Isolation and characterization of methicillin-resistant Staphylococcus

Isolation and characterization of methicillin-resistant Staphylococcus aureus from fire stations in two northwest fire districts Marilyn C. Roberts, PhD, Olusegun O. Soge, PhD, David No, BS, Nicola K. Beck, MS, and John S. Meschke, PhD Seattle, Washington

Background: Methicillin-resistant Staphylococcus aureus (MRSA) strains were isolated and characterized from environmental surfaces of two fire stations from two independent districts in the northwestern United States. After the first sampling and before the second sampling, education was provided, additional signage was added, and changes in disinfection protocols were put in place. Nasal carriage of MRSA was determined at the second sampling. Methods: Environmental samples were collected using SANICULT swabs and RODAC plates. Biochemical tests and 16S rRNA sequencing confirmed MRSA isolates. Antimicrobial susceptibility testing was performed, and the mecA gene, multilocus sequence typing, and SCCmec typing were determined by polymerase chain reaction, sequencing, and pulsed-field gel electrophoresis analysis. Results: MRSA was isolated from 44 of 1,064 samples examined (4.1%) and included USA300 isolates. The same strains of MRSA were found in both the garage (ie, medic and fire trucks and protective clothing) and the living quarters. Nasal carriage of MRSA from one fire district was 22.5%. Conclusion: Community-like and hospital-like MRSA were isolated from the environmental samples. The majority of the nasal MRSA/S aureus isolates were genetically related to the environmental MRSA strains, suggesting possible transmission between personnel and the environmental surfaces. Further research is needed to verify this hypothesis. Key Words: USA300; CA-MRSA; HA-MRSA; environmental surfaces. Copyright ª 2011 by the Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved. (Am J Infect Control 2011;39:382-9.)

Methicillin-resistant Staphylococcus aureus (MRSA) was first identified in the 1960s. Over the last 10 years, MRSA has become a major nosocomial pathogen for patients in hospitals and nursing homes. Known risk factors for MRSA infections include previous antibiotic therapy, extensive hospital stays, surgery, residence in a long-term care facility, dialysis, the presence of invasive medical devices, and colonization.1,2 Moreover, community-acquired MRSA (CA-MRSA) infections are on the rise, and outbreaks of skin and soft tissue infections have been reported in high school, college and professional sports teams; jail inmates; children From the Department of Environmental and Occupational Health Sciences, School of Public Health, University of Washington, Seattle, WA. Address correspondence to Marilyn C. Roberts, PhD, Department of Environmental and Occupational Health Sciences, Box 357234, School of Public Health, University of Washington, Seattle, WA 98195-7234. E-mail: [email protected].

Conflict of interest: None to report. 0196-6553/$36.00 Copyright ª 2011 by the Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.ajic.2010.09.008

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attending day care centers; and military personnel who were previously healthy, with no health care exposure or known classical risk factors.3,4 Identified risk factors associated with CA-MRSA outbreaks include sharing of personal care products, frequent skin-to-skin contact, skin abrasions, and crowded living conditions.3 S aureus and MRSA can be transmitted from fomite to person and from person to person. MRSA has been isolated from whirlpools, razors, towels, gym benches, bar soap, sauna benches, and mops.2,5,6 More recently, a study sampling 35 homes found that 9 had MRSAcontaminated surfaces, including kitchen and bathroom sinks, kitchen and bathroom faucet handles, counter wiping cloths, kitchen counters, kitchen garbage cans, kitchen towels, and bathroom door handles.7 The highest counts were associated with wet sites. A recent study that sampled commonly touched sites at 9 fire-related occupational and training facilities in Tucson found that of 160 sites sampled, 6.9% were MRSA-contaminated with levels of 102-103 cfu/ site.8 Couches made of soft, porous material had the highest percentages of MRSA contamination, whereas no MRSA was found on surfaces on medic or fire trucks. In contrast, a study in southern Maine found that 49%

