A selective and diagnostic medium for use in the enumeration of Listeria spp. in foods

Listeria monocytogenes: diagnostic problems

The first isolation methods for the detection of Listeria spp. were generally based on the direct culture of samples on simple agar media, but isolation of the pathogenic Listeria monocytogenes was difficult. In time, new techniques were developed, based on a variety of selective and elective agents in isolation and enrichment media, which gained better and quicker results. Current reference methods allow the recovery of L. monocytogenes from a variety of foods with relative ease. However, more comparative studies are needed to select one horizontal method. It is suggested that the procedure of the International Organization for Standardization is a good base for such comparisons.

Comparison of selective and non-selective enrichment media in the detection of Listeria monocytogenes from meat containing Listeria innocua

AIMS

This study investigated whether the higher incidence of recovery from meat of Listeria innocua compared with L. monocytogenes could be due to the laboratory media used, leading to an artificially lower detection of the pathogenic species, L. monocytogenes.

METHODS AND RESULTS

Minced beef was inoculated with L. monocytogenes, L. innocua, or a mixture of these species, and stored at 0 or 10 degrees C under vacuum or aerobic conditions for up to 28 days. Listeria were recovered from the minced beef using selective (University of Vermont Medium, UVM) and non-selective (Buffered Peptone Water, BPW) enrichment broths after 0, 14, and 28 days of storage. In general, there were no significant differences (P < 0.05) between the numbers of L. monocytogenes recovered from minced beef samples after 24 h enrichment in BPW and the numbers recovered using UVM. In addition, the presence of L. innocua in meat samples containing L. monocytogenes did not significantly (P < 0.05) affect the numbers of L. monocytogenes recovered using either enrichment broth. In most cases there were no significant differences (P < 0.05) between the numbers of L. innocua recovered from minced beef samples after 24 h enrichment in BPW compared with numbers recovered using UVM.

CONCLUSION

Listeria innocua was found to have no significant competitive advantage over L. monocytogenes in selective or non-selective enrichment media.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results suggest that, in some instances, the use of a selective enrichment broth offers no advantage over a non-selective enrichment broth for the recovery of Listeria species from minced beef.

Thermal resistance of wild-type and antibiotic-resistant Listeria monocytogenes in meat and potato substrates

AIMS

This study aimed to elucidate the relationship, if any, between the acquisition/possession of antibiotic resistance in strains of Listeria monocytogenes and the resistance of such strains to heat stress.

METHODS AND RESULTS

D-values calculated using a linear survival model were used to compare the heat resistance of two wild-type (WT) and two antibiotic (streptomycin)-resistant (AR) mutant strains of L. monocytogenes measured in minced beef and potato substrates at 55 degrees C, with and without prior heat shock at 48 degrees C. In both minced beef and potato, no significant differences (P < 0.05) between D-values of AR and WT strains were noted. Heat shock did not significantly increase D-values of WT or AR strains in minced beef, while in potato slices, D-values in almost all cases were significantly higher in samples which had received heat-shock treatment. In minced beef, the use of a non-selective/overlay recovery medium did not result in higher D-values for any strains, while in potato, significantly higher (P < 0.05) D-values were obtained in most cases.

CONCLUSION

The presence or absence of antibiotic resistance genes did not modulate the heat resistance of the strains examined in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study demonstrated that heat shock, and the type of media used to determine bacterial numbers during heat processing, can significantly affect the D-values obtained.

Risk factors associated with contamination of raw milk by Listeria monocytogenes in dairy farms

A case-control study involving 128 selected dairy farms was conducted to assess the association of several suspected risk factors with the odds of contamination of raw milk by Listeria monocytogenes. Using logistic regression, we found that poor quality of silage (pH > 4.0), inadequate frequency of cleaning the exercise area, poor cow cleanliness, insufficient lighting of milking barns and parlors, and incorrect disinfection of towels between milkings were significantly associated with milk contamination by L. monocytogenes. More attention to preparing silage and good milking and barn hygiene are important for diminishing the risks of exogenous contamination of raw milk by L. monocytogenes.

Detection and prevalence of Listeria monocytogenes in the agricultural ecosystem

The sensitivity of four different enrichment procedures to detect Listeria monocytogenes in the presence of high levels of Streptococcus faecalis was investigated. Defined mixed cultures of Strep. faecalis and L. monocytogenes gave better results with one-stage enrichment techniques. For manure samples, however, two-stage enrichment techniques gave the best performance. The so-called cold enrichment techniques were found to be unsatisfactory for samples from natural environments. The following materials were examined for the presence of L. monocytogenes: fresh pig faeces (16% positive), fresh cattle faeces (20% positive), stored liquid manure (0% positive), manured soil samples (0% positive) and ground water samples (5% positive). After 3 weeks of storage L. monocytogenes could be detected in only one of the initially nine positive fresh faeces samples. Two months after inoculation of stored liquid pig manure, stored liquid cattle manure and soil with L. monocytogenes, this bacterium could not be traced in any of these materials. Radishes (Raphanus sativus) and carrots (Daucus carota), sown in soil inoculated with L. monocytogenes, were gathered after 3 months and examined for the presence of L. monocytogenes. Three of six radish samples were found to be positive. Remarkably, however, all carrot samples (six) were free of L. monocytogenes.

