Loss of methicillin-resistance from resistant strains of Staphylococcus aureus

Severe Staphylococcus aureus infections caused by clonally related community-acquired methicillin-susceptible and methicillin-resistant isolates

We investigated the genetic relatedness of 5 community-acquired (CA) Staphylococcus aureus isolates obtained from 4 consecutive pediatric patients presenting with sepsis syndrome and severe pneumonia during a 3-week period in 2000. Two patients were infected with methicillin-susceptible S. aureus (MSSA), and 2 were infected with methicillin-resistant S. aureus (MRSA). The pulsed-field gel electrophoresis patterns for the 2 CA-MRSA isolates were identical to each other, as were the patterns for the 3 CA-MSSA isolates. A 2-band difference reflecting the presence of a staphylococcal cassette chromosome mec (SCCmec) element distinguished the CA-MRSA isolates from the CA-MSSA isolates. The small, mobile type IV SCCmec element was present in the CA-MRSA isolates. These data suggest that an insertion or, less likely, a deletion of the SCCmec type IV element occurred in a highly virulent S. aureus background. Staphylococcal toxin genes sea, seh, lukS-PV, and lukF-PV were detected in all isolates. Also, in all isolates, was a partial homolog of seo (seo'). The relationship among these patient isolates strengthens the assumption that CA-MRSA infections may be caused by isolates closely related to MSSA isolates.

Distribution of mecA among methicillin-resistant clinical staphylococcal strains isolated at hospitals in Naples, Italy

Two hundred and twenty strains of Staphylococcus isolated in Naples, Italy, were surveyed for the distribution of the mecA, the structural gene for penicillin-binding protein 2a, which is the genetic determinant for methicillin-resistance in staphylococci. Screening by a cloned mecA, revealed that of 220 strains, 43 were methicillin-resistant (19.5%) and 177 were methicillin-susceptible (80.5%). Among the 43 resistant strains 23 (53.5%) carried mecA in their genome and 20 (46.5%) did not carry mecA, in spite of their resistance to methicillin. Every group was submitted to the AP-PCR profiling. A quantitative analysis of the patterns divided strains into four different clusters for methicillin-resistant mecA-negative and two different clusters for methicillin-resistant mecA-positive with primer 1, while no clusters were noted with primer 7. We conclude that these clinical isolates from our area, were not found to belong to a single clone, although the predominance of four methicillin-resistant mecA-negative genotypes were noted.

Characterization of methicillin-resistant Staphylococcus aureus associated with nosocomial infections in the University Hospital, Kuala Lumpur

Methicillin-resistant Staphylococcus aureus (MRSA) as a hospital pathogen has presented many clinical problems in the University Hospital, Kuala Lumpur, Malaysia since 1978. The need for control of spread of these organisms became evident by 1985 when it was noted that the incidence of MRSA among S. aureus isolated from hospital inpatients had increased from 11.5% in 1979 to 18.8% in 1985. The characteristics of 50 MRSA isolates associated with nosocomial infections in the hospital are described here. The predominant strains produced Type IV coagulase and 84% of isolates studied showed moderate to high resistance to methicillin with MIC values of 25 mg l-1 or higher. All the MRSA isolates that could be phagetyped were susceptible to Group III phages, with 76.6% of the isolates being susceptible to phage 85. At least 10 different patterns were distinguishable by plasmid typing, the majority of isolates harbouring up to four small plasmids.

Identification of methicillin-resistant strains of staphylococci by polymerase chain reaction

A simple and reliable method using a polymerase chain reaction (PCR) was devised to identify methicillin-resistant staphylococci. By using lysates of the strain to be tested as templates and 22-mer oligonucleotides as primers, a 533-bp region of mecA, the structural gene of a low-affinity penicillin-binding protein (PBP 2'), was amplified by PCR and detected by agarose gel electrophoresis. Results obtained by this method were compared with those obtained by broth microdilution MIC determination for 210 and 100 clinical isolates of Staphylococcus aureus and coagulase-negative staphylococci, respectively. Of 99 mecA-negative S. aureus isolates, 100% of the strains were methicillin susceptible and 98% of the strains were oxacillin susceptible. Three strains (3%) of 111 mecA-positive S. aureus isolates exhibited almost the same susceptibility to beta-lactams as the mecA-negative ones and did not produce detectable amounts of PBP 2' despite the presence of the mecA gene. One of them yielded typically methicillin-resistant variants at a low frequency with concomitant recovery of PBP 2' production. The mecA gene was also found in coagulase-negative Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus sciuri, Staphylococcus saprophyticus, and Staphylococcus caprae and conferred resistance on most of the bacteria.

