Evidence for a clonal origin of methicillin resistance in Staphylococcus aureus

Stability of genomic DNA fragment patterns in methicillin resistant Staphylococcus aureus (MRSA) during the course of intra- and interhospital spread

The analysis of genomic DNA fragment patterns has revealed as a powerful tool for strain discrimination in Staphylococcus aureus; for use as an epidemiological marker, stability during the course of an outbreak is an essential prerequisite. Genomic DNA fragment patterns (SmaI restriction, pulsed-field electrophoresis) of four different epidemic MRSA strains were compared along with intra- and interhospital and country-wide spread over more than 12 months in Germany. Strain I was isolated from infections in 8 hospitals. In one hospital a subclone arised which differed from the original strain by 4 fragments. Strain II was spread among 4 hospitals, isolates from three of these hospitals exhibited a variability of one to three fragments in the 150-200 kb range. Two hospitals in the Hannover-area were affected by strain III; in 17 isolates of this strain a variability up to three fragments was found in the 170-200 kb range. Strain IV was isolated from 19 cases of infections in 3 hospitals in Berlin. The fragment patterns were completely stable. When S. aureus strains are typed by genomic DNA fragment patterns, a variability in a definite range of molecular masses during the course of an epidemic should be taken into consideration.

Genetic variation in Staphylococcus aureus coagulase genes: potential and limits for use as epidemiological marker

To perform coagulase gene typing, the repeated units encoding hypervariable regions of the Staphylococcus aureus coagulase gene were amplified by the PCR technique; this was followed by AluI restriction enzyme digestion and analysis of restriction fragment length polymorphism (RFLP) patterns. In order to assess the discriminatory power of this typing method, 30 epidemiologically unrelated S. aureus strains which differed by their pulsed-field gel electrophoresis patterns were examined. Although 18 of the 30 strains had unique and unshared AluI RFLP patterns, there were only four observed patterns in the remaining 12 strains. This finding indicated that unrelated strains may share identical AluI RFLP patterns. To elucidate the degree of genetic variation in the C-terminus-encoding loci within the coagulase genes, the PCR products of these 12 strains were subjected to Taq polymerase-mediated sequencing. Sequence analysis confirmed the AluI recognition sites in each of the four RFLP groups and demonstrated that AluI appears to yield the highest RFLP in restriction enzyme analysis. By their DNA sequences the majority of strains sharing common AluI groups could be clearly differentiated from each other and revealed between 93.2 and 98.5% homology. When we determined the nucleotide sequences of two strains after six subcultivations no significant alterations were observed. Because the discriminatory power of the current coagulase gene typing method is not great enough to be used as the sole method to type S. aureus, additional techniques are necessary. Sequence analysis of the repeated unit-encoding region for the typing of S. aureus may be potentially useful as an alternative to other current molecular typing techniques.

Characterization of two different clusters of clonally related methicillin-resistant Staphylococcus aureus strains by conventional and molecular typing

The DNA fragments of 28 distinct isolates of methicillin-resistant Staphylococcus aureus (MRSA) originating from different hospitals in Warsaw and Lodz, were studied. They were obtained by cleavage with restriction endonuclease SmaI and subsequently analysed by pulsed-field electrophoresis. Sixteen different patterns were seen and clusters of related strains were clearly distinguishable. Minor differences in fragment patterns within these clusters and among epidemiologically related strains, revealed genomic rearrangements in the course of clonal dissemination of particular strains. The isolates were also checked for the expression of methicillin resistance. Isolates with heterogenous and homogeneous phenotypes, fell into clearly distinct clusters and thus formed two clonally related MRSA strains. Differences were also seen with phage and biochemical typing, and antimicrobial resistance patterns.

Origin and evolution of DNA associated with resistance to methicillin in staphylococci

The gene mediating resistance to methicillin in staphylococci (mecA) and its flanking sequences (mec DNA) make up a chromosomal DNA locus that is unique to methicillin-resistant bacteria; no equivalent locus exists in methicillin-susceptible cells. The origin of mec DNA is not known, but evidence supports horizontal transfer of mec DNA between different staphylococcal species and of the mecA gene between different Gram-positive genera.

New aspects of old pathogens of pneumonia

Changes in the microorganism or in the host have resulted in exciting new aspects of several old pathogens of pneumonia. Penicillin-resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus, and "toxic strep" are examples of changes in the microorganisms. Host changes have resulted in Mycoplasma pneumoniae now emerging as a cause of pneumonia in the elderly.

