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

Temperate Phages of Staphylococcus aureus

Most Staphylococcus aureus isolates carry multiple bacteriophages in their genome, which provide the pathogen with traits important for niche adaptation. Such temperate S. aureus phages often encode a variety of accessory factors that influence virulence, immune evasion and host preference of the bacterial lysogen. Moreover, transducing phages are primary vehicles for horizontal gene transfer. Wall teichoic acid (WTA) acts as a common phage receptor for staphylococcal phages and structural variations of WTA govern phage-host specificity thereby shaping gene transfer across clonal lineages and even species. Thus, bacteriophages are central for the success of S. aureus as a human pathogen.

Amoxicillin Administration Regimen and Resistance Mechanisms of Staphylococcus aureus Established in Tissue Cage Infection Model

Staphylococcus aureus is a zoonotic pathogen that causes various life-threatening diseases. The mechanisms of action of amoxicillin against S. aureus are unclear. Here, we established a rabbit tissue cage infection model to evaluate the relationship between the pharmacokinetic/pharmacodynamic (PK/PD) parameters of amoxicillin and selective enrichment of resistant strains of S. aureus and to elucidate the evolution of its resistance to amoxicillin. S. aureus was injected into the tissue cages at 10¹⁰ colony forming units (CFU)/mL. We injected different intramuscular concentrations of amoxicillin at doses of 5, 10, 20, and 30 mg/kg body weight once a day for 5 days and 5, 10, 20, and 30 mg/kg body weight twice a day for 2.5 days. Differences in gene expression between two differentially resistant strains and a sensitive strain were evaluated using Illumina sequencing followed by COG and KEGG analysis. RT-qPCR was carried out to validate the difference in protein translation levels. Our results demonstrated that the emergence of resistant bacteria was dose dependent within a given time interval. In the same dosage group, the appearance of resistant bacteria increased with time. The resistant bacteria showed cumulative growth, and the level of resistance increased over time. The resistant bacteria were completely inhibited when the cumulative percentage of time over a 24-h period that the drug concentration exceeded the mutant prevention concentration (MPC) (%T > MPC) was ≥52%. We also found that mecA and femX in S. aureus played a leading role in the development of resistance to amoxicillin. In conclusion, it provide references for optimizing amoxicillin regimens to treat infections caused by S. aureus.

Role of SCCmec type in resistance to the synergistic activity of oxacillin and cefoxitin in MRSA

β-lactam antibiotics target penicillin-binding proteins (PBPs) preventing peptidoglycan synthesis and this inhibition is circumvented in methicillin resistant Staphylococcus aureus (MRSA) strains through the expression of an additional PBP, named PBP2A. This enzyme is encoded by the mecA gene located within the Staphylococcal Chromosome Cassette mec (SCCmec) mobile genetic element, of which there are 12 types described to date. Previous investigations aimed at analysing the synergistic activity of two β-lactams, oxacillin and cefoxitin, found that SCCmec type IV community-acquired MRSA strains exhibited increased susceptibility to oxacillin in the presence of cefoxitin, while hospital-acquired MRSA strains were unaffected. However, it is not clear if these differences in β-lactam resistance are indeed a consequence of the presence of the different SCCmec types. To address this question, we have exchanged the SCCmec type I in COL (HA-MRSA) for the SCCmec type IV from MW2 (CA-MRSA). This exchange did not decrease the resistance of COL against oxacillin and cefoxitin, as observed in MW2, indicating that genetic features residing outside of the SCCmec element are likely to be responsible for the discrepancy in oxacillin and cefoxitin synergy against these MRSA strains.

Applying antibiotic selection markers for nematode genetics

Antibiotic selection markers have been recently developed in the multicellular model organism Caenorhabditis elegans and other related nematode species, opening great opportunities in the field of nematode transgenesis. Here we describe how these antibiotic selection systems can be easily combined with many well-established genetic approaches to study gene function, improving time- and cost-effectiveness of the nematode genetic toolbox.

