Identification of a Staphylococcus aureus transposon (Tn4291) that carries the methicillin resistance gene(s)

Characterization of a complex context containing mecA but lacking genes encoding cassette chromosome recombinases in Staphylococcus haemolyticus

Background

Methicillin resistance determinant mecA is generally transferred by SCCmec elements. However, the mecA gene might not be carried by a SCCmec in a Staphylococcus haemolyticus clinical isolate, WCH1, as no cassette chromosome recombinase genes were detected. Therefore, the genetic context of mecA in WCH1 was investigated.

Results

A 40-kb region containing mecA was obtained from WCH1, bounded by orfX at one end and several orfs of S. haemolyticus core chromosome at the other. This 40-kb region was very complex in structure with multiple genetic components that appeared to have different origins. For instance, the 3.7-kb structure adjacent to orfX was almost identical to that on the chromosome of Staphylococcus epidermidis RP62a but was absent from S. haemolyticus JCSC1435. Terminal inverted repeats of SCC were found but no ccr genes could be detected. mecA was bracketed by two copies of IS431, which was flanked by 8-bp direct target repeat sequence (DR).

Conclusions

The presence of 8-bp DR suggests that the two copies of IS431 might have formed a composite transposon for mobilizing mecA. This finding is of significance as multiple copies of IS431 are commonly present in the contexts of mecA, which might have the potential to form various composite transposons that could mediate the mobilization of mecA. This study also provides an explanation for the absence of ccr in some staphylococci isolates carrying mecA.

Multiple roles of Staphylococcus aureus enterotoxins: pathogenicity, superantigenic activity, and correlation to antibiotic resistance

Heat-stable enterotoxins are the most notable virulence factors associated with Staphylococcus aureus, a common pathogen associated with serious community and hospital acquired diseases. Staphylococcal enterotoxins (SEs) cause toxic shock-like syndromes and have been implicated in food poisoning. But SEs also act as superantigens that stimulate T-cell proliferation, and a high correlation between these activities has been detected. Most of the nosocomial S. aureus infections are caused by methicillin-resistant S. aureus (MRSA) strains, and those resistant to quinolones or multiresistant to other antibiotics are emerging, leaving a limited choice for their control. This review focuses on these diverse roles of SE, their possible correlations and the influence in disease progression and therapy.

Detection of methicillin-resistant Staphylococcus aureus and simultaneous confirmation by automated nucleic acid extraction and real-time PCR

A molecular assay for the simultaneous detection of a Staphylococcus aureus-specific gene and the mecA gene, responsible for the resistance to methicillin in staphylococci, was evaluated. The assay included an automated DNA extraction protocol conducted with a MagNA Pure instrument and real-time PCR conducted with a LightCycler instrument. The performance and robustness of the assay were evaluated for a suspension of methicillin-resistant S. aureus (MRSA) strain with a turbidity equivalent to a McFarland standard of 0.5, which was found to be the ideal working concentration. The specificity of the new molecular assay was tested with a panel of 30 gram-negative and gram-positive bacterial strains other than MRSA. No cross-reactivity was observed. In a clinical study, 109 isolates of MRSA were investigated. All clinical MRSA isolates gave positive results for the S. aureus-specific genomic target, and all but one were positive for the mecA gene. In conclusion, the new molecular assay was found to be quick, robust, and laborsaving, and it proved to be suitable for a routine molecular diagnostic laboratory.

Pathogenicity and resistance islands of staphylococci

Variable genetic elements including plasmids, transposons and prophages are involved in pathogenesis and antibiotic resistance, and are an important component of the staphylococcal genome. This review covers a set of newly described variable chromosomal elements, pathogenicity and resistance islands, carrying superantigen and resistance genes, especially toxic shock and methicillin resistance, respectively.

