Comparative analysis of conjugative plasmids mediating gentamicin resistance in Staphylococcus aureus

Antimicrobial Resistance in ESKAPE Pathogens

Antimicrobial-resistant ESKAPE ( Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens represent a global threat to human health. The acquisition of antimicrobial resistance genes by ESKAPE pathogens has reduced the treatment options for serious infections, increased the burden of disease, and increased death rates due to treatment failure and requires a coordinated global response for antimicrobial resistance surveillance. This looming health threat has restimulated interest in the development of new antimicrobial therapies, has demanded the need for better patient care, and has facilitated heightened governance over stewardship practices.

Antimicrobial Resistance in ESKAPE Pathogens

Antimicrobial-resistant ESKAPE ( Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens represent a global threat to human health. The acquisition of antimicrobial resistance genes by ESKAPE pathogens has reduced the treatment options for serious infections, increased the burden of disease, and increased death rates due to treatment failure and requires a coordinated global response for antimicrobial resistance surveillance. This looming health threat has restimulated interest in the development of new antimicrobial therapies, has demanded the need for better patient care, and has facilitated heightened governance over stewardship practices.

Mobile Genetic Elements Associated with Antimicrobial Resistance

Strains of bacteria resistant to antibiotics, particularly those that are multiresistant, are an increasing major health care problem around the world. It is now abundantly clear that both Gram-negative and Gram-positive bacteria are able to meet the evolutionary challenge of combating antimicrobial chemotherapy, often by acquiring preexisting resistance determinants from the bacterial gene pool. This is achieved through the concerted activities of mobile genetic elements able to move within or between DNA molecules, which include insertion sequences, transposons, and gene cassettes/integrons, and those that are able to transfer between bacterial cells, such as plasmids and integrative conjugative elements. Together these elements play a central role in facilitating horizontal genetic exchange and therefore promote the acquisition and spread of resistance genes. This review aims to outline the characteristics of the major types of mobile genetic elements involved in acquisition and spread of antibiotic resistance in both Gram-negative and Gram-positive bacteria, focusing on the so-called ESKAPEE group of organisms (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp., and Escherichia coli), which have become the most problematic hospital pathogens.

Transfer of Antibiotic Resistance in Staphylococcus aureus

Staphylococcus aureus is a serious human pathogen with remarkable adaptive powers. Antibiotic-resistant clones rapidly emerge mainly by acquisition of antibiotic-resistance genes from other S. aureus strains or even from other genera. Transfer is mediated by a diverse complement of mobile genetic elements and occurs primarily by conjugation or bacteriophage transduction, with the latter traditionally being perceived as the primary route. Recent work on conjugation and transduction suggests that transfer by these mechanisms may be more extensive than previously thought, in terms of the range of plasmids that can be transferred by conjugation and the efficiency with which transduction occurs. Here, we review the main routes of antibiotic resistance gene transfer in S. aureus in the context of its biology as a human commensal and a life-threatening pathogen.

Conjugative Transfer in Staphylococcus aureus

The acquisition of plasmids has led to a significant increase in antimicrobial resistance within the staphylococci. In order to study these plasmids effectively, one must be able move the plasmid DNA into genetically clean backgrounds. While the smaller staphylococcal class I (1-5 kb) and class II (10-30 kb) plasmids are readily transferred using bacteriophage transduction or electroporation, these methods are inefficient at moving the larger class III (30-60 kb) plasmids. This review describes methods to transfer class III plasmids via conjugative mobilization.

An updated view of plasmid conjugation and mobilization in Staphylococcus

The horizontal gene transfer facilitated by mobile genetic elements impacts almost all areas of bacterial evolution, including the accretion and dissemination of antimicrobial-resistance genes in the human and animal pathogen Staphylococcus aureus. Genome surveys of staphylococcal plasmids have revealed an unexpected paucity of conjugation and mobilization loci, perhaps suggesting that conjugation plays only a minor role in the evolution of this genus. In this letter we present the DNA sequences of historically documented staphylococcal conjugative plasmids and highlight that at least 3 distinct and widely distributed families of conjugative plasmids currently contribute to the dissemination of antimicrobial resistance in Staphylococcus. We also review the recently documented "relaxase-in trans" mechanism of conjugative mobilization facilitated by conjugative plasmids pWBG749 and pSK41, and discuss how this may facilitate the horizontal transmission of around 90% of plasmids that were previously considered non-mobilizable. Finally, we enumerate unique sequenced S. aureus plasmids with a potential mechanism of mobilization and predict that at least 80% of all non-conjugative S. aureus plasmids are mobilizable by at least one mechanism. We suggest that a greater research focus on the molecular biology of conjugation is essential if we are to recognize gene-transfer mechanisms from our increasingly in silico analyses.

Linezolid-resistant Staphylococcus aureus strain 1128105, the first known clinical isolate possessing the cfr multidrug resistance gene

The Cfr methyltransferase confers resistance to six classes of drugs which target the peptidyl transferase center of the 50S ribosomal subunit, including some oxazolidinones, such as linezolid (LZD). The mobile cfr gene was identified in European veterinary isolates from the late 1990s, although the earliest report of a clinical cfr-positive strain was the 2005 Colombian methicillin-resistant Staphylococcus aureus (MRSA) isolate CM05. Here, through retrospective analysis of LZD(r) clinical strains from a U.S. surveillance program, we identified a cfr-positive MRSA isolate, 1128105, from January 2005, predating CM05 by 5 months. Molecular typing of 1128105 revealed a unique pulsed-field gel electrophoresis (PFGE) profile most similar to that of USA100, spa type t002, and multilocus sequence type 5 (ST5). In addition to cfr, LZD resistance in 1128105 is partially attributed to the presence of a single copy of the 23S rRNA gene mutation T2500A. Transformation of the ∼37-kb conjugative p1128105 cfr-bearing plasmid from 1128105 into S. aureus ATCC 29213 background strains was successful in recapitulating the Cfr antibiogram, as well as resistance to aminoglycosides and trimethoprim. A 7-kb cfr-containing region of p1128105 possessed sequence nearly identical to that found in the Chinese veterinary Proteus vulgaris isolate PV-01 and in U.S. clinical S. aureus isolate 1900, although the presence of IS431-like sequences is unique to p1128105. The cfr gene environment in this early clinical cfr-positive isolate has now been identified in Gram-positive and Gram-negative strains of clinical and veterinary origin and has been associated with multiple mobile elements, highlighting the versatility of this multidrug resistance gene and its potential for further dissemination.

