Serratia marcescens

Associated infections

• Bloodstream infection and device-associated bacteraemia
• Pneumonia and lower respiratory tract infection, including ventilator-associated pneumonia
• Urinary tract infection, usually catheter-associated
• Surgical site and wound infection
• Conjunctivitis, keratitis and endophthalmitis, including post-operative endophthalmitis
• Meningitis and neonatal sepsis, particularly in intensive care
• Osteomyelitis and septic arthritis following instrumentation or injection
• Peritonitis in patients on peritoneal dialysis

Transmission route

Healthcare transmission occurs by contact, via the hands of staff and contaminated equipment, and by exposure to contaminated fluids. Outbreak investigations have implicated intrinsically or extrinsically contaminated infusates and compounded solutions, contaminated pharmacy water sources, and hospital sink drains acting as persistent reservoirs. Consumption of contaminated tap water during medication administration has also been documented as an acquisition route in critically ill patients. Splash dispersal from colonised drains and sink traps to adjacent surfaces and to items stored near sinks is a recognised mechanism, and has driven design changes in critical care and preparation areas.

S. marcescens is directly relevant to endoscope reprocessing water because it is a genuine wet-environment coloniser rather than a transient contaminant. It can colonise reverse osmosis membranes, storage vessels, distribution pipework, dead legs and the internal wetted components of automated endoscope reprocessors, and once biofilm is established, planktonic counts in a spot sample may substantially understate the burden present. Recovery from final rinse water should therefore be interpreted as evidence of a biofilm-supporting reservoir somewhere in the water path. Effective response generally requires identification and elimination of stagnation points, sanitisation or replacement of affected components, review of filter integrity and change frequency, and confirmation that endoscopes are thoroughly dried before storage, since residual moisture in channels permits regrowth after an otherwise adequate cycle.

Transfer from a contaminated instrument to a patient is direct: organisms present in a channel at the point of use are flushed into the patient with insufflated air, irrigation or instrument passage. Transfer from a contaminated instrument to a specimen is equally direct and is the mechanism behind pseudo-outbreaks, in which the organism appears in cultures of bronchoalveolar lavage fluid or biopsy material without causing infection. Both mechanisms originate in the same failure, and a pseudo-outbreak should be treated as a warning about a process that could equally have produced true infections in a more vulnerable patient group.

Relevance in endoscopy and reprocessing

S. marcescens has a well-documented place in the endoscopy and bronchoscopy contamination literature, most prominently in pseudo-outbreaks. Clusters in which S. marcescens, frequently alongside Pseudomonas aeruginosa, appeared in respiratory specimens from patients undergoing bronchoscopy have been traced to contaminated rinse water, contaminated automated endoscope reprocessors and defective instruments. In one widely reported investigation, contamination of bronchoscopes was linked to a manufacturing defect that permitted organisms to persist in the biopsy port assembly, with visible biofilm recovered from the affected components. Systematic reviews of bronchoscopy-associated outbreaks and pseudo-outbreaks identify contaminated water, reprocessor dysfunction and inadequate disinfection practice as the recurring causes. The pattern across these reports is consistent: the organism was sheltered in a location the disinfectant did not reach, or was reintroduced after disinfection by contaminated rinse water.

Within a reprocessing water system, S. marcescens behaves as a classic premise-plumbing coloniser and will occupy the same locations as Pseudomonas. Terminal segments matter disproportionately: connecting tubes between the circulation loop and individual reprocessors, point-of-use fittings, sample taps that are rarely flushed, and any branch that carries water only intermittently. Inside the reprocessor, rinse water reservoirs, air lines, dosing tubing and the manifolds downstream of the terminal filter are all capable of supporting biofilm, and a colonised component in that position defeats the filter entirely. Because the organism sloughs from biofilm episodically, spot samples can alternate between compliant and non-compliant results without any change in the underlying contamination, and a clean repeat sample should not be read as evidence that the reservoir has been removed.

Against validated high-level disinfection S. marcescens is not intrinsically tolerant, and planktonic cells are inactivated by peracetic acid, glutaraldehyde and ortho-phthalaldehyde at in-use conditions. It does, however, show relative tolerance of some antiseptics and low-level disinfectants at in-use dilutions, particularly when protected within biofilm or in the presence of organic load, and biofilm-associated cells are substantially harder to inactivate than the same organisms in suspension. Drying is therefore critical. An endoscope that has been correctly disinfected but stored with retained moisture provides exactly the conditions this organism exploits, and forced-air drying of channels before storage, together with appropriate storage cabinet conditions, is a control measure of the same order of importance as the disinfection cycle itself. The same reasoning applies to CSD rinse water circuits and to dental unit waterlines, where narrow bore, low flow and long idle periods favour attachment and regrowth.

