Highly selective medium for isolation of Listeria monocytogenes from food

A new selective medium (Al-Zoreky-Sandine listeria medium [ASLM]) was formulated to recover Listeria monocytogenes from food specimens; the medium completely inhibited common food microflora. Recognition of Listeria colonies is evident by black discoloration of the medium due to esculin hydrolysis without need for special illuminating equipment. The medium contains acriflavin, ceftazidime, and moxalactam as selective agents. Compared with Listeria Selective Agar, ASLM was equally effective in recovering L. monocytogenes. However, ASLM inhibited micrococci, enterococci, and gram-negative bacteria, especially a strain that mimicked L. monocytogenes on Listeria Selective Agar. The new medium was able to recover heat injured cells with only 15% less count than the nonselective medium.     

A selective differential medium for the isolation of Listeria monocytogenes

A new medium has been developed for the isolation of Listeria monocytogenes from clinical specimens with a mixed flora. Almost complete inhibition of unwanted organisms was achieved and recognition of colonies of Listeria spp. was usually possible after 24 h using the aesculin-ferric ammonium citrate indicator system. Compared to McBride agar the new medium was more inhibitory to representative contaminating species in pure culture and more successful in isolating small numbers of L. monocytogenes from artificially seeded clinical specimens.     

An improved selective isolation medium for the recovery of Listeria monocytogenes from smoked fish

AIMS: The aim of this study was to improve the selective isolation of Listeria monocytogenes from smoked haddock fillets. METHODS AND RESULTS: Listeria selective agar (LSA)--Oxford formulation was supplemented with 25 microg x ml(-1) of colistin sulphate and 30 microg x ml(-1) of nalidixic acid. Inocula from four smoked haddock fillets produced colonies (approx. 2-13 bacteria x g(-1)), identified as L. monocytogenes, on LSA supplemented with antimicrobial compounds (MLSA). Moreover, there was only negligible evidence of bacteria which were not L. monocytogenes on MLSA. In contrast, LSA supported dense bacterial growth, which was not equated with L. monocytogenes. SIGNIFICANCE AND IMPACT OF THE STUDY: The modified medium permitted the recovery of L. monocytogenes from smoked haddock fillets and reduced the growth of contaminating bacteria.     

Improved Listeria monocytogenes selective agar.

By increasing the LiCl concentration to 5 g/liter and adding 20 mg of moxalactam per liter to modified McBride agar base, it was possible to inhibit the growth of many bacteria which interfered with the recovery of Listeria monocytogenes from beef.     

Optimization of haemolysis in enhanced haemolysis agar (EHA)_a selective medium for the isolation of Listeria monocytogenes

The presence of Listeria monocytogenes in enrichment media can be masked by faster growth of other Listeria spp. Therefore, enhanced haemolysis agar (EHA) is a good alternative for another isolation media, because the presence of a few L. monocytogenes colonies can be detected in a majority of colonies of other listeriae on the basis of haemolysis. In this study the haemolysis reaction in EHA was optimized. In a collaborative study using reference samples, no significant differences in counts on EHA, Palcam and Oxford agar were shown.     

Evaluation of BBL CHROMagar Listeria agar for the isolation and identification of Listeria monocytogenes from food and environmental samples

The performance of BBL CHROMagar Listeria chromogenic agar for the detection of Listeria monocytogenes was evaluated for its ability to isolate and identify L. monocytogenes from food and environmental samples. The medium was compared to non-chromogenic selective agars commonly used for Listeria isolation: Oxford, Modified Oxford, and PALCAM. BBL CHROMagar Listeria had a sensitivity of 99% and 100% for the detection of L. monocytogenes from 200 natural and artificially inoculated food samples, respectively, with a colony confirmation rate of 100%. The sensitivity of non-chromogenic selective media for the detection of L. monocytogenes from these same samples was 97-99% with colony confirmation rates of 65-67.5%. From 93 environmental samples, BBL CHROMagar Listeria agar results correlated 100% with a Listeria spp. visual immunoassay (TECRA) performed on these same samples and the USDA-FSIS standard culture method for the isolation of L. monocytogenes. From environmental samples, the L. monocytogenes confirmation rate was 100% for BBL CHROMagar Listeria as compared to 50% for conventional agars tested. On BBL CHROMagar Listeria, L. monocytogenes forms a translucent white precipitation zone (halo) surrounding blue-pigmented colonies of 2-3 mm in diameter, with an entire border. BBL CHROMagar Listeria offers a high degree of specificity for the confirmation of suspect L. monocytogenes colonies, whereas non-chromogenic selective agars evaluated were not differential for L. monocytogenes from other Listeria species.     

