Synergism of preservative system components: Use of the survival curve slope method to demonstrate anti-Pseudomonas synergy of methyl paraben and acrylic acid homopolymer\/copolymers in vitro
D. S. Orth , C. M. Lutes Anderson , D. K. Smith , S. R. Milstein

Synopsis

The survival curve slope method allows determination of synergy in multicomponent systems when the slope (i.e., rate of death of the population of test organisms) is a larger negative number than the sum of the slopes of each of the components. This method was used to demonstrate anti- Pseudomonas synergy of methyl paraben (MP) and acrylic acid homopolymer\/copolymers in vitro .

Preservative efficacy testing of nonionic lotions containing 0.2% MP and 0.2% acrylic acid homopolymer\/copolymers revealed anti- Pseudomonas synergy against P. aeruginosa, P. putida, P. fluorescent , and P. stutzeri . Addition of 0.1% CaCl ₂ to the lotion caused significant increases in D-values and eliminated the anti- Pseudomonas synergy.

Similar patterns of synergy were observed in lotions containing 0.2% MP + 0.2% carbomer 934, 941 or acrylates\/C10-30 alkyl acrylate cross polymer (1342) and in tap water containing 0.2% MP + 0.01% Na ₂ EDTA. The anti- Pseudomonas synergy observed with MP and neutralized acrylic acid homopolymer\/copolymers is probably related to chelation of divalent metal ions and similar to permeabilization synergy reported for preservative action by EDTA.

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