Three soft cheeses were exposed to quantitative analysis for listeria and found to contain a large number of listeria. Thirty-five of the listeria strains isolated from the three cheeses were characterized by use of biochemical tests, serotyping, phagetyping and DNA restriction enzyme analysis. Seven isolates were identified as Listeria innocuaand 28 as Listeria monocytogenes. Two to four different clones of L. monocytogenescould be identified from each cheese. In contrast, only one clone could be detected among the L. innocua isolates. From an epidemiological point of view the findings of different clones of L. monocytogenes in the same cheese emphasize the need for typing several listeria isolates from one and the same food sample. It is concluded that the best overview of the population of the listeria strains is obtained after direct plating of the sample followed by enumeration, isolation and extensive typing.
In a previous paper, we reported an outbreak of gastrointestinal listeriosis due to consumption of fresh cheese made from raw milk and manufactured on a summer farm. The aim of the present study was to investigate why the cheese harbored Listeria monocytogenes. To our knowledge, this is the first documented outbreak of listeriosis caused by raw milk cheese where the human epidemic strain has been cultured from a dairy animal, whose milk has been used for cheese production. The conditions on a summer farm can hardly fulfil the requirements for hygienic and strictly controlled conditions necessary for safe processing of fresh cheese.
A man died in endocarditis due to listeriosis in the late autumn. He had been looking after two goats during the summer. Listeria monocyto?enes was isolated from a rectal swab from one of the goats. The goat faeces isolate and the human blood isolate were of identical serovar. The two isolates, however, were shown to be different by multilocus electrophoretic enzyme analysis and ribotyping, as well as by biotyping, Thus, these results do not support the hypothesis that the man was infected by the goat.
Listeria monocytogenes was isolated from the brain of a goat, which was euthanized due to listeriosis. A few weeks later a similar subtype of L. monocytogenes was isolated from an on-farm manufactured fresh cheese which did not contain any milk from the goat which had suffered from listeriosis. A similar subtype was also found on 1 of the shelves in the refrigerator where cheeses were stored. Prior to the onset of listeriosis, 1 fresh cheese had been made of milk from the actual goat, which may have excreted L. monocytogenes in her milk. Thus, the cheese made of this milk may have contaminated the shelves in the refrigerator which then has served as a Listeria reservoir for new cheeses during several weeks.
The object of this study was to characterize Swedish human strains of Listeria monocytogenes
The method most often used in Sweden for isolation of Listeria monocytogenes from animal autopsy material is a cold enrichment method. This method is very slow. The International Dairy Federation (IDF) has recently presented a method for detection of L. monocytogenes in milk and milk products that is complete in one week. During a two year period 69 specimens from dead animals with suspected listeriosis were examined for L. monocytogenes in parallel analyses with both the cold enrichment method and the IDF method. Samples derived from different autopsy material representing a variety of animals. L. monocytogenes was isolated in 27.5% of the samples with the IDF method but only in 4.3% with the cold enrichment method. It is concluded that the IDF method was more sensitive than the cold enrichment method.
One hundred and seven strains of Listeria monocytogenes isolated from human cases of listeriosis in Sweden during 1976-1985 were investigated. Sero- and phagetyping showed that 74 strains (69%) belonged to serovar 4b and that 47 of these strains also shared phagovar 2389:2425:3274:2671:47:108:340. Strains sharing this phagovar were further analyzed by REA, and could accordingly be arranged into five REA groups. However, the majority of the 47 strains were distributed in two of the groups. This indicates that two clones of L. monocytogenes were responsible for a major part of the human cases of listeriosis in Sweden during the ten-year period studied.
In Sweden, many Listeria monocytogenes strains belonging to serovar 4b and isolated during the last five years from different sources share the same phagovar - 2389:2425:3274:2671:47:108:340. The object of the present study was to investigate if 31 L. monocytogenes serovar 4b strains belonging to this particular phagovar could be differentiated by use of a simple restriction endonuclease analysis (REA). Among the enzymes tested, Xho I was found to be the most useful, since this enzyme could divide the 31 strains into five groups. The profiles of all human clinical isolates were indistinguishable from each other, which indicates that these strains may represent a single clone. The food isolates and the strains of human origin did not share the same profile. This further characterization may be of epidemiological importance as this phagovar of L. monocytogenes has been associated with at least two outbreaks of human listeriosis in Europe.
An outbreak of listeriosis in Sweden, consisting of nine cases, was investigated by means of molecular typing of strains from patients and strains isolated from suspected foodstuffs, together with interviews of the patients. Listeria monocytogenes was isolated from six of the patients, and all isolates were of the same clonal type. This clonal type was also isolated from a 'gravad' rainbow trout, made by producer Y, found in the refrigerator of one of the patients. Unopened packages obtained from producer y were also found to contain the same clonal type of L. monocytogenes. Based on the interview results and the bacteriological typing, we suspect that at least six of the nine cases were caused by gravad or cold-smoked rainbow trout made by producer Y. To our knowledge, this is the first rainbow trout-borne outbreak of listeriosis ever reported.