The role of the Staphylococcal VraTSR regulatory system on vancomycin resistance and vanA operon expression in vancomycin-resistant Staphylococcus aureus

Vancomycin is often the preferred treatment for invasive methicillin-resistant Staphylococcus aureus (MRSA) infection. With the increase in incidence of MRSA infections, the use of vancomycin has increased and, as feared, isolates of vancomycin-resistant Staphylococcus aureus (VRSA) have emerged. VRSA isolates have acquired the entercoccal vanA operon contained on transposon (Tn) 1546 residing on a conjugal plasmid. VraTSR is a vancomycin and β-lactam-inducible three-component regulatory system encoded on the S. aureus chromosome that modulates the cell-wall stress response to cell-wall acting antibiotics. Mutation in vraTSR has shown to increase susceptibility to β-lactams and vancomycin in clinical VISA strains and in recombinant strain COLVA-200 which expresses a plasmid borne vanA operon. To date, the role of VraTSR in vanA operon expression in VRSA has not been demonstrated. In this study, the vraTSR operon was deleted from the first clinical VRSA strain (VRS1) by transduction with phage harvested from a USA300 vraTSR operon deletion strain. The absence of the vraTSR operon and presence of the vanA operon were confirmed in the transductant (VRS1Δvra) by PCR. Broth MIC determinations, demonstrated that the vancomycin MIC of VRS1Δvra (64 µg/ml) decreased by 16-fold compared with VRS1 (1024 µg/ml). The effect of the vraTSR operon deletion on expression of the van gene cluster (vanA, vanX and vanR) was examined by quantitative RT-PCR using relative quantification. A 2-5-fold decreased expression of the vanA operon genes occured in strain VRS1Δvra at stationary growth phase compared with the parent strain, VRS1. Both vancomycin resistance and vancomycin-induced expression of vanA and vanR were restored by complementation with a plasmid harboring the vraTSR operon. These findings demonstrate that expression in S. aureus of the horizontally acquired enterococcal vanA gene cluster is enhanced by the staphylococcal three-component cell wall stress regulatory system VraTSR, that is present in all S. aureus strains.

Transduction of staphylococcal cassette chromosome mec elements between strains of Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a well-known public health concern. However, the means by which methicillin resistance genes are transferred among staphylococci in nature remains unknown. Older scientific literature suggests transduction as a means of mecA transfer, but the optimal conditions are reported to require plasmids and potentially a lysogenic phage. These reports preceded discovery of the staphylococcal cassette chromosome mec (SCCmec) elements. We undertook studies to confirm and clarify the conditions promoting transduction of SCCmec in S. aureus populations using well-characterized donor and recipient strains primarily of the USA300 lineage. Both bacteriophages 80α and 29 were capable of transducing SCCmec type IV and SCCmec type I to recipient strains of S. aureus. Pulsed-field gel electrophoresis and mec-associated dru typing were used to confirm the identity of the transductants. Transfer of mecA via transduction occurred at low frequency and required extended selection times for mecA gene expression and the presence of a penicillinase plasmid in the recipient. However, interference with the process by clavulanic acid and the necessity of lysogeny with 11 in the recipient or the presence of a small (4-kb) tetracycline resistance plasmid, as previously reported, were not confirmed. SCCmec transduction was occasionally associated with substantial deletions or truncation of SCCmec and the arginine catabolic metabolic element in USA300 recipients. Overall, these data clarify the conditions required for SCCmec transduction and document that rearrangements may occur during the process.

