A multi-centre study on the phage typing of Listeria monocytogenes

A review of the literature of Listeria monocytogenes in Africa highlights breast milk as an overlooked human source

According to the latest WHO estimates (2015) of the global burden of foodborne diseases, Listeria monocytogenes is responsible for one of the most serious foodborne infections and commonly results in severe clinical outcomes. The 2013 French MONALISA prospective cohort identified that women born in Africa has a 3-fold increase in the risk of maternal neonatal listeriosis. One of the largest L. monocytogenes outbreaks occurred in South Africa in 2017-2018 with over 1,000 cases. Moreover, recent findings identified L. monocytogenes in human breast milk in Mali and Senegal with its relative abundance positively correlated with severe acute malnutrition. These observations suggest that the carriage of L. monocytogenes in Africa should be further explored, starting with the existing literature. For that purpose, we searched the peer-reviewed and grey literature published dating back to 1926 to date using six databases. Ultimately, 225 articles were included in this review. We highlighted that L. monocytogenes is detected in various sample types including environmental samples, food samples as well as animal and human samples. These studies were mostly conducted in five east African countries, four west African countries, four north African countries, and two Southern African countries. Moreover, only ≈ 0.2% of the Listeria monocytogenes genomes available on NCBI were obtained from African samples, contracted with its detection. The pangenome resulting from the African Listeria monocytogenes samples revealed three clusters including two from South-African strains as well as one consisting of the strains isolated from breast milk in Mali and Senegal and, a vaginal post-miscarriage sample. This suggests there was a clonal complex circulating in Mali and Senegal. As this clone has not been associated to infections, further studies should be conducted to confirm its circulation in the region and explore its association with foodborne infections. Moreover, it is apparent that more resources should be allocated to the detection of L. monocytogenes as only 15/54 countries have reported its detection in the literature. It seems paramount to map the presence and carriage of L. monocytogenes in all African countries to prevent listeriosis outbreaks and the related miscarriages and confirm its association with severe acute malnutrition.

Phages of Listeria offer novel tools for diagnostics and biocontrol

Historically, bacteriophages infecting their hosts have perhaps been best known and even notorious for being a nuisance in dairy-fermentation processes. However, with the rapid progress in molecular microbiology and microbial ecology, a new dawn has risen for phages. This review will provide an overview on possible uses and applications of Listeria phages, including phage-typing, reporter phage for bacterial diagnostics, and use of phage as biocontrol agents for food safety. The use of phage-encoded enzymes such as endolysins for the detection and as antimicrobial agent will also be addressed. Desirable properties of candidate phages for biocontrol will be discussed. While emphasizing the enormous future potential for applications, we will also consider some of the intrinsic limitations dictated by both phage and bacterial ecology.

Listeria phages: Genomes, evolution, and application

Listeria is an important foodborne pathogen and the causative agent of Listeriosis, a potentially fatal infection. Several hundred Listeria bacteriophages have been described over the past decades, but only few have actually been characterized in some detail, and genome sequences are available for less than twenty of them. We here present an overview of what is currently known about Listeria phage genomics, their role in host evolution and pathogenicity, and their various applications in biotechnology and diagnostics.

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.

Differentiation of the major Listeria monocytogenes serovars by multiplex PCR

A new multiplex PCR assay was developed to separate the four major Listeria monocytogenes serovars isolated from food and patients (1/2a, 1/2b, 1/2c, and 4b) into distinct groups. The PCR test, which constitutes a rapid and practical alternative to laborious classical serotyping, was successfully evaluated with 222 Listeria strains.

Evaluation of the international phage typing set and some experimental phages for typing of Listeria monocytogenes from poultry in Spain

AIMS

The validity of the international phage set and 13 experimental phages for subtyping Listeria monocytogenes strains isolated from poultry in Spain was investigated.