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of ambulances were contaminated with MRSA, and similar levels were found in a second study of ambulances.9,10 None of those studies involved molecular characterization of the MRSA isolates or culturing of personnel for MRSA. The average rate of nasal carriage of MRSA among health care personnel is acknowledged to be 5%12%, compared with ,2% in the general US population, 2.5% in previously hospitalized persons, 7.5% in college students, and 6%-35% in injectable drug users.4,11-13 However, reported MRSA carriage rates in health care professionals have ranged up to 50% depending on the type of patients contacted, patients’ carriage rates, study date and geographic location.3 In a 1999 study, nasal swab samples were obtained from 109 Kansas paramedics to evaluate carriage level, and a second sampling was done 1 year later to determine persistence.14 In the first sampling, 5.5% of the paramedics were colonized with MRSA, and in the second sampling, 5 of these paramedics were colonized with MRSA. Based on pulsed-field gel electrophoresis (PFGE) analysis results, 3 paramedics were thought to carry the same strain in both samples.14 In the current study, of the 1,064 environmental surfaces sampled from two fire stations in two different districts in the northwestern United States, 44 were positive for MRSA. One isolate from each sample was characterized. Nasal samples were collected from personnel in one of the districts, with 9 MRSA-positive personnel. Some of the nasal isolates were genetically related to the fire station surface samples. After the first sampling and before the second sampling, education was provided to the fire personnel and other changes were made.

MATERIALS AND METHODS Surfaces sampled Nine different areas were sampled at each fire station: medic trucks, fire trucks and fire engines, outer fire gear, garages, kitchens, bathrooms, bedrooms, gyms, and other areas. Between the first and second samplings, station 2 acquired a new medic truck, whereas in station 1 the medic trucks are rotated to various stations within the district. Thus, a different medic truck was sampled at each sampling at both fire stations, which might have affected the number of MRSApositive samples detected at the second sampling.

Environmental samples Environmental samples were collected with SANICULT swabs (Starplex Scientific, Etobicoke, Ontario, Canada) and on RODAC plates (Becton Dickinson and Co. Diagnostic Systems, Franklin Lakes, NJ). RODAC plates, which sample an area of ;2.6 cm2, were placed

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on a surface, and a single swab was used to sample a 10cm2 area or a single object, such as the top surface of computer keyboard, microwave keypad, bag handle. The same sample areas were sampled at each sampling period. RODAC plates detect 10%-40% of seeded bacteria, and the swabs detect 10-100 cfu/mL with environmental MRSA, methicillin-susceptible S aureus (MSSA), and/or MSSA ATCC 25923 under laboratory conditions (manuscript in preparation). Swabs were used to sample irregularly shaped, frequently touched surfaces within the fire station living quarters and fire apparatus (medic and fire trucks and fire engines) and outer fire clothes. These surfaces were chosen as those most likely to accumulate bacteria. The sample areas swabbed were the same sizes for both fire stations and both sampling periods. Bacto m Staphylococcus Broth (1.53; Difco Laboratories, Sparks, MD) supplemented with a final concentration of 75 mg/mL of polymyxin B and 0.01% potassium tellurite (Sigma-Aldrich, St Louis, MO) were added to each transport tube. The tubes were incubated in 5% CO2 at 36.58C until turbid (24-96 hours). The positive samples were 10-fold diluted and plated onto Bacto Staphylococcus Medium 110 supplemented with 10 mg/ mL of methicillin and 0.01% potassium tellurite and Bacto Mannitol Salts Agar (MS) (Difco). Tubes were held for 7 days before being labeled as negative for staphylococci. All colonies that grew and had staphylococcal morphology and produced a yellow colony with a yellow zone on MS were screened for b-hemolysin and verified as S aureus by the Remel Staphaurex rapid latex test (Thermo Fisher Scientific, Lenexa, KS).15 RODAC plates with Bacto Staphylococcus Medium 110 supplemented with 10 mg/mL of methicillin and 0.01% potassium tellurite were used to sample smooth frequently touched surfaces. The plates were incubated in 5% CO2 at 36.58C. Black colonies were verified as S aureus screened as described earlier. Plates were held for 7 days. Given the limited data on sampling washing machines, we developed the following method to sample the washing machines using sterile baby washcloths. The baby washcloths were sterilized and placed in the washing machines and washed using the warm wash and rinse cycles with the station’s normal detergent. The wet washcloths were placed into sterile bottles and covered with Bacto m Staphylococcus Broth supplemented with 75 mg/mL of polymyxin B and 0.01% potassium tellurite, and then incubated in 5% CO2 at 36.58C. Control sterile washcloths were also covered with media and incubated for 7 days. Culture-positive samples were 10-fold diluted and plated onto Bacto Staphylococcus Medium 110 supplemented with 10 mg/mL of methicillin and 0.01% potassium tellurite and MS. Colonies were verified as S aureus as described earlier. Bottles were held for 7

