Laboratory diagnosis of oxacillin resistance in Staphylococcus aureus by a multiplex-polymerase chain reaction assay

Clinical Significance and Pathogenesis of Staphylococcal Small Colony Variants in Persistent Infections

Small colony variants (SCVs) were first described more than 100 years ago for Staphylococcus aureus and various coagulase-negative staphylococci. Two decades ago, an association between chronic staphylococcal infections and the presence of SCVs was observed. Since then, many clinical studies and observations have been published which tie recurrent, persistent staphylococcal infections, including device-associated infections, bone and tissue infections, and airway infections of cystic fibrosis patients, to this special phenotype. By their intracellular lifestyle, SCVs exhibit so-called phenotypic (or functional) resistance beyond the classical resistance mechanisms, and they can often be retrieved from therapy-refractory courses of infection. In this review, the various clinical infections where SCVs can be expected and isolated, diagnostic procedures for optimized species confirmation, and the pathogenesis of SCVs, including defined underlying molecular mechanisms and the phenotype switch phenomenon, are presented. Moreover, relevant animal models and suggested treatment regimens, as well as the requirements for future research areas, are highlighted.

Specific real-time polymerase chain reaction places Kingella kingae as the most common cause of osteoarticular infections in young children

BACKGROUND

The use of universal 16S rDNA polymerase chain reaction (PCR) has recently shown that the place of Kingella kingae in osteoarticular infections (OAI) in young children has been underestimated, but this technique is not the most sensitive or the most rapid method for molecular diagnosis. We developed a specific real-time PCR method to detect K. kingae DNA and applied it to the etiologic diagnosis of OAI.

PATIENTS AND METHODS

All children admitted to a pediatric unit for OAI between January 2004 and December 2005 were enrolled in this prospective study. Culture-negative osteoarticular specimens were tested by 16S rDNA PCR and by K. kingae-specific real-time PCR when sufficient sample remained.

RESULTS

By culture alone, a pathogen was identified in 45% of the 131 specimens tested (Staphylococcus aureus, n = 25; K. kingae, n = 17; others, n = 18). 16S rDNA PCR and K. kingae-specific PCR were both applied to 61 of the culture-negative samples. The combination of culture and 16S rDNA PCR identified a pathogen in 61% of cases (K. kingae DNA, n = 16; DNA of other microorganisms, n = 5). Specific real-time PCR identified a further 6 cases caused by K. kingae and confirmed all 16 universal PCR-positive cases, bringing the overall documentation rate to 66%. K. kingae was the leading cause of OAI in this pediatric series (n = 39, 45%), followed by S. aureus (n = 25, 29%)

CONCLUSION

The K. kingae-specific real-time PCR places K. kingae as the leading cause of OAI in children at our hospital.

Rapid detection of antibacterial resistance in emerging Gram-positive cocci

Effective infection control efforts obviously depend on the performance of the laboratory to detect emerging resistant pathogens accurately and confirm resistance patterns by additional methods to conventional or automated systems. Conventional methods still remain the predominant approaches for detection and identification of bacteria and resistance patterns. However, the estimated time for conventional tests to detect resistance is at least 24-48 h for methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci and other epidemiologically important pathogens. Most of the tests used for rapid detection require bacterial growth in culture. There is an important clinical need for rapid detection of bacteria directly from patient samples. Rapid methods based on immunological or molecular technologies have contributed significantly. Molecular assays for several resistance markers are reliable, such as for mecA in staphylococci and vanA in enterococci. However, for other resistance markers, there is a lack of field testing. Cost-effectiveness of rapid detection of antibacterial resistance is another concern. Molecular assays would be useful for tertiary hospitals considering the investment costs and requirement of expert laboratory staff. For smaller centres, rapid tests based on immunological techniques may be a better choice.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Characterization of sparfloxacin-resistant mutants of Staphylococcus aureus obtained in vitro

A sparfloxacin-susceptible clinical isolate of Staphylococcus aureus was grown in increased concentrations of sparfoxacin. The presence of mutations in gyrA, gyrB, grlA and grlB genes was analyzed. The primary point mutation was located in the gyrA gene (Glu-88 to Lys). Two further mutation steps appeared in the amino acid change Ser-80 to Tyr in GrlA. No mutations occurred in the gyrB or grlB genes. Efflux pumps involved in the increase of resistance were also found to affect norfloxacin and ciprofloxacin. This effect may be related to NorA. An overexpression of NorA, may be associated with the increase of the MIC of norfloxacin from 32 mg/l to >200 mg/l in the final mutant. The MICs levels of sparfloxacin were affected by unknown mechanism.