Genetics of antimicrobial resistance in Staphylococcus aureus

Strains of Staphylococcus aureus that are resistant to multiple antimicrobial compounds, including most available classes of antibiotics and some antiseptics, are a major threat to patient care owing to their stubborn intransigence to chemotherapy and disinfection. This reality has stimulated extensive efforts to understand the genetic nature of the determinants encoding antimicrobial resistance, together with the mechanisms by which these determinants evolve over time and are spread within bacterial populations. Such studies have benefited from the application of molecular genetics and in recent years, the sequencing of over a dozen complete staphylococcal genomes. It is now evident that the evolution of multiresistance is driven by the acquisition of discrete preformed antimicrobial resistance genes that are exchanged between organisms via horizontal gene transfer. Nonetheless, chromosomal mutation is the catalyst of novel resistance determinants and is likely to have an enhanced influence with the ongoing introduction of synthetic antibiotics.

The use of bacteriophages in eliminating polyresistant strains of Staphylococcus aureus and Streptococcus agalactiae

Temperate bacteriophages were induced in and released from isolates of Staphylococcus aureus and Streptococcus agalactiae using mitomycin C. Various specific indicator cultures were tested for providing clear plaques after phage infection. Specific lytic mixture of bacteriophages was prepared using the induced, modified and laboratory variants of phages. Under laboratory conditions, the mixture eliminated all isolates from the tested collection of microorganisms. The restriction barrier of some bacterial isolates to bacteriophage infection was overcome either by UV irradiation or in vitro modification of bacteriophage DNA with specific methyltransferases. Conjugative R plasmids, capable of replication in G+ and G- bacteria, were detected and isolated from S. aureus and S. agalactiae antibiotic-resistant strains.

Structural analysis of a defective transfer-like region in a staphylococcal aminoglycoside resistance plasmid

Staphylococcus epidermidis clinical isolate MH6502 contained the 51. 9-kb nonconjugal plasmid pMH6502, which has homology to a major part of the transfer gene region of a known conjugal plasmid. Plasmid pMH6502 mediates aminoglycoside and ethidium bromide resistance. During restriction digest analysis of pMH6502, a double logarithmic regression of marker data gave a better linear relationship than a semi-logarithmic one. The analysis indicated several differences in the transfer-like region of pMH6502 compared to the analogous region of the S. aureus conjugal plasmid pG01. The transfer-like region was in the opposite orientation compared to pG01. An EcoRI site that is within the transfer-like region of pMH6502, has no analogue in pG01. A HindIII site, located outside a 6.3-kb EcoRI fragment in the transfer gene region of pG01, is inside the analogous fragment of pMH6502. A model is proposed to describe how a conjugal ancestral plasmid of pMH6502 could alter to its present form.

Resistance to gentamicin and related aminoglycosides in Staphylococcus aureus isolated in Brazil

Isolates of Staphylococcus aureus obtained from a Brazilian university hospital were characterized in relation to resistance to gentamicin and related aminoglycosides. Thirty-six isolates were susceptible to methicillin (MSSA) and 14 were resistant (MRSA). All isolates were sensitive to nucleic acid-binding compounds. All MRSA isolates and one MSSA isolate were demonstrated to be resistant to gentamicin and were coincidentally resistant to amikacin, kanamycin, neomycin and tobramycin. Among the gentamicin sensitive MSSA isolates, five isolates were found to be resistant only to kanamycin/neomycin. The resistance to gentamicin (and related aminoglycosides: kanamycin and tobramycin) must be due to AAC(6')-APH(2") activity. As these isolates also showed resistance to neomycin, they must carry an additional genetic element, probably the one responsible for APH(3')III activity, which accounts for the high level of resistance to kanamycin and to amikacin. The resistance to kanamycin/neomycin in the gentamicin sensitive isolates could not be attributed to the AAD(4')(4") activity because of the tobramycin sensitivity, and so could be ascribed to the APH(3')III activity. Curing and transfer experiments, as well as electrophoresis procedures, indicate that gentamicin resistance in Staph. aureus strains here studied has, characteristically, chromosomal localization.

A new ketolide, HMR 3004, active against streptococci inducibly resistant to erythromycin

HMR 3004 is a new hydrazono ketolide characterized by a 3-keto function instead of the cladinose moiety. The effect of this antimicrobial agent on inducible and constitutive macrolide-lincosamide-streptogramin B (MLSB) resistance was tested in a lacZ reporter system under control of several ermAM-like attenuator variants. For one constitutively resistant Streptococcus agalactiae strain, three inducibly resistant Streptococcus pneumoniae strains, and one inducibly resistant Enterococcus faecalis strain, the attenuators fused with lacZ were cloned into the shuttle plasmid pJIM2246 and the plasmid was introduced into Staphylococcus aureus RN4220. For the wild-type attenuators, HMR 3004 was a very weak inducer, unlike its cladinose counterpart RU 6652 and erythromycin. As expected, for the fusion originating from the constitutively resistant S. agalactiae strain, the level of uninduced beta-galactosidase synthesis was high. For one S. pneumoniae attenuator, mutations in the 3' end of the attenuator that weakened the stem-loop structure that sequesters the ribosome-binding site and start codon for ermAM methylase could explain the high level of uninduced beta-galactosidase produced. For streptococci, the activity of HMR 3004 correlated with the basal level of beta-galactosidase synthesized. The weak inducer activity of HMR 3004 explained its activity against inducibly MLSB-resistant S. pneumoniae but did not correlate with the moderate activity of the antibiotic against inducibly resistant E. faecalis.

