Synergism versus Additivity: Defining the Interactions between Common Disinfectants

Area of Science:

• Microbiology
• Antimicrobial resistance
• Formulation science

Background:

• Many commercial disinfectants combine multiple antimicrobial agents.
• Claims of synergistic formulations are frequent, but evidence is often lacking.

Purpose of the Study:

• To investigate the antimicrobial interactions of pairwise combinations of common disinfectants.
• To determine if synergistic effects are prevalent in disinfectant formulations.

Main Methods:

• Checkerboard assays were used to classify antimicrobial interactions.
• Time-kill analyses validated the findings from checkerboard assays.
• Combinations were tested against four common bacterial pathogens: Acinetobacter baumannii, Enterococcus faecalis, Klebsiella pneumoniae, and Staphylococcus aureus.

Main Results:

• Synergistic interactions were observed only for chlorocresol with benzalkonium chloride and chlorocresol with polyhexamethylene biguanide.
• Synergism was borderline and not consistently observed across all bacterial species tested.
• Most disinfectant combinations exhibited additive antimicrobial effects rather than synergistic ones.

Conclusions:

• Synergism between common disinfectants is uncommon and context-dependent.
• The broad mechanisms of action for disinfectants limit opportunities for true synergism.
• Accurate classification of additive versus synergistic interactions is vital for regulatory compliance and product development.