Cloning and nucleotide sequence determination of the entire mec DNA of pre-methicillin-resistant Staphylococcus aureus N315

In methicillin-resistant Staphylococcus aureus, the methicillin resistance gene mecA is localized within a large chromosomal region which is absent in the methicillin-susceptible S. aureus chromosome. The region, designated mec DNA, is speculated to have originated from the genome of another bacterial species and become integrated into the chromosome of the S. aureus cell in the past. We report here cloning and determination of the structure of the entire mec DNA sequence from a Japanese S. aureus strain, N315. The mec DNA was found to be 51,669 bp long, including terminal inverted repeats of 27 bp and a characteristic pair of direct repeat sequences of 15 bp each: one is situated in the right extremity of mec DNA, and the other is situated outside the mec DNA and abuts the left boundary of mec DNA. The integration site of mec DNA was found to be located in an open reading frame (ORF) of unknown function, designated orfX. Clusters of antibiotic resistance genes were noted in mec DNA carried by transposon Tn554 and an integrated copy of plasmid pUB110. Both the transposon and plasmid were integrated in the proximity of the mecA gene, the latter being flanked by a pair of insertion sequence IS431 elements. Many ORFs other than those encoding antibiotic resistance were considered nonfunctional because of the acquired mutations or partial deletions found in the ORFs. Two ORFs potentially encoding novel site-specific recombinases were found in mec DNA. However, there was no ORF that might encode mec DNA-specific transposase or integrase proteins, indicating that the mec DNA is not a transposon or a bacteriophage in nature.

Antibiotic resistance in oral/respiratory bacteria

In the last 20 years, changes in world technology have occurred which have allowed for the rapid transport of people, food, and goods. Unfortunately, antibiotic residues and antibiotic-resistant bacteria have been transported as well. Over the past 20 years, the rise in antibiotic-resistant gene carriage in virtually every species of bacteria, not just oral/respiratory bacteria, has been documented. In this review, the main mechanisms of resistance to the important antibiotics used for treatment of disease caused by oral/respiratory bacteria--including beta-lactams, tetracycline, and metronidazole--are discussed in detail. Mechanisms of resistance for macrolides, lincosamides, streptogramins, trimethoprim, sulfonamides, aminoglycosides, and chloramphenicol are also discussed, along with the possible role that mercury resistance may play in the bacterial ecology.

The genome of Staphylococcus aureus: a review

The genome of Staphylococcus aureus consists of a single circular chromosome (2.7-2.8 mbp) plus an assortment of extrachromosomal accessory genetic elements: conjugative and nonconjugative plasmids, mobile elements (IS, Tn, Hi), prophages and other variable elements. Plasmids (1-60 kbp) are classified into 4 classes and there are 15 known incompatibility groups. Mobile elements of the genome (0.8-18 kbp) appear in the chromosome or in plasmids of classes II and III. Prophages (45-60 kbp) are integrated in the bacterial chromosome, and they are UV- or mitomycin-inducible. Temperate bacteriophages of S. aureus are members of the Siphoviridae and the serological groups A, B and F occur most frequently. In the paper presented, the characteristics of chromosome, plasmids, transposons and other genetic elements of S. aureus genome are given and an alphabetical list of known genes of this species is included.

Mobile elements in the evolution and spread of multiple-drug resistance in staphylococci

Since the introduction of antimicrobial chemotherapy, staphylococci have shown a remarkable propensity to develop drug resistance. As a result, strains have evolved that are resistant to most classes of clinically useful antimicrobial agents. The emergence of these multiply-drug-resistant strains is primarily due to the capture of pre-existent resistance genes. In combination with plasmids and gene transfer mechanisms, mobile genetic elements have been central to the acquisition and dissemination of this resistance. In particular they have played a significant role in the assembly of drug-resistance gene clusters in these multiply-resistant staphylococci.

Antibiotic resistance mechanisms in bacteria of oral and upper respiratory origin

Over the past 20 years, antibiotic resistance has increased in virtually every species of bacteria examined. In this paper, the main mechanisms of antibiotic resistance currently known for antibiotics used for treatment of disease caused by oral and upper respiratory bacteria will be reviewed, with an emphasis on the most commonly used antibiotics. The possible role that mercury, which is released from silver amalgams, plays in the oral/respiratory bacterial ecology is also discussed, as it relates to possible selection of antibiotic resistant bacteria.