Control of genes for conjugative transfer of plasmids and other mobile elements

Conjugative transfer is a primary means of spread of mobile genetic elements (plasmids and transposons) between bacteria.It leads to the dissemination and evolution of the genes (such as those conferring resistance to antibiotics) which are carried by the plasmid. Expression of the plasmid genes needed for conjugative transfer is tightly regulated so as to minimise the burden on the host. For plasmids such as those belonging to the IncP group this results in downregulation of the transfer genes once all bacteria have a functional conjugative apparatus. For F-like plasmids (apart from F itself which is a derepressed mutant) tight control results in very few bacteria having a conjugative apparatus. Chance encounters between the rare transfer-proficient bacteria and a potential recipient initiate a cascade of transfer which can continue until all potential recipients have acquired the plasmid. Other systems express their transfer genes in response to specific stimuli. For the pheromone-responsive plasmids of Enterococcus it is small peptide signals from potential recipients which trigger the conjugative transfer genes. For the Ti plasmids of Agrobacterium it is the presence of wounded plants which are susceptible to infection which stimulates T-DNA transfer to plants. Transfer and integration of T-DNA induces production of opines which the plasmid-positive bacteria can utilise. They multiply and when they reach an appropriate density their plasmid transfer system is switched on to allow transfer of the Ti plasmid to other bacteria. Finally some conjugative transfer systems are induced by the antibiotics to which the elements confer resistance. Understanding these control circuits may help to modify management of microbial communities where plasmid transfer is either desirable or undesirable. z 1998 Published by Elsevier Science B.V.

Identification and characterization of the origin of conjugative transfer (oriT) and a gene (nes) encoding a single-stranded endonuclease on the staphylococcal plasmid pGO1

The genes mediating the conjugative transfer of the 52-kb staphylococcal plasmid pGO1 are within a 14.4-kb gene cluster designated trs. However, a clone containing trs alone cannot transfer independently and no candidate oriT has been found within or contiguous to trs. In this study, we identified a 1,987-bp open reading frame (ORF) 24 kb 3' and 13 kb 5' to trs that was essential for conjugative transfer: transposon insertions into the ORF abolished transfer and a plasmid containing the ORF could complement these transposon-inactivated pGO1 mutants for transfer. Analysis of the nucleotide sequence of this ORF revealed significant homology between the amino terminus of its predicted protein and those of several single-stranded endonucleases. In addition, a 12-bp DNA sequence located 100 bp 5' to the ORF's translational start site was identical to the oriT sequences of the conjugative or mobilizable plasmids RSF1010, pTF1, R1162, pSC101, and pIP501. The ability of the ORF, designated nes (for nicking enzyme of staphylococci), to generate a single-stranded nick at the oriT was demonstrated in Escherichia coli by alkaline gel and DNA sequence analysis of open circular plasmid DNA. Plasmids that could be converted to the open circular form by the presence of oriT and nes could also be mobilized at high frequency into Staphylococcus aureus recipients with a second plasmid containing only trs. We propose that the 14.4 kb of trs and the approximately 2.2 kb of the oriT-nes region, coupled with an origin of replication, make up the minimal staphylococcal conjugative replicon.

Antimicrobial drug resistance in Staphylococcus aureus isolated from cattle in Brazil

Isolates of Staphylococcus aureus obtained from apparently healthy cattle in the State of Paraiba, Brazil were characterized in relation to resistance to 21 antimicrobial agents. Among the 46 isolates obtained, resistance to penicillin was most frequent, followed by resistance to cadmium, streptomycin, arsenate, tetracycline, mercury, erythromycin and kanamycin/neomycin. All isolates were susceptible to fusidic acid, ethidium bromide, cetrimide, chloramphenicol, benzalkonium chloride, doxycycline, gentamicin, methicillin, minocycline, novobiocin, rifamycin, tylosin and vancomycin. Only six isolates were susceptible to all the drugs tested. With respect to the antibiotics, multi-resistant isolates were uncommon. These results are probably a consequence of the peculiarities of local drug usage pressures. In relation to metal ions, resistance to mercury was rare while resistance to arsenate was relatively frequent, which contrasts with the situation for human Staph. aureus strains. After treatment with ethidium bromide, elimination of resistance to penicillin, tetracycline, streptomycin, erythromycin and cadmium was observed, which was consistent with the genetic determinants being plasmid-borne.

Characterization of a conjugative staphylococcal mupirocin resistance plasmid

We studied conjugative plasmids encoding high-level mupirocin resistance. These plasmids were found in Staphylococcus aureus isolates from two geographic locations in the United States. Transfer genes on three mupirocin resistance plasmids with different restriction endonuclease profiles were indistinguishable by DNA hybridization from those on pG01, a conjugative aminoglycoside resistance plasmid representative of similar plasmids that are prevalent in the United States. One mupirocin resistance plasmid, pG0400 (34 kb), was smaller than pG01 (52 kb) because of the absence from pG0400 of DNA, found on pG01, that contained genes encoding resistance to aminoglycosides, trimethoprim, and quaternary ammonium compounds flanked by directly repeated copies of the insertion sequence (IS)-like element IS431-IS257. The plasmids pG0400 and pG01 were otherwise indistinguishable except for the presence in pG0400 of a 4.5-kb HinDIII fragment encoding mupirocin resistance. The added mupirocin resistance gene was flanked by two directly repeated copies of IS431/257. The nucleotide sequence of DNA contiguous to the outside of the IS elements, as well as those of the elements themselves, was identical in both pG01 and pG0400, and there were no target site duplications flanking either copy of the element. We conclude that the mupirocin resistance gene was added to an existing conjugative plasmid in conjunction with the deletion of other resistance genes by recombination at IS elements. The construction of conjugative plasmids carrying a mupirocin resistance gene may be a model for the mobility of other resistance genes newly acquired by staphylococci.

