Detection of enterotoxigenic Staphylococcus aureus in dried skimmed milk: use of the polymerase chain reaction for amplification and detection of staphylococcal enterotoxin genes entB and entC1 and the thermonuclease gene nuc

Characterisation and prevalence of community-associated MRSA among horses, dogs, cats and their human handlers: a cross-sectional study

Methicillin-resistant Staphylococcus aureus (MRSA) as an infectious organism of public health significance has evolved to a genetically distinct community-acquired MRSA with extended resistance to other than β-lactams. A cross-sectional study was conducted among 149 participants handling 446 animals (240 horses and 206 companion animals). The isolates were characterised as S. aureus and MRSA based on polymerase chain reaction detection of the nuc, mecA and mecC genes and the pvl gene for differentiation as community associated/livestock associated or hospital associated. The isolation rate of S. aureus from the human handlers' samples was 26 (17.4%) and 170 (38.1%) from the animal samples. The prevalence of MRSA among the isolates was 7 (4.7%) from the human handlers and 19 (4.3%) from the animals. Dogs and dog handlers had the highest isolation rates and were more likely to be colonized by S. aureus and MRSA compared with horses, cats and their handlers. The highest prevalence of MRSA was from horses (5.0%) and dog handlers (10.6%). This study has demonstrated a high prevalence of community associated MRSA in apparently healthy animals and their human handlers. This has important implications for antibiotic selection and use as well as infection control measures.

Sink survey to investigate multidrug resistance pattern of common foodborne bacteria from wholesale chicken markets in Dhaka city of Bangladesh

Antimicrobial resistance (AMR) among foodborne bacteria is a well-known public health problem. A sink survey was conducted to determine the AMR pattern of common foodborne bacteria in cloacal swab of broiler chickens and sewage samples from five wholesale chicken markets of Dhaka city in Bangladesh. Bacteria were identified by culture-based and molecular methods, and subjected to antimicrobial susceptibility testing. Resistance genes were identified by multiplex PCR and sequencing. Multidrug resistance (MDR) was observed in 93.2% of E. coli, 100% of Salmonella spp., and 97.2% of S. aureus from cloacal swab samples. For sewage samples, 80% of E. coli, and 100% of Salmonella and S. aureus showed MDR. Noteworthy, 8.3% of S. aureus from cloacal swab samples showed possible extensively drug resistance. Antimicrobial resistance genes (beta-lactamase—blaTEM, blaSHV; quinolone resistance gene—qnrS) were detected in a number of E. coli and Salmonella isolates from cloacal swab and sewage samples. The methicillin resistance gene (mecA) was detected in 47.2% and 25% S. aureus from cloacal swab and sewage samples, respectively. The findings envisage the potential public health risk and environmental health hazard through spillover of common foodborne MDR bacteria.

Aptasensors for Staphylococcus aureus Risk Assessment in Food

Staphylococcus aureus (S. aureus) is the top ordinary pathogen causing epidemic and food poisoning. The authentication of S. aureus has great significance for pathologic diagnosis and food hygiene supervision. Various biosensor methods have been established for identification. This paper reviews the research progress of aptasensors for S. aureus detection, focusing on the classification of aptamer technologies, including optical aptasensors and electrochemical aptasensors. Furthermore, the feasibility and future challenges of S. aureus detection for aptamer assays are discussed. Combining aptasensors with nanomaterials appears to be the developing trend in aptasensors.

Poly-l-lysine-functionalized magnetic beads combined with polymerase chain reaction for the detection of Staphylococcus aureus and Escherichia coli O157:H7 in milk

Rapid and credible detection of pathogens is essential to prevent and control outbreaks of foodborne diseases. In this study, a poly-l-lysine-functionalized magnetic beads (PLL-MB) strategy combined with a PCR assay was established to detect Staphylococcus aureus. We also detected Escherichia coli O157:H7 to further verify the strategy for gram-negative bacteria detection. Poly-l-lysine has strong positive charges because of its amino groups, which can conjugate with the carboxyl of carboxyl magnetic beads. Furthermore, it can be used to combine with bacteria through electrostatic adsorption. Under optimum conditions, the developed PLL-MB complexes showed 90% capture efficiency in phosphate-buffered saline and 85% capture efficiency in milk for S. aureus detection. The limit of detection of the PLL-MB-PCR assay was 10² cfu/mL (1.8 × 10² cfu/mL for S. aureus and 7 × 10² cfu/mL for E. coli O157:H7) in phosphate-buffered saline and milk samples. The whole assay can be performed within 4 h. The proposed strategy showed a lower limit of detection when compared with the conventional PCR assay without enrichment. In addition, this method exhibited the advantages of a high-efficient, cost-efficient, and simple operation, indicating its potential applications in foodborne pathogen detection.

Prevalence and Multidrug Resistance Pattern of Methicillin Resistant S. aureus Isolated from Frozen Chicken Meat in Bangladesh