Multiplex PCR for detection of genes for Staphylococcus aureus enterotoxins, exfoliative toxins, toxic shock syndrome toxin 1, and methicillin resistance

A multiplex PCR assay for detection of genes for staphylococcal enterotoxins A to E (entA, entB, entC, entD, and entE), toxic shock syndrome toxin 1 (tst), exfoliative toxins A and B (etaA and etaB), and intrinsic methicillin resistance (mecA) was developed. Detection of femA was used as an internal positive control. The multiplex PCR assay combined the primers for sea to see and femA in one set and those for eta, etb, tst, mecA, and femA in the other set. Validation of the assay was performed using 176 human isolates of Staphylococcus aureus. This assay offers a very specific, quick, reliable, and inexpensive alternative to conventional PCR assays used in clinical laboratories to identify various staphylococcal toxin genes.

Clinical impact of rapid oxacillin susceptibility testing using a PCR assay in Staphylococcus aureus bactaeremia

OBJECTIVES

The aim of this work was to establish the clinical impact of rapid oxacillin susceptibility testing in nosocomial Staphylococcus aureus bacteraemia.

METHODS

This study was performed in 145 critically ill patients infected by S. aureus. Patients were randomly assigned to one of two groups: patients for whom susceptibility testing was performed using a rapid same day multiplex PCR assay for detection of the staphylococcal mecA (mean delay of response: 6 h) and those for whom testing was accomplished using traditional overnight techniques (21 h).

RESULTS

The results of this study showed no significant difference between the two groups in terms of age, Simplified Acute Physiologic Score, severity of infection, severity of underlying disease and clinical outcome (control vs. PCR): unfavourable outcome of infection, 12.32 vs. 12.5%; 95% CI for the difference = -11.49 to 11.09 (P = 0.975); unfavourable general outcome, 16.43 vs. 20.83%; 95% CI for the difference = -17.35 to 8.50 (P = 0.497). For the oxacillin-susceptible S. aureus bactaeraemia, results were: unfavourable outcome of infection = 13.04 vs. 11.11%; 95% CI for the difference = -11.38 to 16.18 (P = 0.767); unfavourable general outcome = 13.04 vs. 20.37%; 95% CI for the difference = -22.12 to 8.07 (P = 0.331).

CONCLUSION

This study seemed to demonstrate that rapid oxacillin susceptibility testing using a PCR assay did not have a major impact on the care and outcome of patients with S. aureus bactaeremia.

The reliability of Microscan conventional and rapid panels to identify Staphylococcus aureus and detect methicillin resistance: an evaluation using the tube coagulase test and mecA PCR

Microscan (Dade Diagnostics, Brisbane) Positive Combo Type 6 (312 panels) and Rapid Positive Breakpoint Type 1 (62 panels) were evaluated for Staphylococcus aureus identification, using the tube coagulase test (TC), and oxacillin susceptibility, using mecA. A total of 374 consecutive clinical staphylococci were tested, with TC and Microscan having 100% correlation (335 identified as S. aureus and 39 as coagulase negative staphylococci by both methods). A 93% correlation was observed between Microscan and mecA PCR for oxacillin susceptibility. No very major errors (0/374 false oxacillin susceptibility) and 26 (7%) major errors (26/374 false oxacillin resistance) were found showing false resistance to oxacillin to be a problem in our population. Oxacillin Etest (AB Biodisk, Sweden) was performed on all oxacillin resistant isolates. A bimodal distribution was observed between mecA positive and negative isolates. A testing algorithm (using the Microscan panels and Etest) was developed for this laboratory to detect mecA encoded methicillin resistance. Retrospective application of this algorithm to the 374 isolates gave 100% correlation with mecA detection.