A combined molecular approach to screen for mec gene variants from methicillin-resistant Staphylococcus aureus

Previous reports have suggested a common origin for all methicillin resistance (mec) genes from methicillin-resistant Staphylococcus aureus (MRSA) isolates examined so far. The purpose of this study was to explore several molecular methods for screening MRSA isolates from different sources and, in some cases, with varying phenotypes. Eighty MRSA isolates from three teaching hospitals in the University of Louisville Medical Center were compared with MRSAs from a hospital in southern California and with methicillin-sensitive S. aureus isolates. The methods were used to detect the presence of mec gene and to screen for any polymorphisms in these genes for the respective strains. The mec gene for each isolate was amplified via the polymerase chain reaction, and each polymerase chain reaction product was compared to the others by restriction enzyme digestion, denaturing-gradient gel electrophoresis, and mutation detection enhancement. By these criteria, the mec genes from the 80 MRSA strains in this study seemed to be identical. Such a finding was not unexpected and supported the existing hypothesis of a common ancestor for all mec genes isolated in MRSA isolates. However, the combination of methods used in this study may facilitate screening of MRSA strains in population studies as mec gene variants begin to emerge.

Methicillin resistance in staphylococci: molecular and biochemical basis and clinical implications

Methicillin resistance in staphylococci is determined by mec, composed of 50 kb or more of DNA found only in methicillin-resistant strains. mec contains mecA, the gene for penicillin-binding protein 2a (PBP 2a); mecI and mecR1, regulatory genes controlling mecA expression; and numerous other elements and resistance determinants. A distinctive feature of methicillin resistance is its heterogeneous expression. Borderline resistance, a low-level type of resistance to methicillin exhibited by strains lacking mecA, is associated with modifications in native PBPs, beta-lactamase hyperproduction, or possibly a methicillinase. The resistance phenotype is influenced by numerous factors, including mec and beta-lactamase (bla) regulatory elements, fem factors, and yet to be identified chromosomal loci. The heterogeneous nature of methicillin resistance confounds susceptibility testing. Methodologies based on the detection of mecA are the most accurate. Vancomycin is the drug of choice for treatment of infection caused by methicillin-resistant strains. PBP 2a confers cross-resistance to most currently available beta-lactam antibiotics. Investigational agents that bind PBP 2a at low concentrations appear promising but have not been tested in humans. Alternatives to vancomycin are few due to the multiple drug resistances typical of methicillin-resistant staphylococci.

Isolation of methicillin-resistant coagulase-negative staphylococci from chickens

Methicillin-resistant coagulase-negative staphylococci were isolated from the nares and skin of 1- to 8-week-old healthy chickens in three flocks from a farm. Isolation of methicillin-resistant coagulase-negative staphylococci was positive for 72 (25.7%) of the 280 chickens tested, with the frequency varying from 2.2 to 100% according to flock. A total of 45 appropriate isolates were selected and subjected to identification. Of the 45 methicillin-resistant coagulase-negative staphylococcal isolates selected, 37 were identified as Staphylococcus sciuri, 5 were identified as Staphylococcus epidermidis, and 3 were identified as Staphylococcus saprophyticus. The distribution of the species was different among the flocks. Comparative analysis of the SmaI-digested chromosomal DNA by pulsed-field gel electrophoresis revealed that the isolates could have originated from a single clone of each of S. sciuri and S. saprophyticus and three clones of S. epidermidis. By two methods based on the PCR technique, the mecA gene was detected in all five representative isolates of each methicillin-resistant coagulase-negative staphylococcal clone. The nucleotide sequence of a PCR fragment obtained from an isolate of S. sciuri was completely identical to the corresponding region of mecA genes reported in human methicillin-resistant Staphylococcus aureus isolates and Staphylococcus epidermidis isolates. The representative methicillin-resistant coagulase-negative staphylococcal isolates were resistant to many beta-lactam antibiotics, and some isolates were also resistant to macrolide and aminoglycoside antibiotics. This is the first evidence of the existence of methicillin-resistant coagulase-negative staphylococci from animals possessing the mecA gene.

