Mycobacterium intracellulare

Associated infections

• Nodular-bronchiectatic and fibrocavitary pulmonary disease
• Cervical lymphadenitis in children
• Disseminated infection in severely immunocompromised patients
• Skin and soft tissue infection following inoculation
• Prosthetic device and catheter-associated infection
• Hypersensitivity pneumonitis following heavy aerosol exposure
• Pseudo-infection following contamination of respiratory specimens or instruments

Transmission route

Acquisition is environmental, by inhalation of aerosols from contaminated water, by inhalation of dust from soil, and by direct inoculation. Person-to-person transmission is not an established feature of M. intracellulare. The organism shares the general mycobacterial traits that make control difficult: a hydrophobic, lipid-rich cell wall, strong adherence to pipe and fixture surfaces, growth within plumbing biofilm, survival inside free-living amoebae, and tolerance of chlorine and chloramine at the concentrations maintained in potable supply. Stagnation, warm water and low disinfectant residual in distal plumbing favour regrowth.

The soil route deserves specific mention because it distinguishes this species somewhat from M. avium in community epidemiology. Gardening, potting-mix handling and other activities that disturb dry organic material generate respirable dust that can carry mycobacteria, and case-control work has identified such exposures as risk factors for MAC disease. This does not diminish the water route in healthcare, where the relevant exposure is engineered rather than incidental: water delivered under pressure into the channels of a medical device is a far more direct pathway than ambient dust.

In endoscopy, the concern is contamination of the final rinse. Water drawn from building plumbing may carry MAC unless the treatment train and terminal bacterial-retentive filtration are correctly specified, installed and maintained. Because rinse water contacts the device after high-level disinfection, any breach recontaminates the endoscope directly and may establish biofilm within instrument channels and within the reprocessor. As with M. avium, isolation of M. intracellulare from a bronchial specimen after a procedure performed with a contaminated bronchoscope can be mistaken for genuine pulmonary infection and prompt prolonged multi-drug therapy. AS/NZS 5369 addresses non-tuberculous mycobacteria as a class in reprocessing water monitoring; recovery of M. intracellulare is a critical finding requiring instrument quarantine, investigation of the water system and reprocessor, and remediation before further sampling.

CSD instrument rinse water and dental unit waterlines draw on the same building plumbing and are subject to the same reasoning. Narrow-bore dental tubing with prolonged idle periods is a particularly favourable mycobacterial habitat, and recovery from a dental unit indicates the need for line disinfection and point-of-use control rather than reliance on incoming mains quality.

Relevance in endoscopy and reprocessing

M. intracellulare carries the same weight in a reprocessing service as M. avium, and the two should not be triaged differently on the basis of species. The mechanism of concern is identical: water entering an automated endoscope reprocessor from the facility supply, passing an inadequate or failed barrier, and contacting the interior of every channel at the final rinse — that is, after the disinfection step has completed and can no longer correct anything. Validated high-level disinfection with glutaraldehyde, ortho-phthalaldehyde or peracetic acid is mycobactericidal and will inactivate M. intracellulare on a properly cleaned surface. Documented reprocessing failures involving NTM have turned on inadequate manual cleaning leaving protective organic soil, disinfectant below minimum effective concentration, channels not perfused, damaged instruments, or contaminated rinse water — not on an inherent inability of the chemistry to kill the organism in the open.

Biofilm behaviour explains why a single contamination event can become a persistent one. The organism adheres to polymer and stainless surfaces and, once within an extracellular matrix in a rinse line, storage vessel, connector or endoscope channel, becomes markedly less susceptible than the same strain in suspension. Where the channel lining is scratched, cracked or delaminated, biofilm occupies sites that no chemical cycle can reliably reach, and repeat reprocessing will not resolve a persistent positive history. Borescope inspection is the appropriate diagnostic step, and instruments with confirmed internal damage should be withdrawn from service rather than reprocessed again. This is why rinse water is treated as a control point in its own right: it is the route by which those protected reservoirs are established.

Drying and storage govern how much of any surviving inoculum reaches the next patient. Water retained in a channel supports growth; a channel left wet through a weekend can convert a trivial residual contamination into a substantial load. The controls are an alcohol flush, forced-air drying that demonstrably perfuses each channel rather than merely drying the exterior, and storage that maintains channel dryness — with drying cabinets supplied through filtered air and verified for per-channel perfusion. In CSD, lumened instruments retaining rinse water before packaging present the analogous exposure; steam sterilisation kills the organism but does not remove deposited biofilm material or endotoxin. In dental practice the exposure is chronic rather than episodic, since waterlines stagnate between patients and overnight in warm, small-bore tubing.