METHODS AND RESULTS

Ninety-six L. monocytogenes strains (52 from serogroup 1/2 and 44 from serogroup 4) were phage-typed using the international phage set, 10 experimental phages for typing serogroup 1/2 strains (seven isolated in France: 1313, 9425, 1807, 351, 881, 717 and 586-, and three from Denmark: 5775, 12682 and 6223-) and three experimental phages isolated in France for typing serogroup 4 strains (2425 A, 4286 and 197). Percentages of serogroup 1/2, serogroup 4 and total phage-typeable strains were 57.7%, 52.3% and 55.2%, respectively. Important differences in the behaviour of the phages tested were found. The typeability rate, the specificity index and the percentage of strong reactions were greater in the phages of international set than in the experimental phages. The number of phage typeable strains and the number of phage types (42) were not modified by the use of experimental phages.

CONCLUSIONS

The phage set used was not effective for typing L. monocytogenes strains from poultry in Spain, because a low typeability rate was found.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results suggest the importance of the availability of new phages specific to a geographical area in order to improve the typeability of the system.

Characterization of Listeria monocytogenes isolated from production lines of fresh and cold-smoked fish

AIMS

The aims of this study were to characterize strains of Listeria monocytogenes isolated from cold-smoking fish plants to establish possible routes of contamination through the processing chain.

METHODS AND RESULTS

Listeria monocytogenes from fresh fish suppliers, raw materials, factory sites and finished products isolated in Portugal (162 isolates) and England (28 isolates) were characterized by serotyping, phage typing, tetracycline, cadmium and arsenic resistance, and plasmid profiling. On the basis of serotyping and phage typing, the isolates were categorized into eight groups. Although cultures within some of the groups could be further differentiated on the basis of plasmid profiling and cadmium and arsenite typing, consideration of all typing data predominantly clustered together isolates from a single location. L. monocytogenes strains: from fresh salmon suppliers were not found in the processing lines; from fresh salmon from different locations differed; and from the water where salmon trout were farmed differed from those isolated from the fish samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

No clear source or route of contamination in the cold-smoked processing chain could be established; however, these results highlight the complexity in tracking this bacterium through food chains.

Use of two-dimensional electrophoresis To study differential protein expression in divercin V41-resistant and wild-type strains of Listeria monocytogenes

The use of bacteriocins from food-grade lactic acid bacteria to fight against the food-borne pathogen Listeria monocytogenes has been gaining interest. However, the emergence of resistant cells is frequently reported when Listeria is exposed to such antibacterials. A two-dimensional electrophoresis study of whole-cell protein expression of Listeria monocytogenes variants sensitive or resistant to the action of a bacteriocin produced by Carnobacterium divergens V41, divercin V41, is reported in this paper. The resistant variant obtained from the sensitive strain of L. monocytogenes P was also resistant to piscicocins V1 and SF668, but remained sensitive to nisin. Its growth rate was 50% less than the sensitive strain, and the MIC for it was 10(4) times higher. No reversion of the resistance was observed after 20 successive cultures in the absence of divercin V41. Comparison of the protein patterns by two-dimensional gel electrophoresis analysis showed clear differences. In the resistant variant pattern, at least nine spots had disappeared and eight new ones were observed. One of the newly synthesized proteins was identified as a flagellin of L. monocytogenes. Direct interaction between flagellin and divercin V41 was not evidenced. Intracellular synthesis of flagellin is probably an indirect effect of a modification in transcriptional regulation with widespread effects through a sigma factor. An intense protein, only present in the sensitive strain, was identified as a non-heme iron-binding ferritin displaying strong similarities to Dps proteins. Common modifications in the transcriptional regulation for these two proteins are discussed.

Typing Listeria monocytogenes by random amplified polymorphic DNA (RAPD) fingerprinting

Twenty epidemiologically unrelated Listeria monocytogenes strains isolated from different animals, locations and on different dates in Japan were classified into 18 types by the random amplified polymorphic DNA (RAPD) fingerprinting technique with four primers. Further, seven epidemiologically related L. monocytogenes strains isolated from raw milk and a bulk tank on a dairy farm represented the same RAPD type suggesting that they were all of the same origin. Therefore, RAPD-polymerase chain reaction (PCR) analysis, which is rapid, simple and inexpensive to perform, can be used in surveys as a convenient epidemiological technique.