Infections by methicillin-resistant Staphylococcus aureus (MRSA) are continuously expanding within the community. Chicken meat is usually contaminated by MRSA, and this contaminated chicken meat is an important source of foodborne infections in humans. In this study, a cross-sectional supershop survey was conducted to determine the prevalence and antimicrobial resistance pattern of MRSA in 113 domestic frozen chicken meat samples purchased from nine branded supershops available in five divisional megacities of Bangladesh. The study also focused on the determination of methicillin resistance gene in MRSA isolates. S. aureus was identified by standard culture-based and molecular methods, and subjected to antimicrobial susceptibility testing. MRSA was screened by cefoxitin disk diffusion test. Methicillin resistance gene was identified by PCR. Of samples, 54.9% were positive for S. aureus, and, of these, 37.1% isolates were identified as MRSA. All the isolates were multidrug resistant (MDR): 52.2% were resistant to 6–8 antimicrobial classes, and 47.8% isolates to 9–12 classes. Three (3.2%) isolates of S. aureus were possible extensively drug resistant. The highest rates of resistance were observed against cefoxitin (100%), followed by nalidixic acid, ampicillin and oxacillin (97.7%), colistin (91.3%), amoxicillin-clavulanic acid and amoxicillin (87%), penicillin-G and cloxacillin (82.6%), oxytetracycline (78.3%), and cefixime (73.9%). Screening of methicillin resistance gene revealed that 43.5% isolates of MRSA were positive for mecA gene. The high prevalence of MDR MRSA in frozen chicken meat samples in this study emphasizes the need for better sanitary education of food handlers in hygienic practices focusing on their potential role as reservoirs and spreaders of MRSA.

An “OFF–ON” quantum dot–graphene oxide bioprobe for sensitive detection of micrococcal nuclease ofStaphylococcus aureus

mAb-Strep-QDs-GO probe in an OFF state due to energy transfer from QDs to GO turns into an ON state when the energy transfer is inhibited by MNase, thus allowing the sensing of MNase (Micrococcal Nuclease, an extracellular endonuclease ofStaphylococcus Aureus).

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.

Proteomic Methods of Detection and Quantification of Protein Toxins

Biological toxins are a heterogeneous group of compounds that share commonalities with biological and chemical agents. Among them, protein toxins represent a considerable, diverse set. They cover a broad range of molecular weights from less than 1000 Da to more than 150 kDa. This review aims to compare conventional detection methods of protein toxins such as in vitro bioassays with proteomic methods, including immunoassays and mass spectrometry-based techniques and their combination. Special emphasis is given to toxins falling into a group of selected agents, according to the Centers for Disease Control and Prevention, such as Staphylococcal enterotoxins, Bacillus anthracis toxins, Clostridium botulinum toxins, Clostridium perfringens epsilon toxin, ricin from Ricinus communis, Abrin from Abrus precatorius or control of trade in dual-use items in the European Union, including lesser known protein toxins such as Viscumin from Viscum album. The analysis of protein toxins and monitoring for biological threats, i.e., the deliberate spread of infectious microorganisms or toxins through water, food, or the air, requires rapid and reliable methods for the early identification of these agents.

A method for the direct detection of airborne dispersal in lichens

This study sets out a novel method to determine dispersal distances in lichens. Direct measurement of dispersal often remains difficult for lichens and other small inconspicuous species because of the need to track microscopic reproductive propagules, which even if they can be captured, cannot be identified using traditional morphological approaches. A low-cost device (<£200) was developed to trap the reproductive propagules of lichens, capable of sampling around 0.1 m³ of air per minute. In parallel, molecular techniques were developed to enable species-specific detection of propagules caught by the devices, with identification using novel species-specific primers and optimization of a standard DNA extraction and nested PCR protocol. The methods were tested for both their sensitivity and specificity against a suite of lichen epiphytes, differing in their reproductive mechanisms, dispersal structures and rarity. Sensitivity tests showed that the molecular techniques could detect a single asexual propagule (soredium or isidium), or as few as 10 sexual spores. As proof of concept, propagule traps were deployed into a wooded landscape where the target epiphytes were present. Extractions from deployed propagule traps were sequenced, showing that the method was able to detect the presence of the target species in the atmosphere. As far as we are aware, this is the first attempt to use mechanized propagule traps in combination with DNA diagnostics to detect dispersal of lichens. The tests carried out here point the way for future dispersal studies of lichen epiphytes and other passively dispersed microscopic organisms including fungi or bryophytes.

Advanced molecular diagnostic techniques for detection of food-borne pathogens: Current applications and future challenges

The elimination of disease-causing microbes from the food supply is a primary goal and this review deals with the overall techniques available for detection of food-borne pathogens. Now-a-days conventional methods are replaced by advanced methods like Biosensors, Nucleic Acid-based Tests (NAT), and different PCR-based techniques used in molecular biology to identify specific pathogens. Bacillus cereus, Staphylococcus aureus, Proteus vulgaris, Escherichia coli, Campylobacter, Listeria monocytogenes, Salmonella spp., Aspergillus spp., Fusarium spp., Penicillium spp., and pathogens are detected in contaminated food items that cause always diseases in human in any one or the other way. Identification of food-borne pathogens in a short period of time is still a challenge to the scientific field in general and food technology in particular. The low level of food contamination by major pathogens requires specific sensitive detection platforms and the present area of hot research looking forward to new nanomolecular techniques for nanomaterials, make them suitable for the development of assays with high sensitivity, response time, and portability. With the sound of these, we attempt to highlight a comprehensive overview about food-borne pathogen detection by rapid, sensitive, accurate, and cost affordable in situ analytical methods from conventional methods to recent molecular approaches for advanced food and microbiology research.