Southern hybridization analysis of the mecA deletion from methicillin-resistant Staphylococcus aureus

Genomic organization of methicillin-resistant Staphylococcus aureus strains and their methicillin-susceptible subclones were analyzed by pulsed-field gel electrophoresis and Southern hybridization with DNA fragments of methicillin-resistance gene mecA and an insertion element IS431 as probes. The entire mecA gene was deleted in all the seven methicillin-susceptible subclones studied, and the size of the deletion varied from 20 to 100 kilobases depending on each subclone. In six of the seven subclones, however, the downstream deletion end points were confined within a 2.0 kilobase HindIII-HindIII fragment containing a part of IS431 which was located 2.6 kilobase downstream of mecA gene. The results indicated that the intramolecular transposition of IS431 is responsible for the mecA deletion in methicillin-resistant Staphylococcus aureus.

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.

Genotypic stability of methicillin resistance in Staphylococcus aureus at supraoptimal temperature

Methicillin resistance in a highly resistant mutant of Staphylococcus aureus could not be diminished by elevated temperature. After 100 subcultures at 43 degrees C the resistance to methicillin was retained when determined at 37 degrees C.

Transformation reveals a chromosomal locus of the gene(s) for methicillin resistance in Staphylococcus aureus

The localization of the gene(s) mediating methicillin (mecr) in Staphylococcus aureus was determined by transformation with deoxyribonucleic acid (DNA) from a natural mecr strain (DU 4916) and transformation obtained with DNA from this strain. Streptomycin resistance genes (strr) and novobiocin resistance genes (novr) were used concurrently as representatives for chromosomal genes; penicillinase (PI254) and tetracycline plasmids were used as examples of medium- and small-size extrachromosomal genes, respectively. Superinfection of the lysogenic recipients with the competence-inducing phage phi11 or 83A enhanced transformation for all markers. Phenotypic expression of cadmium (cadr), tetracycline (tetr), or methicillin resistance (mecr) did not appear to require a host recombination system since a recA1 mutant could serve as the recipient provided it was superinfected with a competence-inducing phage. There was, furthermore, no requirement for preexisting plasmids for phenotypic expression. Ultraviolet irradiation of transforming DNA enhanced at low doses the transformation frequency for chromosomal genes strr and novr but not for mecr, cadr, or tetr. The gene(s) for mecr was transformed with chromosomal DNA after sodium dodecyl sulfate-sodium chloride extraction and after neutral sucrose gradient centrifugation of bulk DNA from wild-type strain DU 4916 and the transformats. No cavalently closed circular DNA or open circular DNA carrying the methicillin resistance gene(s) could be detected in the wild type or the transformants either by ethidium bromide-cesium chloride gradient centrifugation or by zonal rate centrifugation of cells directly lysed on top of the gradients. The mecr gene(s) is thus probably of chromosomal nature but possibly under recombinational control of phage genes, since transfer of mecr is independent of the recA1 gene(s) but can be accomplished in this strain after superinfection with a competence-inducing phage. Ultraviolet light inactivation of transforming DNA shows first-order kinetics for mecr transformability similar to that observed for both transfecting and plasmid DNA.

Genetic basis, epidemiology, and future significance of antibiotic resistance in Staphylococcus aureus: a review

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Effect of the prophage and penicillinase plasmid of the recipient strain upon the transduction and the stability of methicillin resistance in Staphylococcus aureus

Transduction of a methicillin-resistance determinant (mec) in Staphylococcus aureus RN450 was dependent on its prior lysogenization with an appropriate temperate phage. In addition, an appropriate transduced penicillinase plasmid was usually required. Some phage 80-resistant variants of RN450 or of its parental lysogenic strain, NCTC 8325, were also effective recipients for transduction of mec. Elimination of prophage from RN450 abrogated its effectiveness as a transductional recipient of mec. Elimination of prophage from a methicillin-resistant transductant of RN450 reduced resistance to undetectable levels in six of seven phage-eliminated strains. In four of these a variable number of clones again became phenotypically resistant after lysogenization alone or lysogenization combined with reintroduction of a penicillinase plasmid. In two prophage-eliminated strains, no evidence of residual mec could be adduced. The establishment, expression, or stability of the transduced mec in strain RN450 appeared to depend on some function determined by a prophage or a prophage and a penicillinase plasmid.

Absence of circular plasmid deoxyribonucleic acid attributable to a genetic determinant for methicillin resistance in Staphylococcus aureus

Plasmid deoxyribonucleic acid was not detected by centrifugal analysis of lysates of penicillinase-negative strains of Staphylococcus aureus harboring a determinant of methicillin resistance derived from strain Villaluz. When these strains contained a penicillinase plasmid, the plasmid deoxyribonucleic acid of methicillin-resistant and methicillin-susceptible strains was indistinguishable by the methods employed. The results indicate that the genetic determinant for methicillin resistance in the strains examined was not associated with a circular plasmid comparable to those that have been shown to determine resistance to benzylpenicillin, tetracycline, and chloramphenicol in S. aureus.