Redefining the role of the β-lactamase locus in methicillin-resistant Staphylococcus aureus: β-lactamase regulators disrupt the MecI-mediated strong repression on mecA and optimize the phenotypic expression of resistance in strains with constitutive mecA expression

In response to β-lactam chemotherapy, Staphylococcus aureus has acquired two resistance determinants: blaZ, coding for β-lactamase, which confers resistance to penicillins only, and mecA, coding for an extra cell wall cross-linking enzyme with reduced affinity for virtually all other β-lactams. The transcriptional control of both resistance determinants is regulated by homologous repressors (BlaI and MecI, respectively) and sensor inducers (BlaR1 and MecR1, respectively). There is a cross-talk between the two regulatory systems, and it has been demonstrated that bla regulators stabilize the mecA acquisition. In a recent study, we have unexpectedly observed that in most MRSA strains, there was no significant change in the resistance phenotype upon the overexpression in trans of a MecI repressor, whereas in those few strains negative for the bla locus, there was a massive decrease of resistance (D. C. Oliveira and H. de Lencastre, PLoS One 6:e23287, 2011). Here, we demonstrate that, contrary to what is currently accepted, the bla regulatory system efficiently disrupts the strong MecI-mediated repression on mecA, enabling the optimal expression of resistance. This effect appears to be due to the formation of MecI::BlaI heterodimers that might bind less efficiently to the mecA promoter and become nonfunctional due to the proteolytic inactivation of the BlaI monomer. In addition, we have also observed that the presence of bla regulators may enhance dramatically the expression of β-lactam resistance in MRSA strains with constitutive mecA expression, compensating for the fitness cost imposed by the large β-lactamase plasmid. These observations point to important unrecognized roles of the bla locus for the expression of the methicillin-resistant S. aureus (MRSA) phenotype.

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.

Bacteriophages of Staphylococcus aureus efficiently package various bacterial genes and mobile genetic elements including SCCmec with different frequencies

Staphylococcus aureus is a serious human and veterinary pathogen in which new strains with increasing virulence and antimicrobial resistance occur due to acquiring new genes by horizontal transfer. It is generally accepted that temperate bacteriophages play a major role in gene transfer. In this study, we proved the presence of various bacterial genes of the S. aureus COL strain directly within the phage particles via qPCR and quantified their packaging frequency. Non-parametric statistical analysis showed that transducing bacteriophages φ11, φ80 and φ80α of serogroup B, in contrast to serogroup A bacteriophage φ81, efficiently package selected chromosomal genes localized in 4 various loci of the chromosome and 8 genes carried on variable elements, such as staphylococcal cassette chromosome SCCmec, staphylococcal pathogenicity island SaPI1, genomic islands vSaα and vSaβ, and plasmids with various frequency. Bacterial gene copy number per ng of DNA isolated from phage particles ranged between 1.05 × 10(2) for the tetK plasmid gene and 3.86 × 10(5) for the SaPI1 integrase gene. The new and crucial finding that serogroup B bacteriophages can package concurrently ccrA1 (1.16 × 10(4)) and mecA (1.26 × 10(4)) located at SCCmec type I into their capsids indicates that generalized transduction plays an important role in the evolution and emergence of new methicillin-resistant clones.

Coagulase-negative staphylococci as reservoirs of genes facilitating MRSA infection: Staphylococcal commensal species such as Staphylococcus epidermidis are being recognized as important sources of genes promoting MRSA colonization and virulence

Recent research has suggested that Staphylococcus epidermidis is a reservoir of genes that, after horizontal transfer, facilitate the potential of Staphylococcus aureus to colonize, survive during infection, or resist antibiotic treatment, traits that are notably manifest in methicillin-resistant S. aureus (MRSA). S. aureus is a dangerous human pathogen and notorious for acquiring antibiotic resistance. MRSA in particular is one of the most frequent causes of morbidity and death in hospitalized patients. S. aureus is an extremely versatile pathogen with a multitude of mechanisms to cause disease and circumvent immune defenses. In contrast, most other staphylococci, such as S. epidermidis, are commonly benign commensals and only occasionally cause disease. Recent findings highlight the key importance of efforts to better understand how genes of staphylococci other than S. aureus contribute to survival in the human host, how they are transferred to S. aureus, and why this exchange appears to be uni-directional.