Advances and Challenges in Viability Detection of Foodborne Pathogens

Foodborne outbreaks are a serious public health and food safety concern worldwide. There is a great demand for rapid, sensitive, specific, and accurate methods to detect microbial pathogens in foods. Conventional methods based on cultivation of pathogens have been the gold standard protocols; however, they take up to a week to complete. Molecular assays such as polymerase chain reaction (PCR), sequencing, microarray technologies have been widely used in detection of foodborne pathogens. Among molecular assays, PCR technology [conventional and real-time PCR (qPCR)] is most commonly used in the foodborne pathogen detection because of its high sensitivity and specificity. However, a major drawback of PCR is its inability to differentiate the DNA from dead and viable cells, and this is a critical factor for the food industry, regulatory agencies and the consumer. To remedy this shortcoming, researchers have used biological dyes such as ethidium monoazide and propidium monoazide (PMA) to pretreat samples before DNA extraction to intercalate the DNA of dead cells in food samples, and then proceed with regular DNA preparation and qPCR. By combining PMA treatment with qPCR (PMA-qPCR), scientists have applied this technology to detect viable cells of various bacterial pathogens in foods. The incorporation of PMA into PCR-based assays for viability detection of pathogens in foods has increased significantly in the last decade. On the other hand, some downsides with this approach have been noted, particularly to achieve complete suppression of signal of DNA from the dead cells present in some particular food matrix. Nowadays, there is a tendency of more and more researchers adapting this approach for viability detection; and a few commercial kits based on PMA are available in the market. As time goes on, more scientists apply this approach to a broader range of pathogen detections, this viability approach (PMA or other chemicals such as platinum compound) may eventually become a common methodology for the rapid, sensitive, and accurate detection of foodborne pathogens. In this review, we summarize the development in the field including progress and challenges and give our perspective in this area.

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.

Limit of detection of genomic DNA by conventional PCR for estimating the load of Staphylococcus aureus and Escherichia coli associated with bovine mastitis

Detection of mastitis-associated bacteria can be accomplished by culturing or by molecular techniques. On the other hand, rapid and inexpensive methods to enumerate bacterial load without culturing can be better achieved by molecular methods. Staphylococcus aureus and Escherichia coli are the predominant bacterial pathogens associated with bovine mastitis. Here, we describe the application of conventional PCR for the limit of detection (LOD) of genomic DNA of S. aureus and E. coli based on single-copy genes. The selected genes were thermonuclease (nuc), aureolysin (aur), and staphopain A (scpA) for S. aureus and β-D-glucuronidase A (uidA), cytochrome d oxidase (cyd), and rodA (a gene affecting cell shape and methicillin sensitivity) for E. coli. The LOD was 5.3, 15.9, and 143 pg for aur, nuc, and scpA genes, corresponding to S. aureus genomic copies of 1.75 × 10(3), 5.16 × 10(3), and 4.71 × 10(4), respectively. The LOD was 0.45, 12.3 and 109 pg for uidA, rodA and cyd genes, corresponding to E. coli genome copies of 8.91 × 10(1), 2.43 × 10(3), and 2.16 × 10(4), respectively. Application of uidA and aur PCRs to field strains revealed that as low as approximately 100 genome copies of E. coli and 1000-10,000 copies of S. aureus could be detected. This study is the first to report LOD of genomic DNA using conventional PCR for aur and scpA genes of S. aureus, and rodA and cyd genes of E. coli. The results should be useful for developing assays to assess bacterial load in milk and to determine the load that contributes to subclinical or clinical mastitis.

Multi-template polymerase chain reaction

PCR is a formidable and potent technology that serves as an indispensable tool in a wide range of biological disciplines. However, due to the ease of use and often lack of rigorous standards many PCR applications can lead to highly variable, inaccurate, and ultimately meaningless results. Thus, rigorous method validation must precede its broad adoption to any new application. Multi-template samples possess particular features, which make their PCR analysis prone to artifacts and biases: multiple homologous templates present in copy numbers that vary within several orders of magnitude. Such conditions are a breeding ground for chimeras and heteroduplexes. Differences in template amplification efficiencies and template competition for reaction compounds undermine correct preservation of the original template ratio. In addition, the presence of inhibitors aggravates all of the above-mentioned problems. Inhibitors might also have ambivalent effects on the different templates within the same sample. Yet, no standard approaches exist for monitoring inhibitory effects in multitemplate PCR, which is crucial for establishing compatibility between samples.

Occurrence of Methicillin Resistant and Enterotoxigenic Staphylococcus aureus in Traditional Cheeses in the North West of Iran

Traditional dairy products are potential sources of a variety of microorganisms which participate in food poisoning. Staphylococcus aureus is a conspicuous example of toxigenic bacteria causative for food-borne diseases. Moreover, resistance to methicillin is a prominent index in food hygiene studies. In the present study, we have aimed at characterization and identification of enterotoxigenic methicillin resistant S. aureus (MRSA) isolated from traditional cheeses in Azerbaijan region in the northwest of Iran during 2012. A number of phenotypical and molecular assays were utilized for screening of S. aureus. Subsequently, the prevalence of the genes responsible for the five staphylococcal enterotoxins (SEA-SEE) and also methicillin resistance gene was assessed. The outcomes of phenotypical methods were in conformity with those of the molecular procedures. The results indicated that 16% of cheese samples were contaminated by S. aureus. 110 isolates were authenticated by both phenotypical and molecular methods. All of the mentioned isolates were positive for coa, nuc, and 16S rDNA primers. 21% of these isolates were mecA positive and 60.8% of these MRSA were positive for SEs. Regarding the frequent outbreaks of enterotoxigenic MRSA, new hygiene policies and management practices should be considered to increase food safety and avoid extra treatment costs.

