Staphylococcus aureus

Associated infections

• Skin and soft tissue infection, including abscess, cellulitis, folliculitis and impetigo
• Bacteraemia and sepsis
• Infective endocarditis, native and prosthetic valve
• Osteomyelitis and septic arthritis
• Prosthetic joint and device-related infection
• Pneumonia, including ventilator-associated and post-influenza necrotising pneumonia
• Toxin-mediated syndromes including toxic shock syndrome, scalded skin syndrome and staphylococcal food poisoning
• Surgical site infection

Transmission route

Transmission of S. aureus is predominantly by direct and indirect contact. The hands of healthcare workers are the dominant vehicle, supplemented by shared and inadequately decontaminated equipment, contaminated environmental surfaces and textiles, and dispersal of skin squames from colonised patients and staff. Nasal carriage is the reservoir that sustains this, with hand contamination following nose touching and with heavy dispersers contributing disproportionately to environmental contamination. Airborne dispersal over short distances is real but secondary to contact spread in most settings.

Waterborne transmission is not a recognised route. S. aureus does not grow in treated water, is not part of plumbing biofilm communities, and is not implicated in the water-associated outbreak literature that concerns Pseudomonas, Legionella, non-tuberculous mycobacteria and the environmental Gram-negatives. Its recovery from a water sample is not evidence of a water treatment failure and should not be treated as one.

In a reprocessing department the transmission pathway that matters is the clean side. An endoscope emerging from a validated high-level disinfection cycle is, at that moment, effectively free of vegetative organisms. Everything that happens subsequently is an opportunity for recontamination: removal from the reprocessor, handling during transfer, connection to drying equipment, placement in a drying cabinet, transport in a container, and hanging or storage. Each of these involves contact with hands, gloves, surfaces and containers, and each is a point at which a shedding or contaminated hand can deposit S. aureus onto a device that has just been rendered clean. The organism's tolerance of drying means that a contaminated cabinet interior, hanging clip, transport tray or bench does not self-decontaminate between uses and can seed successive devices over an extended period.

Sampling is the other pathway. Collection of a channel flush or rinse sample involves manipulation of connectors, syringes and containers in an environment where staff are present and skin squames are in the air, and an unmasked, ungloved or hurried collection is a plausible route by which S. aureus reaches a sample container without ever having been on the device.

Relevance in endoscopy and reprocessing

The significance of S. aureus in endoscope reprocessing is well defined in surveillance guidance and is different in kind from that of the water organisms. ESGE-ESGENA surveillance guidance names staphylococci among the indicator organisms for microbiological surveillance and interprets their recovery from a reprocessed endoscope as evidence of recontamination after disinfection, rather than of failed disinfection or of contaminated source water. This gives the organism a defined diagnostic role: it is the marker that points investigators toward the clean side of the department rather than toward the water treatment plant.

In practice, recovery of S. aureus indicates one of a limited set of post-disinfection events. These include handling of the disinfected device with unclean, ungloved or inadequately decontaminated hands; contact with a contaminated drying cabinet interior, hanging clip, transport container or bench surface; deposition of skin squames or aerosol in a clean area that is inadequately separated from the dirty side or from general traffic; storage in a cabinet whose air filtration or cleaning schedule has lapsed; or contamination introduced at the point of sampling. It does not indicate a water treatment problem, and it does not indicate that the disinfectant failed, because a disinfectant capable of the mycobactericidal and virucidal activity required for high-level disinfection will not spare a vegetative staphylococcus.

The organism has no meaningful biofilm role in rinse water plumbing and does not multiply in a wet channel during storage in the way that P. putida does, so drying is less decisive here than it is for water organisms. What matters instead is the physical cleanliness and traffic control of the clean side. Because S. aureus survives on dry surfaces for extended periods, a contaminated storage cabinet or transport container will continue to recontaminate devices until it is identified and decontaminated, which is why repeated recovery from devices sharing a cabinet is a more informative pattern than repeated recovery from a single device.

S. aureus recovery also carries clinical weight that most recontamination markers do not. Unlike a coagulase-negative staphylococcus or an oral commensal, S. aureus is a primary pathogen capable of causing serious invasive disease in an otherwise healthy host. A device carrying S. aureus into a patient contact is a genuine hazard rather than a theoretical one, which is why the organism should not be recorded as an incidental skin contaminant even though its route of arrival is the same as that of the true contaminants.

