Impact of acid adaptation on attachment of Listeria monocytogenes to stainless steel during long-term incubation under low or moderate temperature conditions and on subsequent recalcitrance of attached cells to lethal acid treatments

Strain Variability in Growth and Thermal Inactivation Characteristics of Listeria monocytogenes Strains after Acid Adaptation

ABSTRACT

Given the importance of strain variability to predictive microbiology and risk assessment, this study aimed to quantify the magnitude of strain variability in growth and thermal inactivation kinetics behaviors after acid adaptation. Thirty-three Listeria monocytogenes strains were exposed to acid-adapted tryptic soy broth supplemented with yeast extract (TSBYE; pH 5.5) and non-acid-adapted TSBYE (pH 7.0) for 20 h. Next, the growth parameters of these adapted and nonadapted strains that grew in nonbuffered TSBYE at 25°C were estimated. The tested strains were inactivated at 60°C in nonbuffered broth to obtain the heat resistance parameters. The results revealed that strain variability was present in the growth and thermal inactivation characteristics. The maximum specific growth rate ranged from 0.21 to 0.44 h-1 and from 0.20 to 0.45 h-1 after acid and nonacid adaptation, respectively. The lag times were from 0.69 to 2.56 h and from 0.24 to 3.36 h for acid-adapted and non-acid-adapted cells, respectively. The apparent D-values at 60°C of the pathogen ranged between 0.56 and 3.93 min and between 0.52 and 3.63 min for the presence and absence of acid adaptation condition, respectively. Acid adaptation significantly (P < 0.05) increased the magnitude of strain variability in the thermal inactivation characteristics of L. monocytogenes, with the coefficient of variation increasing to 0.17, whereas acid adaptation did not significantly (P ≥ 0.05) influence the variabilities in the growth parameters of the tested strains. Furthermore, the subsequent growth behaviors of all strains did not exhibit significant (P > 0.05) changes after exposure to acidic broth. However, the thermal resistance of most (25 of 33) of the tested strains increased (P < 0.05) after growing in acid-adapted broth. The relevant data generated in the present study can be used to describe the strain variability in predictive microbiology and to deeply understand the behavioral responses of different strains to acid adaptation.

HIGHLIGHTS

Advanced Killing Potential of Thymol against a Time and Temperature Optimized Attached Listeria monocytogenes Population in Lettuce Broth

Fresh vegetables and salads are increasingly implicated in outbreaks of foodborne infections, such as those caused by Listeria monocytogenes, a dangerous pathogen that can attach to the surfaces of the equipment creating robust biofilms withstanding the killing action of disinfectants. In this study, the antimicrobial efficiency of a natural plant terpenoid (thymol) was evaluated against a sessile population of a multi-strain L. monocytogenes cocktail developed on stainless steel surfaces incubated in lettuce broth, under optimized time and temperature conditions (54 h at 30.6 °C) as those were determined following response surface modeling, and in comparison, to that of an industrial disinfectant (benzalkonium chloride). Prior to disinfection, the minimum bactericidal concentrations (MBCs) of each compound were determined against the planktonic cells of each strain. The results revealed the advanced killing potential of thymol, with a concentration of 625 ppm (= 4 × MBC) leading to almost undetectable viable bacteria (more than 4 logs reduction following a 15-min exposure). For the same degree of killing, benzalkonium chloride needed to be used at a concentration of at least 20 times more than its MBC (70 ppm). Discriminative repetitive sequence-based polymerase chain reaction (rep-PCR) also highlighted the strain variability in both biofilm formation and resistance. In sum, thymol was found to present an effective anti-listeria action under environmental conditions mimicking those encountered in the salad industry and deserves to be further explored to improve the safety of fresh produce.

Effect of Atmospheric Pressure Plasma on Listeria monocytogenes Attached to Abiotic Surfaces