An outbreak of listeriosis suspected to have been caused by rainbow trout

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.

Pseudo-outbreak of listeriosis elucidated by pulsed-field gel electrophoresis

Listeria monocytogenes was isolated from three patients hospitalized in two departments of the same hospital over a two-week period. A nosocomial outbreak of listeriosis was suspected. Patients presented with tenosynovitis, central venous catheter infection, and bacteremia. The first patient had a community-acquired infection, the second a nosocomial infection, and the source of the third patient's illness was uncertain. Epidemiological investigations failed to identify a common source of contamination within the hospital. The three strains were nontypeable by phage typing, but Smal macrorestriction analysis and pulsed-field gel electrophoresis yielded three distinct profiles. Therefore, the three cases seemed to represent a cluster of sporadic cases as opposed to an outbreak of listeriosis. Rapid typing of isolates is essential in the early investigation of potential outbreaks of listeriosis and may prevent the initiation of expensive and time-consuming epidemiological investigations.

Evaluation of seven experimental phages for inclusion in the international phage set for the epidemiological typing of Listeria monocytogenes

The purpose of our study was to evaluate the inclusion of seven experimental phages into the international phage set for subtyping Listeria monocytogenes. The seven additional phages included the broad-host-range virulent Myoviridae phage A511 (M. J. Loessner, Appl. Environ. Microbiol. 57:1912-1918, 1991), three temperate phages from the Danish subsystem for typing serotype 1/2 strains (12682, 6223, and 5775) (P. Gerner-Smidt, V.T. Rosdahl, and W. Frederiksen, APMIS 101:160-167, 1993), and three temperate phages isolated by this laboratory in France (9425, 1313, and 197). A panel of 395 Listeria monocytogenes isolates (including 180 that were non-phage typeable by the international set) were used in the study for a comparison of the lytic spectra of the various bacteriophages. These results showed that the inclusion of five of the experimental phages contributed greatly to the overall typeability and discriminatory power of the system, especially for strains within serogroup 1/2.

A foodborne outbreak of gastroenteritis involving Listeria monocytogenes

An outbreak of gastroenteritis occurred in Italy among 39 persons who had attended a private supper. All guests were previously healthy, young, non-pregnant adults; 18 (46%) had symptoms, mostly gastrointestinal (78%), with a short incubation period. Four were hospitalized with acute febrile gastroenteritis, two of whom had blood cultures positive for Listeria monocytogenes. No other microorganisms were recovered from the hospitalized patients' specimens. Epidemiological investigation identified rice salad as the most likely vehicle of the food-borne outbreak. L. monocytogenes was isolated from three leftover foods, the kitchen freezer and blender. Isolates from the patients, the foods and the freezer were indistinguishable: serotype 1/2b, same phage type and multilocus enzyme electrophoretic type. Eight (36%) of 22 guests tested were found to have antibodies against L. monocytogenes, compared with none of 11 controls from the general population. This point source outbreak was probably caused by infection with L. monocytogenes. Unusual features included the high attack rate among immunocompetent adults and the predominance of gastrointestinal symptoms.

The use of PCR ribotyping for typing strains of Listeria spp

The potential of PCR ribotyping for discriminating between and within various species of Listeria, as well as strains of Listeria monocytogenes was examined. In total, 49 strains of Listeria monocytogenes and 12 isolates of Listeria spp. were analyzed. The genomic DNA isolated from these strains was subjected to PCR amplification in the regions between 16S and 5S rRNA. Amplifications were performed with both low and high concentrations of Taq polymerase. Length polymorphisms in the amplified DNA products enabled distinction between various strains of Listeria spp. and between various serotypes of L. monocytogenes. Six composite profiles for serotype 4b strains, 8 for 1/2a strains and 11 for 1/2b strains, were observed. In addition, several different PCR ribotyping strategies were evaluated. Restriction fragment length polymorphisms of the spacer region between the 16S and 23S rRNA genes of 16 strains of L. monocytogenes were not observed, except for two isolates. PCR ribotyping analysis displayed promise as an alternative to traditional L. monocytogenes molecular typing methods.

