Oligonucleotide probes for detection and differentiation of Staphylococcus aureus strains containing genes for enterotoxins A, B, and C and toxic shock syndrome toxin 1

Staphylococcal Enterotoxins: Description and Importance in Food

Staphylococcus aureus stands out as one of the most virulent pathogens in the genus Staphylococcus. This characteristic is due to its ability to produce a wide variety of staphylococcal enterotoxins (SEs) and exotoxins, which in turn can cause staphylococcal food poisoning (SFP), clinical syndromes such as skin infections, inflammation, pneumonia, and sepsis, in addition to being associated with the development of inflammation in the mammary glands of dairy cattle, which results in chronic mastitis and cell necrosis. SEs are small globular proteins that combine superantigenic and emetic activities; they are resistant to heat, low temperatures, and proteolytic enzymes and are tolerant to a wide pH range. More than 24 SE genes have been well described (SEA-SEE, SEG, SEH, SEI, SEJ, SElK, SElL, SElM, SElN, SElO, SElP, SElQ, SElR, SElS, SElT, SElU, SElV, SElW, SElX, SElY, and SElZ), being a part of different SFP outbreaks, clinical cases, and isolated animal strains. In recent years, new genes (sel26, sel27, sel28, sel31, sel32, and sel33) from SEs have been described, as well as two variants (seh-2p and ses-3p) resulting in a total of thirty-three genes from Ses, including the nine variants that are still in the process of genetic and molecular structure evaluation. SEs are encoded by genes that are located in mobile genetic elements, such as plasmids, prophages, pathogenicity islands, and the enterotoxin gene cluster (egc), and housed in the genomic island of S. aureus. Both classical SEs and SE-like toxins (SEls) share phylogenetic relationships, structure, function, and sequence homology, which are characteristics for the production of new SEs through recombination processes. Due to the epidemiological importance of SEs, their rapid assessment and detection have been crucial for food security and public health; for this reason, different methods of identification of SEs have been developed, such as liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS), molecular methods, and whole-genome sequencing; providing the diagnosis of SEs and a better understanding of the occurrence, spread, and eradication of SEs. This review provides scientific information on the enterotoxins produced by S. aureus, such as structural characteristics, genetic organization, regulatory mechanisms, superantigen activity, mechanisms of action used by SEs at the time of interaction with the immune system, methods of detection of SEs, and recent biocontrol techniques used in food.

A novel tandem repeat cloning technique for creation of multiple short peptide repeats to differentiate closely related antigens

Antibody cross-reactivity is a problem often associated with closely related antigens. This study was aimed to develop a method enabling differentiation of closely related toxins based on antigen designing strategy. The method involves identification of disparate amino acids (AA) confined to target antigen in comparison with two or more closely related antigens, their assembly into a DNA oligomer and further cloning as six tandem repeats (TR) using restriction and ligation strategy into a desired vector and finally generation of antigen specific antibodies. The practical utility of this method was demonstrated by generating and testing the specificity of polyclonal antibodies against staphylococcal enterotoxin C (SEC). Cross-reactivity is a problem often associated with SEC in immunoassays due to its amino acid sequence identity with staphylococcal enterotoxin B (SEB) (40-60%). To circumvent the same, the above-mentioned strategy was applied. Unique AA of SEC (36 AA) in comparison to SEB were selected, reassembled and with deduced corresponding nucleotides, an oligomer of 117 bases was designed. Using primers with restriction overhangs, three constructs were created each with two repeats using a common restriction site. The resulting three constructs were sequentially cloned into alternating restriction sites of pRSET A vector in directional orientation, expressed in E. coli for rTR/SEC protein which was used to generate specific polyclonal antibodies against SEC. Specificity was compared with antibody raised against whole SEC recombinant protein using Western blot and dot blot assays. High specificity was achieved through the developed strategy signifying its possible application to address cross-reactivity problem associated with closely related antigens.