Listeria monocytogenes can be introduced into food processing plants via raw material of animal or plant origin and can establish endemic populations through formation of biofilms. Biofilms are a continuous source of contamination for food products, and L. monocytogenes cells in biofilms are more resistant to stress and sanitizing agents than are planktonic cells. The use of gas-discharge plasmas may offer a feasible alternative to conventional sanitization methods. Plasmas are a mixture of charged particles, chemically reactive species, and UV radiation and can be used to destroy microorganisms. The purpose of this study was to measure the effectiveness of cold atmospheric pressure plasma (APP) treatments against bacteria attached to a solid surface and to evaluate the individual susceptibility of various L. monocytogenes strains. Attention was focused on the state of the cells after treatment, combining detection by viable counts and quantitative PCR (qPCR). Most of the culturable cells were inactivated after APP treatment, but the qPCR assay targeting the 16S rRNA revealed the presence of injured cells or their entrance into the viable but nonculturable state. These results were at least partly confirmed by a resuscitation experiment. After APP treatment, L. monocytogenes cell suspensions were incubated in brain heart infusion broth; some cells grew in the medium and therefore had survived the treatment. An understanding of the effects of APP on L. monocytogenes can inform the development of sanitation programs incorporating APP for pathogen removal. Methods other than those based of the culturability of the cells should be used to monitor pathogens in food processing plants because cultivation alone may underestimate the actual microbial load.

Purification and Characterization of Plantaricin ZJ316, a Novel Bacteriocin against Listeria monocytogenes from Lactobacillus plantarum ZJ316

Bacteriocins are known to be natural preservatives, which are becoming increasingly necessary in many types of food to control the proliferation of pathogenic bacteria. In this study, a novel bacteriocin produced by Lactobacillus plantarum ZJ316, called plantaricin ZJ316, was purified by ammonium sulfate precipitation, gel chromatography, and high-performance liquid chromatography. By mass spectrometry, the molecular mass of plantaricin ZJ316 was determined to be 2,366.06 Da. No homologous sequences were found in databases based on comparisons with the N-terminal amino acid sequencing. The bacteriocin was heat resistant and stable after incubation at pH 2.0 to 10.0. It was sensitive to α-chymotrypsin, trypsin, and proteinase K. Plantaricin ZJ316 had a broad inhibitory activity against gram-negative and gram-positive bacteria, especially Listeria monocytogenes. Our results suggested that this bacteriocin has the potential to inhibit pathogenic bacteria in food products.

Performance of stress resistant variants of Listeria monocytogenes in mixed species biofilms with Lactobacillus plantarum

Population diversity and the ability to adapt to changing environments allow Listeria monocytogenes to grow and survive under a wide range of environmental conditions. In this study, we aimed to evaluate the performance of a set of acid resistant L. monocytogenes variants in mixed-species biofilms with Lactobacillus plantarum as well as their benzalkonium chloride (BAC) resistance in these biofilms. L. monocytogenes LO28 wild type and acid resistant variants were capable of forming mixed biofilms with L. plantarum at 20°C and 30°C in BHI supplemented with manganese and glucose. Homolactic fermentation of glucose by L. plantarum created an acidic environment with pH values below the growth boundary of L. monocytogenes. Some of the variants were able to withstand the low pH in the mixed biofilms for a longer time than the WT and there were clear differences in survival between the variants which could not be correlated to (lactic) acid resistance alone. Adaptation to mild pH of liquid cultures during growth to stationary phase increased the acid resistance of some variants to a greater extent than of others, indicating differences in adaptive behaviour between the variants. Two variants that showed a high level of acid adaptation when grown in liquid cultures, showed also better performance in mixed species biofilms. There were no clear differences in BAC resistance between the wild type and variants in mixed biofilms. It can be concluded that acid resistant variants of L. monocytogenes show diversity in their adaptation to acidic conditions and their capacity to survive in mixed cultures and biofilms with L. plantarum.

Acquired acid adaptation of Listeria monocytogenes during its planktonic growth enhances subsequent survival of its sessile population to disinfection with natural organic compounds

The abilities of Listeria monocytogenes to adapt to stress and also to attach to surfaces are believed to contribute to its persistence in food processing environments. In this study, the possible influence of acid adaptation of L. monocytogenes cells during their planktonic growth on their subsequent resistance against 3 different natural disinfection treatments (0.05% lactic acid, 1% essential oil or 100% hydrosol of S. thymbra) upon their attachment to stainless steel (SS) was evaluated by simultaneously using the bead vortexing technique and a conductance method. Prior to disinfection, both nonadapted and acid-adapted stationary phase bacteria were left to attach to SS coupons statically incubated for 10days into Brain Heart Infusion (BHI) broth under 3 different temperatures (5, 16 or 30°C). Results revealed that both essential oil and hydrosol presented sufficient bactericidal activity against all formed sessile populations, always resulting in counts following disinfection below the plate counting detection limit. However, the conductance method, able to detect metabolically active sessile bacteria unable to be recovered by the bead vortexing, revealed the positive influence of previously acquired acid adaptation on disinfection resistance of attached cells against these plant extracts. A similar effect of acid adaptation was also evident for lactic acid disinfection.