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Table 1. Samples positive for MRSA, S aureus, MRCoNS, and CoNS Samples, n

No staphylococci, n (%)

MRSA, n (%)

S aureus, n (%)

MRCoNS, n (%)*

CoNS, n (%)*

600 464 1,064

79 (13) 51 (11) 130 (12)

26 (4.3) 18 (3.9) 44 (4.1)

12 (2.0) 13 (2.8) 25 (2.4)

233 (39) 244 (53) 477 (45)

250 (42) 138 (30) 388 (36.5)

First sampling Second sampling Total

*Significant differences between the two sampling periods (MRCoNS: x2 5 20.01; df 5 1; P 5 .00; CoNS: x2 5 16.06; df 5 1; P 5 .0001).

days before being labeled as negative. No control washcloths were positive for either Staphylococcus spp or MRSA. A second set of environmental samples was obtained 7-9 months after the first set of samples. During the period between samplings, educational talks were given to personnel and signs reminding personnel to wash their hands were placed around the stations. Hand sanitizers were installed in all doorways leading from the garage to living quarters, and procedures for daily disinfection of medic and fire trucks were instituted. One isolate/sample from both the first and second sets of samples was used for further characterization. The S aureus isolates that were positive for the mecA gene were considered MRSA. Staphylococci that were not S aureus but grew on the methicillin-supplemented Bacto Staphylococcus Medium 100 were considered methicillin-resistant coagulase-negative Staphylococcus spp (MRCoNS), whereas staphylococci that that produced pink colonies on the MS media and did not grow on methicillin-supplemented Bacto Staphylococcus Medium 110 media were considered coagulasenegative Staphylococcus spp (CoNS), as described previously (Tables 1 and 2).15

Detection of mecA and the Panton-Valentine leukocidin gene, SCCmec typing, and multilocus sequence typing S aureus and presumptive MRSA nasal isolates were tested for the presence of the mecA gene by polymerase chain reaction (PCR) assay as described previously.15 The mecA-positive isolates were tested for the Panton-Valentine leukocidin (PVL) gene and SCCmec type I-V using PCR assays as described previously.17,18 Those isolates that were not type I-V were deemed nontypeable (NT). Positive and negative controls were used as described previously.15 The multilocus sequence typing (MLST) PCR assays were performed using previously published primers and conditions with PCR products sequenced bidirectionally at the University of Washington’s Genome Sciences High-Throughput Sequencing facility. Alleles were assigned by a comparison of their sequences to the corresponding loci in the S aureus Multi Locus Sequence Typing database (www.mlst.net).19 A clinical USA300 isolate was used as the positive control, and negative controls were used for the PCR assays.

Pulsed-field gel electrophoresis Nasal samples Anterior nasal cultures were collected from 40 healthy fire personnel from 13 stations, administration staff, and personnel that move between stations (floaters) using sterile culture SANICULT swabs. Immediately after collection, the samples were transported to the laboratory, and 1.53 Bacto m Staphylococcus Broth (Difco) supplemented with a final concentration of 75 mg/mL of polymyxin B and 0.01% potassium tellurite (Sigma-Aldrich) were added to each tube and incubated in 5% CO2 at 36.58C until turbid (24-48 hours). The enriched nasal samples were plated directly onto MRSASelect Screening Agar (Bio-Rad, Hercules, CA) grown at 36.58C without CO2 and read at 24 hours according to the manufacturer’s instructions. Enrichment of nasal swabs for identification of carriage of MRSA has been described previously.16 The dark-red colonies were tested with the Remel slide agglutination test. A single MRSA isolate from each person was characterized. All subjects’ rights were protected, and the University of Washington’s Institutional Review Board approved the study.

