Methicillin-resistant Staphylococcus aureus susceptibility testing by an automated system, Autobac I

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat.
• Accurate and rapid antibiotic susceptibility testing is crucial for effective treatment.
• The Autobac I system is an automated method for bacterial susceptibility testing.

Purpose of the Study:

• To evaluate the performance of the Autobac I system in determining the antibiotic susceptibility pattern of MRSA isolates.
• To compare the results obtained from the Autobac I system with the conventional disk diffusion method.
• To assess the impact of modified incubation conditions on Autobac I's detection capabilities for MRSA.

Main Methods:

• MRSA isolates were tested using the Autobac I system and the disk diffusion method.
• Antibiotic susceptibility was evaluated against a panel of common antibiotics.
• Modified environmental conditions (5% NaCl broth, 32°C incubation) were tested to improve MRSA detection.

Main Results:

• The Autobac I system demonstrated high agreement (>96%) with disk diffusion for most antibiotics, except methicillin and clindamycin.
• Disk diffusion indicated resistance to methicillin/oxacillin, penicillin, erythromycin, clindamycin, and kanamycin, and susceptibility to cephalothin, chloramphenicol, tetracycline, and gentamicin.
• Autobac I showed discrepancies for methicillin (70% susceptible, 9% intermediate) and clindamycin (14% susceptible, 7% intermediate).
• Prolonged incubation improved agreement for clindamycin (93%) but not methicillin (44%).
• Altering environmental conditions did not enhance MRSA detection by Autobac I.

Conclusions:

• The Autobac I system is reliable for susceptibility testing of MRSA against most antibiotics but requires optimization for methicillin and clindamycin.
• Further research is needed to improve the accuracy of automated systems like Autobac I for detecting specific resistance mechanisms in MRSA.
• Current modifications to Autobac I incubation and media do not sufficiently improve the detection of methicillin resistance.