Comparison of methods for the detection of methicillin resistance in Staphylococcus aureus isolates from food products

Pathogenic and Virulence Factor Detection on Viable but Non-culturable Methicillin-Resistant Staphylococcus aureus

Food safety and foodborne infections and diseases have been a leading hotspot in public health, and methicillin-resistant Staphylococcus aureus (MRSA) has been recently documented to be an important foodborne pathogen, in addition to its recognition to be a leading clinical pathogen for some decades. Standard identification for MRSA has been commonly performed in both clinical settings and food routine detection; however, most of such so-called "standards," "guidelines," or "gold standards" are incapable of detecting viable but non-culturable (VBNC) cells. In this study, two major types of staphylococcal food poisoning (SFP), staphylococcal enterotoxins A (sea) and staphylococcal enterotoxins B (seb), as well as the panton-valentine leucocidin (pvl) genes, were selected to develop a cross-priming amplification (CPA) method. Limit of detection (LOD) of CPA for sea, seb, and pvl was 75, 107.5, and 85 ng/μl, indicating that the analytical sensitivity of CPA is significantly higher than that of conventional PCR. In addition, a rapid VBNC cells detection method, designated as PMA-CPA, was developed and further applied. PMA-CPA showed significant advantages when compared with PCR assays, in terms of rapidity, sensitivity, specificity, and accuracy. Compared with conventional VBNC confirmation methods, the PMA-CPA showed 100% accordance, which had demonstrated that the PMA-CPA assays were capable of detecting different toxins in MRSA in VBNC state. In conclusion, three CPA assays were developed on three important toxins for MRSA, and in combination with PMA, the PMA-CPA assay was capable of detecting virulent gene expression in MRSA in the VBNC state. Also, the above assays were further applied to real samples. As concluded, the PMA-CPA assay developed in this study was capable of detecting MRSA toxins in the VBNC state, representing first time the detection of toxins in the VBNC state.

Characterization of Oxacillin-Susceptible mecA-Positive Staphylococcus aureus from Food Poisoning Outbreaks and Retail Foods in China

In this study, we explored the prevalence of oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) in staphylococcal food poisoning outbreak isolates and foodborne isolates, and then investigated their molecular characteristics, classical staphylococcal enterotoxins (SEs), and drug resistance. Eight (2.9%) of 275 isolates from food poisoning outbreaks and 7 (3.8%) of 184 isolates from retail foods were identified as OS-MRSA isolates. Among the 15 OS-MRSA isolates, the most frequently detected toxin genes were hld (100%), hla (93.3%), pvl (80.0%), and hlb (46.7%) followed by seg and seq (33.3%, each), hlg (26.7%), seb and hlgv (20.0%, each), sec, seh, sel, sep, and tst (13.3%, each), and sei, sem, sen, and seo (6.7%, each). None of isolates carried other tested virulence genes. The most frequently detected classical SEs were SEB and SEC (26.7%, each), followed by SEA and SEE (20.0%, each), and SED (6.7%). Resistance was most frequently observed in ampicillin, penicillin, and cefoxitin (100%, each), followed by trimethoprim/sulfamethoxazole (93.3%), erythromycin (73.3%), amoxicillin/clavulanic acid (46.7%), tetracyclines (26.7%), and ciprofloxacin (6.7%). All isolates were susceptible to other tested antibiotics. A dominant molecular type belonged to ST398-IVa-t034 (26.7%), followed by ST59-IVa-t437 (20.0%), ST88-III-t14340 and ST1-IVa-t114 (13.3%, each), and ST5-II-t002, ST630-t4549, ST5-II, and ST4495-t10738 (6.7%, each). Our findings indicated that OS-MRSA strains had a low prevalence rate among outbreak strains and foodborne strains, which frequently harbored SCCmec IVa, and carried a variety of toxin genes, and also expressed numerous classical SEs. In addition, all OS-MRSA isolates were susceptible to the majority of antibacterial agents except β-lactam. Our study is the first to report that OS-MRSA isolates are associated with food poisoning outbreaks worldwide.