Evidence for a purifying selection acting on the β-lactamase locus in epidemic clones of methicillin-resistant Staphylococcus aureus

Background

The β-lactamase (bla) locus, which confers resistance to penicillins only, may control the transcription of mecA, the central element of methicillin resistance, which is embedded in a polymorphic heterelogous chromosomal cassette (the SCCmec element). In order to assess the eventual correlation between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes, the allelic variation on the bla locus was evaluated in a representative collection of 54 international epidemic methicillin-resistant Staphylococcus aureus (MRSA) clinical strains and, for comparative purposes, also in 24 diverse methicillin-susceptible S. aureus (MSSA) strains.

Results

Internal fragments of blaZ (the β-lactamase structural gene) were sequenced for all strains. A subset of strains, representative of blaZ allotypes, was further characterized by sequencing of internal fragments of the blaZ transcriptional regulators, blaI and blaR1. Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found. In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain. On average, blaZ alleles were more polymorphic among MSSA than in MRSA (14.7 vs 11.4 SNP/allele). Overall, blaR1 was the most polymorphic gene with an average of 24.8 SNP/allele. No correlation could be established between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes. In order to estimate the selection pressure acting on the bla locus, the average dN/dS values were computed. In the three genes and in both collections dN/dS ratios were significantly below 1.

Conclusions

The data strongly suggests the existence of a purifying selection to maintain the bla locus fully functional even on MRSA strains. Although, this is in agreement with the notion that in most clinical MRSA strains mecA gene is under the control of the bla regulatory genes, these findings also suggest that the apparently redundant function of blaZ gene for the MRSA resistant phenotype is still important for these strains. In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

Jumping the barrier to beta-lactam resistance in Staphylococcus aureus

Although the staphylococcal methicillin resistance determinant, mecA, resides on a mobile genetic element, staphylococcus cassette chromosome mec (SCCmec), its distribution in nature is limited to as few as five clusters of related methicillin-resistant Staphylococcus aureus (MRSA) clones. To investigate the potential role of the host chromosome in clonal restriction of the methicillin resistance determinant, we constructed plasmid pYK20, carrying intact mecA, and introduced it into several methicillin-susceptible Staphylococcus aureus strains, five of which were naive hosts (i.e., mecA not previously resident on the host chromosome) and five of which were experienced hosts (i.e., methicillin-susceptible variants of MRSA strains from which SCCmec was excised). We next assessed the effect of the recipient background on the methicillin resistance phenotype by population analysis, by assaying the mecA expression of PBP2a by Western blot analysis, and by screening for mutations affecting mecA. Each experienced host transformed with pYK20 had a resistance phenotype and expressed PBP2a similar to that of the parent with chromosomal SCCmec, but naive hosts transformed with pYK20 selected against its expression, indicative of a host barrier. Either inducible beta-lactamase regulatory genes blaR1-blaI or homologous regulatory genes mecR1-mecI, which control mecA expression, acted as compensatory elements, permitting the maintenance and expression of plasmid-carried mecA.

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.

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.

Methicillin resistance in Staphylococcus epidermidis. Relationship between the additional penicillin-binding protein and an attachment transpeptidase