Typing of Staphylococcus aureus strains by PCR-amplification of variable-length 16S-23S rDNA spacer regions: characterization of spacer sequences

To develop a rapid and accurate method of typing large numbers of clinical isolates of Staphylococcus aureus, the spacer region C of the rRNA operon [1391-507 (16S-23S)] was enzymically amplified from 322 strains. When the products were separated by denaturing PAGE, 15 variable-length rrn alleles were demonstrated, ranging in size from 906 to 1223 bp. The variable-length HpaII-digested region C [(region E; 1446-196 (16S-23S)] amplification products were cloned into M13mp18RF to sequence separate variable-length alleles. A total of 17 region E inserts were sequenced, aligned and divided into nine alleles by length (938-1174) and sequence properties. The 16S-23S spacer rDNA varied in length (303-551 bp) and in properties; three alleles contained a tRNAIle gene alone, two alleles contained a tRNAIle and a tRNAAla gene, and four alleles lacked tRNA genes. The sequences of two alleles showed less than 1% variation when isolated from two or three S. aureus strains. The 48 penicillin- and methicillin-sensitive strains were divided into 26 ribotypes; in contrast, the 274 methicillin-resistant S. aureus (MRSA) strains were divided into nine ribotypes (A-I) with 97% typing as either ribotype A or B (rrnL was missing in B). The sequence conservation of the rrn operons argues for the use of the 16S-23S spacer region as a stable and direct indicator of the evolutionary divergence of S. aureus strains.

Identification of a genetic element (psr) which negatively controls expression of Enterococcus hirae penicillin-binding protein 5

Enterococcus hirae ATCC 9790 produces a penicillin-binding protein (PBP5) of low penicillin affinity which under certain conditions can take over the functions of all the other PBPs. The 7.1-kb EcoRI fragment containing the pbp5 gene of this strain and of two mutants, of which one (E. hirae R40) overproduces PBP5 and the other (E. hirae Rev14) does not produce PBP5, was cloned in pUC18 and sequenced. In the 7.1-kb EcoRI fragment cloned from strain ATCC 9790, an open reading frame (psr) potentially encoding a 19-kDa protein was identified 1 kb upstream of the pbp5 gene. An 87-bp deletion in this element was found in the 7.1-kb EcoRI fragment cloned from strains R40 and Rev14. In addition, several base substitutions were found in the pbp5 genes of strains R40 and Rev14. One of these converted the 42nd codon, TCA, to the stop codon, TAA, in the pbp5 gene of Rev14. Escherichia coli strains were transformed with plasmids carrying the 7.1-kb EcoRI insert or a 2.6-kb HincII insert containing only the pbp5 gene of the three strains. Immunoblotting analysis of proteins expressed by these transformants showed that the 87-bp deletion in psr was associated with the PBP5 overproducer phenotype of strain R40 and the conversion of the TCA codon to the stop codon was associated with the PBP5 nonproducer phenotype of strain Rev14. None of the other nucleotide substitutions had any apparent effect on the level of PBP5 synthesized.

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.

Survey of methicillin-resistant clinical strains of coagulase-negative staphylococci for mecA gene distribution

A total number of 125 methicillin-resistant (MIC, greater than or equal to 16) coagulase-negative Staphylococcus strains isolated in Japan were surveyed for the distribution of the mecA gene, the structural gene for penicillin-binding protein 2', which is the causative genetic element for the intrinsic resistance of methicillin-resistant Staphylococcus aureus. Screening with colony hybridization by using a cloned mecA gene probe revealed that 121 strains (96.8%) belonging to the nine coagulase-negative Staphylococcus species (S. epidermidis, S. haemolyticus, S. saprophyticus, S. sciuri, S. simulans, S. hominis, S. capitis, S. warneri, and S. caprae) carried mecA in their genome, indicating wide distribution of the gene among coagulase-negative Staphylococcus species. Most (93.4%) of the mecA-carrying strains were producers of penicillinase. Four strains, including two S. haemolyticus and two S. saprophyticus strains, did not carry mecA in spite of their resistance to methicillin. One of them was of low-level resistance (MIC, 16), but three of them had moderate- to high-level resistance to methicillin (MIC, 64). Analysis of gel electrophoretic banding patterns of penicillin-binding proteins of these strains showed absence of penicillin-binding protein 2' but some alterations in signal intensities of the other penicillin-binding proteins. The result indicated that about 3% of methicillin-resistant coagulase-negative staphylococci in these hospitals had a resistance mechanism different from that associated with the production of penicillin-binding protein 2', as has been reported in the case of a borderline methicillin-resistant strain of S. aureus.