Basis of Virulence in Enterotoxin-Mediated Staphylococcal Food Poisoning

The Staphylococcus aureus enterotoxins are a superfamily of secreted virulence factors that share structural and functional similarities and possess potent superantigenic activity causing disruptions in adaptive immunity. The enterotoxins can be separated into two groups; the classical (SEA-SEE) and the newer (SEG-SElY and counting) enterotoxin groups. Many members from both these groups contribute to the pathogenesis of several serious human diseases, including toxic shock syndrome, pneumonia, and sepsis-related infections. Additionally, many members demonstrate emetic activity and are frequently responsible for food poisoning outbreaks. Due to their robust tolerance to denaturing, the enterotoxins retain activity in food contaminated previously with S. aureus. The genes encoding the enterotoxins are found mostly on a variety of different mobile genetic elements. Therefore, the presence of enterotoxins can vary widely among different S. aureus isolates. Additionally, the enterotoxins are regulated by multiple, and often overlapping, regulatory pathways, which are influenced by environmental factors. In this review, we also will focus on the newer enterotoxins (SEG-SElY), which matter for the role of S. aureus as an enteropathogen, and summarize our current knowledge on their prevalence in recent food poisoning outbreaks. Finally, we will review the current literature regarding the key elements that govern the complex regulation of enterotoxins, the molecular mechanisms underlying their enterotoxigenic, superantigenic, and immunomodulatory functions, and discuss how these activities may collectively contribute to the overall manifestation of staphylococcal food poisoning.

A Review of the Methods for Detection of Staphylococcus aureus Enterotoxins

Food safety has attracted extensive attention around the world, and food-borne diseases have become one of the major threats to health. Staphylococcus aureus is a major food-borne pathogen worldwide and a frequent contaminant of foodstuffs. Staphylococcal enterotoxins (SEs) produced by some S. aureus strains will lead to staphylococcal food poisoning (SFP) outbreaks. The most common symptoms caused by ingestion of SEs within food are nausea, vomiting, diarrhea and cramps. Children will suffer SFP by ingesting as little as 100 ng of SEs, and only a few micrograms of SEs are enough to cause SPF in vulnerable populations. Therefore, it is a great challenge and of urgent need to detect and identify SEs rapidly and accurately for governmental and non-governmental agencies, including the military, public health departments, and health care facilities. Herein, an overview of SE detection has been provided through a comprehensive literature survey.

Detection of Staphylococcus aureus enterotoxin production genes from patient samples using an automated extraction platform and multiplex real-time PCR

To minimize specimen volume, handling and testing time, we have developed two TaqMan(®) multiplex real-time PCR (rtPCR) assays to detect staphylococcal enterotoxins A-E and Toxic Shock Syndrome Toxin production genes directly from clinical patient stool specimens utilizing a novel lysis extraction process in parallel with the Roche MagNA Pure Compact. These assays are specific, sensitive and reliable for the detection of the staphylococcal enterotoxin encoding genes and the tst1 gene from known toxin producing strains of Staphylococcus aureus. Specificity was determined by testing a total of 47 microorganism strains, including 8 previously characterized staphylococcal enterotoxin producing strains against each rtPCR target. Sensitivity for these assays range from 1 to 25 cfu per rtPCR reaction for cultured isolates and 8-20 cfu per rtPCR for the clinical stool matrix.

Covering all the Bases: Preclinical Development of an Effective Staphylococcus aureus Vaccine

A key aspect of the pathogenesis of the Gram positive bacterium Staphylococcus aureus is its ability to rapidly adapt to the host environment during the course of an infection. To successfully establish infection, the organism deploys a variety of survival and immune evasion strategies, ranging from the acquisition of essential nutrients and expression of adhesins, which promote colonization and survival, to the elaboration of virulence factors such as capsule, which aids host immune evasion. The ability of S. aureus to deploy different virulence factors must be taken into account for S. aureus vaccine design. Here, we present a strategy for designing an effective vaccine against S. aureus disease by evaluating vaccine candidate performance in multiple in vivo models targeted to mimic aspects of human disease, and by co-development of functional in vitro immunoassays that measure the neutralization of relevant S. aureus virulence factors.