The penicillin-binding proteins (PBP) of a methicillin-resistant strain of Staphylococcus epidermidis, 100,604 p+m+ and a non-isogenic sensitive strain, p-m- were characterised. The presence of a novel PBP, produced by the methicillin-resistant strain of S. epidermidis, with an Mr identical to that of PBP2' in Staphylococcus aureus 13,136 p-m+, was revealed by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and subsequent fluorography of solubilised membrane proteins isolated from cells labelled with [3H]benzylpenicillin. This novel PBP was only detected in cells which had been grown at 30 degrees C, in media containing beta-lactam antibiotic and 5% NaCl. The sensitivity of an attachment transpeptidation reaction measured under non-growing conditions in the sensitive and resistant strains indicated that the novel PBP catalysed this reaction. The similarity of radiolabelled peptides resulting from partial proteolytic digestion of the novel PBP in S. epidermidis 100,604 p+m+ and from PBP2' in S. aureus 13,136 p+m+ lends support to the theory that the additional DNA encoding PBP2' in S. aureus and the same protein in S. epidermidis has been passed to both species from an unknown source. Studies of the development and loss of resistance of attachment transpeptidase activity, and the appearance and disappearance of the novel protein when cultures of the resistant strain were transferred from conditions allowing the expression of resistance to those not allowing such expression and vice-versa, indicated that there was a strong correlation between the presence of PBP2' and the degree of resistance of the attachment transpeptidation reaction and that the production of this protein was affected by temperature at a regulatory or genetic level. Studies on the induction and loss of beta-lactamase activity and of the novel PBP when the resistant strain was grown in the presence or absence of beta-lactam antibiotics at either 40 degrees C or 30 degrees C suggests that there is little relationship between the production of this enzyme and of PBP2' other than the fact that beta-lactam antibiotics are common inducers of both.

Methicillin-resistant staphylococci

Strains of staphylococci resistant to methicillin were identified immediately after introduction of this drug. Methicillin-resistant strains have unusual properties, the most notable of which is extreme variability in expression of the resistance trait. The conditions associated with this heterogeneous expression of resistance are described. Methicillin resistance is associated with production of a unique penicillin-binding protein (PBP), 2a, which is bound and inactivated only at high concentrations of beta-lactam antibiotics. PBP2a appears to be encoded by the mec determinant, which also is unique to methicillin-resistant strains. The relationships between PBP2a and expression of resistance and implications for the mechanism of resistance are discussed. The heterogeneous expression of methicillin resistance by staphylococci poses problems in the detection of resistant strains. Experience with several susceptibility test methods is reviewed and guidelines for performance of these tests are given. Treatment of infections caused by methicillin-resistant staphylococci is discussed. Vancomycin is the treatment of choice. Alternatives have been few because methicillin-resistant strains often are resistant to multiple antibiotics in addition to beta-lactam antibiotics. New agents which are active against methicillin-resistant staphylococci are becoming available, and their potential role in treatment is discussed.

Production of low-affinity penicillin-binding protein by low- and high-resistance groups of methicillin-resistant Staphylococcus aureus

Methicillin- and cephem-resistant Staphylococcus aureus (137 strains) for which the cefazolin MICs are at least 25 micrograms/ml could be classified into low-resistance (83% of strains) and high-resistance (the remaining 17%) groups by the MIC of flomoxef (6315-S), a 1-oxacephalosporin. The MICs were less than 6.3 micrograms/ml and more than 12.5 micrograms/ml in the low- and high-resistance groups, respectively. All strains produced penicillin-binding protein 2' (PBP 2'), which has been associated with methicillin resistance and which has very low affinity for beta-lactam antibiotics. Production of PBP 2' was regulated differently in low- and high-resistance strains. With penicillinase-producing strains of the low-resistance group, cefazolin, cefamandole, and cefmetazole induced PBP 2' production about 5-fold, while flomoxef induced production 2.4-fold or less. In contrast, penicillinase-negative variants of low-resistance strains produced PBP 2' constitutively in large amounts and induction did not occur. With high-resistance strains, flomoxef induced PBP 2' to an extent similar to that of cefazolin in both penicillinase-producing and -negative strains, except for one strain in which the induction did not occur. The amount of PBP 2' induced by beta-lactam antibiotics in penicillinase-producing strains of the low-resistance group correlated well with resistance to each antibiotic. Large amounts of PBP 2' in penicillinase-negative variants of the low-resistance group did not raise the MICs of beta-lactam compounds, although these strains were more resistant when challenged with flomoxef for 2 h. Different regulation of PBP 2' production was demonstrated in the high- and low-resistance groups, and factor(s) other than PBP 2' were suggested to be involved in the methicillin resistance of high-resistance strains.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Recipient characteristics in the transduction of methicillin resistance in Staphylococcus epidermidis

Methicillin resistance (Mecr) was transduced into a methicillin-susceptible variant of the Mecr donor Staphylococcus epidermidis BS10. UV irradiation of phage stimulated Mecr transduction frequency. If loss of Cd and Hg ion resistance occurred in this recipient, or if the three markers Mecr, Cdr, and Hgr were co-eliminated from the donor, neither strain acted as a recipient for Mecr.