Interpreting a detection

A positive M. intracellulare result from a final rinse or reprocessing water sample means MAC has crossed every barrier between the facility water supply and the patient-contact surfaces of a device, and it should be escalated immediately rather than held for a confirmatory round. The first physical checks are on the terminal bacterial-retentive filter: correct rating for the application, installation date against validated service life, housing seal integrity, orientation, evidence of the element having dried out and rewetted, and the absence of any bypass. Attention then moves upstream through the treatment train — carbon beds, softeners, reverse osmosis membranes, ultraviolet units, break tanks and storage vessels all offer surface area for biofilm and all remove the residual disinfectant that suppressed growth in the incoming main. The reprocessor is then inspected internally, covering rinse lines, tanks, dosing lines, non-return valves and the self-disinfection cycle record including whether specified temperatures were actually achieved and held. Sampling technique is reviewed, but a slow-growing mycobacterium recovered after weeks on selective media is rarely a sampling artefact and should not be dismissed as one.

A single isolate justifies a full response. Instruments reprocessed since the last satisfactory result should be identified and quarantined, the procedure log reviewed for bronchoscopies performed in that window, a specific remediation action taken, and resampling deferred until afterwards so that the repeat result tests the intervention rather than sampling variability. A trend — repeated positives, positives at multiple points, or recurrence after filter change and line disinfection — indicates established biofilm in a component that cannot be cleaned in place, and the appropriate response shifts from cleaning to replacement of flexible hose, filter housings, storage vessels or reprocessor plumbing. NTM results should be read together with heterotrophic plate counts, Pseudomonas detections, endotoxin results where measured and filter pressure differentials, since all reflect the same underlying condition of the system.

The reason mycobacteria demand a dedicated test is that routine monitoring is blind to them. Total viable count methods use general-purpose media, short incubation periods and temperatures suited to fast-growing waterborne bacteria; M. intracellulare requires decontamination of the sample to suppress competitors, selective mycobacterial media and incubation over several weeks. Unless NTM culture has been separately requested, a file of satisfactory TVC results says nothing at all about mycobacterial status. Escalation should bring infection prevention, the water safety group and facility engineering together, since the corrective actions are engineering ones. Where a bronchoscope is implicated, patient look-back is warranted and molecular comparison of patient and water isolates should be considered — in its absence, MAC recovered from a bronchial specimen following a contaminated procedure will most likely be treated as genuine pulmonary disease, committing the patient to a prolonged and toxic regimen.

Antimicrobial resistance

M. intracellulare is intrinsically resistant to standard anti-tuberculosis agents and requires prolonged combination therapy. Guideline-based management follows that of MAC generally: susceptibility-based treatment for macrolides and amikacin, with a three-drug macrolide-containing regimen for macrolide-susceptible disease. Macrolide resistance, mediated by 23S rRNA mutation, is the principal driver of treatment failure and is selected for by inadequate regimens; macrolide monotherapy must be avoided, as must the addition of a macrolide to a regimen that is already failing. Some evidence suggests M. intracellulare pulmonary disease achieves lower culture-conversion rates than M. avium disease, so species-level identification has therapeutic relevance and affects the intensity of monitoring and the expectation of relapse.

Environmentally, the species tolerates chlorine-based disinfection at potable-supply concentrations and is further protected within biofilm and amoebae. The mycolic-acid cell wall is the principal barrier: thick, hydrophobic and poorly permeable to hydrophilic oxidants, it slows disinfectant access to intracellular targets while also conferring tolerance to desiccation, low-nutrient conditions and low-level disinfectants generally. Cells within an extracellular polymeric matrix are less susceptible again, both because the matrix consumes oxidant and because it limits diffusion, and cells internalised within amoebae are shielded by the amoebal cell or cyst wall from concentrations lethal to free organisms.

The consequence for a water safety programme is that compliance with drinking-water indicator standards does not demonstrate control. Coliform and E. coli testing, chlorine residual measurement and heterotrophic plate counts are all capable of returning satisfactory results from a supply carrying M. intracellulare. Assurance in a reprocessing context comes from physical exclusion at the point of use through correctly specified and maintained bacterial-retentive filtration, from distribution design that eliminates dead legs and stagnation, and from verification by mycobacteria-specific culture rather than by proxy indicators.