Dye labelled monoclonal antibody assay for detection of Toxic Shock Syndrome Toxin -1 from Staphylococcus aureus

OBJECTIVE

The aim of study was to develop a rapid assay, dye labelled monoclonal antibody assay (DLMAA), using non-radioactive organic synthetic dyes for identification of Toxic Shock Syndrome Toxin-1 (TSST-1) producing strains of Staphylococcus aureus.

MATERIALS AND METHODS

The assay protocol required only two simple steps; addition of TSST-1 antigen to a nitrocellulose membrane and then adding a colloidal dye labelled antibody (D/A) suspension detection reagent.

RESULTS

The sensitivity and specificity of the assay was determined relative to positive and negative strains compared to an ELISA assay. Overall 100% agreement was found between both assays. The sensitivity for detection of TSST-1 was 30 ng.

CONCLUSION

The DLMAA did not require handling and disposal of radioactive materials. It is a rapid qualitative technique for detection of TSST-1 toxin at room temperature within a short time.

Detection of SEB gene by bilayer lipid membranes nucleic acid biosensor supported by modified patch-clamp pipette electrode

This work reports a kind of novel bilayer lipid membranes (BLMs) nucleic acid biosensor supported by modified patch-clamp pipette electrode was developed to detect staphylococcus enterotoxins B (SEB) gene. BLMs were formed within 15 min and able to be operated at least 24 h. Hydrophobic dodecane tail (C12) modified 18 bp single-stranded DNA (ssDNA) probe was immobilized on BLMs. The electrochemical currents versus the different concentration of ssDNA probe immobilized on BLMs indicated linearly correlation. The BLMs nucleic acid biosensor was fabricated by selecting the ssDNA probe as the signal sensing element with the concentration of 273.65 ng/mL. The electrochemical performance of the biosensor for the detection of SEB was investigated. The result showed that linear relationship was found between the current and ln(concentration) from 20 to 5000 ng/mL and the detection limit was 20 ng/mL. In addition, the biosensor was specific response to SEB gene and showed no significant current alteration in electrolyte which containing no SEB gene. Finally, Atom Force Microscope (AFM) images could be observed and used to evaluate the superficial microstructure of BLMs, ssDNA immobilized on BLMs and BLMs after hybridization. The BLMs nucleic acid biosensor supported by modified patch-clamp pipette electrode will become a highly sensitive, rapid, selective analytical tool for detection of Staphylococcus aureus, which produce SEB.

USE OF PCR TO DETECT CLASSICAL ENTEROTOXINS GENES (ENT) AND TOXIC SHOCK SYNDROME TOXIN-1 GENE (TST) IN STAPHYLOCOCCUS AUREUS ISOLATED FROM CRUDE MILK AND DETERMINATION OF TOXIN PRODUCTIVITIES OF S. AUREUS ISOLATES HARBORING THESE GENES

ABSTRACT During a 2-year period (2003-2004), 132 strains of Staphylococcusaureus isolated from crude milk (without thermal treatment) collected in different places in Piracicaba, São Paulo State, Brazil, were investigated for the presence of genes for enterotoxins (ent) and toxic shock syndrome toxin-1 (tst). Polymerase-chain reaction (PCR) was performed by using 6 pairs of relevant oligonucleotide primers. Ninety isolates (68.18%) were positive for (47 strains) or 2 (43 strains) toxin genes. The combination of entA and tst showed the highest prevalence (33 strains).The good correlation between PCR results and toxin protein detection and identification by optimum-sensitivity-plate (OSP) test was observed when 44.45% of strains showed positive for toxin production.