Methicillin-resistant Staphylococcus aureus: clinical and laboratory features

Not long after the introduction of penicillin G for anti-staphylococcal therapy, penicillin G resistant strains of staphylococci were isolated and were found to produce penicillinase enzymes. In 1959 methicillin, a penicillinase resistant penicillin (PRP), was introduced in England for therapy of these strains. Within 2 years the first strains of methicillin-resistantStaphylococcus aureus(MRSA) were reported in that country. Subsequently, reports of MRSA infections throughout Europe appeared. Reports from Switzerland and Denmark indicated that 30% to 50% of all nosocomialS. aureusisolates and 40% of allS. aureusbacteremia isolates, respectively, were due to MRSA. Although sporadic reports of MRSA infections appeared in the US after 1960 the first documented outbreak of MRSA infections occurred at Boston City Hospital in 1968. Even today there is not a complete understanding of MRSA and its role in infectious diseases, however, a vast amount of information has accumulated and certain aspects of this information will be discussed.

Molecular relationships among serogroup B bacteriophages of Staphylococcus aureus

The typing bacteriophages 55, 80, 83A, and 85 of Staphylococcus aureus, representative of the three major lytic groups of serological group B aureophages, have been examined for relatedness of their genomes and virion proteins. Phages 11 and 80 alpha were also examined to determine the relationship of phage 80 alpha to phages 11 and 80. Total genome hybridization measurements divided the phages into two groups. Phages 55 and 80, in the first group, had DNA homology of 50%. Phages 11, 80 alpha, 83A, and 85 formed a second group with 27 to 65% homology. Homology between the two groups was in the range of 14 to 22%. Phage 80 alpha is more closely related to phage 11 than to phage 80, though it is probably not a simple recombinant of phages 11 and 80. Restriction enzyme digestion and phage [32P]DNA hybridization analysis of the endonuclease-generated fragments from each phage DNA confirmed the findings of the DNA homology measurements. The endonuclease fragment patterns generated by EcoRI and HindIII were distinctive for each phage, confirming that none of the phages are closely related. Common sequences were present in most fragments from the phage DNAs when the labeled probe DNA was from a different phage in the same group. Cross-group probing of endonuclease fragments revealed both a diminished level of homology when similar sequences were present and the probable absence of some sequences. Virion proteins, examined by polyacrylamide gel electrophoresis, were similar in number and molecular weight for phages 11, 80 alpha, 83A, and 85, reflecting the DNA homology analyses. The virion proteins from phages 55 and 80, however, were more distinctive, and both differed from the phages in the other group.

Occurrence of a beta-lactam-inducible penicillin-binding protein in methicillin-resistant staphylococci

The mechanism of methicillin resistance was investigated in methicillin-resistant staphylococci (MRS) and in variants which had lost methicillin resistance. Phase-contrast microscopy showed that cells swelled at low concentrations of beta-lactam antibiotics in both MRS and variants which had lost methicillin resistance. Cells of variants which had lost methicillin resistance were lysed easily when higher concentrations of antibiotic were used. In contrast, MRS cells remained swollen at even higher concentrations of antibiotics. Furthermore, bacterial growth was inhibited at antibiotic concentrations much lower than MICs for MRS. Examination of the penicillin-binding proteins (PBPs) in MRS revealed that a new PBP-2' (molecular weight, 74,000) was induced in large quantity by exposure to beta-lactams. PBP-2' was produced constitutively in variants of MRS which had lost a penicillinase plasmid. The induction of PBP-2' by beta-lactams was not detected in variants which had lost methicillin resistance. High concentrations of beta-lactam were required for saturation of PBP-2'. The optimum antibiotic concentration for the induction of PBP-2' varied with the beta-lactam used as the inducer, and PBP-2' was produced in a larger amount at 32 degrees C than at 37 degrees C. From these results, we suggest that the mechanism of methicillin resistance depends on the induction of PBP-2', which may function as a detour enzyme for PBP-2 or PBP-3 or may be a particular enzyme involved in peptidoglycan synthesis.

