Pseudomonas aeruginosa

Associated infections

• Ventilator-associated and other hospital-acquired pneumonia
• Bacteraemia and septicaemia
• Urinary tract infection associated with instrumentation
• Surgical site and burn wound infection
• Chronic airway infection in cystic fibrosis and bronchiectasis
• Keratitis, including contact-lens and post-operative cases
• Otitis externa and malignant otitis externa
• Neonatal and paediatric sepsis in intensive care
• Endocarditis and osteomyelitis, typically device- or injection-associated

Transmission route

Acquisition in healthcare is predominantly environmental and indirect, occurring through contact with contaminated water, wet surfaces, respiratory or irrigation fluids, and inadequately reprocessed medical devices, with subsequent transfer to the patient by hands or by the device itself. Patients who are immunocompromised, mechanically ventilated, or undergoing instrumentation of a normally sterile site are at greatest risk. Person-to-person spread occurs but is a secondary route in most healthcare settings; the dominant pattern is repeated independent acquisition from a shared wet reservoir, which is why clusters frequently involve multiple strain types rather than a single clone.

The water-to-patient pathway operates through several distinct mechanisms. Direct contact occurs where water or a water-wetted device touches a mucosal surface, a wound or an instrumented site. Indirect contact occurs where a splash from a contaminated outlet or drain contaminates hands, gloves, a clean surface or stored equipment; splash dispersal from sink bowls and drains has been shown to reach surrounding surfaces at distances that place adjacent preparation areas at risk. Aerosol generation from outlets, spray fittings and drying equipment provides a third route, of particular relevance to the respiratory tract.

The organism has a direct and well-documented relationship to flexible endoscope reprocessing. Endoscopes present long, narrow, wetted lumens that are difficult to dry completely, and residual moisture retained during storage supports proliferation and biofilm development within channels. Contaminated automated endoscope reprocessors, connecting tubing and final rinse water have each been implicated as sources of endoscope recontamination after otherwise adequate high-level disinfection, and the final rinse is the last point at which water contacts the patient-ready device. Thorough drying of channels after reprocessing and before storage is an equally important control, since the organism cannot establish in a circuit that is not left wet.

The same principles apply outside endoscopy. In a central sterile services department, rinse water applied to instruments at the end of a washer-disinfector cycle, and water used to make up detergent or lubricant solutions, are potential vehicles; instruments subsequently passed to a steam steriliser will have their bioburden killed, but wet packaging, condensate and residual endotoxin remain concerns. In dental practice, the small-bore, low-flow, frequently stagnant tubing of dental unit waterlines is a near-ideal biofilm habitat, and P. aeruginosa is among the organisms recovered from untreated units. Dental treatment water contacts oral mucosa directly and is aerosolised by high-speed handpieces and ultrasonic scalers, so control of waterline biofilm is a patient-safety measure rather than a housekeeping matter.

Relevance in endoscopy and reprocessing

P. aeruginosa is the organism around which much of flexible endoscope water-quality practice has been built. It is a specifically named indicator in final rinse water specifications: AS 5369:2023 and the Healthcare Infection Society working party report on final rinse water both require its absence in a 100 mL sample, alongside a low total viable count. That requirement exists because the final rinse follows high-level disinfection and is not itself disinfected. Any viable organism carried in that water is deposited onto a device that has been declared ready for the next patient, and in the case of P. aeruginosa is deposited into channels whose geometry favours its survival and multiplication. Transmission and pseudo-transmission events have been attributed to contaminated reprocessor rinse water, contaminated connecting tubing between the water supply and the machine, and contaminated internal reprocessor components, in each case downstream of an otherwise correctly executed disinfection stage.

Biofilm behaviour within reprocessing circuits explains why these events are often protracted and resistant to simple remediation. Once P. aeruginosa establishes within a washer-disinfector water line, a storage vessel, a filter housing or a section of distribution loop, chemical sanitisation frequently achieves a temporary reduction in counts followed by recovery within days to weeks as surviving matrix-embedded cells repopulate the surface. Repeat contamination after a documented disinfection is therefore a strong indicator of a structural biofilm reservoir rather than a transient ingress. The organism also colonises the endoscope itself: channel biofilm, and the damaged, scored or crazed channel surfaces that develop with instrument age, both protect residual organisms from high-level disinfection and provide an attachment substrate that no chemical process reliably clears.