The Staphylococcus aureus mec determinant comprises an unusual cluster of direct repeats and codes for a gene product similar to the Escherichia coli sn-glycerophosphoryl diester phosphodiesterase

The DNA sequence located between mecA, the gene that codes for penicillin-binding protein PBP2', and insertion sequence-like element IS431mec has been termed hypervariable because of its length polymorphism among different staphylococcal isolates. We sequenced and characterized the hypervariable region of the methicillin resistance determinant (mec) isolated from Staphylococcus aureus BB270. Within the 2,040-bp hypervariable region, we identified an unusual accumulation of long direct repeats. Analysis of the DNA sequence revealed a minimal direct repeat unit (dru) of 40 bp which was repeated 10 times within 500 bp. The dru sequences are responsible for the length polymorphism of mec. Moreover, we identified an open reading frame that codes for 145 amino acids (ORF145), whose deduced amino acid sequence showed 57% amino acid sequence similarity to the N terminus of the glycerophosphoryl diester phosphodiesterase (UgpQ) of Escherichia coli.

Typing of methicillin resistant and susceptible Staphylococcus aureus strains by ribosomal RNA gene restriction patterns using a biotinylated probe

Nine methicillin-sensitive (MSSA) and 37 methicillin-resistant (MRSA) Staphylococcus aureus isolates of various phage types and resistotypes from seven countries were investigated. Chromosomal DNA was restricted with HindIII or EcoRI, Southern blotted and hybridised with a cDNA probe to 16S+23S rRNA derived from MRSA NCTC 10442. Resulting rDNA profiles could be differentiated on the basis of 22 patterns which were unaffected by changes in plasmid, transposon, enterotoxin A or phage content. Percentage similarity values were calculated using the Dice coefficient and UPGA clustering. Australian and epidemic (EMRSA-1) isolates from the UK showed a high degree of similarity, but the pattern was not unique and was also found in MSSA and other MRSA, e.g. NCTC 10442. An MSSA of phage group II was the most distinct isolate. The method shows potential as an additional tool in a complex typing system, types non-phage typable strains and may provide clues to the clonal evolution of MRSA and MSSA.

Beta-lactamases, beta-lactamase inhibitors, and skin and skin-structure infections

beta-Lactamases have been known since the early 1940s when they were recognized as a major mechanism of resistance in Staphylococcus aureus. The synthesis of semisynthetic penicillins provided agents that overcame the resistance of staphylococci, but as gram-negative bacteria became increasingly important as the cause of infections, plasmid-mediated beta-lactamases were recognized in the Enterobacteriaceae, Haemophilus, and chromosomally mediated beta-lactamases in Klebsiella, and Bacteroides were found to be the mechanism of resistance of these species to ampicillin and related penicillins. Two approaches to the problem have been developed. One is to make stable compounds. This has been possible in the cephalosporin family. The other method has been to find inhibitors of beta-lactamases. Clavulanate is a beta-lactamase inhibitor that, in combination with amoxicillin, allows the combination to inhibit many of the organisms that are resistant to amoxicillin. Similarly, clavulanate has been combined with ticarcillin to provide a parenteral agent to inhibit beta-lactamase-producing bacteria and retain activity against Pseudomonas. Sulbactam has been combined with ampicillin. The combination of suicide inhibitors with other beta-lactams has provided agents that inhibit many of the bacteria present in mixed cutaneous infections. Clinical studies have established the efficacy of the clavulanate-amoxicillin and clavulanate-ticarcillin combinations in skin and skin-structure infections. These agents offer an alternative to other drugs when treating cutaneous infections.