Investigations related to the epidemic strain involved in the French listeriosis outbreak in 1992

Two hundred seventy-nine cases of human listeriosis (92 pregnancy-related cases and 187 non-pregnancy-related cases) caused by a serovar 4b and phagovar 2389:2425:3274:2671:47:108:340 strain were identified in France between March and December 1992. Epidemiological investigations included a case-control study (not described here) and microbiological analyses of foods. Results of the case-control study and characterization of food isolates identified pork tongue in jelly, a ready-to-eat meat product, as the major vehicle of this outbreak, and to a lesser extent, delicatessen products contaminated secondarily during handling in food stores. As far as serotyping, phage typing, DNA macrorestriction pattern analysis (obtained by pulsed-field gel electrophoresis [PFGE]), and ribotyping are concerned, this epidemic strain is phenotypically and genomically closely related to strains responsible for major outbreaks of listeriosis previously observed in Europe and North America. The epidemic strain sensu stricto as defined by PFGE (2/1/3) displayed the same serovar, phagovar, ribovar, and ApaI and NotI PFGE patterns as the epidemic strains from outbreaks in Switzerland, California, and Denmark, but it consistently showed differences in the SmaI PFGE profile. This information greatly contributed to the identification of the major food vehicle (pork tongue in jelly) and further allowed exclusion of other foods (cheese) as possible sources of this major listeriosis epidemic.

Proposals for optimization of the international phage typing system for Listeria monocytogenes: combined analysis of phage lytic spectrum and variability of typing results

Combined analysis of 5,179 serial phage reactions of 20 Listeria monocytogenes propagating strains over 14 years and phage typing results from 2,659 further L. monocytogenes strains allowed us to estimate lytic spectrum specificity and the variability of the lytic reactions of 35 phages. These included the 26 phages recommended for the international method for phage typing defined in 1985 by Rocourt et al. (J. Rocourt, A. Audurier, A. L. Courtieu, J. Durst, S. Ortel, A. Schrettenbrunner, and A. G. Taylor, Zentralbl. Bakteriol. Abt. 1 Orig. A 259:489-497, 1985). The results are discussed individually for each phage. Proposals for modifying the present system are made with the aim of producing an optimal bacteriophage set for routine use.

Interactions with lectins and agglutination profiles of clinical, food, and environmental isolates of Listeria

On the basis of preliminary trials with 14 collection strains of Listeria, five lectins (Canavalia ensiformis, concanavalin A; Griffonia simplicifolia lectin I; Helix pomatia agglutinin; Ricinus communis agglutinin; and Triticum vulgaris wheat germ agglutinin) were selected to set up a microtiter agglutination assay. The lectin agglutination profiles of 174 clinical, food, and environmental strains of Listeria monocytogenes, Listeria innocua, and Listeria seeligeri were investigated. Data on the standard determination of the antigenic structure were available for clinical strains; nonclinical isolates were assigned to serogroup 1 or 4 with commercial antisera. The listeria-lectin interaction was related to serological type rather than species; in particular, the strains assigned to serogroup 1 or belonging to serovars 1/2a, 1/2b, 1/2c, 3a, 3b, and 7 were never agglutinated by G. simplicifolia lectin I. The five-lectin set proved to be capable of detecting differences between serologically identical isolates of L. monocytogenes. Of the 150 isolates of this species, 144 were distributed over 15 different lectin agglutination profiles and 6 autoagglutinated, the overall typeability being 96%. However, the profiles encountered among L. monocytogenes isolates were not randomly distributed. With strains assigned to serogroup 1 or belonging to serovars 1/2a, 1/2b, 1/2c, and 3b, the clinical isolates fell into only two of the eight patterns recorded overall; with strains of serogroup 4 and serovar 4b, food and environmental isolates were distributed over eight of the nine patterns found in total, while clinical isolates were distributed over five patterns. In a comparative study of 15 epidemiologically relevant isolates of L. monocytogenes from five distinct outbreaks, strains with identical phage types and/or DNA fingerprints displayed identical lectin profiles. The heterogeneity of agglutination profiles may form the basis of a new approach to L. monocytogenes typing.

