Comparative evaluation of a new molecular method for typing Staphylococcus epidermidis

Molecular tracking of coagulase-negative staphylococcal isolates from catheter-related infections

Three molecular typing methods (pulsed-field electrophoresis, localization of the mecA gene, and probing the vicinity of mec) have been used for the characterization of 40 catheter-related isolates of coagulase-negative staphylococci (CNS) in 14 patients admitted to the same hospital. The 40 isolates yielded 14 different SmaI banding patterns and corresponding unique localizations of mecA, each associated with a unique ClaI mecA polymorph. In 6 of the 14 patients the contaminated skin at the catheter entry site was the source of 4 local infections and 2 cases of bacteremia. A contaminated hub was the origin of 2 local infections and 4 cases of bacteremia in 6 more patients. The remaining 2 patients had positive cultures from both skin and catheter hub. In each bacteremic patient, the CNS recovered from catheter-related sites (tip, skin, and/or hub) and the CNS recovered from blood were identical, but each of these matching isolates was unique to the particular patients, indicating a low rate of cross-infection from patient to patient. Although classical methods for typing CNS (e.g., biotype and antibiotype) are readily available for most hospital laboratories, they have limitations concerning reproducibility and discriminatory power. Molecular epidemiologic techniques can provide powerful support to traditional techniques in determining the etiologic role of CNS in the disease process.

Monitoring Persistence of Coagulase-Negative Staphylococci in a Hematology Department Using Phenotypic and Genotypic Strategies

Objective:

To determine persistence of coagulase-negative staphylococci (CNS) on a hematology-oncology ward and to determine the value of phenotypic and genotypic procedures for establishing clonality among CNS isolates.

Design:

Strains of CNS isolated from bacteremic patients (n=139) were typed by biochemical reactivity, antibiotic susceptibility, DNA macrorestriction analysis by pulsed-field gel electrophoresis (PFGE), and arbitrary primed polymerase chain reaction (AP PCR). Coagulase-negative staphylococci were subgrouped in a random collection (n=20) used for the evaluation of the typing procedures and a collection of 119 CNS isolates from hematologic patients displaying multiple bacteremic episodes.

Results:

Analysis of the reference collection demonstrated the usefulness of the DNA typing procedures, indicating that AP PCR and PFGE can be used for epidemiologic typing of CNS in a concordant fashion. Certain strains appeared to be permanent colonizers of the hematology ward or ward-related personnel. In individual patients, persistent colonization by a single type was demonstrated. However, a number of patients also experienced bacteremic episodes caused by CNS belonging to different types.

Conclusion:

We conclude that monitoring of CNS infections on a hematology ward by various genotypic techniques provides insight into nosocomial epidemiology and elucidates the complexity of the infections taking place. DNA typing is preferred over phenotypic procedures and can identify persistent CNS strains in a given location.

Investigation into the repeated recovery of coagulase-negative staphylococci from blood taken at the end of cardiopulmonary by-pass

Forty-six strains of coagulase-negative staphylococci (CNS) were typed by biochemical reactivity, antibiotic susceptibility pattern, macrorestriction analysis by pulsed-field gel electrophoresis (PFGE) and arbitrary primed polymerase chain reaction (AP PCR). Twenty-four strains were obtained in 1993 from blood cultures of as many patients coupled to a heart-lung machine during cardiac surgery. Since over 30% of the latter belonged to a single type, it was concluded that during the year of analysis a single clone of CNS persisted in this hospital setting. Subsequent epidemiological surveillance putatively identified four possible carriers among surgical personnel. For this reason, 22 strains collected from the hands and nose of two cardiac surgeons and two perfusionists were also tested; none were identical to the persistent clone. Thus either the operation equipment was colonized longitudinally or the causative CNS had disappeared from the suspect individuals' flora. Longitudinal monitoring of CNS infections by various techniques gives a valuable insight into nosocomial epidemiology and elucidates the complexity of the CNS colonization.

Typing of coagulase-negative staphylococci from peritonitis in CAPD-patients by the PhP-CS system and REA

Coagulase-negative staphylococci (CNS) were the most common bacteria causing peritonitis in patients treated with continuous ambulatory peritoneal dialysis (CAPD). In order to investigate if the same clone was responsible for the peritonitis in the different patients and if the exit site was the source of infection we followed 68 patients on CAPD for 2 years. During this period 9 patients had 12 episodes of peritonitis caused by CNS. Cultures were taken from exit site and peritoneal fluid in all patients at peritonitis and during the first study year at monthly intervals. In each culture up to 10 isolates of CNS were randomly collected and frozen. All 437 CNS isolates from the patients with CNS peritonitis were typed using a biochemical typing method and 41 isolates identical by this method were further discriminated by a DNA fingerprinting method. Identical strains were in no case isolated from different patients, indicating that no virulent strain was spread between the patients. The isolates causing the peritonitis were never found at the exist sites before the first day of the peritonitis in any patient. In only two patients was the same strain found at the exit site and in the peritoneal fluid on the first day of peritonitis. It thus seems that no virulent clone of CNS was infecting the patients and we found no evidence of CNS at the exit site causing the peritonitis.