Transduction of methicillin resistance in Staphylococcus aureus: recipient effectiveness and beta-lactamase production

The effectiveness of Staphylococcus aureus strain 8325-4 as a recipient for the transduction of methicillin resistance requires the presence of a penicillinase plasmid but was found to be independent of the lysogenic state of the recipient. Effectiveness is conferred by the plasmid in either the autonomous or integrated states, although the transduction rate is higher in the former. Once established, the maintenance and expression of methicillin resistance were independent of continued carriage of the plasmid deoxyribonucleic acid. Analysis of penicillinase plasmid mutants indicated that beta-lactamase production was the plasmid function responsible for recipient effectiveness. Supportive evidence included the abrogation of recipient effectiveness by the beta-lactamase inhibitor clavulanic acid and the elimination of a plasmid requirement with recipient strains carrying a chromosomal beta-lactamase determinant. A possible role for beta-lactamase production in the transduction of methicillin resistance is discussed.

Modulation of protein A formation in Staphylococcus aureus by genetic determinants for methicillin resistance

Many methicillin-resistant (Mec(r)) strains of Staphylococcus aureus either produce no protein A or secrete it extracellularly (S. Winblad and C. Ericson, Acta Pathol. Microbiol. Scand. Sect. B 81:150-156, 1973). We found that methicillin resistance and protein A production were apparently lost coordinately from the natively Mec(r) strain A676. Restoration of the genetic determinant for methicillin resistance (mec) by transduction or transformation restored protein A production. In two other Mec(r) strains, loss of mec was accompanied by marked reduction in protein A formation. Genetic transfer of mec to derivatives of S. aureus 8325 affected protein A formation differently with different mec determinants. Those derived from strain A676 and two other Mec(r) strains reduced the scanty amount of protein A produced by strain 8325 to even lower or undetectable levels, whereas mec from two more Mec(r) strains increased its protein A content. This "mec-effect," i.e., stimulation or inhibition of protein A formation dependent on the combination of host strain and mec determinant, was reduced in methicillin-susceptible (Mec(s)) mutants produced by ethyl methane sulfonate treatment of Mec(r) strains. The mec-effect reappeared in spontaneous revertants to methicillin resistance. Phenotypic reduction of methicillin resistance in Mec(r) strains grown at 44 degrees C was accompanied by reduction of the mec-effect on protein A, but it had no effect on protein A formation in Mec(s) strains. Two independent mutants of strain 8325 produced large amounts of protein A at rates that were unaffected by growth at 44 degrees C or by the introduction of mec determinants.

Chromosomal map location of the methicillin resistance determinant in Staphylococcus aureus

Three-factor genetic crosses performed by transformation have shown that the methicillin resistance determinant of Staphylococcus aureus strain DU4916 (the mec-4916 marker) is linked to a novobiocin resistance (Novr) marker (nov-142) and mutational sites affecting pyrimidine (pyr-141), purine (pur-102), and histidine (hisG15) biosynthesis in S. aureus strain 8325. The linkage group thus defined is pyr-141-hisG15-nov-142-pur-102-mec-4916. Phage 80alpha previously propagated on a novobiocin-resistant, methicillin-sensitive (Mecs) 8325 strain was used to infect 21 novobiocin-sensitive, methicillin-resistant clinical isolates (including strain DU4916). Among the novobiocin-resistant transductants so obtained from each recipient, between 1 and 5% were methicillin sensitive (reflecting cotransduction of Novr and Mecs). These results are consistent with the genetic determinant of methicillin resistance having a single chromosomal locus in most, if not all, strains of S. aureus.