RAPD typing for distinguishing species and strains in the genus Listeria

The randomly amplified polymorphic DNA (RAPD) technique was employed in the development of a typing protocol for Listeria isolates, particularly Listeria monocytogenes strains. A single strain of L. monocytogenes was used and 200 random decamer primers were screened for their discriminatory abilities by visualizing the amplification products electrophoretically. Three candidate primers displaying potentially useful banding patterns were selected and tested against 52 L. monocytogenes strains, encompassing 11 serotypes, and 12 other strains representing five other Listeria spp. Thirty-four banding profiles were obtained with one particular primer. RAPD analysis allowed differentiation between Listeria spp. and was found to further subdivide strains of the same serotype. Where only one primer was used strains from different serotypes were occasionally found to produce identical banding profiles. RAPD analysis, which in our hands proved to be reproducible, shows much promise as a molecular alternative to traditional L. monocytogenes typing protocols.

Pulsed-field fingerprinting of listeriae: identification of genomic divisions for Listeria monocytogenes and their correlation with serovar

Clamped homogeneous electric field (CHEF) electrophoresis was optimized for genomic analyses of Listeria monocytogenes. Various human, animal, food, and environmental isolates, as well as strains representing other Listeria species, were separately digested with rarely cutting endonucleases. Of 176 L. monocytogenes strains analyzed, the enzymes AscI and ApaI established 63 and 72 unique restriction endonuclease digestion profiles (REDP), respectively. The 22 non-L. monocytogenes strains exhibited 18 AscI and 19 ApaI unique REDP. Statistical analyses of REDP information using the Dice coincidence index and principal component analysis revealed two distinct genomic divisions of L. monocytogenes that also correlated with the flagellar (H) antigen type: division I contained serovar 1/2a, 1/2c, 3a, and 3c stains and division II contained serovar 1/2b, 3b, 4b, 4d, and 4e strains. Division I isolates digested with ApaI were further grouped into cluster IA (serovar 1/2c and 3c) and cluster IB (serovar 1/2a and 3a) strains. Likewise, division II isolates digested with ApaI were further grouped into cluster IIA (serovar 1/2b and 3b) and cluster IIB (serovar 4b, 4d, and 4e) strains. These data indicate that genotypic data generated by CHEF can be directly related to phenotypic data generated by serotyping for establishing the overall relatedness of isolates. Moreover, these data further substantiate that CHEF analysis is a reproducible and highly discriminating method for characterizing L. monocytogenes strains at the molecular level.

Characterization of Listeria strains from a foodborne listeriosis outbreak by rDNA gene restriction patterns compared to four other typing methods

The rDNA gene restriction patterns of 134 isolates of Listeria species were determined with pKK3535--a pBR322 derived plasmid containing an Escherichia coli rRNA operon--used as a probe following digestion of chromosomal DNA by EcoRI endonuclease. Nineteen reference and type strains representing all species and serotypes of Listeria showed 17 distinct ribotypes. One hundred and fifteen wild strains of Listeria monocytogenes were ribotyped and the results were compared to those of serotyping, phage typing, multilocus enzyme electrophoresis (MEE) and restriction endonuclease analysis (REA). Ninety-six Listeria monocytogenes serotype 4b wild strains displayed six distinct ribotypes (I-VI), 72% (69/96) of them clustering in two very close rDNA patterns (I and II) of eight and nine bands, respectively. The same 96 strains displayed six REA patterns and eight MEE electrotypes. Among the 96 Listeria monocytogenes 4b isolates, the 34 epidemic strains defined by phage typing and by epidemiological data all belonged to one ribotype (ribotype I) representing 56% of the strains belonging to this ribotype. These same 34 epidemic strains were also grouped by REA and MEE typing in a unique profile (REA-A) and MEE electrotype (ET 1). Twenty-two Listeria monocytogenes strains of serogroup 1/2 analyzed by rDNA typing showed nine distinct ribotypes. For the 96 Listeria monocytogenes 4b strains studied, the discriminatory index was highest for phage typing and for any combination including phage typing. Ribotyping appears to be a well reproducible molecular typing method and could be a useful complement to other typing methods for the epidemiological study of listeriosis.