Use of gas-liquid chromatography for subgrouping coagulase-negative staphylococci during a nosocomial sepsis outbreak

Gas-liquid chromatographic (GLC) fatty acid profile correlation analysis was applied for the subgrouping of 169 coagulase-negative staphylococci collected during an outbreak of nosocomial sepsis in a hematologic unit. The fatty acid profile similarity index between six ciprofloxacin-resistant Staphylococcus epidermidis septicemia strains was as high as 98.39 +/- 0.68, indicating a high degree of resemblance. This finding corroborated the finding by conventional typing methods that the isolates shared the same strain characteristics and, therefore, could be derived from the same epidemiological origin. Further, the GLC fatty acid profiles were analyzed for coagulase-negative staphylococcal cutaneous isolates recovered from colonization cultures of the patients and personnel in that same unit. The similarity index between 88 ciprofloxacin-resistant Staphylococcus epidermidis skin isolates with similar plasmid profiles was as high as 95.47 +/- 3.78, whereas the correlation coefficient between 45 ciprofloxacin-susceptible Staphylococcus epidermidis skin isolates with different plasmid profiles was only 85.23 +/- 10.82. Cluster analysis grouped the ciprofloxacin-resistant Staphylococcus epidermidis isolates into one distinct cluster, while most of the ciprofloxacin-susceptible Staphylococcus epidermidis isolates were grouped into two separate clusters. When compared with the plasmid profiling, the GLC method congruously grouped 127 (87%) of the 146 Staphylococcus epidermidis isolates, thereby suggesting its potential value in subgrouping coagulase-negative staphylococci during nosocomial outbreaks.

Nosocomial infection by Staphylococcus haemolyticus and typing methods for epidemiological study

A patient with chronic myelogenous leukemia became colonized with a Staphylococcus haemolyticus strain and experienced a septic episode caused by this strain during a cytostatic course. The strain was multiply resistant to antibiotics; the MIC and MBC of vancomycin were 2 and 4 mg/liter, and the MIC and MBC of teicoplanin were 4 and 16 mg/liter, respectively. We performed a surveillance study on the carriage of S. haemolyticus in medical and nursing staff of the hospital ward where the patient was treated. S. haemolyticus was isolated from 18 sites on 12 of the 39 people tested. A number of typing methods were performed in order to investigate the possible relationships among the isolates. Methods used were immunoblotting of staphylococcal peptides, plasmid analysis, restriction fragment length polymorphism of chromosomal DNA, and pulsed-field gel electrophoresis of total DNA. Compared with the immunoblotting technique, the molecular methods were more discriminative. The strain colonizing the patient showed a consistent pattern by all typing methods during isolation. When the immunoblot technique was used, similar patterns were found with isolates from hospital staff and isolates from unrelated sources. With the molecular techniques, no evidence of a local spread of the patient's strain was found. However, plasmid profiles and restriction fragment length polymorphism and pulsed-field gel electrophoresis patterns showed that S. haemolyticus isolates collected from hospital ward personnel were related, which was not the case with isolates collected from unrelated sources. Restriction fragment length polymorphism analysis was more discriminative when IS431 was used as a DNA probe instead of a probe based on the 16S rRNA gene. S. haemolyticus, as in this case, may develop resistance to vancomycin and teicoplanin. These antibiotics are considered the last-resort drugs for the therapy of nosocomial gram-positive infections. Thus, local spread of staphylococci resistant to these drugs is an important problem, which should be prevented by strict hygienic measures and antibiotic policy.

Optimizing testing of methicillin-resistant Staphylococcus species

Selection of the appropriate NaCl concentration for test medium for oxacillin susceptibility testing of Staphylococcus aureus and coagulase-negative staphylococci has been problematic when using different antimicrobial susceptibility testing methods. Broth microdilution, using cation-adjusted Mueller-Hinton broth + 2% NaCl, is the currently recommended reference method. There is currently no recommendation for the addition of NaCl to agar for dilution susceptibility tests when Staphylococcus species are tested with oxacillin. We examined the effects of adding 0, 2%, 4%, and 5% NaCl to Mueller-Hinton agar and broth for agar dilution, Etest, and broth microdilution tests. The results of these tests were compared with the reference broth microdilution results and with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive and had oxacillin minimum inhibitory concentrations (MICs) > or = 4 micrograms/ml. Seven strains of S. aureus were mec probe negative but were oxacillin resistant. Seven coagulase-negative strains (three S. epidermidis, one S. haemolyticus, and three S. simulans) were mec probe positive and were oxacillin susceptible. The MICs for oxacillin-resistant strains increased two- to fourfold with the addition of 2% NaCl, but the MICs for oxacillin-susceptible strains were unchanged. Major and very major interpretative rates ranged from 18.2% to 20.2% for agar dilution and Etest without NaCl added to the medium, and these rates decreased to < 1% with the addition of 2% NaCl to the medium. The addition of 4% or 5% NaCl caused major error rates of > 17% for all test methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Two percent sodium chloride is required for susceptibility testing of staphylococci with oxacillin when using agar-based dilution methods