First Report on the Rapid Detection and Identification of Methicillin-Resistant Staphylococcus aureus (MRSA) in Viable but Non-culturable (VBNC) Under Food Storage Conditions

Formation of viable but non-culturable (VBNC) status in methicillin-resistant Staphylococcus aureus (MRSA) has never been reported, and it poses a significant concern for food safety. Thus, this study aimed to firstly develop a rapid, cost-effective, and efficient testing method to detect and differentiate MRSA strains in the VBNC state and further apply this in real food samples. Two targets were selected for detection of MRSA and toxin, and rapid isothermal amplification detection assays were developed based on cross-priming amplification methodology. VBNC formation was performed for MRSA strain in both pure culture and in artificially contaminated samples, then propidium monoazide (PMA) treatment was further conducted. Development, optimization, and evaluation of PMA-crossing priming amplification (CPA) were further performed on detection of MRSA in the VBNC state. Finally, application of PMA-CPA was further applied for detection on MRSA in the VBNC state in contaminated food samples. As concluded in this study, formation of the VBNC state in MRSA strains has been verified, then two PMA-CPA assays have been developed and applied to detect MRSA in the VBNC state from pure culture and food samples.

Prevalence of Enterotoxigenic Staphylococcus aureus and Shiga Toxin Producing Escherichia coli in Fish in Egypt: Quality Parameters and Public Health Hazard

This study investigated the prevalence of Staphylococcus aureus enterotoxin genes and shiga toxin -producing Escherichia coli (STEC) in fish and evaluated quality parameters of examined fish. A total of 150 fish samples belonging to 6 species (25/each species) were cultured on Baird-Parker agar and eosin methylene blue agar. Staphylococcal enterotoxin genes and virulence genes (stx1, stx2, and eaeA genes) in E. coli serotypes were determined by multiplex PCR. Aerobic plate count (APC), Enterobacteriaceae count, coliform count, and Pseudomonas count were performed. Also, levels of total volatile base nitrogen and histamine in fish were determined. The prevalence of S. aureus ranged from 4% to 36% and count from 2 to 4 log₁₀CFU/g. The sed, sea, and seb genes in S. aureus isolates were detected with percentages of 40%, 26.6%, and 20%, respectively. The E. coli serotype O26 carried stx1, stx2, and eaeA. The APCs, Enterobacteriaceae counts, and Pseudomonas counts ranged from 5.1 to 7.2, from 2.01 to 3.9, and from 2.1 to 3.1 log₁₀ CFU/g, respectively. The most probable number (MPN) of coliform ranged from 1.3 to 3.6 log₁₀/g. Levels of total volatile basic nitrogen and histamine ranged from 29.2 to 12.2 and from 0.6 to 4.6 mg/100 g, respectively. Also, the value of thiobarbituric acid was highly significant (1.1 ± 0.084 mg MDA/kg) in Trachurus mediterraneus samples compared with those levels obtained from other fish species. Our findings concluded that those fish species could constitute a public health hazard as fish are reservoirs for enterotoxigenic S. aureus and Shiga toxin producing E. coli strains. This study highlighted the importance of screening of fish for enterotoxigenic S. aureus strains and STEC isolates, and also assessing the quality parameters of fish.

Characterisation of clinical isolates of oxacillin-susceptible mecA-positive Staphylococcus aureus in China from 2009 to 2014

OBJECTIVES

The aim of this study was to investigate the prevalence and molecular characteristics of clinical oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) isolates in China from July 2009 to June 2014.

METHODS

A total of 2068 non-duplicate S. aureus isolates were collected from 21 hospitals. Antimicrobial susceptibility testing was performed by the agar dilution method. All OS-MRSA strains were screened for the presence of the genes mecA, mecC and nuc as well as the Panton-Valentine leukocidin gene (pvl). Staphylococcal cassette chromosome mec (SCCmec) typing, staphylococcal protein A (spa) typing and multilocus sequence typing (MLST) were performed to analyse the isolate genotypes.