Isolation of Listeria monocytogenes from goat cheese associated with a case of listeriosis in 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.

Plasmids in Listeria monocytogenes in relation to cadmium resistance

One hundred and seventy-three unrelated Listeria monocytogenes strains isolated from humans, animals, the environment, and food were analyzed for the presence of plasmids. Extrachromosomal DNA was found in 28% of the strains. Plasmid DNA was extracted more frequently from L. monocytogenes serogroup 1 strains (35%) than from serogroup 4 strains (15%). Among strains from food and the environment, 40% and 29%, respectively, harbored plasmids, whereas only 13% of the strains from humans and animals with listeriosis bore plasmids. We also investigated the susceptibility of 90 strains to seven antibiotics and four heavy-metal salts. No antibiotic resistance could be detected, but 95.3% of the plasmid-positive strains and only 12.7% of the plasmid-negative strains were resistant to cadmium. The plasmid-determined genetic basis of cadmium resistance was proven by conjugation between strains of L. monocytogenes and by cure of the plasmid. This is the first time that plasmids of L. monocytogenes have been shown to be associated with cadmium resistance.

Classification of virulent and temperate bacteriophages of Listeria spp. on the basis of morphology and protein analysis

A set of 22 phages of Listeria species (listeriaphages) (21 temperate and 1 virulent) were compared on the basis of morphology and protein composition. All 22 phages had icosahedral heads and exhibited either contractile or noncontractile tails. They represented two different morphotypes. Twenty phages belonged to the Siphoviridae family and could be differentiated only on the basis of tail length. Accordingly, they could be assigned to previously defined listeriaphage species. Two other phages were classified as members of the Myoviridae family, one of which (A511) should be regarded as a new species. It was found to be substantially different from all other known listeriaphages. All phages exhibited typical protein profiles, which were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent laser densitometrical analysis of the gels. It was then possible to distinguish eight protein subgroups on the basis of unique protein patterns. This classification corresponds well to the previous groupings based on host range. Most of the phages revealed two or three major proteins ranging from 21 to 24 kDa and 30 to 36 kDa. In addition, at least 10 minor proteins could be observed for each phage. Our results indicate that the major proteins are structural proteins of the capsid and tail, and the protein band ranging from 30 to 35 kDa could clearly be assigned to the proteins of the phage capsid.

Listeria monocytogenes isolates can be classified into two major types according to the sequence of the listeriolysin gene

The nucleotide sequence of a 3.5-kb BamHI fragment from Listeria monocytogenes 12067, a human clinical isolate of serotype 4b, has been determined. The DNA fragment harbors the gene for listeriolysin, part of the gene for a phosphatidylinositol-specific phospholipase C, and part of the gene for a metalloprotease. Comparison of the sequence with corresponding sequences from two other L. monocytogenes isolates revealed a significant number of nucleotide differences. Several of the differences give rise to amino acid substitutions. The most variable region was the examined part of the mpl gene, whereas the lisA gene showed a relatively high degree of conservation, particularly at the amino acid level. To analyze the pattern of sequence variability in the lisA gene, a 160-bp region covering nine nucleotide differences was sequenced from 36 isolates of different origins. This work showed that the strains can be grouped into two major types according to the nucleotide sequences. Oligonucleotide probing of a larger number of L. monocytogenes isolates showed that the observed differences can be used to subdivide the species. The data suggest a correspondence between the sequence type of the lisA gene and flagellar antigens. Assays based on hybridization or the polymerase chain reaction with type-specific oligonucleotides may provide fast and easy alternative methods for strain typing.

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.