The need to add NaCl to agar media to ensure accuracy of results when testing staphylococci with oxacillin was investigated. The results of four antimicrobial susceptibility testing methods (agar and broth dilution, E test, and disk diffusion) in which the growth medium contained 0, 2, 4, or 5% NaCl were compared with the results of a hybridization assay using a mec gene probe. We tested 223 strains of staphylococci, 128 of which were mec gene positive. A total of 7 of the 128 positive strains were coagulase-negative staphylococci with 24-h oxacillin MICs of < or = 2 micrograms/ml. Ninety-five isolates were mec gene negative, including seven strains of Staphylococcus aureus with oxacillin MICs of > or = 4 micrograms/ml. The oxacillin MICs for mec gene-positive, oxacillin-resistant strains of staphylococci increased two- to fourfold with the addition of NaCl to the test medium, while the MICs for mec gene-negative strains did not change in the presence of added salt. Very major error rates for the agar dilution and E test methods in the absence of salt ranged from 18.2 to 20.2%. Major error rates for mec gene-negative S. aureus isolates were > 17% for all test methods when 4 or 5% NaCl was added to the test medium. The addition of 2% NaCl to Mueller-Hinton agar for testing of oxacillin resulted in very major error rates of < 1% for the agar dilution and E test methods although the major error rates for the two methods with added NaCl were 8.5 and 6.9%, respectively. The disk diffusion test did not perform well in this study, showing essential error rates of > or = 18.3%. We recommended the addition of 2% NaC1 to Mueller-Hinton agar when testing staphylococci with oxacillin by either the agar dilution or E test method. NaC1 should not be added for the disk diffusion test.

Genes involved in the regulation of beta-lactamase production in enterococci and staphylococci

In enterococci, the structural gene for beta-lactamase (blaZ) is identical to blaZ from Staphylococcus aureus. However, in the enterococci studied to date, beta-lactamase is produced constitutively, whereas in staphylococci it is often inducible. Recent reports have revealed the presence of two adjacent genes upstream of the staphylococcal blaZ thought to be the antirepressor (blaR1) and repressor (blaI) genes. In the present study, beta-lactamase expression mutants of the staphylococcal beta-lactamase plasmid pI524 were generated by transposon mutagenesis with the transposon Tn917. Tn917 insertions upstream of blaZ in either blaR1 or blaI resulted in constitutive beta-lactamase production, indicating that the repressor function is lost with insertion of Tn917 into either gene. This finding supports the concept that the staphylococcal beta-lactamase regulatory genes are encoded on a polycistronic mRNA. The corresponding region upstream of the enterococcal blaZ from Enterococcus faecalis HH22 was sequenced and compared with the staphylococcal blaR1 sequence. The two sequences were identical for 893 nucleotides, and then the sequences diverged completely. Therefore, in strain HH22, only 51% of the putative antirepressor gene is present and the repressor gene is also absent. In conclusion, constitutive beta-lactamase production in HH22 appears to be due to a lack of the regulatory genes blaR1 and blaI which regulate expression of blaZ in staphylococci.

DNA sequence and units of transcription of the conjugative transfer gene complex (trs) of Staphylococcus aureus plasmid pGO1

The conjugative transfer genes of 52-kb staphylococcal R plasmid pGO1 were localized to a single BglII restriction fragment and cloned in Escherichia coli. Sequence analysis of the 13,612-base transfer region, designated trs, identified 14 intact open reading frames (ORFs), 13 of which were transcribed in the same direction. Each ORF identified was preceded by a typical staphylococcal ribosomal binding sequence, and 10 of the 14 proteins predicted to be encoded by these ORFs were seen when an E. coli in vitro transcription-translation system was used. Functional transcription units were identified in a Staphylococcus aureus host by complementation of Tn917 inserts that abolished transfer and by Northern (RNA) blot analysis of pGO1 mRNA transcripts. These studies identified three complementation groups (trsA through trsC, trsD through trsK, and trsL-trsM) and four mRNA transcripts (trsA through trsC [1.8 kb], trsA-trsB [1.3 kb], trsL-trsM [1.5 kb], and trsN [400 bases]). No definite mRNA transcript was seen for the largest complementation group, trsD through trsK (10 kb). Comparison of predicted trs-encoded amino acid sequences to those in the data base showed 20% identity of trsK to three related genes necessary for conjugative transfer of plasmids in gram-negative species and 32% identity of trsC to a gene required for conjugative mobilization of plasmid pC221 from staphylococci.

Occurrence of quinolone resistance in Staphylococcus aureus from nosocomial infection

Among 63 Staphylococcus aureus isolates (one isolate per one patient) counted from infections (from August to November 1991) in hospital T., eight exhibited resistance to fluoroquinolones. Seven of these quinolone-resistant isolates were multiply- and methicillin-resistant S. aureus (QR-MRSA). The results of phage-, plasmid- and genotyping (pulsed field electrophoresis) revealed that six different strain-clones of these MRSA were spread in the hospital. In vitro spontaneous mutants resistant to fluoroquinolones are 10-100-fold more frequent in MRSA than in other S. aureus when selected on isosensitest-agar containing 1 microgram/ml of ciprofloxacin. However, the same mutant frequencies were found in strain 8325-4 with and without the mecA-determinant. The resistance phenotype was stable over 30 generations of subculture in nutrient broth as well in natural quinolone resistant MRSA as in mutants of other types of S. aureus selected in vitro. The phenotypic association of quinolone resistance and MRSA is rather likely due to a higher frequency of spontaneous resistant mutants which are present in natural populations of MRSA. Data of chemotherapy prior to the isolation of S. aureus show that three of seven patients from whom QR-MRSA were isolated were treated with a quinolone. In eight cases of infections with non-MRSA and quinolone treatment the isolated S. aureus strains were in vitro sensitive to quinolones.