Spread and maintenance of a dominant methicillin-resistant Staphylococcus aureus (MRSA) clone during an outbreak of MRSA disease in a Spanish hospital

It was not until November 1989 that the 1,000-bed University-affiliated Hospital de Bellvitge "Princeps d'Espanya" in Barcelona first acquired methicillin-resistant Staphylococcus aureus (MRSA). Since that time, the outbreak of MRSA disease has continued. We have analyzed by genomic DNA fingerprinting 189 MRSA isolates collected between late 1989 and the end of 1993. The isolates include both invasive and colonizing strains as well as isolates from health-care workers and environmental sources. In addition, 52 clinical isolates of methicillin-susceptible S. aureus (MSSA) collected in the same hospital were also analyzed. Isolates were classified into clonal types on the basis of molecular typing techniques. A single MRSA clone (I::B::a) belonging to ClaI type I, pulsed-field gel electrophoretic pattern B, and Tn554 pattern a was responsible for the great majority of infections (73% of blood cultures and 79% of specimens from other clinical sources). This clone appeared at the very beginning of the outbreak, spread throughout the hospital wards, and was also carried by inpatients and health-care workers and on environmental surfaces. In contrast, no dominant lineage was apparent among MSSA isolates (33 distinct pulsed-field gel electrophoretic patterns among 52 isolates). Two MSSA isolates seem to have originated from the dominant clone by deletion of the mecA gene and some additional DNA. In several isolates, different mecA polymorphs were present in identical chromosomal backgrounds or cells with distinct chromosomal backgrounds carried the same mecA polymorph, suggesting horizontal transfer of the mecA gene.

Clonal dissemination of two MRSA strains in Germany

Clonal dissemination of two different MRSA strains, both clumping factor negative, has been observed in Germany for more than a year. Both strains possess the mec-A determinant and each exhibits a characteristic genomic DNA fragment pattern. One strain has spread in the north, the other in the south-west of Germany. Intensive care units are mainly affected by MRSA-infections and probably play a special role in further intra- and inter-hospital spread.

Comparison of the virulence of methicillin-resistant and methicillin-sensitive Staphylococcus aureus

The virulence of methicillin-resistant Staphylococcus aureus (MRSA) was compared with that of methicillin-sensitive S. aureus (MSSA), using 13 MRSA and 7 MSSA strains isolated from clinical specimens. The infectivity and lethality of the two groups were examined as to the inoculum required to infect 50% of guinea pigs (ID50) and to kill 50% of mice (LD50), respectively. The mean ID50 [log10 colony forming units (CFU)] for MRSA strains was 7.1 +/- 0.60 standard deviation, which was 1.5 higher than that for MSSA strains (P < 0.001). The mean LD50 (log10 CFU) for MRSA strains was 9.0 +/- 0.42, being 1.1 higher than that for MSSA strains (P = 0.001). Pretreatment of mice with cyclophosphamide decreased the mean LD50 for MRSA strains more than that for MSSA strains, resulting in the difference in the mean LD50 being insignificant (P = 0.502). These results indicate that MRSA is less virulent than MSSA in normal hosts, but that they are equally virulent in immunocompromised hosts. The growth of MRSA strains was much slower than that of MSSA strains in the lag phase, although their growth rates were almost the same in the exponential growth phase, suggesting that the difference in virulence between them may be at least partly due to such a difference in growth.

Cloning and characterization of a gene affecting the methicillin resistance level and the autolysis rate in Staphylococcus aureus

Tn918 mutagenesis of a high-level methicillin-resistant Staphylococcus aureus (methicillin MIC, 800 micrograms/ml) led to the isolation of a low-resistance mutant. The Tn918 insert was transferred back to the parent to produce strain SRM563 (methicillin MIC, 12.5 micrograms/ml), which showed heterogeneous resistance. Twenty-two clinical isolates of methicillin-resistant S. aureus were transformed with DNA of SRM563. In the transformants of most strains, instances of reduced resistance were observed with concomitant increases of autolysis rate induced by Triton X-100 and were generally more profound in high-resistance strains. Two transformants exhibited a decrease of the autolysis rate and little reduction of resistance. In the transformant of methicillin-susceptible strain RN2677, an increase of the autolysis rate and little reduction of resistance were observed. The production of low-affinity penicillin-binding protein (PBP2') did not significantly decrease in the mutants. Insertion of Tn918 occurred within the 3'-terminal region of a novel gene designated llm, which was cloned and sequenced. RNA blot analysis demonstrated that the gene was transcribed. The encoded protein was composed of 351 amino acid residues with a molecular weight of 38,512 and was hydrophobic, suggesting its location on the membrane. The gene was detected by PCR in all S. aureus strains tested but not in the other 26 staphylococcal species. Comparison of the 3'-terminal sequences of the gene among several S. aureus strains showed that, whereas nucleotide substitutions occurred at the third position in seven of eight 3'-terminal codons, only C-terminal amino acid variation of glutamate or aspartate was observed.