Listeria monocytogenes, a food-borne pathogen

The gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been implicated within the past decade as the causative organism in several outbreaks of foodborne disease. Listeriosis, with a mortality rate of about 24%, is found mainly among pregnant women, their fetuses, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. Epidemiological investigations can make use of strain-typing procedures such as DNA restriction enzyme analysis or electrophoretic enzyme typing. The organism has a multifactorial virulence system, with the thiol-activated hemolysin, listeriolysin O, being identified as playing a crucial role in the organism's ability to multiply within host phagocytic cells and to spread from cell to cell. The organism occurs widely in food, with the highest incidences being found in meat, poultry, and seafood products. Improved methods for detecting and enumerating the organism in foodstuffs are now available, including those based on the use of monoclonal antibodies, DNA probes, or the polymerase chain reaction. As knowledge of the molecular and applied biology of L. monocytogenes increases, progress can be made in the prevention and control of human infection.

Use of the polymerase chain reaction for direct detection of Listeria monocytogenes in soft cheese

The polymerase chain reaction (PCR) amplification technique was investigated as a tool for direct detection of Listeria monocytogenes in soft cheeses. Different sets of oligonucleotide primers were used, and parts of the L. monocytogenes Dth 18-gene could be amplified specifically when either a plasmid vector carrying the cloned gene or chromosomal DNA was used a template. The detection limit for L. monocytogenes in dilutions of pure cultures was between 1 and 10 colony-forming units. In extracts from soft cheeses containing L. monocytogenes DNA, the amplification was strongly inhibited. This inhibition could be reduced by an additional purification step. Despite this the detection limit showed a large variation, depending on the brand of cheese used. In some cheeses 10(3) cfu/0.5g could be visualized whereas in others the presence of 10(8) cfu/0.5 g did not yield a detectable quantity of amplified product.

Simplified Henry technique for initial recognition of Listeria colonies

The Henry oblique transmitted-light viewing technique was modified to provide a more precise, convenient, and familiar manner with which to read (score) and recognize colonies of listeriae by their distinct bluish cast. The simplified technique involved illuminating each colony directly with a high-intensity lamp while viewing it with a hand lens at a precise angle in place of a scanning light microscope.

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.

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. PALCAM 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 test strains of L. monocytogenes and other Listeria spp quantitatively and suppressed most other bacteria of common occurrence 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.

Listeria monocytogenes in foods. Isolation, characterization and control

The intent for examining foods for Listeria monocytogenes, i.e. for surveying, for epidemiological purposes, or to inspect consignments of foods for microbiological safety, determines which analytical method is to be used. For instance resuscitation of debilitated cells may be required, and the degree of accuracy and precision necessary should be considered. Moreover, in the case of acceptance-or-rejection monitoring target values for 'absence' of the pathogen have been amply but not always effectively discussed. Recommendations are given for assessing adequate repair of sublethally damaged populations of L. monocytogenes and for performance testing of selective enrichment and isolation media to be used for the isolation and enumeration of L. monocytogenes. An approach to the empirical assessment of reference values for L. monocytogenes in foods processed for safety is also presented. This relies on a data base of results obtained when examining foods manufactured and distributed according to practices previously validated by longitudinally integrated ('holistic') quantitative risk analysis.

A comparison of two procedures for the isolation of Listeria monocytogenes from raw chickens and soft cheese

A cold enrichment method and a modified FDA procedure were compared for the isolation of Listeria monocytogenes from raw chickens and soft cheese. L. monocytogenes was isolated from a total of 23 of 222 cheese and 70 of 160 chicken samples by either one or both methods. Neither method alone yielded all isolates from the two food types. Only 12 cheese and 13 chicken samples were shown to be positive by both methods, although the serotypes isolated were not always identical. On some occasions one method yielded L. monocytogenes while the other produced a different Listeria sp. Reasons for differences in the performance of the two procedures and various points of technical interest are discussed.

Comparison of seven plating media for enumeration of Listeria spp

The suitability of seven media for the enumeration of Listeria spp. was evaluated at 30 degrees C for 48 h. The media tested were (i) the original McBride Listeria agar formulation (with glycine); (ii) modified McBride agar containing glycine anhydride; (iii) LiCl-phenylethanol-moxalactam (LPM) agar; (iv) acriflavine-ceftazidime agar; (v) Rodriguez isolation agar (RISA); (vi) modified Vogel-Johnson (MVJ) agar; (vii) cyclohexanedione-nalidixic acid-phenylethanol agar; and tryptose agar as control. A total of 66 organisms were used including 11 Listeria monocytogenes strains and 5 other Listeria spp. For L. monocytogenes strains only, all media performed highly similarly. Of the other Listeria spp., only two grew on MVJ agar and three each grew on LPM and RISA. Only LPM agar inhibited the 50 non-listeriae, including five yeasts, while MVJ agar inhibited all but one yeast. The McBride Listeria agar formulation that contained glycine anhydride was less selective than the original. When pure cultures of 10 bacteria (including one L. monocytogenes strain) were combined and plated on four media, L. monocytogenes colonies were easiest to enumerate on MVJ agar, followed by LPM and RISA. These media ranked in the same order when plated with homogenates of various foods to which was added L. monocytogenes Scott A, but LPM agar was the best overall since Scott A was inhibited by MVJ. Upon microscopic examination of listerial colonies from the plating media, atypical cell morphology was noted with cells being about twofold in size on LPM, MVJ, and acriflavine-ceftazidime agars. Overall, LPM agar was the most suitable of the media tested even though it was inhibitory to Listeria grayi and Listeria murrayi.