Influence of Protein Type on the Antimicrobial Activity of LAE Alone or in Combination with Methylparaben

Abstract

The cationic surfactant Lauric arginate (LAE) has gained approval for utilization in meat products (limit: 200 mg/kg). However, as for other antimicrobials, its activity is reduced when applied to complex food matrices. The current study therefore aims to better understand protein-antimicrobial agent-interactions and their influence on the antimicrobial activity of (i) LAE and (ii) methylparaben against Listeria innocua and Pseudomonas fluorescens in defined model systems (pH 6). Antimicrobials were utilized alone or in combination with nutrient broth containing either no protein or 2% bovine serum albumin, whey protein isolate, or soy protein hydrolysate. LAE was found to form complexes with all proteins due to electrostatic attraction, determined using microelectrophoretic and turbidity measurements. Minimal lethal concentrations of LAE were remarkably increased (4-13 fold) in the presence of proteins, with globular proteins having the strongest impact. Combinations of LAE (0-200 µg/mL) with the less structure-sensitive component methylparaben (approved concentration 0.1%) remarkably decreased the concentrations of LAE needed to strongly inhibit or even kill both, L. innocua and P. fluorescens in the presence of proteins. The study highlights the importance of ingredient interactions impacting microbial activity that are often not taken into account when examining antimicrobial components having different structure sensitivities.

Keywords: Lauric arginate; cationic surfactant; food antimicrobials; methylparaben; protein interaction; structure sensitivity.

Figure 1

Viable counts of L. innocua LTH3096 (A) and P. fluorescens DSM50091 (B) determined in a nutrient broth in the absence or presence of 2% bovine serum albumin (BSA), whey protein isolate (WPI), or soy protein hydrolysate (SPH), and 0%–0.15% methylparaben after 12 days of storage at 4 °C; all solutions have been adjusted to pH 6.0 ± 0.1; initial inoculation level: ~10³ CFU/mL.

Figure 2

Growth behavior of L. innocua LTH3096 (A, A1) and P. fluorescens DSM50091 (B, B1) in a nutrient broth examined over 12 days of storage at 4 °C in the presence of 2% whey protein isolate (WPI; A and B) or 2% soy protein hydrolysate (SPH; A1 and B1) and 0–200 µg/mL LAE (applied as LAE/Tween 80 solution, ratio 1:1); all solutions have been adjusted to pH 6 ± 0.1; initial inoculation level: ~10³ CFU/mL.

Figure 3

Influence of LAE (applied as LAE/Tween 80 solution, ratio 1:1) concentration (0–250 µg/mL) on the Zeta potential of solutions containing either 2% bovine serum albumin (BSA), whey protein isolate (WPI), or soy protein hydrolysate (SPH) at pH 6 ± 0.1.

Figure 4

Dependency of sample turbidity on complex formation between 2% bovine serum albumin (BSA), whey protein isolate (WPI), or soy protein hydrolysate (SPH) and 0–1000 µg/mL LAE (applied as LAE/Tween 80 solution, ratio 1:1) at pH 6 ± 0.1. Sample turbidity was recorded directly after mixing and again after 24 h of incubation at 4 °C.

Figure 5

Growth behavior of L. innocua LTH3096 (A, A1) and P. fluorescens DSM50091 (B, B1) in a nutrient broth containing either 2% bovine serum albumin (BSA; A and B) or 2% soy protein hydrolysate (SPH; A1 and B1) and 0.1% methylparaben. Growth was examined over 12 days of storage at 4 °C in presence of 0–200 µg/mL LAE (applied as LAE/Tween 80 solution, ratio 1:1); all solutions have been adjusted to pH 6 ± 0.1; initial inoculation level: 10³ CFU/mL.

Figure 6

Graphical abstract: Mechanistic insights in LAE–protein interactions and their consequences on the antimicrobial activity of structure-sensitive LAE in combination with less structure-sensitive methylparaben against Gram-positive and Gram-negative bacteria.