Molecular techniques and their potential application in monitoring the microbiological quality of indoor air

Health effects associated with poor indoor air quality have created a need for accurate, reproducible methods of monitoring the microbiological content of indoor air. Improved methods of detection may allow researchers to clarify the effect of individual species present in the indoor environment on human health. This review discusses the shortcomings of current methods of identification and detection and focuses on the potential for molecular techniques in this emerging field. Probe techniques, restriction endonuclease analysis, karyotyping, and DNA and polymerase chain reaction fingerprinting methods available to detect and identify bacteria and fungi significant in the indoor air environment are discussed. Problems that may be encountered using these techniques are also considered. The authors have included a brief discussion on current air sampling techniques as well as adapting these techniques for use with molecular detection methods.

Use of ribotyping in epidemiological surveillance of nosocomial outbreaks

Over the past few years, genotypic methods based on the study of bacterial DNA polymorphism have shown high discriminatory power for strain differentiation and superiority over most phenotypic methods commonly available in the clinical microbiology laboratory. Some of the methods used, however, required either a high level of technology and sophisticated equipment (e.g., pulsed-field gel electrophoresis) or species-specific reagents of restricted availability (randomly cloned DNA probes or gene-specific probes). Because ribotyping uses a universal probe (rRNA) and is a rather simple technology, particularly since the advent of nonradioactive labelling systems, it has been widely used for strain differentiation of most bacterial species involved in nosocomial outbreaks. In vitro and in vivo stability of the markers studied has been demonstrated. Although there may be limitation to this approach, ribotyping was found to be highly discriminative, particularly for typing members of the family Enterobacteriaceae, Pseudomonas cepacia, and Xanthomonas maltophilia. In many cases, it has improved the understanding of the mechanism of nosocomial acquisition of organisms by allowing a distinction between endogenous and exogenous infections. Among exogenous infections, it has distinguished between individual and epidemic strains, thus differentiating cross-infection from independent acquisition.

Genotyping of Staphylococcus epidermidis by small-fragment restriction endonuclease analysis and pulsed-field gel electrophoresis of genomic restriction fragments

Small-fragment restriction endonuclease analysis (SF-REA) was established as a typing tool for Staphylococcus epidermidis. A total of 60 isolates comprising 48 epidemiologically nonrelated strains and 12 putatively linked isolates from 7 patients in 2 wards were analyzed. Nonrelated isolates were characterized by unique fingerprints when DNA was cleaved with EcoRI or ClaI, electrophoretically separated in a polyacrylamide gel, and silver stained. Three blood culture isolates from one patient in an intensive care unit, 4 isolates obtained from a child over a span of 2 weeks, and 5 isolates from 5 newborns in the same ward were grouped into 3 DNA pattern types, indicating identity of sequential isolates from 2 patients and nosocomial transmission of one Staphylococcus epidermidis strain between 5 babies. Results from pulsed-field gel electrophoresis of SmaI and SacII DNA digests and conventional marker systems such as antibiogram and plasmid profile were in accordance with these interpretations, whereas slight variation was observed in the biotypes of several strains. From the results of this study, we conclude that SF-REA is a precise and efficient method for the genotypic characterization of Staphylococcus epidermidis strains that can be used as a rapid and reliable typing tool.

Update on clinical significance of coagulase-negative staphylococci

The clinical significance of coagulase-negative Staphylococcus species (CNS) continues to increase as strategies in medical practice lead to more invasive procedures. Hospitalized patients that are immunocompromised and/or suffering from chronic diseases are the most vulnerable to infection. Since CNS are widespread on the human body and are capable of producing very large populations, distinguishing the etiologic agent(s) from contaminating flora is a serious challenge. For this reason, culture identification should proceed to the species and strain levels. A much stronger case can be made for the identification of a CNS etiologic agent if the same strain is repeatedly isolated from a series of specimens as opposed to the isolation of different strains of one or more species. Strain identity initially can be based on colony morphology, and then one or more molecular approaches can be used to gain information on the genotype. Many of the CNS species are commonly resistant to antibiotics that are being indicated for staphylococcal infections, with the exception of vancomycin. The widespread use of antibiotics in hospitals has provided a reservoir of antibiotic-resistant genes. The main focus on mechanisms of pathogenesis has been with foreign body infections and the role of specific adhesins and slime produced by Staphylococcus epidermidis. Slime can reduce the immune response and opsonophagocytosis, thereby interfering with host defense mechanisms. As we become more aware of the various strategies used by CNS, we will be in a better position to compromise their defense mechanisms and improve treatment.

Characterization of unrelated strains of Staphylococcus schleiferi by using ribosomal DNA fingerprinting, DNA restriction patterns, and plasmid profiles

The molecular characteristics of 31 unrelated strains of Staphylococcus schleiferi isolated from 13 hospitals between 1973 and 1991 were determined by ribosomal DNA fingerprinting by using a digoxigenin-labeled DNA probe, genomic DNA restriction patterns, and plasmid profiles. Only six strains harbored one or two plasmids. DNA restriction analysis, which was carried out with five endonucleases (EcoRI, HindIII, PstI, PvuII, and ClaI), did not allow us to discriminate between isolates. Ribotyping with HindIII, ClaI, or EcoRI enzymes generated six, seven, and nine distinct patterns, respectively. With the combination ClaI-EcoRI, 13 ribotypes were obtained among the 31 strains, suggesting a relative heterogeneity within the species. Moreover, all strains shared two or three common bands, according to the endonuclease used, which were relatively specific for S. schleiferi in comparison with the ribosomal banding patterns described for other coagulase-negative staphylococci. These results illustrate that ribotyping can be used for the epidemiological investigation of S. schleiferi isolates and possibly for taxonomic analysis in this species.