DNA polymorphism in strains of Listeria monocytogenes

DNA polymorphism in 35 Listeria monocytogenes strains belonging to serovars 1/2a, 1/2b, 1/2c, and 4b was studied by genomic DNA digestion. The restriction endonucleases ApaI and NotI, which cleave DNA at rare sequences, were used, and DNA fragments were analyzed by pulsed-field gel electrophoresis. Restriction fragment length polymorphism varied among different serovars and was used for epidemiological studies, but serovar 1/2c isolates could not be analyzed because their restriction patterns were indistinguishable. The genome sizes were calculated by addition of the sizes of the ApaI fragments and were found to be about 2,660 kb for serovar 1/2a strains, 2,640 kb for serovar 1/2b strains, and 2,710 kb for serovar 4b strains but only 2,340 kb for serovar 1/2c strains. This last group therefore appears to differ from the other serovar strains by the absence of restriction fragment length polymorphism and a chromosome that is 15% shorter, suggesting that strains of serovar 1/2c have quite recently emerged.

Restriction enzyme analysis of Listeria monocytogenes strains associated with food-borne epidemics

Listeria monocytogenes (serotype 4b) has caused four major food-borne epidemics in North America. In this study, L. monocytogenes isolates from the Nova Scotia (Canada), Boston (Mass.), and Los Angeles (Calif.) outbreaks were examined by restriction enzyme analysis with the endonuclease HhaI. Human isolates (n = 32) from the 1981 Canadian outbreak were compared with a strain recovered from coleslaw, which was epidemiologically incriminated as the vehicle of infection. After HhaI digestion, 29 of 32 isolates exhibited the restriction enzyme pattern of the reference coleslaw isolate. The restriction enzyme patterns of the nine clinical isolates from the 1983 Massachusetts outbreak were identical to each other but differed from those of raw milk isolates recovered from sources supplying the pasteurizer. Isolates (n = 48) from the 1985 California outbreak were evaluated. The restriction enzyme patterns of the L. monocytogenes isolates from humans and from the suspect cheese samples were identical to those of four of five cheese factory environmental isolates. Isolates from each of these outbreaks exhibited a restriction enzyme pattern that was characteristic of that outbreak. The case with which restriction enzyme analysis can be applied to all serotypes of L. monocytogenes argues for its use in the epidemiology of L. monocytogenes.

Characterization by DNA restriction endonuclease analysis of Listeria monocytogenes strains related to the Swiss epidemic of listeriosis

Listeria monocytogenes strains responsible for outbreaks of listeriosis were studied by using serotyping and phage typing. An additional approach based on restriction endonuclease analysis (REA) of the chromosomal DNA was used to characterize L. monocytogenes strains collected from various sources during and after a Swiss outbreak of listeriosis (1983 to 1987). Among the 169 wild-type strains of Listeria spp. that were examined, 161 (95%) belonged to the species L. monocytogenes, of which 109 were of human origin. Ten different REA profiles were obtained from the 120 L. monocytogenes serotype 4b strains tested. All 57 serotype 4b strains that were identified as Swiss epidemic strains by phage typing clustered in two closely related REA profiles. In particular, 10 L. monocytogenes 4b strains isolated from the brand of soft cheese responsible for the outbreak and from its direct environment were indistinguishable from isolates from 40 patients by both phage typing and REA analysis. However, 5 of the 17 non-phage-typeable L. monocytogenes strains and 18 L. monocytogenes strains with a phage type different from those of the Swiss epidemic types showed the same profile. REA enabled the characterization of non-phage-typeable strains and, thus, seems a promising tool for L. monocytogenes typing, especially during epidemiological investigations.

Analysis of clinical and food-borne isolates of Listeria monocytogenes in the United States by multilocus enzyme electrophoresis and application of the method to epidemiologic investigations

To investigate the microbiology and epidemiology of the 1,700 sporadic cases of listeriosis that occur annually in the United States, we developed a multilocus enzyme electrophoresis (MEE) typing system for Listeria monocytogenes. We studied 390 isolates by MEE. Eighty-two electrophoretic types (ETs) were defined. Two distinct clusters of ETs, ET group A (ETGA) and ET group B (ETGB), separated at a genetic distance of 0.440, were identified. Strains of ETGB were associated with perinatal listeriosis (P = 0.03). All strains of H antigen type a were in ETGA, while all strains of H antigen type b were in ETGB. Among 328 clinical isolates from cases of literiosis, 55 ETs of L. monocytogenes were defined. Thirty-four ETs were identified among 62 isolates from food products. The mean number of strains per ET (5.2) was significantly higher among clinical isolates than among food-borne isolates. Examination of isolates from outbreaks further documented the link between cases and contaminated food products. In one investigation, we found 11 different ETs, ruling out a single common source as a cause of that outbreak. By examining a large number of isolates collected over a specified time in diverse geographic locations in the United States, we have begun to establish a baseline for the study of the epidemiology of listeriosis by MEE.