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.

Typing of Staphylococcus aureus by amplification of the 16S-23S rRNA intergenic spacer sequences

The possibility of using polymorphisms in the spacer regions between 16S and 23S rRNA genes in order to type Staphylococcus aureus has been evaluated. To this purpose, DNA extracted from 74 independent isolates was amplified making use of a pair of primers complementary to conserved regions in the 16S and 23S genes. We have demonstrated that the method provides a good discrimination between unrelated isolates, giving better results when methicillin-sensitive strains are considered. Moreover, the amplification profiles were reproducible and all strains were typable. Given these results, and the technical simplicity of the process, we propose PCR-ribotyping to be taken into consideration as a method for typing S. aureus.

Comparison of four genotyping assays for epidemiological study of methicillin-resistant Staphylococcus aureus

Twenty-six methicillin-resistant Staphylococcus aureus strains were genetically differentiated by interrepeat PCR and the results compared with those of ribotyping, pulsed-field gel electrophoresis (PFGE) and random amplified polymorphic DNA analysis obtained in a previous study for the same strains. The comparison showed that the PCR-mediated assays were as discriminatory as PFGE, whereas ribotyping was the least powerful genotyping method. Due to the ease of performance, PCR fingerprinting may become the method of choice for establishing clonal relationship among Staphylococcus aureus isolates.

Cell surface hydrophobicity and adherence to extra-cellular matrix proteins in two collections of methicillin-resistant Staphylococcus aureus

Non-specific and specific mechanisms of adherence have been examined in two collections of methicillin-resistant Staphylococcus aureus (MRSA). Determination of hydrophobicity by salt aggregation, hydrophobicity indices and of adherence to the extra-cellular matrix proteins fibronectin, vitronectin, laminin and collagen type 1 have failed to reveal any correlation with phage-type, plasmid profile or antibiogram. Further, the strain collections, made over a period of years in two countries, differ markedly in their adherence characteristics; MRSA are heterogeneous in this respect. Such heterogeneity may explain the polarization of views on the epidemicity or 'virulence' of MRSA. With the exception of adherence to collagen a small group of methicillin sensitive S. aureus had characteristics intermediate between the two groups of MRSA.

Analysis of nosocomial outbreaks with multiply and methicillin-resistant Staphylococcus aureus (MRSA) in Germany: implications for hospital hygiene

Two outbreaks of nosocomial infections with MRSA, one in a urological unit in connection with transurethral prostatectomy and the other in an orthopaedic clinic with infections after implantation of prosthetic hips, have been analyzed on the basis of typing MRSA by phage-patterns, plasmid profiles and genomic DNA fragment patterns. Main reasons for these outbreaks were obviously mistakes in hospital hygiene and an inappropriate antibiotic prophylaxis (in the first outbreak a quinolone over about 7 days, in the second a third generation cephalosporin). Both outbreaks could be stopped by measures of hospital hygiene including isolated or cohort nursing of affected patients, and change in antibiotic prophylaxis. Intensive care units (ICUs) are more often affected by MRSA than other clinical settings. As described by the example of an outbreak with MRSA in a municipal hospital, ICUs can play a special role in intrahospital spread of MRSA. The recently observed inter-regional clonal interhospital dissemination of MRSA in Germany is mainly due to a transfer of patients between hospitals; prewarning of the hospital of destination and a number of hygiene measures can prevent further spread.

Dissemination among staphylococci of DNA sequences associated with methicillin resistance