Detection of Staphylococcus aureus enterotoxin A and B genes with PCR-EIA and a hand-held electrochemical sensor

Two electrochemical assays for detecting Staphylococcus aureus enterotoxin A and B genes were developed. The assays are based on PCR amplification with biotinylated primers, hybridization to a fluorescein-labeled probe, and detection with horseradish peroxidase-conjugated anti-fluorescein antibody using a hand-held electrochemical detector. The limit of detection (LOD) for both assays was approximately 16 copies of the sea and seb genes. The assays were evaluated in blinded studies, each with 81 samples that included genomic and cloned S. aureus DNA, and genomic DNA from Alcaligens, Bacillus, Bacteroides, Bordetella, Borkholderia, Clostridium, Comanonas, Enterobacter, Enterococcus, Escherichia, Francisella, Haemophilus, Klebsiella, Listeria, Moraxella, Neisseria, Proteus, Pseudomonas, Salmonella, Serratia, Shigella, Streptococcus, Vibrio and Yersinia species. Both assays showed 100% sensitivity. The specificity was 96% for the SEA assay and 98% for the SEB assay. These results demonstrate the feasibility of performing probe-based detection of PCR products with a low-cost, hand-held, electrochemical detection device as a viable alternative to colorimetric enzyme-linked assays of PCR products.

Molecular diagnostics of clinically important staphylococci

Bacterial species of the genus Staphylococcus known as important human and animal pathogens are the cause of a number of severe infectious diseases. Apart from the major pathogen Staphylococcus aureus, other species until recently considered to be nonpathogenic may also be involved in serious infections. Rapid and accurate identification of the disease-causing agent is therefore prerequisite for disease control and epidemiological surveillance. Modern methods for identification and typing of bacterial species are based on genome analysis and have many advantages compared to phenotypic methods. The genotypic methods currently used in molecular diagnostics of staphylococcal species, particularly of S. aureus, are reviewed. Attention is also paid to new molecular methods with the highest discriminatory power. Efforts made to achieve interlaboratory reproducibility of diagnostic methods are presented.

Staphylococcal accessory gene regulator (sar) as a signature gene to detect enterotoxigenic staphylococci

AIMS

To evaluate the use of a staphylococcal accessory gene regulator (sar) as a means of detecting enterotoxigenic staphylococci.

METHODS AND RESULTS

SarA gene-specific primers were designed and applied in PCR, which resulted in the detection of 49 sar-positive isolates from a total of 67 natural food isolates of staphylococci. Colony hybridization using PCR-generated Digoxigenin (DIG)-labelled sarA probe tested in spiked samples of khoa (a traditional heat-concentrated milk product) comprising a mixed microflora ensured the specificity of the probe. Validation experiments with the commercial samples of khoa also demonstrated the specificity of the probe. PCR characterization for enterotoxins A-D revealed the presence of at least one of the toxin-encoding genes in all the sarA-positive isolates tested.

CONCLUSION

The study indicated that sarA gene could be an ideal marker gene either in colony hybridization or in PCR, for an effective detection of potentially enterotoxigenic strains of staphylococci in a food system.

SIGNIFICANCE AND IMPACT OF THE STUDY

As an alternative to targeting the individual toxin genes, a regulatory gene responsible for controlling the synthesis of various virulence factors may be a suitable target gene for screening potentially toxigenic staphylococci in food system using nucleic acid-based methods.

Development of a single-reaction multiplex PCR toxin typing assay for Staphylococcus aureus strains

We describe here the development of a single-reaction multiplex PCR assay for the enterotoxin genes from Staphylococcus aureus that utilizes a universal toxin gene primer in combination with toxin-specific primers to amplify characteristic toxin gene products. In combination with a new DNA purification method, the assay can detect enterotoxin genes A to E from a pure culture within 3 to 4 h. The test was used to characterize a diverse set of environmental S. aureus isolates, and a 99% correlation with toxin typing using standard immunological tests was found. The design of the assay allows it to be extended to include both newly characterized and as-yet-unknown toxin genes.

Mitogenic activities of amino acid substitution mutants of staphylococcal enterotoxin B in human and mouse lymphocyte cultures

