Antimicrobial Sub-MIC induces Staphylococcus aureus biofilm formation without affecting the bacterial count

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Antimicrobial Resistance

Background:

• Biofilm formation is a key virulence factor for Staphylococcus aureus (S. aureus), enabling survival in harsh conditions.
• Sub-minimal inhibitory concentrations (sub-MICs) of antibiotics can alter bacterial virulence, impacting pathogenesis and treatment outcomes.
• Standard antimicrobial therapies may not eradicate biofilm-associated infections completely.

Purpose of the Study:

• To investigate the impact of sub-MICs from five different antimicrobial classes on the biofilm-forming capacity of S. aureus clinical isolates.
• To compare the effectiveness of three distinct biofilm quantification techniques in assessing these effects.

Main Methods:

• Tested five antimicrobial agents (azithromycin, gentamicin, ciprofloxacin, doxycycline, imipenem) at varying sub-MIC levels (12.5%, 25%, 50%) on five S. aureus clinical isolates.
• Assessed biofilm formation using crystal violet (CV) staining, quantitative PCR (qPCR), and spread plate method (SPM).

Main Results:

• Crystal violet staining indicated significant biofilm induction in 64% of tested conditions.
• Quantitative PCR and spread plate methods showed significant induction in only 24% and 17.3% of conditions, respectively, suggesting increased biomass without proportional growth.
• Sub-MICs of imipenem and ciprofloxacin showed the highest biofilm-inducing effect (80% significance), while doxycycline, azithromycin, and gentamicin had lower rates.

Conclusions:

• Exposure to sub-MIC doses of various antibiotics can enhance Staphylococcus aureus biofilm formation.
• This enhancement is primarily due to an increase in total biofilm biomass, rather than a significant increase in viable bacterial count.
• The findings underscore the complex relationship between sub-inhibitory antibiotic concentrations and bacterial virulence mechanisms.