Listeria monocytogenes

Associated infections

• Invasive listeriosis with bacteraemia
• Meningitis and meningoencephalitis
• Rhombencephalitis and brain abscess
• Maternal-fetal listeriosis, chorioamnionitis and fetal loss
• Early-onset and late-onset neonatal sepsis (granulomatosis infantiseptica)
• Febrile gastroenteritis
• Endocarditis and, rarely, focal infections including osteomyelitis, septic arthritis and prosthetic device infection

Transmission route

The dominant transmission route is ingestion of contaminated food, particularly ready-to-eat products that are not cooked before consumption: soft and unpasteurised cheeses, deli meats and pate, smoked seafood, pre-prepared salads, rockmelon and other cut fruit, and chilled products with extended shelf lives. Vertical transmission across the placenta or during delivery accounts for the neonatal burden. Direct person-to-person spread is not a feature outside the maternal-fetal route, and healthcare transmission is essentially confined to rare nursery cross-infection reports.

The organism's persistence in food-processing environments is attributed in large part to biofilm formation on equipment surfaces, conveyors, floors and drains, where it survives cleaning cycles and can recontaminate product over long periods. Environmental reservoirs including soil, water and biofilm are recognised contributors to that persistence, and persistent factory strains have been shown by whole-genome sequencing to survive in a single plant for years, causing intermittent contamination events. Tolerance of cold, salt and desiccation, and the ability to grow across a wide pH range, are what allow the organism to occupy those niches.

That environmental adaptability is the reason a healthcare facility cannot dismiss the organism out of hand simply because it is a food-associated pathogen. Listeria is capable of persisting in wet, intermittently cleaned equipment and in drains, and it can be introduced into a building on produce, footwear, soil or untreated water. What it does not have is any demonstrated capacity to establish within a purified water treatment train or to survive the disinfection processes applied in instrument reprocessing. It is a vegetative organism with no spore stage and no unusual biocide tolerance, and it is inactivated by high-level disinfectants and by thermal processes at ordinary parameters.

Relevance in endoscopy and reprocessing

L. monocytogenes is not an established organism of endoscope reprocessing water systems and has no recognised association with endoscopy-related infection transmission. There are no documented endoscopy outbreaks or pseudo-outbreaks attributed to it, it is not among the organisms that reprocessing water surveillance programmes are designed to detect, and it is not a documented constituent of purified-water or automated endoscope reprocessor biofilm in the way that non-tuberculous mycobacteria and Pseudomonas species are. This should be stated plainly so that a facility does not respond to an isolate as though a known reprocessing pathway existed.

The organism does have two properties that make it worth taking seriously rather than dismissing. The first is severity: in a pregnant patient, a neonate, an elderly patient or an immunosuppressed patient, invasive listeriosis carries a high case fatality, and endoscopy units and CSDs serve exactly those populations. The second is its documented capacity to establish persistent biofilm on wet processing equipment in the food industry, which demonstrates that the organism can occupy a niche functionally similar to a poorly drained reprocessing sink, a drain, an ice machine or a hand-wash basin. Reprocessing rooms contain all of these.

Against the disinfection step itself, Listeria presents no particular challenge. It is not sporicidal-resistant, is inactivated by glutaraldehyde, ortho-phthalaldehyde and peracetic acid at their validated conditions, and is destroyed by the thermal disinfection and steam sterilisation cycles used in CSD. Drying and storage remain relevant because the organism tolerates desiccation better than many vegetative Gram-positives and survives on dry surfaces long enough to be transferred by hands or equipment. Practically, its relevance to a reprocessing facility is as an indicator that untreated water, environmental material, soil or drainage has reached a location it should not have reached, rather than as a device-borne infection risk in its own right.

Interpreting a detection

Recovery of L. monocytogenes from a final rinse or supply water sample is an atypical result and should be treated first as a question of validity and provenance. Routine reprocessing water surveillance uses heterotrophic plate count and membrane filtration on non-selective media; Listeria is not a target organism and would ordinarily only be identified if a colony happened to be picked and worked up, or if selective enrichment was specifically requested. Establish what method produced the result, whether the laboratory concurrently handles food or clinical Listeria specimens, and whether the isolate has been confirmed by a reference method rather than by a preliminary identification. Confirm sample labelling, collection time and transport conditions, since an extended hold at ambient temperature permits growth of any organism introduced at collection.

If the identification is confirmed, the investigation should look for a route by which environmental or untreated material could enter the circuit rather than assume a failure of the treatment plant. The lines of enquiry are the integrity of backflow prevention and any cross-connection between treated and untreated lines, recent plumbing or building works, the condition of the sampling port, whether the sample was taken near a floor drain or hand-wash basin where splashing is possible, the condition of reprocessor drain-back and waste pathways, and whether any non-clinical water source, ice machine or cleaning equipment has been used in the vicinity. Footwear, trolleys and externally sourced items that carry soil into the reprocessing room are also plausible contributors.

A single confirmed isolate warrants escalation to the infection prevention team, given the severity of the organism in vulnerable patients, together with repeat sampling under observed aseptic conditions before any conclusion is drawn about the water system. Where the organism recurs across sampling rounds, is recovered from more than one point, or is accompanied by elevated total viable counts or other environmental Gram-negative organisms, the finding should be treated as evidence of persistent environmental ingress and biofilm establishment, and the response should extend to disinfection of the distribution loop, review of terminal filtration, and inspection or replacement of components that cannot be effectively disinfected in place. Isolates should be retained. If any patient exposed to the affected pathway develops a compatible illness, the isolate and the patient isolate should be submitted for whole-genome comparison, since sequencing is the standard method for resolving Listeria relatedness.

Antimicrobial resistance

Listeria monocytogenes is intrinsically resistant to all cephalosporins and to fosfomycin, a property of practical importance because empirical regimens for bacterial meningitis based on a third-generation cephalosporin alone will not cover it. This is the reason amoxicillin or ampicillin is added for patients in at-risk groups, specifically neonates, adults over fifty, pregnant women and the immunosuppressed, and the omission of that addition is a recognised cause of treatment failure in listerial meningitis. The organism is also intrinsically resistant to older quinolones and shows reduced susceptibility to some other agents that would otherwise be considered for central nervous system infection.

Acquired resistance in clinical isolates remains uncommon, and high-dose amoxicillin or ampicillin, usually combined with gentamicin for serious infection, and trimethoprim-sulfamethoxazole for penicillin-allergic patients, remain effective. Prolonged courses are required for meningitis, rhombencephalitis and endocarditis because the intracellular location of the organism limits the effectiveness of agents with poor cell penetration and because relapse is described with short courses. Multidrug-resistant and biofilm-forming Listeria isolates have been reported from environmental and food-chain surveillance, including resistance associated with mobile genetic elements, and continued monitoring is warranted given the potential for transfer into clinically relevant lineages.

Biocide behaviour is more directly relevant to a reprocessing facility than antimicrobial behaviour. Listeria has no spore stage and no intrinsic tolerance to the high-level disinfectants or thermal processes used in instrument reprocessing, so it presents no challenge to a correctly executed cycle. What is documented is tolerance to sub-lethal biocide exposure in the food industry, including adaptation to quaternary ammonium compounds through efflux mechanisms, and markedly increased survival within established biofilm compared with planktonic cells. The operational conclusion is that Listeria will not survive a validated disinfection step but may persist indefinitely in a wet niche that the disinfectant does not reach at full strength, such as a drain, an under-sink void, a dead leg in a distribution loop, or a component with an inaccessible internal surface. Remediation of a confirmed environmental isolate should therefore be directed at eliminating that niche rather than at increasing disinfectant concentration.