DNA probes consisting of pUC19 containing cloned Staphylococcus aureus chromosomal fragments were constructed from two methicillin-resistant S. aureus strains with different DNA sequences 5' to mecA, the gene that mediates methicillin resistance. The probe from one strain, BMS1, contained a portion of the regulatory sequences (the terminal 641 bp of mecR1 and all of mecI) associated with the induction and repression of mecA transcription (pGO195). The second probe, from strain COL (pGO198), contained DNA not found in strain BMS1. This DNA was within the sequences added at the site of a mecR1 deletion. Genomic digests of 14 S. aureus isolates recovered between 1961 and 1969 all hybridized with pGO198. In contrast, 78% (36 of 46) of the S. aureus organisms isolated since 1988 hybridized with pGO195 but not with pGO198; the remainder hybridized with pGO198. No S. aureus isolates hybridized with both probes. Staphylococcus epidermidis digests hybridized with pGO198 (46%), pGO195 (14%), or both probes (35%); all 20 Staphylococcus haemolyticus isolates hybridized with pGO198. The restriction fragment length polymorphism patterns of all pGO198-hybridizing regions in S. aureus were identical to those in strain COL. In addition, the mecR1 deletion junction nucleotide sequences of eight S. aureus and six S. epidermidis isolates were identical. However, 21 of 23 S. epidermidis and all 20 S. haemolyticus isolates had from 5 to more than 20 additional chromosomal bands that hybridized with pGO198; none of 21 S. aureus isolates had additional hybridizing bands. These data suggest that the additional DNA responsible for the mecR1 deletion was part of a repetitive, and possibly mobile, element resident in coagulase-negative staphylococci but not in S. aureus. These data also support a hypothesis that the deletion event occurred in a coagulase-negative staphylococcus with subsequent acquisition of the interrupted sequences by S. aureus.

Comparison of traditional and molecular methods of typing isolates of Staphylococcus aureus

Fifty-nine Staphylococcus aureus isolates and 1 isolate of Staphylococcus intermedius were typed by investigators at eight institutions by using either antibiograms, bacteriophage typing, biotyping, immunoblotting, insertion sequence typing with IS257/431, multilocus enzyme electrophoresis, restriction analysis of plasmid DNA, pulsed-field or field inversion gel electrophoresis, restriction analysis of PCR-amplified coagulase gene sequences, restriction fragment length polymorphism typing by using four staphylococcal genes as probes, or ribotyping. Isolates from four well-characterized outbreaks (n = 29) and a collection of organisms from two nursing homes were mixed with epidemiologically unrelated stock strains from the Centers for Disease Control and Prevention. Several isolates were included multiple times either within or between the sets of isolates to analyze the reproducibilities of the typing systems. Overall, the DNA-based techniques and immunoblotting were most effective in grouping outbreak-related strains, recognizing 27 to 29 of the 29 outbreak-related strains; however, they also tended to include 3 to 8 epidemiologically unrelated isolates in the same strain type. Restriction fragment length polymorphism methods with mec gene-associated loci were less useful than other techniques for typing oxacillin-susceptible isolates. Phage typing, plasmid DNA restriction analysis, and antibiogram analysis, the techniques most readily available to clinical laboratories, identified 23 to 26 of 29 outbreak-related isolates and assigned 0 to 6 unrelated isolates to outbreak strain types. No single technique was clearly superior to the others; however, biotyping, because it produced so many subtypes, did not effectively group outbreak-related strains of S. aureus.

Methicillin-resistant Staphylococcus aureus disease in a Portuguese hospital: characterization of clonal types by a combination of DNA typing methods

Fifteen pediatric patients as well as the five nursing staff of the Burn Unit of the Hospital D. Estefania in Lisbon, Portugal, were assayed at weekly intervals over a five-month period in order to identify the nature and number of methicillin-resistant Staphylococcus aureus (MRSA) clones associated with colonization and wound infection. Methicillin resistance was confirmed by a mec-specific DNA probe. MRSA isolates were classified into chromosomal types (clones) on the basis of a variety of techniques: (i) ribotyping; (ii) restriction digestion by the endonuclease ClaI followed by Southern hybridization with the mecA-specific DNA probe and (iii) by hybridization with Tn554; and (iv) pulsed-field electrophoresis (PFE) of SmaI digests followed by (v) Southern hybridization with the mecA DNA probe. A sixth, physiological technique (population analysis) was used to define the mode of phenotypic expression of methicillin resistance in each isolate. All isolates carried a single, common polymorph (ClaI type III) of the mecA gene. Hybridization with Tn554 resolved these isolates to two novel patterns (alpha and beta), of which one (Tn554 alpha) was predominant (90%). This pattern could be further resolved to four closely related PFE types (A through D). In contrast, all isolates with the Tn554 beta pattern belonged to an additional, grossly different PFE type E. The Tn554 beta class was also unique in that these bacteria carried the mecA gene in a SmaI fragment smaller (about 170 kb) than that found in the alpha type strains (194 kb). Most isolates (83%) showed a single heterogeneous (population analysis Class 3) mode of resistance expression. The data demonstrate the full capacity of the globally rare (ClaI type III) MRSA clone for colonization and virulence. The results also document the stability of the complex heterogeneous resistance phenotype as well as the stability of the chromosomal types under conditions of in vivo carriage over a period of several months. In a few isolates the same mecA polymorph was present in several, grossly different genetic backgrounds, suggesting horizontal transfer of the mecA gene.