Development and application of loop-mediated isothermal amplification assays on rapid detection of various types of staphylococci strains

A loop-mediated isothermal amplification (LAMP) method for rapid detection of various Staphylococcus strains and associated antibiotic resistance determinant had been developed and evaluated in this study. Six primers, including outer primers, inner primers and loop primers, were specially designed for recognizing eight distinct sequences on three targets: 16SrRNA, femA and mecA.. Forty-one reference strains, including various species of gram-negative and -positive isolates, were included in this study to evaluate and optimize LAMP assays. The optimal reaction condition was found to be 65 °C for 45 min, with detection limits at 100 fg DNA/tube and 10 CFU/reaction for 16S rRNA, 100 fg DNA/tube and 10 CFU/reaction for femA, 1 pg DNA/tube and 100 CFU/reaction for mecA, respectively. Application of LAMP assays were performed on 118 various types of Staphylococcus isolates, the detection rate of LAMP assays for the 16SrRNA, femA and mecA was 100% (118/118), 98.5% (64/65) and 94.3% (66/70), and the negative predictive value (NPV) was 100%, 98.1% and 92.3% respectively; with a 100% positive predictive value (PPV) for all three targets. In conclusion, LAMP assays were demonstrated to be useful and powerful tools for rapid detection of various Staphylococcus strains, and undoubtedly, the rapidness, technical simplicity, and cost-effectiveness of LAMP assays will demonstrate broad application for bacteriological detection of food-borne Methicillin-resistant Staphylococcus (MRS) isolates.

ESTAFILOCOCOS EM ALIMENTOS

RESUMO Este artigo de revisão tem como objetivo descrever as principais características do Staphylocococus aureus e a relação deste micro-organismo com as doenças transmitidas por alimentos. S. aureus é a principal espécie do gênero e o homem e os animais são os reservatórios. Este micro-organismo se multiplica com facilidade em vários alimentos e produz enterotoxinas (EE) termorresistentes. Leite cru e derivados, creme, tortas recheadas com creme, salada de batata, atum, carne de frango, presunto, carnes e produtos a base de ovos já foram incriminados nos surtos alimentares envolvendo os estafilococos. Os principais sintomas são náuseas, vômito, cãibras abdominais e diarréia. O período de incubação varia de 2 a 4 horas. São conhecidos 20 tipos diferentes de EE e sua produção é influenciada pela temperatura, pH, atividade de água, tamanho do inóculo, fonte de carbono e nitrogênio, concentração de sal e condições atmosféricas do substrato. A maioria das intoxicações é produzida por EEA e EED. As EE são detectáveis nos alimentos que apresentam populações de S. aureus acima de 105 UFC/mL de alimento. A dose mínima de EE ingerida para causar intoxicação é 100 ng. Para detecção de EE, existem métodos como Sensibilidade Ótima em Placas (OSP- Optimun Sensitivity Place), microslide, Aglutinação Reversa Passiva em Látex (RPLA- Reversed Passive Latex Agglutination) e ELISA. A Reação em Cadeia pela Polimerase (PCR- Polimerase Chain Reaction), apesar de não diferenciar células viáveis de não viáveis, pode ser aplicada para detecção de diversos tipos de estafilococos enterotoxigênicos.

Rapid and reliable detection of 11 food-borne pathogens using thin-film biosensor chips

Traditional methods for identifying food-borne pathogens are time-consuming and laborious, so it is necessary to develop innovative methods for the rapid identification of food-borne pathogens. Here, we report the development of silicon-based optical thin-film biosensor chips for sensitive detection of 11 food-borne pathogens. Briefly, aldehyde-labeled probes were arrayed and covalently attached to a hydrazine-derivatized chip surface, and then, biotinylated polymerase chain reaction (PCR) amplicons were hybridized with the probes. After washing and brief incubation with an antibiotin immunoglobulin G-horseradish peroxidase conjugate and a precipitable horseradish peroxidase substrate, biotinylated chains bound to the probes were visualized as a color change on the chip surface (gold to blue/purple). Highly sensitive and accurate examination of PCR fragment targets can be completed within 30 min. This assay is extremely robust, sensitive, specific, and economical and can be adapted to different throughputs. Thus, a rapid, sensitive, and reliable technique for detecting 11 food-borne pathogens was successfully developed.

A high sensitive and specific QDs FRET bioprobe for MNase

We used CdSe/ZnS quantum dots-ssDNA-fluorescent dye conjugates as bioprobes to detect micrococcal nuclease with high specificity and sensitivity, and further utilized the bioprobe to monitor the micrococcal nuclease activity in the culture medium of Staphylococcus aureus by fluorescence microscopy.

Evaluation of three different molecular markers for the detection of Staphylococcus aureus by polymerase chain reaction

The aim of this study was to target three genes of Staphylococcus aureus-fmhA (coding for a factor of unknown function), catalase and femA (coding for a factor essential for methicillin resistance) to establish and validate a PCR assay for the detection of this pathogen. Two pairs of primers were designed for fmhA and one pair each for catalase and femA genes. The PCR assays were standardized and found to give specific amplicons under similar reaction parameters. Target specificity of the primers was confirmed by DNA sequencing of the amplicons. While the initial inclusivity and exclusivity test reactions were in agreement in case of three of the primer pairs, one pair based on fmhA gene produced a non-specific product with a template DNA used in exclusivity test reactions. Forty-five strains of S. aureus were subjected to these PCR assays for their evaluation. Three among the four pairs of primers, one against each gene detected all the 45 strains precisely whereas one of the PCR assays using primers targeting the fmhA gene did not generate the specific amplicon with several of the strains. Seven unidentified strains of Gram-positive cocci subjected to these PCR assays produced negative results for each culture. Six of the strains were identified as Staphylococcus haemolyticus and one strain as Staphylococcus arlettae by 16S ribosomal gene analyses. All the three assay systems showed a detection limit of 100 cells per 20mul reaction assay. For validation of these assay systems, 80 coded samples of 11% skimmed milk spiked with different pathogens were received from NICED (National Institute of Cholera and Enteric Diseases), Kolkata and subjected to these PCR assays. All the three assays could detect S. aureus correctly in two of the samples. Amongst 150 raw milk samples, 36 (24%) were found positive for S. aureus. We conclude that fmhA, catalase and femA genes are conserved in S. aureus and, therefore, could be used as specific targets for its detection and identification by PCR. The protocols developed herein could be used for rapid and specific detection of this pathogen in food, clinical and environmental samples, especially milk.