MRSA isolates with the same MLST types were PFGE-typed as described previously.20 The different PFGE patterns were labeled A-S (Tables 3 and 4), and the genetic relatedness of the isolates with the same ST type was analyzed by the Dice coefficient– unweighted pair-group method with arithmetic averages using GelCompar II software (Applied Maths, Austin, TX). Two different characterized and verified USA300 strains, provided by the Washington State Public Health Laboratory, were used as positive controls for determining whether the MRSA isolates were USA300 as recommended.20

Detection of antibiotic resistance genes PCR assays were used to detect the presence of kanamycin-resistance gene aadD; macrolide-resistance genes erm(A), erm(C), and msr(A); and tetracyclineresistance genes tet(M) and tet(K).15 The PCR products were verified as described previously.20 Plasmids with cloned erm, msr(A), and tet genes were used as positive controls.15 The MRSA clinical strain MS361 was used as


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Table 2. MRSA-positive surfaces at the first and second samplings Number of positive samples Site

First sampling (n 5 600)

Second sampling (n 5 464)

13 2 1 1 3 1 1 1 3 26 (4.3%)

3 1 4 1 4 1 1 1 2 18 (3.9%)

Medic truck Fire truck/engine Outer fire gear Garage Kitchen Bathrooms Bedrooms Gym Others Total

the positive control for the aadD PCR assay, and Enterococcus faecalis JH2-2 was used as a negative control.21

Statistical analysis Both 2 3 5 and individual Pearson x2 tests were performed on the data presented in Table 1 to identify any differences in the percentage of samples positive for MRSA, S aureus, MRCoNS, or CoNS or negative for staphylococci between the two time periods.

RESULTS Identification and characterization of MRSA-positive samples Of the 1,064 samples collected (600 samples in the first sampling and 464 samples in the second sampling), a total of 44 samples (4.1%) were MRSA-positive, 25 samples (2.4%) were S aureus‒positive, 477 samples (45%) were MRCoNS-positive, 388 samples (37%) were CoNS-positive, and 130 samples (12%) were staphylococci-negative after 7 days of incubation. Of the 44 MRSA-positive samples, 18% were from the RODAC plates, which had Staphylococcus spp counts ranging from 3 to 41 cfu/2.6 cm2 (median, 3.5 cfu/2.6 cm2) with 1-4 colonies confirmed as a MRSA/RODAC plate. A 2 3 5 Pearson x2 analysis detected a statistically significant difference in the distribution of isolates between sampling periods (x2 5 23.1; df 5 4; P 5 .0001). However, individual x2 analyses determined that only MRCoNS (x2 5 20.01; df 5 1; P 5 .00) and CoNS (x2 5 16.06; df 5 1; P 5 .0001) differed significantly between the two sampling periods, with no statistical difference between the two time periods for the no staphylococci, MRSA, and S aureus groups (Table 1). At the first sampling, 26 of the 600 surface samples (4.3%) were MRSA-positive, with MRSA-positive samples found in all 9 areas sampled (Table 2). The most common area of MRSA contamination was the medic trucks (13; 50%), the kitchens (3; 11.5%), and other

Type of MRSA First sampling

Second sampling

4 IV ST8 USA300; 9 NT 2 NT 1 NT 1 NT 1 IV ST5 USA300; 1 IV ST34; 1 IV ST30 1 NT 1 NT 1 NT 1 IV ST30; 2 NT 8 IV; 18 NT 5 USA300

