DNA PROBE SPECIFIC FOR HEMOLYTIC LISTERIA

Abstract: A DNA probe is disclosed which is capable of hybridizing to a portion of the genome of pathogenic Listeria monocytogenes but which does not hybridize to portions of the genomes of other Listeria species and other humilytic bacteria. This probe is useful to identify food sources infected with Listeria monocytogenes and to distinguish these food services from those infected nearly with non-pathogenic Listeria species. In addition, methods for the detection of pathogenic Listeria in samples using the disclosed probes are provided.

Description: DNA PROBE SPECIFIC FOR HEMOLYTIC LISTERI A Background of the Invention Listeria monocytogenes is a facultative intracellular gram positive bacterium. In the genus Listeria, only L. monocytogen and L. ivanovii are pathogens for human and animals. L. monocytogenes is increasingly recognized as responsible for se¬ vere infections in both animals and humans. Pregnant women, new-born and im unocompromised patients are especially suscepti¬ ble to infection. To avoid human infection, food sources have routinely been screened for the presence of Listeria organisms. Potential foo sources infected with any species of Listeria have routinely bee discarded to avoid infecting the consumer because of the time an expense involved in determining whether the infecting organisms are or are not pathogenic. Thus, there is a need for a means fo identifying and distinguishing pathogenic Listeria. In addition, another need has arisen for means to distin¬ guish readily and efficiently between pathogenic and non-pathogenic Listeria species. This information is necessary to determine the course of treatment of suspected listeria infec tions and for the development of data for epidemiological studies. The present inventors have developed a DNA probe capable of distinguishing pathogenic Listeria species from non-pathogenic Listeria species. These probes and their uses are described mor fully below. Brief Description of the Drawings Figure 1 is a restriction map of plasmid pCL loz. The re¬ striction site BamHI is indicated by "B n ; Sail by "S"; and Hindlll by "H". The inverted triangle corresponds to the inser¬ tion site of transposon Trl545. Figure 2 depicts the nucleotide sequence of the hlyA gene and its proximal region. Figure 3 depicts the schematic comparison of the amino-acid sequences of listeriolysin 0 (LLO) , streptolysin (SLO) and pneumolysin (PLY)r sequences, represented by lines, are aligned on the unique cysteine (C) in the monologous position. Signal sequences are indicated by thick lines. ( ) indicates deletions of one amino acid, and the numbers, the coordinates in the pro¬ tein sequence. Detailed Description of the Preferred Embodiments Among the virulence factors which enable Listeria organism to enter, survive and grow within cells, including macrophages, the SH-activated hemolysin is a serious candidate. The first o servation which suggested a correlation between hemolysin and virulence was that all non-hemolytic listeria strains are experi mentally non-pathogenic and that all pathogenic strains of Listeria produce zones of hemolysis on blood agar plates. Hemolysin production being a phenotype easily identified on blo agar plates, genetic studies were undertaken and transposon mutagenesis was performed to obtain a non hemolytic (Hl -) mutant. This mutant was avirulent? insertion of a single copy the transposon had inactivated the hemolytic phenotype. Sponta neous loss of the transposon led to the recovery of the Hly + phenotype and virulence. It was further shwon that the Hly " mu tant, although phagocytosed at the same rate as that of the hemolytic revertant strain, stayed in the phagolysosome and failed to replicate significantly within the human enterocyte-like cell line Caco-2: electron microscopic study de onstrated that bacteria from the Hly- mutant remained inside the phagosomes during cellular infection, whereas hemolytic bacteri from L. monocytoqene became free in the cytoplasm. These data were a strong indication that disruption of vacuole membranes b hemolysin-producing strains of L. monocytogenes might be a key mechanism allowing bacteria to escape from phagosome and to mul¬ tiply within the cell cytoplasm. The SH-activated hemolysin referred to herein as "Listeriolysin 0" has been purified from L. monocytogenes cultur supernatants. It shares the typical properties of other bacte¬ rial sulfhydryl-activated toxins: (i) inhibition with very low amounts of cholesterol, (ii) activation by reducing agents and suppression of the lytic activity by oxidation, (iii) cross-reactivity with streptolysin O. Antiserum raised against the purified protein allowed the demonstration that the Hly " mu¬ tant produced a truncated protein, indicating that the transposo had inserted in the listeriolsin 0 structural gene. Accordingly the region of insertion of the transposon was cloned and sequenc analysis showed that it had inserted in an