Role of penicillinase plasmids in the stability of the mecA gene in methicillin-resistant Staphylococcus aureus

The stability of methicillin resistance (Mcr) in three independent clinical isolates, MR108, MR6, and MR61, of methicillin-resistant Staphylococcus aureus (MRSA) was studied. The Mcr phenotype was stably maintained in the progeny of all three MRSA clones carrying penicillinase plasmids. However, when the clones were tested after elimination of the plasmids, methicillin-susceptible (Mcs) subclones appeared at various frequencies. Seven Mcs subclones were classified into two groups based on their stabilities. Five Mcs subclones, which were derived from homogeneous strains MR108 and MR61, were stably susceptible. They lost penicillin-binding protein 2' production, and moreover, the mecA gene was deleted in four of five subclones. Two subclones were derived from heterogeneous strain MR6. They were very unstable, and more than half of their progeny were Mcr revertants. However, the remainder were stably Mcs and had lost penicillin-binding protein 2' and the mecA gene. We propose that penicillinase plasmids, which are present in most MRSA strains, play an important role in the stability and phenotypic expression of the mecA gene.

Homology of mecA gene in methicillin-resistant Staphylococcus haemolyticus and Staphylococcus simulans to that of Staphylococcus aureus

A penicillin-binding protein of molecular weight 76,000 inducible by beta-lactams was detected in methicillin-resistant Staphylococcus haemolyticus and Staphylococcus simulans. DNA from these strains hybridized to the mecA gene from Staphylococcus aureus; however, the chromosomal HindIII fragments containing the mecA genes were 3.4 kilobases in S. haemolyticus and 4.3 kilobases in S. simulans.

New mechanism for methicillin resistance in Staphylococcus aureus: clinical isolates that lack the PBP 2a gene and contain normal penicillin-binding proteins with modified penicillin-binding capacity

Seventeen clinical isolates of Staphylococcus aureus (from the United States and Europe) selected for low (borderline)-level methicillin resistance (MIC of methicillin, 2 to 4 micrograms/ml; MIC of oxacillin, 0.5 to 8 micrograms/ml) were examined for their mechanisms of resistance. Five strains were typical of heterogeneous S. aureus: they gave positive reactions with a DNA probe specific for mec and contained a small fraction (10(-6] of highly resistant cells (MIC, greater than 100 micrograms/ml). The rest of the 12 strains were homogeneous with respect to their methicillin resistance: the MIC of methicillin for all cells was 2 to 4 micrograms/ml, and no cells for which MICs were 50 micrograms/ml or higher were detectable (less than 10(-9]. None of these strains reacted with the mec-specific DNA probe. One representative strain of each group was characterized in more detail. Strain CDC-1, prototype of heterogeneous methicillin-resistant S. aureus, contained penicillin-binding protein (PBP) 2a; its DNA could transform a methicillin-susceptible and novobiocin-resistant recipient to methicillin resistance with ca. 35% linkage to Novr. Introduction of the "factor X" determinant (K. Murakami and A. Tomasz, J. Bacteriol. 171:874-879, 1989) converted strain CDC-1 to high, homogeneous resistance. Strain CDC-6, prototype of the second group of isolates, showed completely homogeneous MICs of methicillin, oxacillin, and cefotaxime. The strain contained modified "normal" PBPs: PBPs 1 and 2 showed low drug reactivity (and/or cellular amounts), and PBP 4 was present in elevated amounts. No PBP 2a could be detected. DNA isolated from strain CDC-6 could transform the methicillin-susceptible and novobiocin-resistant strain to methicillin resistance in a multistep fashion, but this resistance showed no genetic linkage to the Nov marker. We suggest that staphylococci with borderline resistance may contain at least three different classes of mechanism: heterogeneous, methicillin-resistant S. aureus, PBPs of modified drug reactivities, and the previously reported hyperproduction of beta-lactamase (L.K. McDougal and C. Thornsberry, J. Clin Microbiol. 23:832-839, 1986).