1 II ST105; 2 NT 1 II ST1 4 NT 1 II ST105 4 NT 1 NT 1 NT 1 NT 2 NT 3 II; 15 NT 0 USA300

areas (ie, computer keyboard and computer desks, 2; 7.7%). Five MRSA isolates were USA300, the most common cause of CA-MRSA infection in North America. Three other MRSA isolates were SCCmec type IV but were not related to USA300 by PFGE analysis (Table 2). The USA300 isolates were from samples taken from the medic trucks and kitchen surfaces. The remaining 22 MRSA isolates were not SCCmec type I-V (NT). All 26 MRSA carried tetracyclineresistance genes, with tet(M) detected in 19 isolates, both tet(M) and tet(K) detected in 6 isolates, and tet(K) detected in 1 isolate. Fifteen MRSA isolates carried one or more macrolide-resistance gene(s) [erm(A), erm(C), and/or msr(A)], and one MRSA isolate carried the kanamycin-resistance gene, aadD (Table 3). At the second sampling, 18 of the 464 surface samples (3.9%) were MRSA-positive, with MRSA-positive samples found in all 9 areas sampled (Table 2). The kitchen and outer gear both had 4 MRSA-positive samples (22%), the medic truck had 3 MRSA-positive samples (16.6%), other areas had 2 MRSA-positive samples (11%), and the other 5 sampling areas had one MRSA-positive sample each. Two (11%) isolates of MRSA SCCmec type II, which is commonly found in hospitals, were isolated from the fire truck/engine and garage areas. The remaining 16 MRSA isolates (89%) were NT and were found in all of the areas sampled (Table 2). Five MRSA isolates (28%) were negative for the other antibiotic resistance genes examined, 8 (44%) carried a tetracycline-resistance gene [4 tet(M), 3 tet(M) and tet(K), and 1 tet(K), 11 (61%) carried one or more macrolide-resistance gene(s), and 6 (33%) carried kanamycin-resistance gene aadD (Table 3).

MLST and PFGE typing Three isolates (SNS2100-2218, SNS2100-2313, and SNS2100-2380) had MLST patterns not shared with other isolates and were considered to represent unique

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Isolate(s)

Station

Sample set

Surface(s)

Sccmec type

PFGE

USA300

ST

Genotype

5 8

tet(K), tet(M), erm(C), aadD 1 tet(K), tet(M), erm(C); 2 tet(M), erm(C); 1 tet(K), tet(M), erm(C) tet(M), erm(A), erm(C) None tet(M) 4 tet(M) 1 tet(M), erm(C), msr(A); 1 tet(M); 1 tet(K), tet(M), msr(A) 1 tet(M), erm(C), msr(A); 1 tet(M), mef(A) 1 tet(M), erm(A), msr(A); 1 tet(K), tet(M), erm(A), msr(A) tet(M), msr(A) 1 tet(M), msr(A); 1 tet(K), tet(M), msr(A), aadD 1 tet(K), tet(M), msr(A); 1 tet(K), erm(C), aadD; 1 tet(M), erm(A), msr(A), aadD 1 tet(M), erm(C), msr(A); 1 tet(K), tet(M)

RVS700-814 RVS700-808, -810, -841, -851

1 1

First First

Kitchen Medic truck*

IV IV

A B

Yes Yes

RVR800-1151 RVS2700-2992 SNS500-445 SNS500-407, -413, -455, -550 SNS500-408, -443, -427

1 1 2 2 2

First Second First First First

IV NT NT IV NT

Cz C C D D

30 30 30 34 34

SNR600-5326, -5321 RVS2700-2810, -2905

2 1

Second Second

NT II

D E

34 105

RVS2700-2832 SNS500-430, -2327 SNS2100-2258, -2359, -2392

1 2 2

Second First, second Second

Othery Other Kitchen Medic truck Medic truck, bathroomx Medic truck Garage floor, medic truck Fire engine Garage, bed Bathroom, kitchen{

II NT NT

F G H**

1 5 5

RVR800-951, -1087

1

First

NT

Iyy

15

RVR800-921, -1125 SNWC1, SNS2100-2297 RVS2700-2828 RVS2700-2914 RVS700-820 SNS2100-2404 RVS700-859, 973 RVS700-856 SNS2100-2345, -2350, -2346 SNS2100-2218 SNS2100-2313 SNS2100-2380

1 2 1 1 1 2 1 1 2 2 2 2

First First, second Second Second First Second First First Second Second Second Second

Fire engine, outer fire gear Medic truck, bathroom Other, kitchen Medic truck Outer fire gear Medic truck Kitchen Medic and fire trucks Other Outer fire gearzz Medic truck Gym floor Other

NT NT NT NT NT NT NT NT NT NT NT NT

I I J K L L M N O Pxx Q R

15 15 8 8 45 45 59 59 72 97 1710{{ 1711{{

1 tet(M); 1 tet(M), erm(C) 1 tet(M); 1 msr(A), aadD None None tet(M) msr(A) 1 tet(K), tet(M), erm(A); 1 tet(M) tet(K), tet(M), erm(A) 1 msr(A), aadD; 1 aadD; 1 none None msr(A) tet(M), aadD American Journal of Infection Control June 2011

