Sir,

Further to the case described by Pinto and Merlino1 in their recent correspondence, we report our experience of an outbreak of CO2-dependent methicillin-resistant Staphylococcus aureus (MRSA) on a UK regional liver transplant unit (RLU). Six patients and one staff member were infected or colonized with a strain of MRSA, which would not have been reliably detected by routine screening methods. The laboratory characteristics of this strain, infection control strategies and the implications for screening are discussed.

In early 2008, a liver transplant recipient was screened for MRSA on admission to the critical care unit from the RLU. Small poorly growing green colonies were isolated from a nasal swab on a chromogenic MRSA identification medium (chromID MRSA; bioMérieux) after aerobic incubation for 24 h. The colonies were slide-coagulase positive (Slidex Staph-Plus reagent; bioMérieux). The strain repeatedly failed to grow after 24 and 48 h of aerobic incubation on Iso-Sensitest agar (IST; Oxoid) or Columbia agar supplemented with 5% horse blood (BA; Oxoid). When subcultured onto BA and incubated overnight at 37°C in air plus 5% CO2, however, a heavy growth of an isolate with typical S. aureus colony morphology was observed. Susceptibility testing performed on IST according to BSAC guidelines,2 but in CO2-enriched atmospheric conditions (5% CO2), confirmed resistance to cefoxitin, erythromycin, clindamycin, moxifloxacin and trimethoprim.

A possible outbreak was suspected as the microbiologist and infection prevention and control team were aware of another liver transplant recipient on the RLU who was found, 3 weeks earlier, to be colonized with a strain of MRSA that grew much better in 5% CO2 than aerobically. This index case was a male patient in his 60s, 5 months into an admission for decompensated alcohol-related liver disease. MRSA was not detected from his routine admission screening swabs. Following an orthotopic liver transplant his post-operative course was complicated by acute rejection, hospital-acquired pneumonia and Clostridium difficile-associated diarrhoea. The following month he suffered an acute clinical deterioration and underwent an exploratory laparotomy. MRSA, as described above, was isolated from enrichment culture of intra-abdominal tissue. Repeat superficial swabs from this patient were incubated in CO2-enriched atmospheric conditions, demonstrating carriage of the organism at multiple clinical sites. Despite topical decolonization attempts, throat and perineum swabs remained positive at discharge and also at follow-up 4 weeks later.

Following the identification of the second case, all patients and staff on the RLU were screened for carriage of CO2-dependent MRSA using chromID MRSA medium in 5% CO2. A deep clean of the ward was carried out, and infection prevention and control practices were reinforced. Environmental screening swabs prior to the deep clean and post-decontamination were negative. Four further cases (three patients and one staff member—a student nurse) were found to be colonized at one or more sites by a CO2-dependent strain of MRSA. Ongoing targeted screening revealed a seventh case 5 weeks after the initial outbreak. This patient was admitted to the RLU during March 2008, but had been discharged 2 days prior to the recognition of the outbreak and screening swabs were found to be positive when he was readmitted to the RLU in May 2008. Table 1 summarizes the admission date and location of hospitalization for each case, along with the site of first isolation of MRSA and the length of time on the unit when MRSA was first isolated.

Table 1.

Date and location of hospitalization on the RLU, and timing and site of isolation of CO2-dependent MRSA for each case involved in the outbreak

Case Admission date Length of time on unit (days) Length of time on the unit when MRSA first isolated (days) Site of colonization or infection first detected Location on RLU 
Sep 2007 217 168 intra-abdominal tissue cubicle 
Feb 2008 15 15 nasal swab cubicle 
Feb 2008 28 28 abdominal drain site bay A 
Mar 2008 33 17 abdominal drain site bay A 
NA NA NA nasal swab staff 
Mar 2008 13 13 nasal/perineum swab cubicle 
Mar 2008 NA NA bay A 
May 2008 nasal swab NA 
Case Admission date Length of time on unit (days) Length of time on the unit when MRSA first isolated (days) Site of colonization or infection first detected Location on RLU 
Sep 2007 217 168 intra-abdominal tissue cubicle 
Feb 2008 15 15 nasal swab cubicle 
Feb 2008 28 28 abdominal drain site bay A 
Mar 2008 33 17 abdominal drain site bay A 
NA NA NA nasal swab staff 
Mar 2008 13 13 nasal/perineum swab cubicle 
Mar 2008 NA NA bay A 
May 2008 nasal swab NA 

NA, not applicable.

Molecular testing was performed locally at the Institute of Cell and Molecular Biosciences, University of Newcastle upon Tyne. PFGE showed that strains generated identical profiles to that of EMRSA-15, except for one strain in which a single band was reduced in size. Diagnostic PCR was used to detect the presence of the coa, mecA and Panton–Valentine leucocidin (PVL) genes. All strains were coa and mecA positive, but PVL negative. Multilocus sequence typing and staphylococcal cassette chromosome mec (SCCmec) typing indicated that the strains were MRSA ST22 with SCCmec type IV, confirming that they were variants of EMRSA-15.

Following prompt recognition, this outbreak was successfully brought under control by reinforcement of good infection control practices. Despite its unusual growth characteristics, the strain was identified as a clone of EMRSA-15, the predominant UK epidemic strain.3,4 Transmission of CO2-dependent S. aureus has been previously described in the literature as early as 1955.5 Gómez-González et al.6 recently observed nosocomial transmission of a strain of S. aureus that was auxotrophic for CO2, and described the clinical and molecular characteristics of such small-colony variants of S. aureus in their institution. To our knowledge, ours is the first reported outbreak of a CO2-dependent MRSA. Similar outbreaks may be missed if screening swabs are processed by conventional methods. Establishing the local prevalence of CO2-dependent MRSA is necessary to determine whether targeted screening is required.

Funding

This study was conducted as part of our routine work.

Transparency declarations

None to declare.

Acknowledgements

The main findings were presented in abstract form in a poster (P855) at the European Congress of Clinical Microbiology and Infectious Diseases, Helsinki, 2009.

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