Performance of a new chromogenic plating medium for the isolation of Listeria monocytogenes from marine environments

AIMS: This study investigated the performance of a new chromogenic plating medium for the detection of Listeria monocytogenes from naturally contaminated samples obtained from marine environments in Morocco in comparison with the conventional plating media PALCAM and Oxford. METHODS: A total of 479 marine samples (sea water, sediment and mussels) were collected from 16 littoral sites in the region of Agadir (western centre of Morocco). They were examined for the presence of L. monocytogenes using a slight modification of the standardized French method (AFNOR V 08-055) for the detection of L. monocytogenes from food and three different isolation media: PALCAM, Oxford and a new chromogenic plating medium. RESULTS AND SIGNIFICANCE OF THE STUDY: The Oxford and the new chromogenic plating media were found relatively more efficient than the PALCAM medium for the isolation of L. monocytogenes (chi-square test, P < 0.05) from marine samples. However, the new chromogenic plating medium was significantly more selective for L. monocytogenes (P < 0.005) than the two other isolation media as 87.5% of the suspect colonies on this medium were indeed confirmed through identification of the isolates vs 12.7% for Oxford and only 3.8% for the PALCAM medium.     

Selective-enrichment procedure for isolation of Listeria monocytogenes from fecal and biologic specimens.

A selective-enrichment procedure (SEP) was developed to isolate Listeria monocytogenes from fecal and biologic specimens. This procedure was compared with direct plating with McBride listeria agar and 2-, 4-, and 8-week cold-enrichment procedures in recovering L. monocytogenes from mouse fecal, liver, and brain specimens. Although the SEP occasionally did not isolate the organism from specimens proved positive by the other procedures, the SEP isolated L. monocytogenes from about two and five times as many specimens as the cold-enrichment and direct-plating procedures, respectively.     

免疫磁珠富集技术联合选择性培养基快速检测单增李斯特菌

单增李斯特菌是一种危害极大的食源性致病菌,建立快速及特异的检测方法对于食品安全监控尤为重要。文中联合免疫磁珠与选择性培养基对不同浓度(101~105CFU/mL)单增李斯特菌进行检测,并对3种李斯特菌、金黄色葡萄球菌及副溶血弧菌进行交叉试验;同时模拟食物污染,探索免疫磁珠-平板法检测样品的检测限以及该方法的最快检测时间。结果显示特异性免疫磁珠联合选择性平板法可检出浓度为103CFU/mL及以上的单增李斯特菌;牛奶样品仅需6 h增菌能被检出,检测限为0.7 CFU/mL。联合使用免疫磁珠富集技术与选择性培养基,能在30 h内完成对牛奶样品的检测,较国标法减少38 h以上,且具有同等的灵敏度。     

Glycine-containing selective medium for isolation of Legionellaceae from environmental specimens.

Glycine, at a final concentration of 0.3%, has been shown to be an excellent selective agent for the isolation of Legionellaceae. Stock cultures of Legionella pneumophila were not inhibited on buffered charcoal-yeast extract agar containing the amino acid. Among the other Legionellaceae tested, only one of two strains of L. dumoffii and two of six strains of L. micdadei were appreciably inhibited. This medium permitted the isolation of L. pneumophila from environmental specimens with marked inhibition of many non-Legionellaceae bacteria. The selectivity of the medium was subsequently improved by the incorporation of vancomycin (5 microgram/ml) and polymyxin B (100 U/ml). This selective medium, glycine-vancomycin-polymyxin B agar, should facilitate the recovery of Legionellaceae from environmental sources.     

Selective-enrichment procedure for isolation of Listeria monocytogenes from fecal and biologic specimens

A selective-enrichment procedure (SEP) was developed to isolate Listeria monocytogenes from fecal and biologic specimens. This procedure was compared with direct plating with McBride listeria agar and 2-, 4-, and 8-week cold-enrichment procedures in recovering L. monocytogenes from mouse fecal, liver, and brain specimens. Although the SEP occasionally did not isolate the organism from specimens proved positive by the other procedures, the SEP isolated L. monocytogenes from about two and five times as many specimens as the cold-enrichment and direct-plating procedures, respectively.     

Selective plating medium for quantitative recovery of food-borne Listeria monocytogenes.

A new plating medium (lithium chloride-ceftazidime agar [LCA]) was designed to quantitatively recover food-borne Listeria monocytogenes in the form of large colonies while inhibiting most other food-borne microorganisms. This medium included brain heart infusion agar as the nutritive agar base and a combination of selective agents (LiCl, glycine anhydride, and ceftazidime). Comparison of LCA and lithium chloride-phenylethanol-moxalactam agar (LPM) indicated that both were equally effective for the enumeration of the cold-tolerant pathogen in artificially and naturally contaminated foods. However, LCA was more effective than LPM in the recovery of sublethally heat-injured cells. Moreover, Listeria colonies on LCA exhibited a more distinct bluish hue than those on LPM when viewed by the Henry oblique transillumination technique.     

Listeria monocytogenes isolation from human faecal specimens: experiments with the selective media, PALCAM and L-PALCAMY

The selective media PALCAM and L-PALCAMY were evaluated for their potential ability to detect Listeria monocytogenes in faeces. Recovery on PALCAM was almost total, and similar at 30°C and 35°C with or without CO₂ incubation. Warm enrichment in L-PALCAMY was necessary in order to detect low numbers (<10²/ml faeces). Faeces in excess of 0.25 ml/10 ml L-PALCAMY was inhibitory. The results point to L-PALCAMY and PALCAM as an epidemiological tool.