Interpreting a detection

A detection of S. aureus in a reprocessed endoscope or final rinse water sample is not a plausible water-system finding and should not initiate remediation of the water treatment train. The organism does not colonise treated water. The two hypotheses worth distinguishing are recontamination of the device after disinfection and contamination of the sample during collection, and both point to the same part of the department: the clean side and the people working in it.

The first checks are of clean-side workflow and of sampling technique, taken together because they share most of their controls. Establish who handled the device between the end of the reprocessing cycle and the collection of the sample, whether gloves were worn and changed, and whether hand hygiene was performed at the transition from dirty to clean handling. Review the physical separation of clean and dirty zones, airflow direction and traffic through the clean area. Inspect and review the cleaning schedule for drying cabinets, including their interiors, hanging clips and filters, and for transport containers, trays and covers, since these are the reservoirs most likely to sustain a recurring problem. For the sample itself, confirm that the operator wore gloves and a mask, that the container was sterile and opened only at the moment of collection, that the sampling port and connectors were disinfected and dried, and that the sample was not collected in a busy or aerosol-generating area. Confirm the identification, since coagulase testing and rapid methods can confuse S. aureus with S. lugdunensis in both directions.

A single low-count isolate with an identifiable handling explanation is still not dismissible in the way a coagulase-negative staphylococcus might be. The appropriate response to a first detection is quarantine and reprocessing of the affected device, a documented review of the clean-side workflow and sampling technique, decontamination of the implicated cabinet or container if one is identified, and repeat sampling of that device together with others stored in the same location. Escalation to infection prevention is warranted for any confirmed isolate, and the strength of the response should scale with count and with the patient population served by the device.

A trend indicates a systematic breakdown rather than an incident. Repeated recovery from the same device points to handling of that device or to a defect in it; repeated recovery from devices sharing a cabinet, container or storage area points squarely at that item; repeated recovery across the whole clean side points to workflow, zoning or hand hygiene rather than to any single object. Recovery of a methicillin-resistant strain does not change the reprocessing implications, since MRSA is inactivated by validated processes exactly as a susceptible strain is, but it does raise the clinical stakes of a device reaching a patient and should be escalated accordingly, with isolates retained in case typing against any patient isolate becomes necessary. Where a heavy disperser among staff is suspected on the basis of a persistent pattern that survives environmental remediation, that question belongs with occupational health and infection prevention rather than with the reprocessing unit.

Antimicrobial resistance

Methicillin resistance, conferred by the mecA gene or its homologue mecC encoding an altered penicillin-binding protein with low beta-lactam affinity, removes the entire beta-lactam class with the exception of the anti-MRSA cephalosporins such as ceftaroline. MRSA remains a major healthcare-associated and community-associated pathogen internationally, with community-associated lineages differing from healthcare lineages in their toxin profiles, susceptibility patterns and clinical presentation. In methicillin-susceptible strains, penicillinase production is near universal, so penicillin itself is rarely usable and penicillinase-stable agents such as flucloxacillin remain the treatment of choice, retaining superiority over vancomycin for susceptible strains.

Resistance to macrolides, lincosamides, fluoroquinolones, tetracyclines and trimethoprim-sulfamethoxazole is common and varies regionally. Inducible clindamycin resistance requires specific laboratory detection to avoid treatment failure. Reduced susceptibility to vancomycin in the form of vancomycin-intermediate and heterogeneous vancomycin-intermediate strains is documented and is associated with thickened cell walls and with treatment failure at apparently susceptible minimum inhibitory concentrations. Fully vancomycin-resistant isolates carrying vanA acquired from enterococci have been reported but remain rare. Resistance to daptomycin and linezolid occurs but is uncommon.

None of this affects reprocessing. Antimicrobial resistance in S. aureus operates through altered antibiotic targets, enzymatic drug destruction and efflux, none of which confers tolerance of the oxidising or alkylating chemistry used in high-level disinfection, or of thermal disinfection. Methicillin-resistant strains show no increased tolerance of validated high-level disinfectants, and recovery of MRSA from a reprocessing environment reflects exactly the same recontamination pathways as recovery of a susceptible strain, with exactly the same remedies. Reduced susceptibility to quaternary ammonium compounds and to chlorhexidine mediated by qac efflux genes has been described in some staphylococci and is worth noting in relation to low-level surface disinfection and skin antisepsis, but it does not extend to the high-level disinfectants or to thermal processes used in device reprocessing.