pH-dependent oxacillin tolerance of Staphylococcus aureus

Area of Science:

• Microbiology
• Antimicrobial Resistance
• Bacteriology

Background:

• A subset of Staphylococcus aureus clinical isolates displays tolerance, characterized by resistance to antibiotic bactericidal activity despite normal susceptibility to growth inhibition.
• Existing laboratory methodologies for assessing bacterial tolerance vary, potentially impacting study outcomes and clinical relevance.
• Understanding the factors influencing tolerance detection is crucial for accurate antimicrobial susceptibility testing and effective treatment strategies.

Purpose of the Study:

• To investigate the impact of different laboratory methodologies on the detection of Staphylococcus aureus tolerance.
• To clarify the relationship between inoculum preparation methods and the observed phenomenon of bacterial tolerance.
• To explore the potential role of culture pH in inducing or influencing bacterial tolerance in Staphylococcus aureus.

Main Methods:

• Determined minimal bactericidal concentrations (MBCs) and minimal inhibitory concentrations (MICs) for 20 clinical Staphylococcus aureus isolates using two distinct inoculum preparation methods.
• Method 1: Inocula prepared by 3-hour growth in Mueller-Hinton broth (mean pH 7.15).
• Method 2: Inocula prepared by overnight (22-24 hour) incubation in Trypticase soy broth (mean pH 6.22). Colony counts and pH were assessed for all inocula. Tolerance was defined as MBC/MIC ratio ≥ 100.

Main Results:

• Using the 3-hour Mueller-Hinton broth method, none of the 20 isolates exhibited tolerance.
• Employing the overnight Trypticase soy broth method, 35% (7 of 20) of the isolates demonstrated tolerance.
• A significant association (P < 0.01) was observed between overnight incubation in Trypticase soy broth and decreased bactericidal activity at high oxacillin concentrations, indicating pH-induced tolerance.

Conclusions:

• Laboratory methodology, specifically inoculum preparation and incubation time, significantly influences the detection of tolerance in Staphylococcus aureus.
• Overnight incubation in Trypticase soy broth, resulting in a lower pH, is associated with a higher prevalence of detected tolerance.
• The findings suggest that pH-induced tolerance in staphylococci may have clinical implications in sequestered infections characterized by acidic environments.