Comparison of the gentamicin resistance transposon Tn5281 with regions encoding gentamicin resistance in Enterococcus faecalis isolates from diverse geographic locations

The genetic determinant encoding gentamicin resistance (Gmr) on the beta-lactamase encoding plasmid pBEM10 of Enterococcus faecalis HH22 is carried on a transposon, termed Tn5281, that is highly related to the staphylococcal Gmr transposons Tn4001 found in Australian isolates of Staphylococcus aureus and Tn4031 found in United States isolates of Staphylococcus epidermidis. We have now studied plasmid DNA from Gmr strains of E. faecalis isolated from diverse geographical locations (Houston, Pennsylvania, Thailand, and Chile) by using restriction endonuclease analysis and DNA-DNA hybridization to determine whether other Gmr E. faecalis carry Tn5281 or a similar type of element. We also compared these enterococci to several United States isolates of Staphylococcus aureus with nonmobile Gmr determinants. Three E. faecalis isolates (from Houston and Chile) carried Tn5281-like elements, whereas two isolates (from Houston and Pennsylvania) had restriction endonuclease and DNA-DNA hybridization patterns more similar to those of the Tn4001-IS257 hybrid found in the nonmobile Gmr determinants in United States isolates of S. aureus. A strain from Thailand had a third pattern unrelated to either Tn5281 or the nonmobile Gmr determinants present in United States isolates of S. aureus. Our results demonstrate that there is both similarity and diversity between the Gmr determinant of strains of E. faecalis isolated in diverse geographic locations.

Lysogeny of methicillin-resistant Staphylococcus aureus and the role of prophages in transfer of conjugative and non-conjugative plasmids

The lysogenicity of 49 strains of methicillin-resistant S. aureus (MRSA) isolated in Moscow clinics in the 1970s and '80s was studied by the method of mitomycin C induction. It was found that one strain had phage of serogroup B, 33 strains had serogroup F phages and 15 strains had phages of both serogroups. In the course of genetic crossing on nitrocellulose filters it was demonstrated that serogroups B and F prophages contained in recipient cells 1) increase the frequency of transfer of conjugative plasmid pG873 and 2) mobilize transfer of non-conjugative plasmids pE994 and rms7.

Comparative sensitivity to antibiotics and biocides of methicillin-resistant Staphylococcus aureus strains isolated from Saudi Arabia and Great Britain

Methicillin-resistant Staphylococcus aureus (MRSA) isolated in Saudi Arabia and Great Britain were examined for susceptibility to antibiotics and biocides. The strains differed in their sensitivity patterns. None of the Saudi strains showed resistance to propamidine isethionate, but most of the British gentamicin methicillin-resistant Staph. aureus (GMRSA) strains were highly resistant to this compound and to some other nucleic acid-binding (NAB) compounds. Both groups showed a low level of resistance towards quaternary ammonium compounds (QACs), but resistance to these compounds was not associated with resistance to gentamicin in the Saudi strains. The aminoglycoside-resistant determinants were non-conjugative in these strains. Natural MRSA strains were good recipients for pWG613, but transferred this plasmid in reciprocal crosses at significantly lower rates.

Nucleotide sequence of the beta-lactamase gene from Enterococcus faecalis HH22 and its similarity to staphylococcal beta-lactamase genes

The nucleotide sequence of the constitutively produced beta-lactamase (Bla) gene from Enterococcus faecalis HH22 was shown to be identical to the published sequences of three of four staphylococcal type A beta-lactamase genes; more differences were seen with the genes for staphylococcal type C and D enzymes. One hundred forty nucleotides upstream of the beta-lactamase start codon were determined for an inducible staphylococcal beta-lactamase and were identical to those of the constitutively expressed enterococcal gene, indicating that the changes resulting in constitutive expression are not due to changes in the promoter or operator region. Moreover, complementation studies indicated that production of the enterococcal enzyme could be repressed. The genes for the enterococcal Bla and an inducible staphylococcal Bla were each cloned into a shuttle vector and transformed into enterococcal and staphylococcal recipients. The major difference between the backgrounds of the two hosts was that more enzyme was produced by the staphylococcal host, regardless of the source of the gene. The location of the enzyme was found to be host dependent, since each cloned gene generated extracellular (free) enzyme in the staphylococcus and cell-bound enzyme in the enterococcus. On the basis of the identities of the enterococcal Bla and several staphylococcal Bla sequences, these data suggest the recent spread of beta-lactamase to enterococci and also suggest the loss of a functional repressor.

Emergence of quinolone resistance among clinical isolates of methicillin-resistant Staphylococcus aureus in Ontario, Canada

One hundred two isolates of methicillin-resistant Staphylococcus aureus (MRSA) randomly selected from across the Canadian province of Ontario were tested for their susceptibility to ciprofloxacin, norfloxacin, and nalidixic acid by the agar dilution method. Forty-nine percent (50 of 102) had high levels of resistance to these quinolone compounds. For the 50 resistant isolates, ciprofloxacin and norfloxacin had high MICs for 90% of isolates (MIC90s) of 128 micrograms/ml and greater than 128 microgram/ml, respectively; for these isolates, the nalidixic acid MIC90 was greater than 640 micrograms/ml. The majority (98%) of the 50 isolates were also resistant to tobramycin (MIC90, greater than 128 micrograms/ml), while 42% of the isolates were resistant to gentamicin (MIC90, 64 micrograms/ml). Quinolone-resistant MRSA isolates were susceptible to bacteriophages from several groups, indicating independent selection of resistant strains. These results suggest that a reappraisal of the use of fluoroquinolones against MRSA in Canada is necessary.

4',4'' adenyltransferase activity on conjugative plasmids isolated from Staphylococcus aureus is encoded on an integrated copy of pUB110

In staphylococci, linked resistance to the aminoglycosides kanamycin, neomycin, paromomycin, and tobramycin (KmNmPmTmr) is generally mediated by an aadD determinant which encodes production of an adenyltransferase aminoglycoside modifying enzyme, AAD(4',4''). The aadD resistance determinant is located on small multicopy plasmids such as pUB110, and has also been found on large multiresistance plasmids and on the chromosome in some strains. Examination of two conjugative plasmids from strains of Staphylococcus aureus isolated in North America indicated that the aadD determinant on these plasmids is located on an integrated copy of pUB110. The integrated pUB110 is flanked by direct repeats of the staphylococcal insertion sequence IS257. Analysis of the conjugative plasmid pSK41 showed an 8-bp duplication of the pUB110 sequence immediately adjacent to flanking IS257 elements, suggesting that integration of pUB110 was mediated by IS257.