open reading frame (ORF) . The deduced sequence of this ORF shared homologies with streptolysin 0 and pneumolysin. From this, the identity of the locus of insertion of the transposon in the listeriolysin 0 gene, called hlyA has been determined. In order to elucidate the role of listeriolysin 0 in pathogenicity, and to clarify the conflicting views on the nature and the number of Listeria hemolysin(s) , a structural and func¬ tional analysis of the chromosomal region carrying hlyA was un¬ dertaken. In the Examples which follow, the complete nucleotide sequence of hlyA and an extensive analysis of the deduced protein sequence, especially a comparison with other membrane-damaging thiol-activated he olysins is set forth. In addition, DNA-DNA hybridization studies which indicate that the disclosed probes are present only in Listeria monocytogenes are provided. These analysis lead to the identification of the DNA se¬ quence of a 2.8 kb portion of the Listeria monocytogenes genome. From this piece of chromosonal DNA, hylA has been identified. Within this gene, a $-§3. bp Hindi11 --(oligo 1309-1965) is . preferred for use as a probe. In addition, certain smaller oligos are contemplated for use as probes. Preferrably, oligo 1 . Z75-1805 would make a suitable probe. In addition, oligos 1412-1441; 1643-1665; and 1810-1835 are contemplated for use as probes. As one of ordinary skill in the art will note, the desired characteristics of these probes include their ability to hybridization with Listeria monocytogenes DNA while not hy¬ bridizing with DNA from other Listeria species. In addition, i is a preferred characteristic of these probes that they not cr ■ react with DNA from other hemolytic bacterial species such as Streptococcus. In addition to the probes discussed above, various improv probes are contemplated. In these probes, certain modificatio are made which will not affect their ability to hybridize with the target DNA but will increase the strength of the hy¬ bridization bond and/or the ability of the hybridized probe to detected. In part, these improvements include the replacement certain base pairs with analoges or the modification of the pro to incorporate certain detection compounds. In certain situations, it might be desired to replace some of the bases within the probe with non-base analoges. For exa ple, the base adenine may be replaced with diaminopurine. This modification would result in a stronger bond between the probe and any homologous DNA present in a sample. To facilitate detection of hybridized probe, various chemi cal modifications to the DNA sequence are possible. One exampl of these modifications includes sulfonating the DNA. Sulfonate DNA allows for increased detectability in reaction such as ELIS methods. In addition, through biotinylation, or the use of a biotin analoge of a particular base, it is possible to incorpo¬ rate a biotin site into the probe. The incorporation of a biot site would facilitate detection of the hybridized probe in an avidin-biotin system. Further, radiolabels may be incorporated into the probes for use and detection in labs. At the present, 32p is preferred for use as a labeling method. Various methods may be used with these probes to detect the presence of Listeria monocytogenes in a sample. These include radioimmunoassays (RIA) ; enzyme-linked immunosorbent assays (ELISA) and other types of diagnostic, qualitative assays com¬ monly known to those of ordinary skill in the art. Example 1 - Creation of pCL102 Identification of a DNA region encoding listeriolysin 0. Chromosomal DNA of hemolytic L. monocytogenes strain L028 digested with Mbol had been cloned in the BamHI site of the cosmid vector pHC79. After transformation in E. co i HB101, sev¬ eral hemolytic clones were obtained, identified as ampicillin re¬ sistant colonies producing a halo of lysis on ampicillin blood agar plates. Spontaneous deletion of the original clones led to a stable derivative pCLlOl. Two deletions were created on the cosmid pCLlOl between identical restriction sites (BamHI and Sail) to give pCL102 which contained an insert of 8.5 kilobases of L. monocytogenes DNA. Its restriction map is shown on Fig. 1. The hemolytic activity detected in the extracts of Listeria monocytogenes strain HB101 as described by Boyer in J. Mol. Biol. 41:459-464 (1969), harboring pCLlOl or pCL102 (7.5 HU/ml of over¬ night culture) is lower than that measured in the Listeria rupernatants (64 HU/ml of overnight culture). By western blot .analysis of the extracts, using an antiserum against listeriolysin 0, a protein of the same molecular weight than th listeriolysin 0 (60-kilodaltons) was detected. Antilisteriolysin O serum preparation: Female albino rab¬ bits were immunized by repeated sucutaneous inoculations of highly purified hemolysin from L. monocytogens. The protocol w based on 3 injections of 75 g of toxin in complete Freund's adjuvant and one injection of the same dose in incomplete Freud' adjuvant. Blood was