Site-directed mutagenesis has been used to introduce amino acid substitutions at specific residues of the staphylococcal enterotoxin B (SEB) gene cloned from Staphylococcus aureus 10-275. The mitogenic activities of these derivatives were determined in two assay systems: (i) mouse spleen cells and (ii) a mixture of human peripheral blood mononuclear cells and lymphocytes. Substitution of either His-12, His-32, His-121, His-166, Lys-152, or Gly-205 did not significantly alter the mitogenic activity from that of the wild-type toxin in either proliferation assay. Substitution of either residue Asn-23, Phe-44, or Cys-93 reduced the mitogenicity of SEB by a degree that depended upon the assay system used. Similar to the results reported by others measuring toxin activation of mouse lymphoid cells, we found that substitutions of these three residues of SEB caused at least 800-fold reductions of mitogenic activity from that of the wild-type toxin. When tested for toxicity in vivo in D-galactosamine-treated mice, the reduced activities of these mutant toxins, however, were not as pronounced. In contrast, when tested in the human cell mitogenicity assay, these mutant toxins were active. Small alterations in activity (two- to fivefold reduction) were observable only at low concentrations. Our findings reveal the importance of using human lymphocytes in addition to the traditional mouse spleen cell assay when assessing biological activities of staphylococcal enterotoxins.

Coagulase-negative staphylococci isolated from two cases of toxic shock syndrome lack superantigenic activity, but induce cytokine production

Two strains of Staphylococcus epidermidis isolated from patients with toxic shock symptoms have been reported to carry genes related to S. aureus enterotoxins B and C by dot-blot hybridisation, although the corresponding superantigenic toxins were not detected immunologically. We here show that these strains produce no superantigens capable of stimulating proliferation of human mononuclear leukocytes or rabbit splenocytes, and that no DNA homologous to the seb or sec genes can be detected by PCR. However, stimulation of human monocytes by whole killed bacteria induced dose-dependent production of the cytokines TNF alpha, IL-1 beta and IL-6, which may be responsible for the clinical symptoms in these patients.

Genotypic identification of staphylococcal enterotoxins and toxic shock syndrome toxin 1 by the enzymatic detection of polymerase chain reaction-amplified DNA

The enzymatic detection of a polymerase chain reaction product (ED-PCR), a new detection method of PCR-amplified DNA, was evaluated for the identification of staphylococcal enterotoxin (SE) and toxic shock syndrome toxin 1 (TSST-1) genes. A total of 61 Staphylococcus aureus strains, including reference strains and strains isolated from clinical specimens and food poisoning outbreaks, were examined by ED-PCR and by reverse passive latex agglutination (RPLA) phenotypic identification. There was 100% agreement between the genotypic and phenotypic identification of SEA, SEB, SEC, SEE strains and TSST-1. In the case of SED, however, 4 strains were positive by ED-PCR and negative by RPLA. ED-PCR offers an accurate alternative to traditional immunoassays or conventional PCR using electrophoresis for the detection of SE and TSST-1 production yielding results that are more precise than with older techniques.

Probes and polymerase chain reaction for detection of food-borne bacterial pathogens

DNA-hybridization and the polymerase chain reaction (PCR) are techniques commonly used to detect pathogenic bacteria. In this paper, the use of these techniques for detection of Salmonella, E. coli, V. cholerae, non-O1 Vibrio, Yersinia enterocolitica, Campylobacter, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and C. botulinum is reviewed with emphasis on application in food microbiology. In food control, DNA-techniques have most often been used in a 'culture confirmation' fashion, i.e. bacteria are enriched and sometimes even purified by traditional culture procedures and thereafter identified by the use of DNA-based methods. The most desirable approach is, however, to detect organisms directly in the food, but major problems remain to be solved before this can be routinely performed.

Identification of staphylococcal enterotoxin B sequences important for induction of lymphocyte proliferation by using synthetic peptide fragments of the toxin

