Use of plasmid analysis and determination of aminoglycoside-modifying enzymes to characterize isolates from an outbreak of methicillin-resistant Staphylococcus aureus

Detection of genes encoding aminoglycoside-modifying enzymes in staphylococci by polymerase chain reaction and dot blot hybridization

Dot blot hybridization and the polymerase chain reaction (PCR) were used to study aminoglycoside-modifying enzymes in aminoglycoside-resistant staphylococci isolated in hospitals in Kuwait. DNA encoding the acetyltransferase (AAC) (6')-phosphotransferase (APH) (2"), nucleotidyltransferase (ANT) (4') and APH (3') enzymes were detected in Staphylococcus aureus and coagulase negative staphylococci. ANT (4') was the most common enzyme detected. The majority of isolates contained genes for all three modifying enzymes, AAC (6')-APH (2"), ANT (4') and APH (3'); only few isolates carried genes for a single modifying enzyme. Genes encoding the AAC (6')-APH (2") were detected in all except two gentamicin-resistant isolates. In these isolates the genes for the AAC (6')-APH (2") enzyme could not be detected by PCR and dot blot hybridization. Whereas antibiotic resistance testing could be used to predict the presence of the AAC (6')-APH (2") enzyme it was not useful in predicting the presence of the ANT (4') or APH (3') enzymes in gentamicin-resistant isolates. Results obtained with dot blot hybridization were comparable to those obtained with PCR. However, PCR was fast and results were obtained within the same day. Therefore PCR would be preferred for the detection and confirmation of the presence of aminoglycoside-modifying enzymes in clinical microbiology laboratories.

Molecular genotyping of methicillin-resistant Staphylococcus aureus via fluorophore-enhanced repetitive-sequence PCR

Methicillin resistance in Staphylococcus aureus is a frequent cause of nosocomial and community-acquired infections. Accurate, rapid epidemiologic typing is crucial to the identification of the source and spread of infectious disease and could provide detailed information on the generation of methicillin-resistant S. aureus (MRSA) strains. The high degree of genetic relatedness of MRSA strains has precluded the use of more conventional methods of genetic fingerprinting. A rapid DNA fingerprinting method that exploits PCR amplification from a DNA repeat sequence in MRSA is described. The random chromosomal distribution of this repeat sequence provides an ideal target for detecting DNA fragment patterns specific to individual MRSA strains. Two PCR fingerprinting methods which use an oligonucleotide primer based on a repetitive sequence found in Mycoplasma pneumoniae are presented. The repetitive element sequence-based PCR (rep-PCR) and fluorophore-enhanced rep-PCR (FERP) can identify epidemic strains among background MRSA. The combination of oligonucleotide primers labeled with different fluorescent dyes allowed simultaneous FERP fingerprinting and mecA gene detection. Eight different fingerprint patterns were observed in MRSA strains collected from different sources. These techniques provide a rapid discriminatory means of molecular epidemiologic typing of MRSA involved in nosocomial infections.

Random amplified polymorphic DNA assay is less discriminant than pulsed-field gel electrophoresis for typing strains of methicillin-resistant Staphylococcus aureus

Twenty-six strains of methicillin-resistant Staphylococcus aureus with different pulsed-field gel electrophoresis fingerprints were tested by random amplified polymorphic DNA assay with three primers, resulting in 15 to 20 different random amplified polymorphic DNA fingerprints. By summing the results for the three primers, the number of different fingerprints increased to 25, but two strains could not be differentiated. We conclude that pulsed-field gel electrophoresis remains the best method of typing methicillin-resistant S. aureus strains.

Methicillin-resistant Staphylococcus aureus: a consensus review of the microbiology, pathogenesis, and epidemiology with implications for prevention and management

Methicillin-resistant Staphylococcus aureus (MRSA) has become a major nosocomial pathogen in community hospitals, long-term-care facilities, and tertiary care hospitals. The basic mechanism of resistance is alteration in penicillin-binding proteins of the organism. Methods for isolation by culture and typing of the organism are reviewed. MRSA colonization precedes infection. A major reservoir is the anterior nares. MRSA is usually introduced into an institution by a colonized or infected patient or health care worker. The principal mode of transmission is via the transiently colonized hands of hospital personnel. Indications for antibiotic therapy for eradication of colonization and treatment of infection are reviewed. Infection control guidelines and discharge policy are presented in detail for acute-care hospitals, intensive care and burn units, outpatient settings, and long-term-care facilities. Recommendations for handling an outbreak, surveillance, and culturing of patients are presented based on the known epidemiology.

Characterization of methicillin-susceptible and methicillin-resistant Staphylococcus aureus isolates by molecular typing methods

Two groups of clinical Staphylococcus aureus isolates were examined by molecular typing methods. Detailed protocols for the technical procedures are described. By means of plasmid DNA profiles, plasmid DNA restriction enzyme digestion, as well as whole-cell DNA endonuclease digestions subjected to conventional agarose gel electrophoresis and pulsed-field gel electrophoresis, eight methicillin-susceptible S. aureus strains from three patients segregated into three different groups. Consecutive isolates from the same patient revealed identical fingerprints. Generally, good agreement between different molecular typing methods also was achieved in the investigation of 28 methicillin-resistant isolates. However, compared to whole-cell DNA restriction endonuclease analysis, plasmid DNA profiling (all but one strain had detectable plasmid DNA) showed a somewhat greater discriminatory ability. The techniques evaluated were reproducible and relatively easy to perform and provided valuable tools for studying the epidemiology of S. aureus in hospitals.