Altogether, 133 strains of Listeria monocytogenes serovar 4b were investigated, A segment of 2,916 bp containing parts of the two genes inlA and inlB in L. monocytogenes was amplified by the PCR technique. The PCR product obtained was cleaved with the restriction enzyme AluI, and the fragments generated were separated by gel electrophoresis, leading to two distinct groups: PCR-restriction enzyme analysis groups I and II, containing 37 and 96 strains, respectively, The PCR-restriction enzyme analysis method described in this paper could be a useful tool for the subtyping of L. monocytogenes serovar 4b strains.
The major part of the gene inlB was sequenced in 24 strains of Listeria monocytogenes belonging to serovars 1/2a, 1/2b, 1/2c, 3b and 4b. A phylogenetic analysis based on the inlB nucleotide sequences showed that strains of serovars 1/2a and 1/2c were closely related, as well as those of serovars 1/2b and 3b. Strains sharing serovar 4b could be divided into two distinct groups. There were differences in amino-acid sequence between all serovars except between serovars 1/2b and 3b. Differences in amino-acid sequence were also seen within each of the serovars 1/2a and 4b. The data presented indicate that the inlB gene may be useful for typing purposes as an alternative or complement to serotyping.
The multiple-locus variable-number tandem-repeats analysis (MLVA) method for genotyping has proven to be a fast and reliable typing tool in several bacterial species. MLVA is in our laboratory the routine typing method for Salmonella enterica subsp. enterica serovar Typhimurium and Escherichia coli O157. The gram-positive bacteria Listeria monocytogenes, while not isolated as frequent as S. Typhimurium and E. coli, causes severe illness with an overall mortality rate of 30%. Thus, it is important that any outbreak of this pathogen is detected early and a fast trace to the source can be performed. In view of this, we have used the information provided by two fully sequenced L. monocytogenes strains to develop a MLVA assay coupled with high-resolution capillary electrophoresis and compared it to pulsed-field gel electrophoresis (PFGE) in two sets of isolates, one Norwegian (79 isolates) and one Swedish (61 isolates) set. The MLVA assay could resolve all of the L. monocytogenes serotypes tested, and was slightly more discriminatory than PFGE for the Norwegian isolates (28 MLVA profiles and 24 PFGE profiles) and opposite for the Swedish isolates (42 MLVA profiles and 43 PFGE profiles).
Two hundred strains ofListeria monoctyogenespreviously isolated from 19 soft and semi-soft cheeses by enrichment were characterized by serotyping and pulsed-field gel electrophoresis (PFGE). Strains of serogroup 1/2 predominated, with 33.5% of all strains belonging to serovar 1/2a, 58.5% to serovar 1/2b and 5% to serovar 1/2c. By using a PFGE method, 16 different clonal types were obtained. All 10 isolates ofL. monocytogenesrecovered from one white mould cheese were serovar 1/2a, but displayed two different PFGE profiles. Another 10L. monocytogenesisolates from white mould cheese recovered after a 48-h enrichment broth procedure revealed two clonal types, only one of which could be detected after 5 additional days of incubation in the same enrichment broth. Cross-contamination in dairy plants and/or retail stores may play an important role in the incidence and diversity ofL. monocytogenesclonal types recovered from soft and semi-soft cheeses. Therefore, it is necessary to characterize several isolates from the same sample when conducting epidemiological investigations.
Copyright © 1998 Academic Press. All rights reserved.
A 70-year-old woman fell seriously ill overnight with meningitis and was admitted to hospital. Cerebrospinal fluid culture yielded Listeria monocytogenes. One of the first problems in solving a human case of listeriosis suspected to be foodborne is to find the foods likely to have been transmitting L. monocytogenes. Two enrichment procedures and a direct plating procedure were used for isolation of the bacteria from different food items collected from the patient's refrigerator, local retail store and producer. Samples of vacuum-packed products of sliced pork brawn, sliced cooked medwurst and berliner wurst of the same brand harboured L. monocytogenes. Serotyping and restriction enzyme analysis (REA) with pulsed-field gel electrophoresis (PFGE) were used to characterize and compare 41 isolates, including the human strain. At least three clones were present in the foods investigated, and one of these was identical to the human clone. This clone was present in samples of medwurst from the patient's refrigerator and the local retail store. This is, to our knowledge, the first proven foodborne case of listeriosis reported in Sweden.
Samples of 333 retail cheeses produced in or imported into Sweden were examined for the presence of Listeria monocytogenes. Listeria monocytogenes was isolated from 6% of the cheese samples. Cheeses made from raw milk were more frequently contaminated with L. monocytogenes (42%) than cheeses made from heat-treated milk (2%). The incidence of the organism in whole cheeses and pre-cut wedges was similar (6%). L. monocytogenes was only found in imported cheeses (18 from France, and one from Germany and Italy, respectively). The numbers of L. monocytogenes varied from < 1 x 10(2) to 1 X 10(5) cfu/g. All L. monocytogenes strains belonged to serogroup 1/2, except isolates from two samples that belonged to serogroup 4.