eap Gene as novel target for specific identification of Staphylococcus aureus

The cell surface-associated extracellular adherence protein (Eap) mediates adherence of Staphylococcus aureus to host extracellular matrix components and inhibits inflammation, wound healing, and angiogenesis. A well-characterized collection of S. aureus and non-S. aureus staphylococcal isolates (n = 813) was tested for the presence of the Eap-encoding gene (eap) by PCR to investigate the use of the eap gene as a specific diagnostic tool for identification of S. aureus. Whereas all 597 S. aureus isolates were eap positive, this gene was not detectable in 216 non-S. aureus staphylococcal isolates comprising 47 different species and subspecies of coagulase-negative staphylococci and non-S. aureus coagulase-positive or coagulase-variable staphylococci. Furthermore, non-S. aureus isolates did not express Eap homologs, as verified on the transcriptional and protein levels. Based on these data, the sensitivity and specificity of the newly developed PCR targeting the eap gene were both 100%. Thus, the unique occurrence of Eap in S. aureus offers a promising tool particularly suitable for molecular diagnostics of this pathogen.

Optimization of polymerase chain reaction for detection of Clostridium botulinum type C and D in bovine samples

Botulism is a rare but serious paralytic illness caused by a nerve toxin that is produced by the bacterium Clostridium botulinum. The economic, medical and alimentary consequences can be catastrophic in case of an epizooty. A polymerase chain reaction (PCR)-based assay was developed for the detection of C. botulinum toxigenic strains type C and D in bovine samples. This assay has proved to be less expensive, faster and simpler to use than the mouse bioassay, the current reference method for diagnosis of C. botulinum toxigenic strains. Three pairs of primers were designed, one for global detection of C. botulinum types C and D (primer pair Y), and two strain-specific pairs specifically designed for types C (primer pair VC) and D (primer pair VD). The PCR amplification conditions were optimized and evaluated on 13 bovine and two duck samples that had been previously tested by the mouse bioassay. In order to assess the impact of sample treatment, both DNA extracted from crude samples and three different enrichment broths (TYG, CMM, CMM followed by TYG) were tested. A 100% sensitivity was observed when samples were enriched for 5 days in CMM followed by 1 day in TYG broth. False-negative results were encountered when C. botulinum was screened for in crude samples. These findings indicate that the current PCR is a reliable method for the detection of C. botulinum toxigenic strains type C and D in bovine samples but only after proper enrichment in CMM and TYG broth.

Occurrence of enterotoxic Staphylococcus aureus in raw milk from yaks and cattle in Mongolia

Staphylococcal food poisoning is considered one of the leading foodborne illnesses in humans worldwide and is associated with contaminated foods of animal origin, such as milk and dairy products. In this study, we investigated the occurrence of staphylococci and the enterotoxigenic properties of Staphylococcus aureus isolated from raw milk from yaks (Bos mutus) and cattle in Mongolia. Staphylococci were isolated from 72 (74%) of the 97 raw milk samples. Of the samples containing staphylococci, 69% (50 of 72) were from yaks and 30.5% (22 of 72) were from cattle. S. aureus was detected in 10% of yak (7 of 72) and 21% of cattle (15 of 72) milk samples. Staphylococcal enterotoxin C was detected in 23% (5 of 22) of the S. aureus strains investigated, based on the reverse passive latex agglutination technique. Three of the five enterotoxigenic strains were from yaks and two were from cattle. None of the S. aureus strains tested produced staphylococcal enterotoxins A, B, or D. To our knowledge, this is the first report of the occurrence of staphylococci and enterotoxigenic S. aureus in milk from yaks and cattle in Mongolia.

Real-time PCR method for quantification of Staphylococcus aureus in milk

A reproducible real-time PCR method that targets the putative transcriptional regulator gene of Staphylococcus aureus was developed to quantify this microorganism in milk samples. On the basis of partial sequences of this gene determined from S. aureus strains, we designed the specific primers and probe for use in a quantitative PCR assay. These specificities were confirmed with 25 strains of S. aureus and 35 strains of other bacteria. A real-time PCR assay with serial 10-fold dilutions of purified DNA and pure culture was conducted. It was possible to construct standard curves with a high correlation coefficient (r2 = 0.99) in the range of 50 ng to 50 fg for purified DNA and 10(7) to 10(1) CFU/ml for a pure culture. The constructed standard curve for milk samples was similar to that for the pure culture, and the quantification of S. aureus in the range of 10(7) to 10(1) CFU/ml was possible. Moreover, to determine how our real-time PCR method would perform under actual analytical conditions, we quantified the DNA from S. aureus after two types of heat treatments were used for the pasteurization of milk. The amount of DNA found was affected after heat treatment at 63 degrees C for 30 min (low-temperature long-time method) but not at 72 degrees C for 15 s (high-temperature short-time method). The results indicate that the real-time PCR method developed in this study is effective for monitoring S. aureus contamination in milk because of its high specificity and sensitivity.

PCR-based procedures for detection and quantification of Staphylococcus aureus and their application in food

AIMS

To evaluate the specificity of nuc targeted primers for PCR detection of Staphylococcus aureus in different food matrices and to establish a RTQ-PCR procedure suitable for the routine detection and quantification of this pathogen in food.