Plasmid-pattern analysis in the differentiation of Staphylococcus epidermidis isolates

Plasmids carried by 29 Staphylococcus epidermidis isolates from two different origins (invasive and carrier, non-hospital-associated strains) were analysed by agarose gel electrophoresis in order to evaluate any association with virulence. Plasmid patterns were found to be characteristic of individual strains and no particular molecular size band was found to correlate with either of the two categories of isolates. It is concluded that the plasmid pattern analysis carried out could not differentiate between carrier and invasive S. epidermidis, but did provide a useful tool for epidemiological investigation.

Clonal analysis of methicillin-resistant Staphylococcus aureus strains from intercontinental sources: association of the mec gene with divergent phylogenetic lineages implies dissemination by horizontal transfer and recombination

Genetic relationships among 254 isolates of Staphylococcus aureus resistant to methicillin recovered between 1961 and 1992 from nine countries on four continents were determined by analyzing electrophoretically demonstrable allelic variation at 15 chromosomal enzyme loci. Fifteen distinctive electrophoretic types, marking clones, were identified. The mec gene is harbored by many divergent phylogenetic lineages representing a large portion of the breadth of chromosomal diversity in the species, a result that is interpreted as evidence that multiple episodes of horizontal transfer and recombination have contributed to the spread of this resistance determinant in natural populations. Isolates recovered in the United Kingdom, Denmark, Switzerland, Egypt, and Uganda in the 1960s are of a single multilocus enzyme genotype and probably are progeny of an ancestral methicillin-resistant clone. There is geographic variation in the frequency of recovery of the common methicillin-resistant clones, an observation that may in part explain reported regional differences in natural history correlates of resistant organisms.

DNA fingerprinting by pulsed-field gel electrophoresis is more effective than ribotyping in distinguishing among methicillin-resistant Staphylococcus aureus isolates

Pulsed-field gel electrophoresis (PFGE) after SmaI restriction of DNA from 239 methicillin-resistant Staphylococcus aureus isolates (from 142 patients) produced 26 different fingerprints. The deduced chromosome sizes ranged from 2,200 to 3,100 kb (+/- 100 kb). A total of 81 isolates taken from 65 patients were then typed by PFGE and ribotyping with ClaI, EcoRI, and HindIII. Ribotypes were less discriminating than PFGE. Ribotyping did not discriminate isolates from a given PFGE fingerprint into different subsets. PFGE may be a more effective epidemiological tool than ribotyping for the typing of methicillin-resistant S. aureus strains.

Correlation between aminoglycoside resistance profiles and DNA hybridization of clinical isolates

DNA hybridization data and aminoglycoside resistance profiles (AGRPs) were determined for 4,088 clinical isolates from three studies (United States, Belgium, and Argentina). The correlation between susceptibility profiles and hybridization results was determined with nine DNA probes. For each of the seven aminoglycoside resistance profiles which we were able to test, the data suggested at least two distinct genes could encode enzymes which lead to identical resistance profiles. Furthermore, the DNA hybridization data showed that individual strains carried up to six unique aminoglycoside resistance genes. DNA hybridization revealed interesting differences in the frequencies of these genes by organism and by country.

Pulsed field gel electrophoresis as a new epidemiological tool for monitoring methicillin-resistant Staphylococcus aureus in an intensive care unit

Pulsed field gel electrophoresis (PFGE) of bacterial DNA was used in a 1-month epidemiological study of methicillin-resistant Staphylococcus aureus (MRSA) in a 15-bed Intensive Care Unit (ICU). Patient and hospital staff carriage as well as distribution of MRSA in the ICU environment were investigated, and a total of 3802 samples produced 175 isolates. The stability and the reproducibility of the PFGE method were satisfactory. Moreover, the plasmid content of the strains so far examined had no influence on the PFGE profiles of the MRSA strains. The polymorphic profiles observed also account for the use of this method as an epidemiological tool for investigating MRSA. Among 30 patients who stayed more than 4 days in the unit, PFGE analysis showed 11 episodes of colonization in nine patients, whereas lysotyping and plasmid DNA analysis demonstrated only eight and seven such episodes in the same patients, respectively. The combination of PFGE with lysotyping and plasmid analysis may provide a greater discriminatory capacity between MRSA isolates.

Comparison of genomic DNAs of different enterococcal isolates using restriction endonucleases with infrequent recognition sites

Epidemiologic evaluation of enterococci has been limited by the lack of a simple and effective method for comparing strains. In this study, we have compared chromosomal restriction endonuclease digestion patterns of 27 isolates of Enterococcus faecalis from three different locations by using pulsed-field electrophoresis of large chromosomal fragments (14 to 1,000 kilobases). All but two isolates generated a clear, evaluable pattern with a single lysis and digestion, and the remaining two were visualized when a larger quantity of bacteria was used. All isolates from different locations generated different restriction patterns, as did most isolates within a single location; there was also evidence of spread of strains between individuals in each location. The ease with which this analysis can be performed, together with the clarity and polymorphism seen in the patterns, suggests that this technique will be very useful for epidemiological evaluations of nosocomial enterococcal infections.