Mutations in prophage phi11 that impair the transducibility of their Staphylococcus aureus lysogens for methicillin resistance

Methicillin resistance (mec) is not transduced into Staphylococcus aureus 8325-4, but is transduced into this host after it has been lysogenized with phage phi11 and has acquired the penicillinase plasmid pI524 by a separate transduction (Cohen and Sweeney, 1970, 1973). Strain 8325-4 is competent for transformation of typical plasmid or chromosomal markers and for mec only if it is lysogenic for phi11 or a related prophage (Sjöström et al., 1974, 1975). A mutant strain of phi11 that was temperature sensitive (Ts) for vegetative multiplication did not mediate competence for transformation of its 8325-4 lysogen if the lysogen had been grown at a nonpermissive temperature (Sjöström and Philipson, 1974). We isolated four Ts mutants of phi11 that did not mediate transducibility of their 8325-4(pI524) lysogens for mec after growth at nonpermissive temperatures (40 to 42 degrees C). Transduction of typical plasmid or chromosomal markers was not affected. These phi11-Ts mutants mediated normal competence of their lysogens for transformation of a tetracycline resistance plasmid. Similarly, phi11-Ts mutants that rendered their lysogens temperature sensitive for transformation did not depress the frequency of transduction of mec. These two types of phi11-Ts mutants fell into two different genetic complementation groups that differed in the physiology of deoxyribonucleic acid synthesis and in the time of expression of the mutations during a single-burst growth cycle at a nonpermissive temperature. A virulent mutant of phi11, which plaqued with 100% efficiency on 8325(phi11), also failed to condition strain 8325-4 for transducibility of mec but retained the ability to confer competence for transformation of a tetracycline resistance plasmid. Different genetic loci and physiological functions are involved in phi11 mutations that affect transducibility of mec and those that affect competence for transformation of markers generally in S. aureus 8325-4.

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.

Role of the phi 11 phage genome in competence of Staphylococcus aureus

Both phage ø11 and 83A, when present as prophage or when used as helper phage, induce competence for transfection and transformation to the same level in Staphylococcus aureus, strain 8325-4. Cells lysogenized with certain temperature-sensitive (ts) mutants of phage ø11 show competence at the nonpermissive temperature (41 C) without production of infectious phages. Phage ø11ts allele 31 can neither as a prophage nor as a helper phage develop competence under nonpermissive conditions. This mutant appears, therefore, to be mutated in the region of the phage genome controlling competence. The competence level for both transfection and transformation is increased by superinfecting strain 8325-4 (ø11) or 8325-4 (83A) at high multiplicities with phage ø11 with some of its mutants or with phage 83A. This superinfection enhancement appears to require protein synthesis but not deoxyribonucleic acid synthesis as judged from studies with inhibitors of macromolecular synthesis. Besides the phage particle, no extracellular or cell-bound factors so far detected can induce competence. The phage-induced product conferring competence is rapidly synthesized by strain 8325-4 (tsø11(31)) after shift to permissive conditions, but requires deoxyribonucleic acid and protein synthesis to be expressed. Recombination between the sus mutants of phage ø11 of Kretschmer and Egan and tsø11(31) indicate that competence is controlled by an early gene in the lytic cycle which may be expressed also in lysogenic cells. The phage product inducing competence appears to have a half-life of 10 to 15 min in the conditional lethal mutant at shift to nonpermissive temperature. Ultraviolet inactivation of phage ø11 infectivity occurs more rapidly than inactivation of competence induction. In fact, the number of transformants is increased at low doses of irradiation. Competence induction is, however, decreased at high does of ultraviolet irradiation.