A series of 13 synthetic peptides, approximately 30 amino acids each, which spanned the entire sequence of staphylococcal enterotoxin B (SEB) were tested to evaluate their effects on T-cell proliferation in a culture system containing elutriated human peripheral blood lymphocytes incubated with a specific ratio of mononuclear cells. Four peptide regions were found to inhibit SEB-induced proliferation; they included sequences 1 to 30 (previously thought to be involved in major histocompatibility complex class II binding), 61 to 92 (sequences which relate to the T-cell receptor site), 93 to 112 (a linear sequence corresponding to the cysteine loop), and 130 to 160 (containing a highly conserved sequence, KKKVTAQEL). Antisera raised to this last peptide were capable of neutralizing SEB-induced proliferation. Antisera raised against the peptides which overlapped this sequence also were somewhat inhibitory. Neutralizing antisera were not produced from any other peptide sequence tested. To determine if any of these effects were nonspecific with regard to SEB-induced proliferation, the peptides were tested for inhibition of phorbol dibutyryl ester-induced proliferation, and only the sequence 93 to 112 (corresponding to the cysteinyl loop region) was consistently inhibitory (40%). Of the regions which displayed inhibition of SEB-induced proliferation, the peptide 130 to 160 inhibited binding of 125I-SEB to lymphocytes. These data suggest that the residues containing and surrounding the sequence KKKVTAQEL may be critical in the SEB-induced proliferation and may be useful for developing neutralizing antisera to SEB.

Phage-associated differences in staphylococcal enterotoxin A gene (sea) expression correlate with sea allele class

Staphylococcus aureus strains which produced either high or low levels of staphylococcal enterotoxin A (SEA) with a minimal eightfold difference between the two groups were identified. For FRI100 and FRI281A (prototypes for each group), strain differences in the expression of the SEA-encoding gene (sea) were found to occur at the level of sea mRNA concentration, and part of the difference in expression was associated with the sea-containing phages. Southern blot analysis revealed that this phage-associated difference was not due to differences in the copy number of sea. Nucleotide sequence analysis of sea from FRI281A revealed a new allele of sea, with the majority of the sequence differences occurring in the upstream promoter region. Although a strict correlation was observed between the level of SEA production and sea allele class for several strains, the sequence differences observed in the upstream region were not sufficient in themselves to alter the expression level of sea.

Potential problems in the use of oligonucleotide probes for staphylococcal enterotoxin genes

Oligonucleotide probes unique to the five major enterotoxin genes of Staphylococcus aureus were synthesized and used to detect DNA sequences homologous to these genes in 27 non-clinical isolates of Staph. aureus isolated from nasal swabs of 74 healthy human volunteers. Genomic DNA from all 27 isolates reacted with at least one of the probes. In a phenotypic assay for toxin production by a reverse passive latex agglutination test however, only 15 of the 27 isolates produced enterotoxin in culture. The results raise the possibility that a number of Staph. aureus isolates harbour DNA sequences that are apparently silent or mutant copies of the enterotoxin genes. This complicates the identification of enterotoxin producers by tests which depend on oligonucleotide or DNA hybridization.

Investigation of a staphylococcal food poisoning outbreak in a centralized school lunch program

The trend in many communities toward centralized school lunch preparation potentially increases the risk of foodborne illness. Foods often are prepared long before serving and may be distributed to satellite schools by persons with little formal training in safe techniques of food preparation or food service. In May 1990, an outbreak of staphylococcal food poisoning occurred in elementary schools in a Rhode Island community participating in such a program. In the investigation of the outbreak, students in schools that reported cases were interviewed. Food preparation, handling, and distribution were reviewed. At School E, 662 lunches were prepared and distributed to 4 additional schools (schools A-D). Schools A and B accounted for nearly all cases of the food poisoning, with rates of 47 percent and 18 percent. Eating ham increased the risk of illness (62 percent of those consuming ham and 3 percent of those who did not, relative risk = 18.0, 95 percent confidence interval = 4.0, 313.4). Large amounts of Staphylococcus aureus were cultured, and preformed enterotoxin A was identified in leftover ham. A food handler, who tested positive for the implicated enterotoxic strain S. aureus, reported having removed the casings from two of nine warm ham rolls 48 hours prior to service. Because of improper refrigeration, prolonged handling, and inadequate reheating, the ham was held at temperatures estimated at 10-49 degrees Celsius (50-120 degrees Fahrenheit) for a minimum of 15 hours. The potential for larger outbreaks prompted a statewide training program in safe food preparation for school lunch personnel, which may have applications for other communities.