METHODS AND RESULTS

Specificity of nuc targeted primers (Pri1-Pri2 and the newly designed RTQ-PCR primers) was tested on a total of 157 strains of genetically confirmed identity, including reference and food isolates. PCR detection on artificially inoculated beef samples by DNA extraction using a DNeasy Tissue Kit (Qiagen GmhH, Hilden, Germany) showed a sensitivity value around 10(3) CFU g(-1). The two RTQ-PCR systems, incorporating SYBR-Green I and TaqMan, respectively, applied in the present work improved the sensitivity of conventional PCR by lowering the detection level to 10 and 100 cells, respectively. Out of 164 naturally contaminated foods tested for the presence of Staph. aureus, 74 were positive by conventional PCR and 69 by the traditional culture method with a high degree of result agreement between both methodologies (93.3%).

CONCLUSIONS

PCR approaches, using nuc targeted primers, have proved specific and combined with growth techniques may improve detection of Staph. aureus in different types of food.

SIGNIFICANCE AND IMPACT OF THE STUDY

The SYBR-Green I real-time PCR approach established allows the sensitive, automated and quantitative detection of Staph. aureus for routine analysis at a reasonable cost.

Identification and typing of food-borne Staphylococcus aureus by PCR-based techniques

The possibility of using PCR for rapid identification of food-borne Staphylococcus aureus isolates was evaluated as an alternative to the API-Staph system. A total of 158 strains, 15 S. aureus, 12 other staphylococcal species, and 131 isolates recovered from 164 food samples were studied. They were phenotypically characterized by API-Staph profiles and tested for PCR amplification with specific primers directed to thermonuclease (nuc) and enterotoxin (sea to see) genes. Disagreement between the PCR results and API-Staph identification was further assessed by the analysis of randomly amplified polymorphic DNA (RAPD) profiles obtained with three universal primers (M13, T3, and T7) and 16S rDNA sequencing. Forty out of 131 isolates (31%) tested positive for PCR enterotoxin. Of these, 14 (11%) were positive for sea, 22 (17%) for sec, one (0.8%) for sed, and three (2.2%) for sea and sec. No amplification corresponding to seb nor see was obtained. Cluster analysis based on RAPD profiles revealed that most of the sec positive food isolates grouped together in three clusters. Cluster analysis combining the three RAPD fingerprints (M 13, T3, and T7), PCR-enterotoxin genotype and API-Staph profiles, grouped the nuc PCR positive isolates together with S. aureus reference strains and the nuc PCR negative isolates with reference strains of other staphylococcal species. The only nuc PCR positive food isolate that remained unclustered was a sed positive strain identified by 16S rDNA sequence as S. simulans. The high concordance between S. aureus and nuc PCR positive strains (99%) corroborates the specificity of the primers used and the suitability of nuc PCR for rapid identification of S. aureus in routine food analysis.

Thermonuclease gene as a target for specific identification of Staphylococcus intermedius isolates: use of a PCR-DNA enzyme immunoassay

A PCR-DNA enzyme immunoassay (PCR-DEIA) was developed for identification of the coagulase-positive species Staphylococcus intermedius. Two PCR primers and a hybridization probe were designed to target specific sequences of the S. intermedius thermonuclease (nuc) gene. In addition to S. intermedius reference strains, the PCR-DEIA was tested using 295 veterinary and human S. intermedius isolates. A specific 933-bp DNA fragment was successfully amplified in 281 (94.9%) S. intermedius isolates. Five canine isolates showed an unexpected 2.8-kbp band. Except for 10 amplicons derived from equine, camel, and pigeon isolates, all positive PCR results (n = 288, 96.6%) were confirmed by the colorimetric microtiter plate DEIA hybridization. Isolates that failed both in amplification and DEIA hybridization were only observed in equine isolates (10/23, 43.5%). Except for the limitations with isolates of hoofed animals, the S. intermediusnuc PCR assay has potential for rapid identification of S. intermedius and differentiation from other coagulase-positive staphylococci including S. aureus.

Simultaneous and sensitive detection of three foodborne pathogens by multiplex PCR, capillary gel electrophoresis, and laser-induced fluorescence

The simultaneous detection of Staphylococcus aureus, Listeria monocytogenes, and Salmonella spp. has been approached by a new multiplex PCR-based procedure followed by capillary gel electrophoresis with laser-induced fluorescence detection (multiplex-PCR-CGE-LIF). As compared to slab gel electrophoresis, the use of CGE-LIF improved from 10- to 1000-fold the sensitivity of the multiplex PCR analysis, allowing the detection of 2.6 x 10(3) cfu mL(-1) of S. aureus, 570 cfu mL(-1) of L. monocytogenes, and 790 cfu mL(-1) of Salmonella in artificially inoculated food, without enrichment. Following 6 h of enrichment, as low as 260, 79, and 57 cfu mL(-1) of S. aureus, L. monocytogenes, and Salmonella, respectively, were detected. The CGE-LIF method is shown to be reproducible, providing relative standard deviation (RSD) values lower than 0.8% for analysis time and lower than 5.8% for peak areas. The multiplex-PCR-CGE-LIF proved a powerful analytical tool to detect various food pathogens simultaneously in a fast, reproducible, and sensitive way.

PCR-based detection of enterotoxigenic Staphylococcus aureus in the early stages of raw milk cheese making

AIMS

To define PCR-based detectability of Staphylococcus aureus in raw milk and intermediate products of raw milk cheese making in the presence of a complex background microflora by targetting different specific genes harboured by a single strain.

METHODS AND RESULTS

The strain Staph. aureus FRI 137 harbouring nuc, sec, seg, seh and sei genes was used in this study. Raw milk artificially contaminated by different concentrations of Staph. aureus FRI 137 was employed in dairy processing resembling traditional raw milk cheese making. Samples of milk and curds were PCR-analysed after DNA extraction by targetting all the above genes. The pathogen was detected when the initial contamination was 10(4) CFU ml(-1) by amplification of nuc and seh genes. 10(5) and 10(7) CFU ml(-1) were needed when seg or sei and sec genes were targetted, respectively. Enrichment cultures from raw milk and curd samples proved to increase the detection limit of 1 log on average.