RESULTS

A total of 34 S. aureus isolates were mecA-positive but were susceptible to oxacillin [minimum inhibitory concentration (MIC)≤2mg/L]. All OS-MRSA isolates were resistant to cefoxitin and most were also multiresistant to other antibiotics besides β-lactams. Among the 34 OS-MRSA isolates, nine spa and three SCCmec types were detected and, combined with MLST, ST338/59-t437-SCCmecV (47%; 16/34) was the predominant clone. In addition, 17 strains (50%) carried the pvl gene.

CONCLUSIONS

The most frequent clone of OS-MRSA isolates in China was ST338-t437-SCCmecV. Most of the OS-MRSA isolates were susceptible to the majority of antibacterial agents except macrolides, clindamycin and chloramphenicol.

Prevalence and molecular characteristics of Staphylococcus aureus, including methicillin resistant strains, isolated from bulk can milk and raw milk products in pastoral communities of South-West Uganda

Background

Staphylococcus aureus strains are now regarded as zoonotic agents. In pastoral settings where human-animal interaction is intimate, multi-drug resistant microorganisms have become an emerging zoonotic issue of public health concern. The study of S. aureus prevalence, antimicrobial resistance and clonal lineages in humans, animals and food in African settings has great relevance, taking into consideration the high diversity of ethnicities, cultures and food habits that determine the lifestyle of the people. Little is known about milk carriage of methicillin resistant S. aureus strains (MRSA) and their virulence factors in Uganda. Here, we present the prevalence of MRSA in bulk can milk and raw milk products in pastoral communities of south-west Uganda. We also present PFGE profiles, spa-types, as well as frequency of enterotoxins genes.

Methods

S. aureus was identified by the coagulase test, susceptibility testing by the Kirby-Bauer disc diffusion and E-test methods and MRSA by detection of the mecA gene and SCCmec types. The presence of Panton – Valentine Leucocidin (PVL) genes and staphylococcal enterotoxins was determined by PCR, while genotyping was by PFGE and spa typing.

Results

S. aureus were isolated from 30/148 (20.3%) milk and 11/91(12%) sour milk samples. mecA gene carriage, hence MRSA, was detected in 23/41 (56.1%) of the isolates, with 21 of the 23 (91.3%) being SCCmec type V; while up to 30/41 (73.2%) of the isolates were resistant to tetracycline. Only five isolates carried the PVL virulence gene, while PFGE typing revealed ten clusters (ranging from two seven isolates each) that comprised 83% of the sample, and only eight isolates with unique pulsotypes. The largest PFGE profile (E) consisted of seven isolates while t7753, t1398, and t2112 were the most common spa-types. Thirty seven of the 41 strains (90.2%) showed at least one of the eight enterotoxin genes tested, with sem 29 (70.7%), sei 25 (61%) and seg 21 (51.2%) being the most frequently observed genes.

Conclusion

This is the first study to demonstrate MRSA and enterotoxin genes in raw milk and its products in Uganda. The fact that over 90% of the isolates carried at least one gene encoding enterotoxins shows a high risk of spread of foodborne diseases through milk in this setting.

Occurrence of methicillin-resistant Staphylococcus aureus in dairy cattle herds, related swine farms, and humans in contact with herds

