identification and pathogenicity, not an SM-level AST worksheet.

Organism pathogenicity

Match virulence to the disease states in 14.1.

M. tuberculosis is an intracellular pathogen of alveolar macrophages. Cord factor (trehalose dimycolate) promotes serpentine growth, granuloma architecture, and persistence. Aerosol droplet nuclei deliver small numbers of bacilli to alveoli; cavitation later creates the high-burden, highly infectious patient. Cell-mediated immunity contains latent infection; waning immunity reactivates disease.

MAC is an environmental organism of modest virulence in intact hosts. When cell-mediated immunity collapses (advanced AIDS), MAC survives in macrophages, seeds blood and marrow, and produces disseminated disease. In damaged airways (COPD, bronchiectasis, CF) it causes chronic pulmonary infection without person-to-person spread.

The M. abscessus group combines rapid growth with biofilm-like persistence in CF and bronchiectatic airways and with traumatic or iatrogenic skin inoculation. Pathogenicity is local destruction and chronicity, not airborne person-to-person TB.

M. marinum prefers cool temperatures, which matches acral skin after aquatic trauma; it is not a pulmonary droplet-nuclei disease.

Nocardia is a soil aerobic actinomycete. After inhalation it causes necrotizing pulmonary infection in compromised hosts and seeds brain abscesses hematogenously. The partially acid-fast, branching filament is the laboratory correlate of that pyogenic, abscess-forming lifestyle.

M. gordonae remains the control case: a water organism whose “pathogenicity” on a single sputum is usually none. Identification prevents both under-calling TB and over-calling a contaminant as MDR-TB.

On exam day: inactivate before MALDI, believe a smear-positive MTB NAAT with an rpoB alert as rifampin-resistant TB until culture says otherwise, start from RIPE not monotherapy, define MDR as isoniazid plus rifampin, and leave the AST tables to SM.