CONCLUSIONS

The direct detection of the pathogen in the raw material and dairy intermediates of production can provide rapid results and highlight the presence of loads of Staph. aureus potentially representing the risk of intoxication. However, every target gene to be used in the analysis has to be studied in advance in a system similar to the real case in order to determine the level of contamination potentially predictable.

SIGNIFICANCE AND IMPACT OF THE STUDY

The detection in real dairy systems of significant loads of Staph. aureus by multiple targets PCR can be more accurate.

Increased sensitivity in PCR detection of tdh-positive Vibrio parahaemolyticus in seafood with purified template DNA

PCR is an important method for the detection of thermostable direct hemolysin gene (tdh)-positive (pathogenic hemolysin-producing) strains of Vibrio parahaemolyticus in seafood because tdh-negative (nonpathogenic) V. parahaemolyticus strains often contaminate seafood and interfere with the direct isolation of tdh-positive V. parahaemolyticus. In this study, the use of PCR to detect the tdh gene of V. parahaemolyticus in various seafoods artificially contaminated with tdh-positive V. parahaemolyticus was examined. PCR was inhibited by substances in oysters, squid, mackerel, and yellowtail but not by cod, sea bream, scallop, short-necked clam, and shrimp. To improve detection, DNA was purified by either the silica membrane method, the glass fiber method, or the magnetic separation method, and the purified DNA was used as the PCR primer template. For all samples, the use of the silica membrane method and the glass fiber method increased detection sensitivity. The results of this study demonstrate that the use of properly purified template DNA for PCR markedly increases the effectiveness of the method in detecting pathogenic tdh-positive V. parahaemolyticus in contaminated seafood.

Application of a convenient DNA extraction method and multiplex PCR for the direct detection of Staphylococcus aureus and Yersinia enterocolitica in milk samples

The application of PCR for the direct and sensitive detection of food-borne pathogens is largely affected by the quality of the template DNA prepared from food samples. In the present study, a chemical extraction method of bacterial DNA from spiked milk samples for the direct detection of Staphylococcus aureus and Yersinia enterocolitica was evaluated by PCR. Gene specific primers were designed to target the nuclease (nuc) and the attachment invasion locus (ail) genes of S. aureus and Y. enterocolitica, respectively and used in PCR. A combination of organic solvents, detergents and alkali in the DNA extraction method permitted a detection limit of 10 cfu ml(-1) milk for both the pathogens. When equal numbers of S. aureus and Y. enterocolitica were spiked in milk samples, the individual detection limit was determined to be 10(3) cfu ml(-1) milk. Simultaneous amplification of 482 and 359 bp fragments of the nuc and ail genes was obtained using the primer pairs in a single reaction. Multiplex PCR enabled the detection of 10(4) cfu ml(-1) milk of S. aureus and Y. enterocolitica without any pre-enrichment step. A combination of conventional isolation technique and PCR using DNA extracted by the proposed method was used to test raw milk samples for possible contamination with S. aureus and Y. enterocolitica. The presence of S. aureus in the tested samples was indicated by both the methods while Y. enterocolitica could not be detected in any of the samples. The template DNA extraction method developed in this study is rapid, sensitive and avoids interference from potential PCR inhibitors and demonstrates the potential of detecting multiple pathogens in milk samples without any enrichment.

Development of two multiplex polymerase chain reactions for the detection of enterotoxigenic strains of Staphylococcus aureus isolated from foods

Two multiplex polymerase chain reactions were developed for the detection of enterotoxigenic strains of Staphylococcus aureus: one multiplex reaction for the simultaneous detection of enterotoxigenic strains type A (entA), type B (entB), and type E (entE) and another for the simultaneous detection of enterotoxigenic strains type C (entC) and type D (entD). Both reactions were standardized with the use of the reference enterotoxigenic strains of S. aureus: FRI 722, producer of staphylococcal enterotoxin (SE) type A (SEA); FRI 1007, producer of SEB; FRI 137, producer of SEC1; FRI 472, producer of SED; and FRI 326, producer of SEE. Optimized methods were used to determine the presence of enterotoxigenic types for 51 S. aureus strains isolated from meat (sausage, ham, and chorizo) and dairy (powdered milk and cheese) products by the Baird-Parker technique. The enterotoxigenic capacities of the strains were determined by the indirect enzyme-linked immunosorbent assay (ELISA) with the use of reference staphylococcal toxins and antitoxins. Fifty of the 51 strains isolated were enterotoxigenic and produced one to four enterotoxin types, with the most frequently produced types being SEA and SED. Levels of correlation between the presence of genes that code for the production of SE (as determined by polymerase chain reaction) and the expression of these genes (as determined by the indirect ELISA) were 100% for SEA and SEE, 86% for SEC, 89% for SED, and 47% for SEB.

PCR-based detection of non-indigenous microorganisms in 'pristine' environments

PCR-based technologies are widely employed for the detection of specific microorganisms, and may be applied to the identification of non-indigenous microorganisms in 'pristine' environments. For 'pristine' environments such as those found on the Antarctic continent, the application of these methods to the assessment of environmental contamination from human activities must be treated with caution. Issues such as the possibility of non-human dispersal of organisms, stability and survival of non-indigenous organisms in vivo, the sensitivity, reproducibility and specificity of the PCR process (and particularly primer design) and the sampling regime employed must all be considered in detail. We conclude that despite these limitations, PCR and related technologies offer enormous scope for assessment of both natural and non-indigenous microbial distributions.