Molecular analysis of a gentamicin resistance transposonlike element on plasmids isolated from North American Staphylococcus aureus strains

Plasmid-encoded resistance to the aminoglycosides gentamicin (Gm), tobramycin (Tm), and kanamycin (Km) (GmTmKmr) in strains of Staphylococcus aureus isolated in Australia and North America appears to be mediated by one resistance determinant. In Australian isolates, this determinant is flanked by inverted copies of a 1.3-kb insertion sequence, IS256, thereby forming a composite transposon, Tn4001. Analysis of two conjugative plasmids and a related nonconjugative plasmid from strains of S. aureus isolated in North America showed that the GmTmKmr determinant on these plasmids is also flanked by inverted repeats. In the nonconjugative plasmid, these repeats include only 425 bp of IS256 immediately adjacent to the GmTmKmr region and identical to that on Tn4001. This truncated Tn4001 element is flanked by copies of the insertion element IS257, and together these elements form a truncated Tn4001-IS257 hybrid transposonlike structure. A third copy of IS257 was located 418 bp from the hybrid structure. The truncated Tn4001 and three repeats of IS257 were present at a conserved site on the plasmids studied. Four additional copies of IS257 were identified on the two conjugative plasmids. These elements flank determinants for resistance to the aminoglycosides neomycin and paromomycin and to ethidium bromide and quaternary ammonium compounds, as well as the region involved in conjugative plasmid transfer.

Relationship of staphylococcal conjugative plasmids to large gentamicin-resistance nonconjugative plasmids in clinical isolates of Staphylococcus epidermidis

Self-mobilization of large plasmids was not observed in 15 of 15 strains of gentamicin-resistant (Gmr) Staphylococcus epidermidis clinical isolates that were taken from bacteremic newborns in a neonatal intensive care unit. Alternatively, nine Gmr Staphylococcus aureus clinical isolates, which were isolated along with the S. epidermidis strains, were all shown previously to contain Gmr conjugative plasmids. All of the nonconjugative strains had plasmids that were similar in size to the S. aureus conjugative plasmids. Transfer of seven of these nonconjugative plasmids by protoplast transformation indicated that they carried Gmr determinants. The rationale of these studies was to detect the presence of genes for conjugation (tra) on the large Gmr nonconjugative plasmids. It was thought that these plasmids might contain either defective or deleted tra gene sequences. To gain insight into these possibilities, a 6.3-kb probe, which contained a major tra gene region, was hybridized with EcoRI and XbaI digests of nonconjugative plasmid DNA. Hybridization occurred with only one of eight plasmids. It was concluded that seven of the large S. epidermidis plasmids were not self-transmissible because they lacked tra genes. However, pMH6502 contained an excess of tra gene regions compared to prototype conjugative plasmids and was still not self-transmissible.

Rapid field inversion gel electrophoresis in combination with an rRNA gene probe in the epidemiological evaluation of staphylococci

A rapid field inversion gel electrophoresis (FIGE) protocol was combined with an rRNA gene probe in the analysis of staphylococci that were difficult to study epidemiologically by conventional means. The following groups of clinical isolates were examined: (i) predominantly antibiotic-susceptible Staphylococcus aureus strains containing no detectable plasmids and unresponsive to bacteriophage typing and (ii) methicillin-resistant Staphylococcus epidermidis strains carrying a single plasmid ca. 30 kilobases in size. The results indicated that strain interrelationships could be established on the basis of SmaI-generated chromosomal restriction fragment length polymorphisms (RFLPs) analyzed by FIGE. RFLP analysis of strains known to be unrelated established the importance of minor differences in DNA banding patterns as indicators of strain dissimilarities. Hybridization studies with an rRNA gene probe confirmed this conclusion. These results suggest that FIGE analysis of chromosomal RFLPs (especially in combination with molecular probes) is an important addition to the armamentarium of molecular epidemiology.

Staphylococcus aureus in patients with cystic fibrosis: an epidemiological analysis using a combination of traditional and molecular methods

An epidemiological analysis of Staphylococcus aureus was conducted in a study group of 157 cystic fibrosis patients cultured over a 30-month period. The resulting S. aureus isolates were categorized by bacteriophage type, plasmid profile, and (in some instances) chromosomal restriction fragment pattern of the culture-positive patients with S. aureus (34 of 157) 44% only were sporadically infected while 68% shared identical strains with one or more other patients. Six patients exhibited persistent infection (for up to ten months) which, in three individuals, occurred as cycles of carriage and reappearance. By contributing toward our understanding of the persistence and spread of S. aureus in cystic fibrosis patients these data should aid in clarifying the role this organism may play in the course of the disease.

Mobility of gentamicin resistance genes from staphylococci isolated in the United States: identification of Tn4031, a gentamicin resistance transposon from Staphylococcus epidermidis

Homologous genes encoding resistance to gentamicin, tobramycin, and kanamycin through the bifunctional acetylating [AAC(6')] and phosphorylating [APH(2")] aminoglycoside-modifying enzyme were identified in staphylococci isolated from patients in the United States. The mobility of gentamicin resistance (Gmr) genes found on a prototype conjugative plasmid (pGO1) was compared with that of genes cloned from chromosomal sites. Plasmid-encoded Gmr genes and flanking sequences were introduced onto a temperature-sensitive plasmid (pRN3208) from pGO1 by homologous recombination between insertion sequence-like elements present on both replicons. Growth of Staphylococcus aureus strains containing the temperature-sensitive recombinant (pGO161) at the nonpermissive temperature for plasmid replication (42 degrees C) revealed no translocation of Gmr from its plasmid location. A transposon (Tn551) resident on the same replicon did translocate. Chromosomal Gmr determinants were cloned, together with the gene for trimethoprim resistance (dfrA), from three geographically distinct S. epidermidis isolates; two were subcloned onto temperature-sensitive Escherichia coli-S. aureus shuttle plasmids as 7.2-kilobase BglII fragments. Growth of both recombination-deficient and-proficient S. aureus strains containing the cloned genes at 42 degrees C allowed detection of transposition of Gmr sequences and identification of insertion into random chromosomal sites. We have designated this 5-kilobase transposon from S. epidermidis as Tn4031.