*Medic truck surfaces include gurney handle and straps, equipment bag, blood pressure cuff, electronic equipment, inside door, floor, passenger seat, and arm rest. y Other surfaces include washing machine, telephone handle, and cloth chair. z Related ($90%) to nasal isolates 301 and 305. x Bathroom surfaces include counter and door handle. { Kitchen surfaces include dishwasher handle, toaster handle, refrigerator door handle, and floor. **Related ($90%) to nasal isolates 300 and 340. yy Related ($90%) to nasal isolate 312. zz Outer fire gear includes inside and outside of jacket and helmet, all from one firefighter. xx Related ($90%) to nasal S aureus isolate 327. {{ Novel ST types.

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Table 3. Genotypic characteristics of MRSA isolates from frequently touched fire station surfaces


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Table 4. Genotypic characteristics of MRSA and S aureus isolated from nasal swabs Isolate MRSA (n 5 9) 300, 340 312 301, 305 305 309, 325 303 341 S aureus (n 5 3) 313 324 327

Station

PFGE type

ST

Genotype

A Floater B, D D A, C C A

H* Iy Cz C S ND ND

5 15 30 30 30 256 291

1 msr(A); 1 none msr(A) erm(C) msr(A) 1 msr(A); 1 erm(A) msr(A) None

A A E

H ND Px

5 34 97

msr(A) None msr(A)

ND, not done. *Related ($90%) to surface isolates SNS2100-2359, SNS2100-2392, and SNS21002258. y Related ($90%) to surface isolates RVR800-921, RVR800-951, RVR800-1087, RVR800-1125, SNWC1, and SNS2100-2297. z Related ($90%) to surface isolates RVR800-1151, SNS 500-445, and RVS2700-2992. x Related ($90%) to surface isolate SNS2100-2218.

strains. Of these strains, SNS2100-2313 and SNS21002380 had novel ST types not previously characterized (PFGE patterns Q and R). The remaining 41 MRSA isolates were analyzed by PFGE and classified into 19 different strains (A-S) based on MLST type and PFGE pattern (Table 3). Five PFGE patterns (A, F, J, K, and N) had a single isolate represented in the study. The remaining PFGE patterns had multiple isolates identified on multiple surface samples. Four PFGE patterns were genetically related to 5 of the MRSA and 2 of S aureus isolates obtained from the fire personnel nasal swabs (Tables 3 and 4). Four isolates found on medic truck surfaces at the first sampling, were PFGE pattern B and USA300. These isolates were SCCmec IV and MLST type ST8, one of the most frequently found MRSA MLST clones associated with CA-MRSA in North America.22 Two isolates (both pattern E) obtained from the medic truck and garage floor near the medic truck were SCCmec type II. Two isolates of pattern M were found in the medic and fire trucks. The 3 isolates with pattern O came from the bunk gear, pants, jacket, and helmet of a single firefighter. The remaining 6 PFGE pattern isolates (C, D, G, H, I, and L) were found on truck/engine and garage and living quarters surfaces in both fire stations at both the first and second samplings. Three PFGE pattern C isolates included two isolates that were SCCmec type IV, one found on a computer desk at one station and the other found on a computer keyboard located in the kitchen of the other station at the first sampling. A third isolate that was genetically related but NT was isolated from a computer desk at the second sampling. All 3 of these isolates were ST30, which is a widely dispersed clone of MRSA.22 Nine isolates with PFGE pattern D

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included one SCCmec type IV isolate obtained from the kitchen and one obtained from the bathroom. Of the 2 isolates with PFGE pattern G, one was obtained from the garage water cooler at the first sampling and the other was obtained from a bed at the second sampling. The 3 pattern H isolates were obtained from the bathroom and kitchen and from nasal cultures from 2 personnel (Tables 3 and 4). The 6 pattern I isolates were found on a fire engine gas pedal, outer fire gear, the medic truck, and a bathroom from station 1 and in the washing machine of station 2 at the first sampling, and in the kitchen of station 2 at the second sampling. The two pattern L isolates were found in the medic truck and kitchen. None of the environmental isolates carried the PVL gene.