Rapid detection and identification of Staphylococcus aureus from blood culture specimens using real-time fluorescence PCR

Molecular surveillance of pathogens has shown the need for rapid and dependable methods for the detection and identification of organisms of clinical and epidemiologic importance. Staphylococcus aureus, one of the most frequent causes of human infections, was used as a model organism to develop and refine a real-time fluorescence PCR assay and enhanced DNA purification method. One hundred clinical isolates of S. aureus, verified by biochemical reactions and latex agglutination and 90 negative control clinical isolates were screened in the assay. Moreover, fifty blood broth samples from blood culture bottles showing Gram-positive cocci in clusters on direct Gram's stain and 25 showing Gram-negative bacilli were screened. The probes, constructed from the nuc gene, correctly detected all S. aureus genomes present without cross-reaction to negative controls. The speed and ease of this approach will make it adaptable to identification of many bacterial pathogens and provide potential for adaptation to direct detection from other types of clinical specimens.

Multiplex polymerase chain reaction assay for the differential detection of trichothecene- and fumonisin-producing species of Fusarium in cornmeal

The genus Fusarium comprises a diverse group of fungi including several species that produce mycotoxins in food commodities. In this study, a multiplex polymerase chain reaction (PCR) assay was developed for the group-specific detection of fumonisin-producing and trichothecene-producing species of Fusarium. Primers for genus-level recognition of Fusarium spp. were designed from the internal transcribed spacer regions (ITS1 and ITS2) of rDNA. Primers for group-specific detection were designed from the TRI6 gene involved in trichothecene biosynthesis and the FUM5 gene involved in fumonisin biosynthesis. Primer specificity was determined by testing for cross-reactivity against purified genomic DNA from 43 fungal species representing 14 genera, including 9 Aspergillus spp., 9 Fusarium spp., and 10 Penicillium spp. With purified genomic DNA as a template, genus-specific recognition was observed at 10 pg per reaction; group-specific recognition occurred at 100 pg of template per reaction for the trichothecene producer Fusarium graminearum and at 1 ng of template per reaction for the fumonisin producer Fusarium verticillioides. For the application of the PCR assay, a protocol was developed to isolate fungal DNA from cornmeal. The detection of F. graminearum and its differentiation from F. verticillioides were accomplished prior to visible fungal growth at <10(5) CFU/g of cornmeal. This level of detection is comparable to those of other methods such as enzyme-linked immunosorbent assay, and the assay described here can be used in the food industry's effort to monitor quality and safety.

Airborne coagulase negative staphylococci produced by a sewage treatment plant

Since some coagulase-negative staphylococci species are involved in clinical and environmental situations, the authors carried out a study on the spread of these bacteria in the air of a sewage treatment plant. For these purposes a total of 196 samples were taken from 16 sampling stations, 11 during the day and 5 at night (only at 4 points), using the settle plate technique. Altogether 13 species of coagulase negative staphylococci were isolated: S. haemolyticus, S. xylosus and S. cohnii were particularly common. Station no. 2 (an area almost always kept closed containing the fine screens) was found to be the most contaminated. The presence of coagulase negative staphylococci was favoured by high temperatures and low humidity. No differences were found due to variations in wind speed and direction or between day and night.

Development of a multiplex polymerase chain reaction assay for detection and differentiation of Staphylococcus aureus in dairy products

A multiplex polymerase chain reaction (PCR) assay was developed for the detection and differentiation of enterotoxigenic Staphylococcus aureus in dairy products. A solvent extraction procedure was successfully modified for extraction of S. aureus DNA from 10 ml of artificially contaminated skim milk or 20 g cheddar cheese. Primers targeting the enterotoxin C gene (entC) and thermostable nuclease gene (nuc) were used in the multiplex PCR. PCR products were confirmed using restriction fragment length polymorphism analysis. DNA was consistently quantified and amplified by uniplex PCR from 10 CFU/ml of S. aureus in skim milk or 10 CFU/20 g cheddar cheese. The sensitivity of the multiplex PCR was 100 CFU/ml of skim milk or 100 CFU/20 g cheddar cheese. The developed methodology allows presumptive identification and differentiation of enterotoxigenic S. aureus in less than 6 h.

Optimization of the PCR for detection of Staphylococcus aureus nuc gene in bovine milk

Staphylococcus aureus is an economically important and a major mastitis-causing pathogen that also poses food safety and antimicrobial resistance threats. Substances in mastitic milk inhibit the Taq DNA polymerase reaction (Taq PCR) making it of limited use for detecting S. aureus mastitis. In the study reported here, a set of oligonucleotide primers of 21 and 24 bases was used in Taq-PCR to amplify DNA from S. aureus (isolates from bovine mastitis). A specific amplicon of 270 bp was generated as predicted. Replacing Taq DNA polymerase with Thermus thermophilus (Tth) DNA polymerase alone (Tth-PCR) raised the sensitivity of S. aureus detection in milk from experimentally infected cows from 65 to 80%. Combining the use of Tth DNA polymerase and the purification of crude DNA extract using Chelex-100 before PCR raised the sensitivity to 100%. In a random survey involving 100 milk samples from cattle not infected with S. aureus, the test was 100% specific. With milk samples from clinical cases of bovine mastitis, 100% sensitivity and specificity were also observed. It is concluded that Tth-PCR on milk samples with the purification of crude DNA extracts using Chelex-100 is as sensitive as but faster than conventional milk bacteriological culture techniques and is highly specific. The modified PCR correlates with elevated somatic cell counts, detects evidence of chronic and resolving infection based on S. aureus-specific DNA and circumvents the endogenous inhibitory effects of milk.