Bacteriophage typing of Listeria species

A bacteriophage typing scheme for differentiating Listeria isolates from dairy products and various other foodstuffs was developed. Sixteen selected phages isolated from both environmental sources and lysogenic strains were used for typing and, according to their lytic spectra, divided into four groups. Thus far, 41 distinct patterns of lysis were seen when this set was used in typing 57 defined reference strains, representing all five confirmed species and 16 serotypes in addition to 454 Listeria isolates of primarily foodborne origin. Overall, typability was 84.5%; i.e., a strain was lysed by at least one phage at 100x routine test dilution. Strains belonging to serovar 3 were mostly resistant to lysis by the phages employed. The results were highly reproducible, as determined in retyping trials several weeks later. Some phages isolated from environmental sources showed a wider lytic spectrum than did those isolated from lysogenic strains. In accordance with this, the phages were found in different clusters within a computer-generated linkage map. Species specificity and serovar specificity of the lytic reaction were not found. None of the phages was able to lyse strains of Listeria grayi, Listeria murrayi or Jonesia denitrificans. This phage typing system may provide important information for a means of recognizing and eliminating sources of contamination by Listeria spp. within dairy plant equipment.

Genetic characterization of clones of the bacterium Listeria monocytogenes causing epidemic disease

One hundred and seventy-five isolates of the pathogenic bacterium Listeria monocytogenes recovered from human clinical (blood and cerebrospinal fluid), animal, and environmental sources in Europe, North America, and elsewhere were analyzed electrophoretically for allelic variation at 16 genetic loci encoding metabolic enzymes. Forty-five distinctive allele profiles (electrophoretic types, ETs) were distinguished, among which mean genetic diversity per locus (H) was 0.424. Cluster analysis of a matrix of genetic distances between paired ETs revealed two primary phylogenetic divisions of the species separated at a distance of 0.54. ETs in division I were presented by strains of serotypes 4b, 1/2b, and 4a, whereas strains of ETs in division II were of serotypes 1/2a and 1/2c. Human and animal isolates did not represent distinctive subsets of ETs. The occurrence of linkage disequilibrium between enzyme loci and the widespread distribution of certain ETs indicate that the genetic structure of L. monocytogenes is clonal. One clone, marked by ET1, caused major epidemics of human disease in western Switzerland in the period 1983-1987 and in Los Angeles County, California, in 1985, both of which were attributed to contamination of soft cheese. ET 1 is closely related to the clone (ET7) that caused two large outbreaks of listeriosis in Massachusetts in 1979 and 1983.

Perinatal listeriosis in Dresden 1981-1986: clinical and microbiological findings in 18 cases

Between 1981 and 1986 Listeria monocytogenes was isolated from blood cultures, CSF, meconium/stools or external swabs from 18 newborn infants of two neonatal intensive care units (ICU) in adjacent pediatric clinics of Dresden. The epidemiological and clinical data of infants and their mothers, as well as microbiological and laboratory, x-ray, EEG and ultrasonic findings, are presented. All infants had an early onset of their disease. Cases were classified as granulomatosis infantiseptica (three cases), sepsis (three cases), meningitis (eight cases) and listerial infection without distinct organ manifestations (four cases), respectively. As far as the predominant symptoms at admission were concerned, no typical clinical signs of neonatal listeriosis could be evaluated. Cases with manifest clinical infections had an overall mortality rate of 21% (3/14) despite the immediate initiation of antibiotic therapy; at discharge, a further five patients showed neurological residuals. Serotyping and phagetyping have proved to be methods for recognition or exclusion of epidemiological relationships.