Evaluation of a Cost Effective Broth and Selective Agar Combination for the Detection of MRSA and Staphylococcus aureus from Surveillance Specimens Using Regular Workflow

Abstract

OBJECTIVE: To evaluate the use of selective agar and broth combination in a regular laboratory daily workflow.

DESIGN: Swabs from 173 surveillance specimens were inoculated onto half of the Bio-Rad MRSASelect (M), SaSelect (S) and Sheep Blood agars (SBA) and the swab placed in the LIM broth. After overnight incubation, 10 μL of the LIM broth was inoculated onto the other half of the three agars and incubated overnight. All the agars were examined and worked up after approximate14-18 hours of incubation for day one and two according to the regular workflow of the laboratory, without incubating for the full 24 hours for each incubation day. M agar and SBA were evaluated for Methicillin-Resistant Staphylococcus aureus (MRSA), while the S agar was evaluated for Staphylococcus aureus (SA) based on typical colony morphology development. Colonies on the SBA were picked and processed for definitive identification and cefoxitin susceptibility result.

SETTING: Trinity Medical Center, a community hospital with network hospitals

PATIENTS: Patient admitted to the hospital submitted swab for surveillance culture

RESULTS: There were a total of 29 MRSA isolated in the study. On day one, both M agar and SBA detected 14 MRSA (48.3%) and on day two, M agar detected 10 (82.7%), while SBA detected 8 (75.8%) additional MRSA. LIM broth added 5 more MRSA to both agars on day 2, to give M agar a total of 29 (100%) and SBA agar a total of 27 (93.1%) of MRSA from the 173 specimens.

There were a total of 62 SA isolated. Both the S agar and SBA isolated 34 (54.8%) on day one and 15 more (79%) on day two. The LIM broth added an additional 13 SA for both agars on day two.

CONCLUSION: Using half of the agar plate for the initial swab and the other half for the broth creates an economic strategy for the detection of MRSA using the M agar and SA using the S agar. Both the M and S agars provided excellent identification and recovery of MRSA or SA based on color and colony morphology unless the colony was too young for color development. The color morphology from the M and S agars was distinguishable overnight after being subcultured from LIM broth. Working up the specimen according to the workflow of the laboratory without having to wait for each plate to incubate a full 24 hours, can still detect all the targeted organisms within 2 workdays using this cost effective strategy.

ABBREVIATIONS: MRSA = Methicillin resistant Staphylococcus aureus, MSSA = Methicillin susceptible Staphylococcus aureus, SA = Staphylococcus aureus