harvested 2 weeks after the last injection Western blot analysis: For western blot analysis, protein were electrophoreticall transferred to nitrocellulose sheets in a Trans-Blot cell appar tus with blotting buffer containing 20% (v/v) methanol. The sheets were incubated for 1 h at room temperature with shaking 50 mM Tris, 150 mM NaCl solution (pH 8.0) and 5% (w/v) skim mil prior to 1 h incubation in anti-listeriolysine 0 immune serum diluted (1:20) in the above buffer. The sheets were washed eig times in buffer before addition of 20 ml of milk buffer contain ing 1 Ci (0.37 kBq) of 125 I-protein A. Shaking was continued f a further 1 h and then the filters were washed six times in buffer supplemented with 0.1% Triton X-100. The filters were dried at 80°C and then autoradiographed using Kodak X-O-mat (SO-282) film. Example 2 - Localization of h- j -yA in pCLl02 Localization of hly.A in pCLl02 was achieved by using 400-base-pair DNA probe carrying part of the listeriolysin 0 gen called hlyA and part of Tnl545: a 410-base-pair Hindi11 fragment of pCLl02 hybridized to the probe (Fig.l). It revealed the presence of a stop codon, presumably the end of the hly.A gene. Nucleotide secn-tence determination: The dideoxy chain terminator sequencing method was used with the modification of Biggin et al. The technique shich generates sequential deletions of the insert starting at the cloning site using T4 polymerase was used as described by IBI. Nucleotide sequence of h-^-Y.A. The nucleotide sequence of hly.A and its upstream region is indicated on Fig.2. The open reading frame which ends in the 410-base-pair HindiII fragment (see below) starts in the 1670-base-pair Hindi11 fragment and is 1617-base-pair long. The first ATG in the sequence is located 30 nucleotides downstream from the beginning of this ORF. It is preceded 10 nucleotides upstream by an hexanucleotide (AAGGAG) complementary to the 3' end of the 16 S RNA of L. monocytogenes. If this ATG is con¬ sidered as the start codon of hlyA, the gene is 36% GC rich, a value which is in agreement with that calculated for L. monocytogenes (36-38%). This value is higher than that of the sequences adjacent to the gene. We looked for the presence of gram positive promoter-like sequences: upstream from the ORF, two TATAAT sequences are detected but these sequences are not preceded by a π -35" close t the Consensus TTGACA. The hemolysin production known to be regu lated by factors such as iron was examined for sequences similar to the consensus identified as target for iron binding regulator proteins but none was detected. The exact location of the prometer is under study. The codon usage in h--yA is reported in Table 3 where we al report the average codon usage in six other genes from gram pos tive bacteria, i.e. Streptococcus and Staphylococcus and in 25 highly expressed genes of E. coli. Codons ending by an A or a are more widely used (72%) in hlyA than in E. coli genes (44%). This is in agreement with the different GC% of these different organisms. In addition, in most of the cases, the codons rarel used in hlyA are rarely used in genes of gram positive bacteria Codons which are rarely used in E. coli genes but which are use in staphylococcal or streptococcal genes are also used in hly.A. As already proposed, this probably reflects differences in the distribution of tARNs between bacteria of different GC content and is probably a genome strategy to modulate gene expressivity Analysis of the deduced protein sequence. The gene hlyA encodes for a protein of 529 amino-acids, which corresponds to a protein of 58.6 kDa. The amino-terminal sequence presents all the characteristics of signal sequences o qra positive bacteria: the first residues are hydrophilic and positively charged. They are followed by about twenty hydrophobic residues. The putative cleavage site of the signal peptidase lies probably after lysine 25 as the sequence startin at residue 26 is highly homologous to the amino-terminal sequen of the SH-dependant hemolysin secreted by Listeria ivanovii . The signal sequence of listeriolysin 0 has a length comparable the average length of signal sequences of gram positive bacteria genes. Consequently, the secreted listeriolysin 0 (without the signal sequence) contains 504 amino-acids and has a molecular mass of 55.8kDa. This value is in agreement with the molecular weight of the protein purified from L. monocytogenes culture supernatants. The amino-acid composition of the protein does not reveal any special feature except the presence of a unique cysteine: this residue is located in the carboxy-terminal part of the se¬ quence, in position 484. It is known that listeriolysin 0, lik all cytotoxins of the streptolysion 0 family, is thiol-activabl Reagents which block the SH-group inhibit the activity of this toxin, and in particular, inhibit the binding to cholesterol which is thought to be the first step in the cytolytic activity -■f this protein. Thus the carboxy-terminal region