Mechanisms of methicillin resistance in staphylococci

The continuously high prevalence of methicillin-resistant staphylococci (MRS) throughout the world is a constant threat to public health, owing to the multiresistant characteristics of these bacteria. Methicillin resistance is phenotypically associated with the presence of the penicillin-binding protein 2a (PBP2a) not present in susceptible staphylococci. This protein has a low binding affinity for beta-lactam antibiotics. It is a transpeptidase which may take over cell wall synthesis during antibiotic treatment when normally occurring PBPs are inactivated by ligating beta-lactams. PBP2a is encoded by the mecA gene, which is located in mec, a foreign DNA region. Expression of PBP2a is regulated by proteins encoded by the plasmid-borne blaR1-bla1 inducer-repressor system and the corresponding genomic mecRl-mecl system. The blaRl-blal products are important both for the regulation of beta-lactamase and for mecA expression. Methicillin resistance is influenced by a number of additional factors, e.g. the products of the chromosomal fem genes which are important in the synthesis of normal peptidoglycan precursor molecules. Inactivation of fem-genes results in structurally deficient precursors which are not accepted as cell wall building blocks by the ligating PBP2a transpeptidase during antibiotic treatment. This may result in reduced resistance to beta-lactam antibiotics. Inactivation of genes affecting autolysis has shown that autolytic enzymes are also of importance in the expression of methicillin resistance. Methicillin resistance has evolved among earth microorganisms for protection against exogenous or endogenous antibiotics. Presumably the mec region was originally transferred from coagulase negative staphylococci (CNS) to Staphylococcus aureus (SA). A single or a few events of this kind with little subsequent interspecies transfer had been anticipated. However, recent data suggest a continuous horizontal acquisition by S. aureus of mec, being unidirectional from CNS to SA. Methicillin resistance may also be associated with mechanisms independent of mecA, resulting in borderline methicillin resistance. These mechanisms include beta-lactamase hyperproduction, production of methicillinases, acquisition of structurally modified normal PBPs, or the appearance of small colony variants of SA. Most MRS are multiresistant, and the mec region may harbour several resistance determinants, resulting in a clustering of resistance genes within this region.

European multicentre evaluation of a commercial system for identification of methicillin-resistant Staphylococcus aureus

A commercial system for the rapid detection of methicillin-resistant Staphylococcus aureus, the BBL Crystal MRSA test (C-MRSA ID; Becton Dickinson, USA), was evaluated prospectively and compared with a polymerase chain reaction test for the presence of the mecA gene. Ten European centres tested a total of 676 isolates of Staphylococcus aureus from blood cultures. The system correctly identified 661 (97.8%) isolates within 4 h. All but three mecA gene-negative isolates (99.4% specificity) yielded a negative C-MRSA ID reaction, and 158 of 170 mecA gene-positive isolates were accurately detected (92.9% sensitivity). After repeated testing of discrepant results, sensitivity and specificity increased to 99% and 100%, respectively.

Comparison of an improved RAPD fingerprinting with different typing methods for discriminating clinical isolates of Staphylococcus spp

Different epidemiological markers were used to characterize 2 Staphylococcus epidermidis and 8 Staphylococcus aureus strains isolated from patients with severe infections. We compared random amplified polymorphic DNA (RAPD) fingerprints, biotypes, antibiotic assays, plasmid profiles and chromosomal DNA restriction endonuclease analysis (REA). Data analysis based on numerical taxonomy methods indicates that RAPD and REA give similar results allowing a good discrimination of the two species and of each isolate. The RAPD method is easier and faster than REA, but the reproducibility of RAPD fingerprints obtained in independent experiments can be problematic. We have found simple technical devices to improve the reproducibility of the RAPD procedure which is therefore a very useful tool in epidemiology for identification and characterization of Staphylococcus spp.

Applications of DNA amplification techniques in veterinary diagnostics

An overview of the principles of the polymerase chain reaction, ligase chain reaction, self-sustained sequence replication and Q beta replicase is given. The application of these methods for the diagnosis of veterinary infectious and hereditary diseases as well as for other diagnostic purposes is discussed and comprehensive tables of reported assays are provided. Specific areas where these DNA-based amplification methods provide substantial advantages over traditional approaches are also highlighted. With regard to PCR-based assays for the detection of viral pathogens, this article is an update of a previous review by Belák and Ballagi-Pordány (1993).