Intrinsic and Acquired Methicillin-Resistance Detection in Staphylococcus aureus and Its Relevance in Therapeutics

Staphylococcus aureus (S. aureus) is a bacterium responsible for a variety of clinical conditions, ranging from skin diseases and pneumonia to bloodstream infections. Methicillin-resistant S. aureus (MRSA) is a perpetual concern in medical settings due to its resistance to many antibiotics; accordingly, the periodic surveillance of antibiotic susceptibility patterns is an everlasting concern in the clinical arena.
This quantitative study aimed to detect intrinsic and acquired methicillin resistance precisely, testing for the effect of beta-lactamase inhibitors and assessing the antibiotic sensitivity patterns of clinical isolates for S. aureus.
One hundred and five isolates of S. aureus were ascertained for their resistance toward methicillin by agar screening and agar dilution (minimal inhibitory concentration [MIC]) methods. Acquired resistance was detected in methicillin-resistant isolates by the addition of beta-lactamase inhibitors. The tolerance phenomenon in these isolates was determined by a ratio of their minimal bactericidal concentration (MBC) over their MIC.
Methicillin resistance was detected in 38% of S. aureus isolates, confirmed by the agar dilution method and MIC > 8 µg/mL (intrinsic resistance). These isolates were mostly collected from ICU burn patients (32.5%). No isolate was found to have acquired resistance or modified (MOD) type resistance. Tolerance was not observed in any isolate. Antibiotic susceptibility toward other therapeutic agents showed that all intrinsic methicillin-resistant isolates were susceptible to vancomycin; however, methicillin-susceptible isolates were the least sensitive (30%) to penicillin.
Methicillin resistance is a specialized concern in clinical studies, and it is mandatory to detect this mechanism by an accurate method that is updated periodically. Increasing the resistance of S. aureus to multiple antibiotics underlines the need for intermittent resistance surveillance.