Restriction maps of the regions coding for methicillin and tobramycin resistances on chromosomal DNA in methicillin-resistant staphylococci

Chromosomal BamHI DNA fragments containing both the mecA gene encoding the penicillin-binding protein responsible for methicillin resistance and the aadD gene encoding 4',4"-adenylyltransferase responsible for tobramycin resistance were cloned from three methicillin- and tobramycin-resistant strains of Staphylococcus aureus and one strain of Staphylococcus epidermidis. Physical maps of the fragments were similar, suggesting their unique origin.

Expression and inducibility in Staphylococcus aureus of the mecA gene, which encodes a methicillin-resistant S. aureus-specific penicillin-binding protein

A beta-lactam-sensitive strain of Staphylococcus aureus could be converted to methicillin resistance by the introduction of a plasmid carrying the 4.3-kilobase HindIII chromosomal DNA fragment which encoded the mecA gene from a methicillin-resistant S. aureus. Transformant cells produced methicillin-resistant S. aureus-specific penicillin-binding protein constitutively, and additional insertion of an inducible penicillinase plasmid caused production of the pencillin-binding protein to become inducible.

Characterization of a novel insertion of the macrolides-lincosamides-streptogramin B resistance transposon Tn554 in methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis

Macrolides-lincosamides-streptogramin B resistance in staphylococci can result from a gene, ermA, that comprises part of transposon Tn554. Tn554 is unusual in (i) its high specificity for a primary chromosomal attachment site, att554, and (ii) the variability of its 3'-terminal six or seven nucleotides, which appear to copy the six or seven chromosomal nucleotides 5' to the parent transposon during transposition. We characterized a novel Tn554 insert in the chromosomes of methicillin-resistant Staphylococcus aureus strains involved in a current outbreak. This insert was found to resemble an insert recently discovered in S. epidermidis in its junctional fragment restriction pattern. Sequence analysis of the junctional regions showed that the attachment site, att155, exhibited 78% similarity to att554 (39 of the 50 nucleotides flanking the insertion sites) for both S. aureus and S. epidermidis inserts and that the 3' hexanucleotide of the S. epidermidis transposon (GACATC) resembled the reverse complement (TACATC) of its commonly occurring S. aureus counterpart (GATGTA). Epidemiologic and molecular data indicated that att155 is harbored by extra DNA characteristic of methicillin-resistant strains and absent from methicillin-susceptible ones. Further, Southern hybridization showed that, even in the absence of Tn554 inserts, some methicillin-resistant strains contain DNA related to att155 and Tn554.

Cloning and expression of methicillin resistance from Staphylococcus epidermidis in Staphylococcus carnosus

A 6.2-kilobase chromosomal DNA fragment from a methicillin-resistant Staphylococcus epidermidis strain was cloned into Staphylococcus carnosus by using staphylococcal plasmid pCA44 as the vector. The recombinant plasmid obtained, pBBB21, conferred methicillin resistance on its host and was responsible for the synthesis of a low-affinity penicillin-binding protein (PBP), PBP 2'. PBP 2' determined by the S. epidermidis DNA and expressed as a membrane-bound PBP in S. carnosus reacted with monoclonal antibodies directed against PBP 2' of Staphylococcus aureus origin, and the cloned S. epidermidis DNA hybridized to the methicillin (mec)-specific DNA from S. aureus. These findings point to a common origin of the methicillin resistance determinant in staphylococci.

Sequential transposition of Tn916 among Staphylococcus aureus protoplasts

Transposition of the Streptococcus faecalis conjugal tetracycline-resistance transposon Tn916 between S. aureus strains occurred when protoplasts of donor and recipient strains were regenerated together without prior fusion. Under these conditions, only Tn916 was transferred; spontaneous fusion of parental protoplasts is therefore unlikely to be responsible for Tn916 transfer. While the exact nature of this transfer remains unclear, it appears to resemble Tn916 conjugal transposition reported in S. faecalis. Evidence for sequential transpositions of Tn916 was obtained by 3-factorial transformation analyses and confirmed by DNA-DNA hybridizations. The ability of Tn916 to transpose within S. aureus and occupy diverse chromosomal sites demonstrates the value of this transposon in genetic studies of S. aureus.