Drying and storage are as important as the water. A correctly reprocessed endoscope that is stored wet provides a nutrient-poor but liquid environment in which P. aeruginosa multiplies over hours, so a device sampled at the end of a cycle may be compliant while the same device sampled after overnight storage is not. Forced-air channel drying, verified drying cabinet performance and controlled storage intervals are the practical controls. In CSD and dental settings the same organism carries different consequences: in CSD its presence in rinse water indicates a wet-system control failure and raises questions about instrument-borne endotoxin and about wet packs, while in dental unit waterlines it represents a directly patient-contacting and aerosolised exposure that requires waterline treatment, purging protocols and periodic verification.

Interpreting a detection

A detection of P. aeruginosa in final rinse water, washer-disinfector water or a dental or CSD water sample should be treated as a genuine finding, not as a probable artefact. The organism is a true water and biofilm organism, it is not a common laboratory or skin contaminant of water samples, and it is not plausibly explained by ordinary sampling handling error. Under the Healthcare Infection Society action framework, the presence of an organism of significance in final rinse water places the affected endoscope washer-disinfector out of use for endoscope reprocessing until satisfactory results are obtained, irrespective of whether the accompanying total viable count is within limits. A single isolate is therefore actionable on its own; it does not require confirmation by a trend before the machine is stopped.

The first investigative question is where in the circuit the organism entered. Sampling should be extended beyond the failing outlet to establish a location: incoming mains or tank supply, post-treatment product water, storage vessel, the distribution loop at both near and far points, the connecting hose between the wall outlet and the machine, and the machine's own internal water path and rinse-water filter. Contamination confined to the connecting hose or to a machine-side fitting points to a local, replaceable component; contamination present in the loop or storage vessel points to a system-level biofilm problem. Practical points to examine are filter integrity and change interval, whether filters are being challenged beyond their rated service life, water residence time and stagnation, dead legs and capped-off branches left from earlier plant changes, storage vessel design and turnover, loop temperature, the sanitisation method and its verified contact time and concentration, and whether the loop can actually be brought into contact with the sanitant at every point. Where a reprocessor has been under-used or idle over a weekend or a shutdown, stagnation within the machine is a common precipitant.

Escalation is warranted immediately on first detection. The affected reprocessor should be withdrawn from use for high-risk endoscope reprocessing, the incident recorded, and a decision made on look-back: the relevant question is which devices were reprocessed through the affected water path since the last satisfactory result, and whether any patient exposure warrants clinical notification under local policy. Remediation should be verified by repeat sampling after sanitisation and again after a defined interval, because a single post-sanitisation clear result is a well-recognised false reassurance where biofilm is present. Two or more consecutive satisfactory results, separated in time and taken under normal operating conditions rather than immediately after a flush, provide a more defensible basis for return to service. Concurrent review of total viable count and endotoxin trends, and of endoscope drying and storage practice, should accompany the water investigation, since a contaminated rinse and a wet storage practice compound one another.

Antimicrobial resistance

P. aeruginosa exhibits high intrinsic resistance to many antimicrobial classes through low outer membrane permeability, constitutive and inducible efflux systems such as MexAB-OprM, and the inducible chromosomal AmpC cephalosporinase. Acquired mechanisms including carbapenemases, extended-spectrum beta-lactamases, aminoglycoside-modifying enzymes and porin loss produce multidrug-resistant and difficult-to-treat phenotypes. Loss or downregulation of the OprD porin is a particularly common route to carbapenem resistance and can arise during therapy, so a susceptible isolate at the start of treatment does not guarantee continued susceptibility. Carbapenem-resistant P. aeruginosa is listed as a high-priority pathogen in the WHO bacterial priority pathogens list 2024.

Of more direct relevance to water systems and reprocessing is tolerance of biocides, which is a separate phenomenon from antibiotic resistance and is not predicted by susceptibility testing. Biofilm-associated cells display phenotypic tolerance to both antibiotics and chemical disinfectants arising from several concurrent mechanisms: restricted diffusion of the agent through the exopolysaccharide matrix, chemical quenching of oxidising agents in the outer layers of the biofilm before they reach deeper cells, and the presence within the biofilm of slow-growing or metabolically quiescent subpopulations against which most biocides are far less effective. The consequence is that a sanitant concentration and contact time validated against planktonic organisms may achieve only partial kill against the same organism in an established biofilm.

This has practical implications for water system management. Chemical sanitisation of a distribution loop should be specified with biofilm, not planktonic, targets in mind, and should be paired with measures that address the physical reservoir: elimination of dead legs, replacement of compromised flexible hoses and fittings, correction of stagnation, and where necessary replacement of components that cannot be reliably cleaned. Repeated recovery of the organism after apparently adequate chemical treatment should be interpreted as evidence that the reservoir has not been reached, rather than as evidence of an emerging biocide-resistant strain.