Nasal colonization with MRSA Nine MRSA-positive nasal cultures (22.5%) were from fire personnel (representing 4 different stations [stations A-D] and a floater who moved among fire stations within the same district) (Table 4). All 9 strains were NT, and 7 (78%) were macrolide resistant. However, unlike the environmental surface samples, none of the nasal isolates were tetracycline resistant (Tables 3 and 4). Seven of the isolates were MLST type ST5, ST15, or ST30, which also were found among the MRSA strains isolated from environmental surfaces in both fire stations. Five of these isolates were .90% related to environmental isolates with PFGE pattern C, H, or I (Tables 3 and 4). Nasal isolates 309 and 325 were highly related to each other, but were obtained from personnel assigned to different fire stations. Nasal isolates 300, 340, 312, 301, and 305 were related to environmental MRSA strains, whereas nasal isolates 303 and 341 had ST types not found in the environmental samples and were not related to environmental MRSA isolates (Table 4). None of the isolates was PVL-positive. Of the 3 S aureus isolates recovered from nasal samples, 2 were from station A (Table 4). One was .90% related to MRSA nasal isolates 300 and 340, as well as to environmental MRSA SNS2100-2258, SNS2100-2359, and SNS2100-2392 isolated from kitchen and bathroom surfaces. The second isolate, 327, was .90% related to MRSA isolate SNS21002218 obtained from the medic truck. The third isolate did not have the same ST type as any of the other MRSA isolates and was not analyzed by PFGE, given that it likely was not related to any of the surface MRSA isolates identified. Both of these S aureus isolates carried the msr(A) gene, making this the most commonly carried macrolide-resistance gene among the MRSA/S aureus nasal isolates. In addition, like the nasal MRSA isolates, the nasal S aureus isolates were tetracycline susceptible.

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DISCUSSION This is the first study to molecularly characterize MRSA isolates from fire station environmental surfaces, the first to sample both fire station surfaces and personnel, as well as one of the first to characterize non–health care environmental MRSA. In this study, 6.5% of the environmental samples were MRSApositive (4.1%) and/or MSSA-positive (2.4%), whereas the majority (.80%) of the environmental samples were positive for CoNS or MRCoNS. The percentage of MRSA-positive isolates was similar in the samples obtained from fire apparatus (medic trucks, fire engines, fire trucks) and the garage and in the samples obtained from living quarters (57% vs 43%). This finding differs from the results of the Tucson study, in which no fire apparatus samples were found to be MRSA-positive.8 However, other studies have found MRSA in ambulances in Maine and Colorado and in the United Kingdom (http://news.bbc.co.uk/1/hi/wales/ 4213670.stm).9,10 Fire personnel interact with both hospital and community populations as part of their job, and thus face potential exposure to MRSA from both sources. MRSA USA300 isolates, currently the major cause of CAMRSA in North America, were identified in samples obtained from medic trucks and kitchen surfaces in both stations. MRSA SCCmec type II isolates, commonly found in the hospital, also were identified, demonstrating that both community-like and hospital-like MRSA can contaminate fire station surfaces. The isolation of the same strain in the fire apparatus and garage and in the living quarters suggests possible transmission of MRSA between these two areas. Some 30% of the nasal cultures obtained from personnel from 13 fire stations, floaters, and headquarters personnel were positive for MRSA (9 samples) or S aureus (3 samples). The majority (58%) of the nasal MRSA and S aureus isolates were genetically related to environmental surface isolates, suggesting possible transmission between personnel and the environmental surfaces. These results differ from those found in a previous study of paramedics in Kansas, where 54% of nasal cultures were S aureus, but only 10% of S aureus cultures were MRSA.14 However, this earlier study was conducted 8 years ago, and the prevalence of CAMRSA in the United States has increased since that time.3,4,11 Clearly, more research is needed to determine whether our findings are representative of surfaces and personnel in fire stations throughout the country. Funding for this work was provided by the Washington State Safety and Health Investment Projects and by Snohomish County Fire District 1. We thank the Fire District’s Advisory Board for facilitating access to the fire stations and for their advice throughout the project.

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