containing t unique cysteine is probably essential for activity. Two lines evidence are in favor of this statement. (i) In the Hly-mutant the transposon had inserted in codon 481 of hlγ_A, three codons upstream from the cysteine codon, giving rise to a truncated pr tein devoid of hemolytic activity. (ii) Comparison of .listeriolysin O with two other SH-dependent hemolysins, streptolysins, streptolysin 0 and pneumolysin reveals the pres¬ ence of a conserved undecapeptide containing the unique cystein in the three proteins (see next Example). , Example 3 - Comparison of listeriolysin O with streptolysi O and pneumolysin. It is well known that SH-activated hemolysins immunologically cross-react. It has been recently shown that streptolysin 0 from Streptococcus pyogenes and pneumolysin from Streptococcus pneumoniae share homologies. These proteins have identical molecular weight, if one takes into account the se¬ creted form of streptolysin O, the pneumolysin being a non se¬ creted protein. When one aligns the sequences at the unique cysteine present in the carboxy-terminal end of these proteins, the highest homology lies in the region of this unique cysteine. We compared the amino-acid sequence of listeriolysin 0 (LLO) to the sequences of streptolysin 0 (SLO) and pneumolysin (PLY). These three proteins have similar sizes. The secreted forms of LLO and SLO are respectively 504 and 471 amino-acids long, pneumolysin is 471 amino-acids long. The putative signal se¬ quence of LLO (25 aminoacids) is shorter thatn that of SLO (33 amino-acids). The three sequences can be completely aligned at the unique cysteine, if one introduces two deletions of one amino-acid for SLO and one deletion of one amino-acid for PLY. This alignment reveals strong homologies: the percentage of amino-acid identity between two sequences compared two by two, i the common 469 residue region, is about 42-43%. Interestingly, the signal sequence of listeriolysin 0 corresponds to the N-terminal part of streptolysin 0, the hydrophobic amino-acids o listeriolysin 0 being changed for hydrophilic ones. This region is the region of lowest homology. Homologies between the three proteins are evident along the whole sequence but they are stronger towards the carboxy-terminal end. In particular, aroun the unique cysteine, an eleven-amino-acid peptide is conserved i the three sequences. If one compares the sequences in terms of similarity (and not identity), one observes even stronger homologies, illustrated by the superimposition of the hydrophobicity profiles. At the DNA level, the genes do not show substantial homolog (between 52% to 54% homologies when comparing any two of the three genes). Nevertheless, in the region cod_mg for the con¬ served undecapeptide, the homology at the DNA level is 73% between ply and hlyA, 88% between slo and hlyA and 82% between slo and ply. These results are in agreement with previous results: when, using a DNA tragment internal to slo as a probe, the homolc-gies between S. pyogenes and L. monocytogenes could n be detected. Indeed, the probe used did not correspond to the r gion of highest homology and the conditions used could only detect 80% homology. It is highly probable that use of a probe corresponding to the nucleotide sequence coding for the common eleven-amino-acid region would have let to different results. Example 4 - Detection of hlyA in the different species of the genus Listeria As listeriolysin O is now considered as a major virulence factor, it seemed interesting to test if hlyA was present in th different species of the genus Listeria. By Southern blot anal sis, we looked for the presence of hlyA in several Listeria monocytogenes strains and in the other species of the genus, i. L. ivanovii. L. seeliqeri, L.innocua, L. welshimeri, L. murravi. When a probe consisting of a DNA fragment internal to the gene, particularly the 651-base- pair-Hindll fragment which extends from codon to codon was used, the results were unambiguous, even at low stringencies. Particularly the above-said 651-base-pair-HindiI or any part contained therein and comprising as from 15, preferably 18 nucleotides (up to the maximum number of nucleotides contained in any strand of the 651-base-pair-HindiI fragment), hybridizes selectively with the DNA of pathogenic Listeria monocytogenes and not with the DNA of other Listeria under both low and high stringency conditions. The same results are achieved under stringencies with DNA fragments corresponding to sequences contained in fig.2 but outside of the DNA of fig.2, particularly those next to the 651-base-pair-HindII fragment downstream thereof in fig.2 (in the direction of reading of the sequence) . Under low stingencieε fragments originating from the latter part recognize DNA of Listeria pathogenic in cattle (e.g. L. ivanovii) or of another Listeria seldomly encountered in man (L. seeligeri) . By way of examples: - Conditions of high stringencies are: .hybridization in 50 formamide, 5xSSPE,10x