Synthetic DNA probes for detection of genes for enterotoxins A, B, C, D, E and for TSST-1 in staphylococcal strains

A dot blot hybridization technique with oligonucleotide probes was developed for the specific detection of the TSST-1 gene and the staphylococcal enterotoxin (SE) genes A, B, C, D and E. For each toxin gene a probe sequence was chosen from the previously determined sequence. A total of 145 staphylococcal strains (133 Staphylococcus aureus and 12 coagulase-negative staphylococci (CNS) were studied by this genotypic method and by two phenotypic assays (gel immunodiffusion and ELISA). An excellent correlation (96%) was observed between the genotypic and phenotypic assays. DNA from two CNS strains hybridized with a probe without detection of the corresponding toxin (SEB for one strain and SEC for the other strain). One Staph. aureus strain was shown to be an SEC producer, but was not detected by the corresponding probe. Gene probe and immunological assays seem to be complementary methods for studies of staphylococcal strains producing (or potentially producing) TSST-1 or enterotoxins.

New microbial diagnosis

Rapid methods for comprehensive nucleic acid analysis are essential for the progress in basic research. In microbial diagnosis nucleic acid analyses are useful for achieving quick, sensitive results with new dimensions of specificity. As to specificity, hybridisation tests are easily constructed for genus or species specific microbial typing or for the detection of genes accounting for pathogenic properties. Modern sequence analysis allows rapid detection of mutations eg., those conferring antiviral resistance. A new level of sensitivity comparable to that of the conventional enrichment culture is obtainable by enzymatic amplification (PCR) of microbial nucleic acids before detection. PCR-technology has proved particularly useful in the diagnosis of microbial infections when the organisms are impossible or tedious to cultivate. For reliable diagnostic use the performance of in vitro amplification requires extreme care to prevent false positive results owing to carry over problems. In conclusion, nucleic acid analyses are expanding the potential for diagnosing microbial infections and may well replace some conventional diagnostic methods.

Screening of staphylococci for the toxic shock syndrome toxin-1 (TSST-1) gene

Dot blot hybridization was used to screen 820 staphylococci for the presence of the gene coding for TSST-1. The DNA of 33 strains among 70 Staph. aureus strains isolated from suspected toxic shock syndrome (TSS) cases hybridized with the probe. These results agreed perfectly with those obtained with a phenotypic method (immunodiffusion). Among 608 Staph. aureus strains isolated over a period of one month from hospitalized patients, 66 (11%) hybridized with the probe; of these strains, 64 (97%) were found to produce TSST-1 in vitro. None of 145 coagulase-negative staphylococcal strains harboured DNA hybridizing with the probe. The data indicate that this genotypic assay is suitable for epidemiological studies.

Detection of genes for enterotoxins, exfoliative toxins, and toxic shock syndrome toxin 1 in Staphylococcus aureus by the polymerase chain reaction

Eight pairs of synthetic oligonucleotide primers were used in a polymerase chain reaction (PCR) protocol to detect genes for staphylococcal enterotoxins A to E, exfoliative toxins A and B, and toxic shock syndrome toxin 1 in Staphylococcus aureus strains isolated from clinical specimens and contaminated foods. Primers were targeted to internal regions of the toxin genes, and amplification fragments were detected after the PCR by agarose gel electrophoresis. Unequivocal discrimination of toxin genes was obtained by the PCR by using nucleic acids extracted from 88 strains of S. aureus whose toxigenicity was established biologically and immunologically. In immunological assays, two strains of S. aureus produced equivocal results for production of enterotoxin C or toxic shock syndrome toxin 1, giving an overall concordance between phenotypic and genotypic identification of 97.7%. Primer specificity was established in the PCR by using nucleic acids from known toxin-producing bacterial pathogens and from nontoxigenic S. aureus. Strains of Streptococcus spp., including some producers of pyrogenic exotoxin A carrying the speA gene, were negative by the PCR designed to detect staphylococcal toxins. The detection limits were established for all the staphylococcal toxin genes within their respective PCR protocols. The identification of staphylococcal toxin genes in strains of S. aureus by the PCR offers a very specific, sensitive, relatively rapid, and inexpensive alternative to traditional immunological assays which depend on adequate gene expression for reliability and sensitivity.