Site-directed mutagenesis of the mecA gene from a methicillin-resistant strain of Staphylococcus aureus

The mecA-27r gene from Staphylococcus aureus 27r encodes penicillin-binding protein 2a (PBP2a-27r), which causes this strain to be methicillin resistant. Removal or replacement of the N-terminal transmembrane domain had no effect on binding of penicillin, but removal of portions of the putative transglycosylase domain (144, 245, or 341 amino acids after the transmembrane region) destroyed penicillin-binding activity. The SXXK, SXN, and KSG motifs, present in all penicillin-interacting enzymes, were found in the expected linear spatial arrangement within the putative transpeptidase region of PBP2a-27r. Alterations of amino acids in all three of these motifs resulted in elimination of penicillin-binding activity, confirming their roles in the interaction with penicillin.

Phenotypical and genotypical characterization of epidemic clumping factor-negative, oxacillin-resistant Staphylococcus aureus

A total of 50 oxacillin-resistant Staphylococcus aureus (ORSA) strains that were clumping factor negative (CFN) and protein A negative by latex agglutination were collected from patients in six different hospitals at different locations in Germany during 1991 and 1992. Antibiograms, bacteriophage typing, and plasmid analysis were performed. The antibiograms showed that, besides oxacillin, all CFN ORSA strains were resistant to gentamicin, clindamycin, erythromycin, ciprofloxacin, and fosfomycin. All these isolates were nontypeable with an international set of phages, and an additional experimental phage set indicated that the strains were phage type 16, 192. Moreover, all isolates possessed a single plasmid of 30 kb, and restriction analysis of those plasmids revealed identical patterns. For genotyping, these 50 isolates were also analyzed by pulsed-field gel electrophoresis (PFGE) and polymerase chain reaction (PCR) of the coagulase and protein A genes and then by restriction enzyme digestion and analysis of restriction fragment length polymorphisms (RFLPs). With 49 strains, electrophoresis of SmaI-digested chromosomal DNA revealed identical PFGE patterns regarding the number and size of the DNA fragments, which could be differentiated from those of clumping factor-positive ORSA strains. Typing for the coagulase gene by PCR revealed PCR products of identical sizes. The AluI restriction digestion patterns of the PCR products were identical. PCR with primers derived from the region of that part of the protein A gene that encodes the immunoglobulin G-binding domains showed a PCR product that was about 170 bp smaller than that of the protein A gene from strains that were positive in the protein A latex agglutination test. Since it is precisely this size that is required in order to encode one immunoglobulin G-binding region, we assume that this is not present in the CFN ORSA strains. The phenotypical and genotypical features identify these very unusual CFN ORSA stains as being of clonal origin.

Construction and overexpression in Escherichia coli of genetically engineered derivatives of penicillin-binding protein 2' of Staphylococcus epidermidis

Removal of the putative amino-terminal membrane spanning region of penicillin-binding protein 2' (PBP-2') of Staphylococcus epidermidis WT55 was carried out by truncating the amino terminus-coding end of the mecA gene. PCR and site directed mutagenesis were used to introduce unique restriction sites at position 68 (HindIII) and at position 80 (NcoI) of the mecA gene, respectively. The coupling of the shortened coding regions to the trc promoter and gene fusion to the lacZ gene, aimed to facilitate subsequent protein purifications, resulted in strong expression in the cytoplasm of Escherichia coli and partial sequestration into insoluble protein granules. The truncated PBP-2' retained its penicillin-binding ability and also bound the monoclonal antibody directed against PBP-2' of Staphylococcus aureus.

An introduction to molecular hospital epidemiology

One of the primary goals of the hospital epidemiologist is the identification and characterization of nosocomial outbreaks. Outbreaks usually are identified by the recovery of a unique strain from a cluster of patients infected with a nosocomially acquired pathogen. Until recently, the microbiologic tools available to any hospital epidemiologist permitted identification of novel strains by speciation and antibiogram. Thus, most outbreak descriptions consisted of the identification of an unusual species or the appearance of a new antibiotic resistance phenotype in a recognized nosocomial pathogen. The emergence of enterococci in this decade or the emergence of methicillin-resistantStaphylococcus aureus(MRSA) in the 1970s am notable examples. Where resources have existed, additional methods have been used for further discrimination. Outbreak and endemic strains have been compared by phage typing, serologic typing, and capsular typing. For example, beta-hemolytic streptococci may be grouped by Lancefield antisera, andStaphylococcus aureusmay be grouped by phage typing. However, certain Lancefield groups and certain phage groups are common among epidemic and endemic isolates, which may prevent identification of a specific epidemic strain.

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.