DENHARDT, 200ug/ml of sonicated DNA at 42 * C and washing twice, each time 30 minutes in 1SSC, 0.1%SDS at room temperature; - conditions of low stringencies are: hybridization in 35% formamide, 5xSSPE, 10xDenhart, 200 mg/ml of sonicated DNA at 37 * C and . washing twice, each time during 30 minutes, in 1SSC,0.1% SDS at room temperature. Standard hybridization techniques can be relied upon for the detection of pathogenic Listeria in a biological sample or food sample. In the case of the serum sample a suitable amount is incubated in a suitable enrichment culture medium to promote growth of the Listeria possibly present. In the case of food, one may resort to conventional techniques, e.g. homogenizing the food under study in the enrichment culture medium and also incubating under conditions as indicated above. By way of example suitable medium is LPM disclosed by Lee, W.H. r and Mcclain 1986. Improved Listeria monocytogenes selective agar. Appl Environ. Microbiol. 52,1215-1217. Samples from the medium possibly containing the pathogenic Listeria are then treated to authorize for the release of DNA and, after optional fractionation according to the sizes of the nucleotides, hybridization experiments are carried out with a probe according to the invention for instance either under the low or high stringency conditions wherein use is made of a probe consisting of the 651-base-pair-Hindll fragment or part thereof or under high stringency conditions when use is made of another part of the DNA of fig.2, particularly (and in both of the instances hereabove) when the hybridization tests aim at detecting Listeria monocytogenes which are pathogenic in man - or under low stringency conditions when use is made of said another part of the DNA of fig.2 and the hybridization test aims at detecting Listeria pathogenic in cattle. DNA probe fragments, particularly those included within the 651-base-pair-Hindll fragment are suitable for use in polymerase gene reaction techniques; see for instance for reference: 1) Saϊ i RK, Scharf S. Faloona F. et al Enzymatic amplification of Beta-globin genomic sequences and restriction site analysis for diagnosis of sickle cell anemia. SCIENCE 1985; 230: 1350-4; 2) Saϊki RK, Gelfand D H, Stoffel S, et al. Primer-directed amplification of DNA with a thermostable DNA polymerase. SCIENCE 1988; 239:487-91. It is understood that the claims which define DNAs by reference to the drawings also extend to any equivalent DNA which may depart from the first by localized mutations but which still hybridizes with them under the stringent conditions as defined herein.

Claims: . WHAT IS CLAIMED IS: 1. A DNA probe which is capable of hybridizing to a por¬ tion of the genome of Listeria monocytogenes and which does not hybridize to portions of the genomes of other Listeria species and other hemolytic bacteria. 2. The DNA probe of claim 1 wherein said probe comprises oligo 1309-1966 of the nucleotide sequence of Figure 2. 3. The DNA probe of claim 1 wherein said probe comprises oligo 1775-1805 of the nucleotide sequence of Figure 2. 4. The DNA probe of claim 1 wherein said probe comprises .oligo 1412-1441 of the nucleotide sequence of Figure 2. 5. The DNA probe of claim 1 wherein said probe comprises oligo 1643-1665 of the nucleotide sequence of Figure 2. 6. The DNA probe of claim 1 wherein said probe comprises oligo 1810-1835 of the nucleotide sequence of Figure 2. 7. A method for the detection of pathogenic Listeria in a sample comprising: (a) placing said sample in a media capable of supporting growth of bacteria of the genus Listeria; (b) treating the sample in the media in a manner sufficient to disrupt any bacterial cells present in the sample and to re¬ lease bacterial DNA; (c) additionally treating said treated sample to denature the bacterial DNA; (d) contacting said denatured bacterial DNA with a DNA probe capable of hybridizing to a portion of the genome of Listeria monocytogenes and which is not capable of hybridizing portions of the genomes of other Listeria species, said con¬ tacting occurring under conditions sufficient to cause said DNA probe to hybridize to any homologous denatured bacterial DNA; an (e) detecting any hybridized probe. 8. The DNA probe of claim 7 wherein said probe comprises oligo 1309-1966 of the nucleotide sequence of Figure 2. 9. The DNA probe of claim 7 wherein said probe comprises oligo 1775-1805 of the nucleotide sequence of Figure 2. 10. The DNA probe of claim 7 wherein said probe comprises oligo 1412-1441 of the nucleotide sequence of Figure 2. 11. The DNA probe of claim 7 wherein said probe comprises oligo 1643-1665 of the nucleotide sequence of Figure 2. 12. The DNA probe of claim 7 wherein said probe comprises oligo 1810-1835 of the nucleotide sequence of Figure 2.

Inventors: BAQUERO FERNANDO, COSSART PASCALE, BAQUERO, FERNANDO, COSSART, PASCALE
Applicants: PASTEUR INSTITUT, INSTITUT PASTEUR
Application year: 1989
Main classification: C12Q1
IPC codes: C12N15, C12Q1, C12R1