Interpreting a detection

Recovery of S. marcescens from final rinse water, reprocessor supply water or an endoscope channel sample should be treated as a genuine water-system finding rather than a sampling artefact. It is not a skin organism, it is not a common airborne laboratory contaminant, and its ecology is precisely that of a colonised wetted circuit. The presumption should therefore be that a reservoir exists somewhere in the water path or in the reprocessor, and the investigation should be designed to locate it. Under AS 5369 the applicable total viable count limit for endoscope final rinse water is not more than 10 CFU per 100 mL, but the identification of this organism is significant on qualitative grounds even where the numeric count is within limit.

The first checks concentrate on locating the reservoir rather than on repeating the sample. Establish which outlet and which stage the sample represents, and immediately extend sampling to upstream points, including the reverse osmosis product water, the storage vessel, the return of the circulation loop and each reprocessor served by the same loop, so that the boundary of contamination can be drawn. Inspect and, where practical, replace the flexible connecting tubes and point-of-use fittings, which are frequently the colonised component and are rarely reached by loop sanitisation. Review loop design for dead legs, blind branches and capped-off spurs left by previous plant modifications, and check flow velocity and whether continuous circulation is genuinely maintained out of hours. Review the sanitisation regime, including agent, concentration, contact time, temperature and whether every branch is exposed. Review terminal filter type, integrity testing and change interval, noting that a filter cannot compensate for contamination downstream of itself. Finally, review endoscope drying and storage, and the frequency and duration of reprocessor idle periods.

The single-isolate versus trend distinction is less informative for this organism than for others, because episodic sloughing from biofilm produces intermittent positives from a stable underlying problem. A single isolate should still trigger a full water path review rather than a simple resample. Where the organism has been recovered on more than one occasion, from more than one outlet, or over more than one sampling round, the presence of an established biofilm reservoir should be assumed and the response should move from chemical sanitisation to physical intervention, meaning replacement of the affected tubing, fittings or reprocessor components. Repeated sanitisation of a system that recolonises within weeks is not a corrective action, and documenting successive cycles of treatment and recurrence without structural change is a common and avoidable failure mode.

Escalation to infection prevention is warranted when the organism is recovered from a patient-ready endoscope, when counts exceed action limits, when contamination persists after a documented intervention, or when S. marcescens is isolated from clinical specimens taken from patients who underwent endoscopy or bronchoscopy at the facility. Given the organism's documented role in pseudo-outbreaks, the microbiology laboratory should be asked whether there has been any unexplained increase in S. marcescens from respiratory or biopsy specimens, since that pattern may be the first indication of a reprocessing problem. Affected reprocessors and endoscopes should be quarantined pending investigation and isolates retained for typing so that water, device and clinical isolates can be compared.

Antimicrobial resistance

S. marcescens is intrinsically resistant to ampicillin, first-generation cephalosporins, cefoxitin, colistin and nitrofurantoin, and carries an inducible chromosomal AmpC beta-lactamase that can be derepressed during therapy with third-generation cephalosporins. As with Enterobacter, an isolate reported susceptible to ceftriaxone may develop resistance during treatment, so these agents are generally avoided as definitive therapy for serious infection. Intrinsic colistin resistance is of practical note because it removes an agent that would otherwise be a fallback in multidrug-resistant Gram-negative infection.

Acquired extended-spectrum beta-lactamases, aminoglycoside-modifying enzymes and carbapenemases are increasingly reported, and multidrug-resistant S. marcescens has been described in neonatal and adult critical care outbreaks. Because outbreak strains can persist in a water reservoir for extended periods, a resistant clone introduced into a colonised system may continue to be delivered to patients long after the index cases have resolved.

The organism also shows relative tolerance of some antiseptics and disinfectants at in-use dilutions, particularly when protected within biofilm. Historical reports of S. marcescens surviving in dilute chlorhexidine and other in-use antiseptic solutions reflect a combination of low intrinsic susceptibility to certain agents, dilution errors and the protective effect of organic material. This does not extend to high-level disinfectants used for endoscope reprocessing at validated concentrations and contact times, and no acquired resistance to peracetic acid, glutaraldehyde or ortho-phthalaldehyde has been established in this species.

The operationally important form of resistance for a reprocessing department is therefore physical rather than genetic. Cells embedded in a mature biofilm are protected by restricted diffusion of the biocide, by reaction of the biocide with the extracellular matrix, and by reduced metabolic activity in the deeper layers, and can survive exposures that would inactivate the same organism in suspension by many orders of magnitude. Control depends on preventing biofilm establishment through continuous circulation, elimination of stagnation, appropriate materials and effective drying, and on physically removing colonised components once biofilm is present.