Enumeration Of Listeria Monocytogenes Research Articles

Comparison of Selective Direct Plating Media for Enumeration and Recovery of L. monocytogenes from Cold-process (smoked) Fish

Three selective media were evaluated for direct plating recovery and enumeration of Listeria monocytogenes in the presence of high levels of a variety of microorganisms occurring on cold-process (smoked) salmon products. Sliced salmon was brined to contain either no added salt, 3, or 5% water-phase NaCl, or 3 or 5% NaCl plus 140 ppm NaNO2. The slices were packaged in oxygen-permeable film or sealed under vacuum in oxygen-impermeable film, and stored at 10°C or 5°C until total microbial loads reached 106 to 109 CFU/g. Oxford formulation of Listeria selective agar and Lee's modification of Listeria selective agar achieved quantitative recovery of 102 cells per ml of L. monocytogenes strain Scott A in the presence of diluted slurries of these fish containing 104 to 108 CFU/ml of background organisms. A modification of lithium chloride-phenylethanol-moxalactam agar containing an iron-esculin indicator system sometimes failed because of interfering growth by the background microflora.

Influence of Reconstitution on Isolation and Enumeration of Listeria monocytogenes from Milk Powder Used for Reference Samples

To test the performance of the Listeria isolation methods, reference samples consisting of gelatin capsules filled with spray-dried milk powder containing Listeria have been developed. During the spray-drying process the Listeria cells are exposed to heat stress and are susceptible to osmotic stress during the reconstitution procedure. To limit the effect of osmotic shock, the milk powder has to be encapsulated in gelatin in order to guarantee slow dissolution. Furthermore, the capsules have to be preen-riched in a nonselective medium. The practical consequences of these findings are discussed.

Liquid and solid selective differential media for the detection and enumeration of L. monocytogenes and other Listeria spp.

A selective and differential medium (PALCAM agar) was elaborated for the isolation and enumeration of Listeria monocytogenes. PALCOM is based on Columbia agar with 0.05% glucose made selective by the addition of 0.001% polymyxin B, 0.0005% acriflavin, 1.5% lithium chloride and 0.002% ceftazidime. The diagnostic traits were attained by the incorporation of (i) 0.08% aesculin and 0.05% ferric salt; and (ii) 1% mannitol plus 0.008% phenol red. PALCAM recovered testrains of L. monocytogenes and other Listeria spp quantitatively and suppressed most other bacteria of common occurence in fresh food. L. monocytogenes colonies were approximately 2 mm grey-green with a black sunken centre and a black halo on a cherry-red background. The occasional Enterococcus or Staphylococcus strains developing on the medium gave rise to grey colonies with a brown-green halo or yellow colonies with a yellow halo. PALCAM was the preferred medium out of 13 tested Listeria selective agars in current use. A similar differential enrichment broth, L-PALCAMY was developed based on peptone yeast extract broth with 2.5% egg yolk emulsion. The diagnostic traits and inhibitors used in this medium were the same as in PALCAM agar, through in different concentrations. Growth rate and cellcrop of L. monocytogenes in L-PALCAMY were of the same order as in Columbia broth. The growth of the majority of other bacteria of common occurrence in fresh foods was inhibited. The medium recovered L. monocytogenes more effectively from severely contaminated food than other current enrichment media.

Evaluation of three newly developed direct plating media to enumerate Listeria monocytogenes in foods

LiCl-phenylethanol-moxalactam Agar (LPMA), ARS Modified McBride Agar, and Modified Vogel Johnson Agar were compared with previously tested plating media in the enumeration of Listeria monocytogenes from pasteurized whole milk, chocolate ice cream mix, Brie cheese, and raw cabbage. LPMA was most suitable for analyzing Brie cheese and cabbage. Gum base-nalidixic acid-tryptone-soya medium (previously tested) was most suitable for analyzing milk and chocolate ice cream mix.

Methods and media for the isolation and cultivation of Listeria monocytogenes from various foods

Many methods and media currently exist for the detection and enumeration of Listeria monocytogenes. However, the suitability of any specific method or media is influenced by the purpose of the analysis and the type of food being analyzed. Food which are likely to contain high populations of contaminating microorganisms require highly selective media for enumeration. Moreover, media which contain indicator systems are also helpful in distinguishing L. monocytogenes colonies. In contrast, less selective media may be adequate for less contaminated foods.

Evaluation of ten selective direct plating media for enumeration of Listeria monocytogenes in hams and oysters

The suitability of ten direct plating media for isolating and enumerating four strains of uninjured and injured Listeria monocytogenes from dry- and country-cured hams, and raw oysters was evaluated. Five different lots of each food were inoculated with approximately 10 2 ,10 4 , or 10 5 –10 6 viable L. monocytogenes cells g −1 . Populations of the organism were determined by surface plating appropriate dilutions onto the test media. Inoculated media were incubated at 30 °C for 48 h before L. monocytogenes colonies were counted and confirmed. LiCl Phenylethanol Moxalactam Agar (LPMA) was selected as the most suitable medium for isolating and enumerating L. monocytogenes from both types of ham, while Dominguez Rodriguez Isolation Agar (DRIA) was selected as most suitable for raw oysters. Selection of the most suitable media was based on relative recovery of viable cells as well as ease of recognizing and counting L. monocytogenes colonies.

Enumeration of Listeria monocytogenes in raw milk

Enumeration of Listeria monocytogenes in raw milk was compared by direct plating and Most Probable Number (MPN) techniques involving both direct and two‐stage enrichments. Direct plating was found to lack sensitivity for the enumeration of low numbers of listerias found in milk but an MPN technique with direct selective enrichment was found to be more suitable than one with two‐stage enrichment.

Direct Plating Technique for Enumeration of Listeria monocytogenes in Foods

The advantages and disadvantages of various techniques for detecting and enumerating Listeria monocytogenes in foods are reviewed, and results from a study designed to compare 14 direct plating media for their suitability to recover uninjured cells of L. monocytogenes from 4 foods are summarized. McBride Listeria agar (MLA), gum base nalidixic acid tryptone soy agar (GBNTSA), modified Despierres agar (MDA), and modified MLA (MMLA) performed best for recovering all inoculum populations from milk and ice cream mix. For Brie cheese, MLA, MDA, MMLA, and Dominguez Rodriguez isolation agar were superior for recovering L. monocytogenes; GBNTSA, MDA, MMLA, and Donnelly's Listeria enrichment agar were best for recovering the organism from cabbage. Direct plating procedures without prior enrichment can be utilized successfully for recovering L. monocytogenes from foods such as pasteurized milk and ice cream mix, which contain low populations of background microflora. However, recovery of L. monocytogenes from foods such as raw cabbage and Brie cheese, which contain high populations of other microorganisms, was not satisfactory using direct plating procedures.