JSTOR

Methods

Three types of fomites were chosen to evaluate the survival time of MRSA and MSSA on their surfaces: a plastic patient chart, a portable plastic laminated bedside table, and a 100% polyester cloth curtain used to separate patient beds. The chart and tabletop were thoroughly cleaned with a solution of N–alkyl dimethyl benzyl ammonium chloride (0.105%) and N‐alkyl dimethyl ethylbenzyl ammonium chlorides (0.105%) (ConFlikt; Deacon Laboratories). These surfaces were allowed to dry for 12 hours. Staphylococcus aureus obtained from 3 unique patients (2 MRSA isolates and 1 with MSSA isolate) each were diluted in 200 μL of saline to create three 0.5‐McFarland unit suspensions. The surfaces were marked with a series of circles that were the size of our agar contact plates (6 cm in diameter). From each 0.5‐McFarland unit suspension, 10⁷ organisms were extracted and evenly inoculated with a wooden dowel into designated circles on the chart and the tabletop. The 2 fomites were then left in ambient air at room temperature, to mimic hospital environment conditions. At 24‐hour intervals, tryptic soy agar contact plates (Remel) were used to sample 1 of the circular areas on each of these fomites. This was done to avoid duplicate sampling of a single area, so that any organisms removed from the surface as a result of the sampling would not bias the results. Colonies of each morphologic type were subcultured on blood agar plates.

Three –cm swatches of polyester cloth curtain were cleaned with chlorhexidine gluconate 2% (Ecolab) and allowed to dry for 12 hours. Each swatch was immersed into a 0.5‐McFarland suspension of either MRSA or MSSA. The cloth swatches were left in ambient air at room temperature, to mimic hospital environment conditions. At 24‐hour intervals, –mm pieces of cloth were removed with sterile scissors and placed into 3 mL of thioglycolate broth (Remel). These cultures were placed in CO₂ incubator at 37°C. After 24 hours, if the broth was cloudy (indicating bacterial growth) a McFarland scanner was used to quantitate the density of growth, and a sterile pipette was then used to inoculate a blood agar plate with 1 drop of the broth.

Coagulase‐positive β‐hemolytic colonies were taken from the blood agar plates after 24 hours of incubation at 37°C to be subcultured on mannitol salt agar plates containing oxacillin at a concentration of 4 μg/mL (Remel). Colonies that grew on the oxacillin plates after 24 hours of incubation at 37°C were considered MRSA; coagulase‐positive colonies that did not grow on the oxacillin containing plates were considered MSSA.

Results

The Table shows the duration of survival of each microbial strain on the 3 different fomites. MRSA strain 1 survived in a dry, room‐air environment for 11 days on a plastic chart, for 9 days on a laminated tabletop, and for 9 days on a polyester cloth curtain. MRSA strain 2 survived for more than 12 days on a plastic chart, for 11 days on a laminate tabletop, and for 8 days on a polyester cloth curtain. The MSSA strain survived for more than 12 days on a plastic chart, for 12 days on a laminated tabletop, and for more than 14 days on a polyester cloth curtain.

Table 1.  Duration of Survival of 2 Strains of Methicillin‐Resistant Staphylococcus aureus (MRSA) and 1 Strain of Methicillin‐Susceptible S. aureus (MSSA) on Hospital Fomites

Open New Window

The Figure shows the survival curves for the 3 strains on each of the 3 types of fomite. The number of colony‐forming units of S. aureus on the tabletop declined with time. Although the amount of growth on the plastic chart also trended down with time, it clearly did not demonstrate the smooth decline in the number of colony‐forming units that was demonstrated on the tabletop. Similarly, the duration of microbial survival on the curtain fluctuated with time, as measured by broth turbidity (McFarland units).

(34 KB)

Figure  Duration of survival of 2 strains of methicillin‐resistant Staphylococcus aureus (MRSA 1 and MRSA 2) and 1 strain of methicillin‐susceptible S. aureus (MSSA) on a plastic patient chart (A), a plastic laminate tabletop (B), and a polyester cloth curtain (C). CFU, colony‐forming units.

Open New Window

Discussion

Our pilot study shows that MRSA can survive for up to at least 9 days on 3 common hospital fomites. These results vary from results of prior studies in which MRSA survival on various surfaces was ascertained using a number of different methods, including keeping the contaminated surfaces moistened and isolating the surface area or fomite in an incubator and/or hood. These conditions are less representative of in‐hospital, patient room conditions, and thus the survival times we obtained may more accurately reflect clinically relevant MRSA survival times.

The expected quantitative decline in the number of colony‐forming units was seen in staphylococcal growth on the tabletop, which has a smooth surface without any variation in its texture. On the other hand, the plastic charts that were used for our study had a corrugated surface, which likely explains the variation in the number of colonies observed; the bacteria that lived in the nooks of the irregular surface and could proliferate in a slightly different environment than the completely smooth surface of the tabletop. This also explains the similar quantitative growth findings for the curtain, which also has an irregular surface because of weave in the fabric. The results suggest that smooth surfaces, in addition to being easier to clean, may be less favorable to the survival of bacteria (including MRSA), and should be preferred for frequently used hospital fomites.

The fact that MRSA can survive in dry conditions at room temperatures for the periods we demonstrated has several implications. During outbreaks of MRSA colonization and infection, infection control teams generally focus on maintaining contact precautions with use of gowns and gloves and an emphasis on hand hygiene. Previous studies have shown that environmental contamination with MRSA occurs regularly^3‐4 and we have added to the body of literature that demonstrates the potential for MRSA to survive in the hospital environment for significant periods.^8‐10 Therefore, environmental contamination may be an important and overlooked reservoir of MRSA through which noncolonized patients can acquire the organism. In this instance, careful room cleaning with an effective disinfectant and cleaning of all potentially contaminated surfaces could be an important means to controlling MRSA transmission and halting outbreaks.