Acinetobacter radioresistens

Associated infections

• Rarely reported opportunistic infection, principally bacteraemia in immunocompromised or device-associated patients; not an established significant human pathogen

Transmission route

Acquisition is from environmental sources or from the skin flora rather than by recognised person-to-person or device-mediated outbreak transmission. There is no substantive published record of A. radioresistens being transmitted by flexible endoscopy, and the species is not among the indicator organisms named in the AS 5369:2023 final rinse water microbiological criteria. The rare reports of clinical infection concern profoundly immunocompromised or heavily instrumented patients and generally involve intravascular devices, consistent with a low-virulence skin commensal reaching a normally sterile site rather than with an environmental transmission event.

The organism's dual habitat is the feature of most practical interest. As a skin commensal it is present on the hands of healthcare workers and on patients, which makes it a plausible contaminant of samples and of surfaces handled during collection. As an environmental organism it persists in soil and in aqueous environments and participates in mixed biofilm on wetted surfaces in the same way as other Acinetobacter species. Both origins are credible explanations for its appearance in a water sample, and distinguishing between them is the central interpretive task.

The secondary transmission consideration concerns genes rather than organisms. Genetic exchange between A. radioresistens and A. baumannii has been hypothesised to occur in aquatic environments where the two species may be in close contact, which is an argument for controlling Acinetobacter populations in healthcare water systems generally rather than an assertion that such exchange has been demonstrated within a reprocessing circuit. No published evidence establishes that a reprocessing water system has acted as the site of blaOXA-23 mobilisation, and the point should not be overstated. It is a reason to take persistent Acinetobacter colonisation of a healthcare water system seriously as a general matter, not a reason to treat a single A. radioresistens isolate as a resistance emergency.

Relevance in endoscopy and reprocessing

A. radioresistens has no established relevance to flexible endoscope reprocessing as a transmissible pathogen. It has no published record of endoscopy-associated transmission, is not a named indicator organism in AS 5369:2023, and is not associated with duodenoscope, bronchoscope or gastroscope incidents. Presenting its recovery from final rinse water as an infection risk to patients would not be supportable.

Its relevance is instead as evidence about the water. Final rinse water contacts the endoscope after high-level disinfection is complete and is not itself disinfected, so any viable organism recovered at that stage indicates that the water applied to a patient-ready device is not of the required microbiological quality. A. radioresistens is a genuine environmental organism capable of persisting in aqueous systems and contributing to mixed biofilm, so its recovery is consistent with a water treatment, filtration or distribution problem, particularly if repeated or accompanied by other heterotrophic growth. That justification for investigation stands on its own and does not require the organism to be a pathogen.

A third consideration, applicable equally in CSD and dental water systems, is the general case for limiting Acinetobacter populations in healthcare water. The resistance-gene reservoir role described above provides a background rationale for not tolerating persistent genus-level colonisation of engineered water systems, on the principle that dense mixed populations in biofilm create the conditions in which horizontal gene transfer occurs. This is a reason for sound water system management rather than a specific reprocessing hazard, and it should be framed as such in any report. In dental practice, where treatment water is directly applied and aerosolised, the practical response to any persistent heterotrophic population, including this one, is waterline treatment and verification rather than species-specific action.

Interpreting a detection

The first question on receiving an A. radioresistens result is whether it came from the water or from the sampling process. The species is a recognised human skin commensal as well as an environmental organism, which makes introduction during collection a genuinely plausible explanation. A single low-count isolate, reported alone, with a compliant total viable count and no prior history, should prompt a repeat sample with attention to aseptic technique before any system intervention. Outlet preparation, glove and hand contact with the container rim, whether the container was opened only at the point of filling, and transport and hold times should all be reviewed. The second question is identification, since routine phenotypic systems do not always distinguish this species reliably and confirmation may be warranted before the result is acted upon.

If confirmed and genuine, the finding should be interpreted as a general water quality signal and assessed on trend. It is not a named indicator organism, so a single isolate does not carry the mandatory consequences attached to detection of P. aeruginosa, Legionella or environmental mycobacteria, and it does not by itself require the affected washer-disinfector to be withdrawn. It gains weight when it recurs, when it is accompanied by an elevated total viable count or endotoxin result, when it is present at multiple points in the circuit, or when it is recovered alongside other water-associated organisms, which together indicate biofilm shedding rather than an isolated event. Where the total viable count itself falls within the unsatisfactory or unacceptable ranges defined by the Healthcare Infection Society working party, the actions attached to those ranges apply irrespective of the species named.

Investigation follows the standard wet-system path: filtration integrity, rating and change interval; storage vessel design and turnover; loop temperature; water residence time and stagnation, including idle periods over weekends and shutdowns; dead legs and capped branches; the sanitisation regime and whether it contacts every part of the circuit; the condition of connecting hoses; and verification that reprocessed endoscopes are thoroughly dried before storage. Escalation beyond the department is warranted where the identification proves unreliable and resolves to a clinically significant Acinetobacter species, where the quantitative results indicate a system failure in their own right, or where persistent genus-level colonisation is demonstrated across the water system and warrants an engineering rather than a sampling response. Reporting should be careful to describe the finding as a water quality indicator and not as a patient risk, since the evidence does not support the latter framing.

Antimicrobial resistance

Clinical isolates of A. radioresistens are typically susceptible to carbapenems despite carrying chromosomal blaOXA-23-like genes, because these genes are not expressed or are only weakly expressed in their native host. Co-detection of blaOXA-23-like and blaOXA-58 genes has also been reported in the species. The significance of the organism lies in its role as a silent reservoir: mobilisation of blaOXA-23 into A. baumannii, where it is expressed, has produced one of the most widely disseminated mechanisms of carbapenem resistance in that pathogen. Carbapenem-resistant Acinetobacter baumannii is classified as a critical-priority pathogen in the WHO bacterial priority pathogens list 2024.

The distinction between carriage and expression is the important one and is frequently lost in summary. A carbapenem-susceptible A. radioresistens isolate carrying blaOXA-23 poses no direct therapeutic problem; the concern arises only if the gene is mobilised onto a plasmid and acquired by a species in which it is strongly expressed. Reporting should reflect this. An A. radioresistens isolate from a water sample is not a carbapenem-resistant organism detection and should not be recorded or communicated as one.

With respect to biocides, A. radioresistens is not known to possess unusual disinfectant tolerance, and the WHO Guidelines for Drinking-water Quality note that Acinetobacter species generally are readily inactivated by chlorine and comparable disinfectants under normal water treatment conditions. The radiation tolerance from which the species takes its name relates to ionising radiation and confers no established advantage against the chemical sanitants or thermal treatments used in water system management, and it should not be read as implying general robustness. Persistence in a treated water system therefore indicates biofilm protection, loss of residual disinfectant, or a treatment or design failure, and remediation should be directed at the physical reservoir rather than at the choice of chemistry.