In this study we investigated the circulation of methicillin-resistant Staphylococcus aureus (MRSA) in 2 dairy cattle farms (farm A and B), previously identified as MRSA-positive in bulk tank milk samples, and epidemiologically related to swine farms. Collected specimens included quarter milk samples and nasal swabs from dairy cows, pig nasal swabs collected at both the farm and slaughterhouse level, environmental dust samples, and human nasal swabs from the farms' owners and workers. The prevalence of MRSA was estimated at the herd level by testing quarter milk samples. The prevalence of MRSA was 4.8% (3/63; 95% confidence interval=0-10.2%) and 60% (33/55; 95% confidence interval=47.05-72.95) in farm A and B, respectively. In farm A, MRSA was also isolated from humans, pigs sampled at both farm and slaughterhouse level, and from environmental samples collected at the pig facilities. The dairy cattle facilities of farm A tested negative for MRSA. In farm B, MRSA was isolated from environmental dust samples in both the cattle and pig facilities, whereas nasal swabs collected from cows and from humans tested negative. Sixty-three selected MRSA isolates obtained from different sources in farm A and B were genetically characterized by multilocus sequence typing, spa-typing, ribosomal spacer-PCR, and also tested for the presence of specific virulence genes and for their phenotypical antimicrobial susceptibility by broth microdilution method. Different clonal complex (CC) and spa-types were identified, including CC398, CC97, and CC1, CC already reported in livestock animals in Italy. The MRSA isolates from quarter milk of farm A and B mostly belonged to CC97 and CC398, respectively. Both lineages were also identified in humans in farm A. The CC97 and CC398 quarter milk isolates were also identified as genotype GTBE and GTAF by ribosomal spacer-PCR respectively, belonging to distinct clusters with specific virulence and resistance patterns. The GTBE and GTAF clusters also included swine, environmental, and human isolates from both farms. A high heterogeneity in the genetic and phenotypic profiles was observed in environmental isolates, in particular from farm B. These results demonstrate the possibility of a dynamic sharing and exchange of MRSA lineages or genotypes between different species and farm compartments in mixed-species farms. The risk of transmission between swine and related dairy cattle herds should be considered. Our findings also confirm the zoonotic potential of livestock-associated MRSA and underline the importance of applying biosecurity measures and good hygiene practices to prevent MRSA spread at the farm level and throughout the food production chain.

Working across the veterinary and human health sectors

Antibiotics are widely used in human and veterinary medicine for the prevention and treatment of infectious diseases. This practice has led to the emergence of antimicrobial-resistant bacteria in both humans and animals. The potential role that animals, particularly livestock, might play as potential reservoirs of antibiotic resistance genes has been recognized, and it is currently a cause of public health concern. The impact of animal and human antibiotic usage on the emergence and persistence of resistant bacteria and the precise transfer pathways for resistance genes between humans and animals are not currently fully understood. As part of the remit of the UK Advisory Committee on Antimicrobial Resistance and Healthcare-Associated Infection (ARHAI), two main areas were addressed, namely methicillin-resistant Staphylococcus aureus (MRSA) and multidrug-resistant Gram-negative bacteria, where both the human and veterinary health sectors share interests. We review the current knowledge of MRSA and resistant Gram-negative bacteria, and provide guidance on occupational risks for veterinary healthcare workers relating to animals infected or colonized with MRSA. Findings and recommendations for further work across disciplines and future research in multidrug-resistant Gram-negative bacteria are also presented. Working collaboratively across disciplines is essential in order to better understand and challenge an important human and animal health problem: antimicrobial resistance.

A magnetic bead-based assay for the rapid detection of methicillin-resistant Staphylococcus aureus by using a microfluidic system with integrated loop-mediated isothermal amplification

This study reports a new diagnostic assay for the rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) by combing nucleic acid extraction and isothermal amplification of target nucleic acids in a magnetic bead-based microfluidic system. By using specific probe-conjugated magnetic beads, the target deoxyribonucleic acid (DNA) of the MRSA can be specifically recognized and hybridized onto the surface of the magnetic beads which are then mixed with clinical sample lysates. This is followed by purifying and concentrating the target DNA from the clinical sample lysates by applying a magnetic field. Nucleic acid amplification of the target genes can then be performed by the use of a loop-mediated isothermal amplification (LAMP) process via the incorporation of a built-in micro temperature control module, followed by analyzing the optical density (OD) of the LAMP amplicons using a spectrophotometer. Significantly, experimental results show that the limit of detection (LOD) for MRSA in the clinical samples is approximately 10 fg μL(-1) by performing this diagnostic assay in the magnetic bead-based microfluidic system. In addition, the entire diagnostic protocol, from bio-sample pre-treatment to optical detection, can be automatically completed within 60 min. Consequently, this miniature diagnostic assay may become a powerful tool for the rapid purification and detection of MRSA and a potential point-of-care platform for detection of other types of infections.