Identification and cloning of the conjugative transfer region of Staphylococcus aureus plasmid pGO1

The conjugative transfer (tra) genes of a 52-kilobase (kb) staphylococcal plasmid, pGO1, were localized by deletion analysis and transposon insertional inactivation. All transfer-defective (Tra-) deletions and Tn551 or Tn917 transposon insertions occurred within a 14.5-kb BglII fragment. Deletions and insertions outside this fragment all left the plasmid transfer proficient (Tra+). The tra region was found to be flanked by directly repeated DNA sequences, approximately 900 base pairs in length, at either end. Clones containing the 14.5-kb BglII fragment (pGO200) and subclones from this fragment were constructed in Escherichia coli on shuttle plasmids and introduced into Staphylococcus aureus protoplasts. Protoplasts could not be transformed with pGO200E (pGO200 on the staphylococcal replicon, pE194) or subclones containing DNA at one end of the tra fragment unless pGO1 or specific cloned tra DNA fragments were present in the recipient cell. However, once stabilized by sequences present on a second replicon, each tra fragment could be successfully introduced alone into other plasmid-free S. aureus recipients by conjugative mobilization or transduction. In this manner, two clones containing overlapping fragments comprising the entire 14.5-kb BglII fragment were shown to complement each other. The low-frequency transfer resulted in transconjugants containing one clone intact, deletions of that clone, and recombinants of the two clones. The resulting recombinant plasmid (pGO220), which regenerated the tra region intact on a single replicon, transferred at frequencies comparable to those of pGO1. Thus, all the genes necessary and sufficient for conjugative transfer of pGO1 are contained within a 14.5-kb region of DNA.

Complex typing of methicillin-resistant Staphylococcus aureus (MRSA)

To discriminate between methicillin-resistant Staphylococcus aureus from 5 nosocomial outbreaks and from sporadic nosocomial infections, the efficacy of a complex typing scheme by phage typing, biochemical typing, resistance phenotype, plasmid profiles, plasmid patterns and attribution of resistance determinants to the chromosome was studied. In addition to the International Basic Set and experimental phages 88-93, 10 experimental phages from the Public Health Laboratory Service, Colindale, London, were used for phage-typing. The 10 experimental phages from PHLS in particular, in combination with plasmid profiles and plasmid patterns, were of special discriminative value.

Biotyping coagulase-negative staphylococci

The biochemical profiles obtained with Staph-Ident (Analytab Products, Plainview, N.Y.) panels were combined with the results of adherence and synergistic hemolysis tests to define biotypes among 1,064 clinical isolates representing eight species of coagulase-negative staphylococci. The 672 isolates of Staphylococcus epidermidis were aligned in 69 of 144 potential biotypes in our scheme because of 18 different biochemical profiles and the eight physiologic subtypes. Isolates of most other species were in fewer biotypes because of more uniform adherence and synergistic hemolysis data, as well as fewer biochemical profiles. Since adherence and synergistic hemolysis may prove to be related to virulence and pathogenicity, biotyping with these test results would help evaluate the reliability of adherence and synergistic hemolysis as possible indices of the clinical significance of some of these organisms. When the antimicrobial susceptibility and plasmid profiles obtained on two clusters of S. epidermidis isolates were compared with the biotyping results, one cluster was not further differentiated by plasmid profiles, but was by antimicrobial profiles; the other cluster with only two biotypes was further divided into five distinct types by plasmid profiles but was not separated at all by antimicrobial profiles.

Restriction endonuclease analysis of Staphylococcus epidermidis DNA may be a useful epidemiological marker

We compared the epidemiological markers of 13 Staphylococcus epidermidis strains isolated from an adult inpatient during a febrile episode and 23 S. epidermidis strains isolated during a presumptive outbreak of nosocomial infection in a neonatal ward. The total DNA restriction endonuclease analysis (REA) was processed along with the following conventional markers: biotyping, serotyping, phage typing, antibiotic susceptibility profiles, and plasmid profiles. The REA method was reproducible, giving stable results both in vitro and in vivo. For the hospitalized adult patient, the conventional markers of the 13 strains were concordant and the restriction profiles were identical. Five restriction groups were demonstrated during the course of the outbreak. Within two of the groups, the identities of all of the markers were used to verify whether all of the isolates belonged to the same cell clone. In a third group, combined analysis of the conventional markers and REA had to be used to demonstrate isolate similarity. On the other hand, in another group, none of the markers were similar; interpretation was not easy. An epidemiological study of S. epidermidis infections in hospitals must take into account all of the epidemiological markers: biotypes, serotypes, phage types, antibiograms, plasmid profiles, and REA.

Genetic analysis of gentamicin resistance in methicillin- and gentamicin-resistant strains of Staphylococcus aureus isolated in Dublin hospitals

Methicillin- and gentamicin-resistant strains of Staphylococcus aureus isolated in Dublin hospitals have been classified into groups I, II, and III based on resistance to antimicrobial agents and plasmid profiles. Each group expresses a characteristic level of resistance to gentamicin, tobramycin, and sisomicin. Enzyme assays showed that resistant strains expressed 2"-aminoglycoside phosphotransferase-6'-aminoglycoside transferase activity by a determinant which is known to be chromosomally located. The gentamicin resistance (Gmr) determinants were transferred from group I, II, or III strains by transduction into a laboratory strain where each expressed the same low level of resistance. This finding suggests that high-level resistance in some clinical strains is due to a second, unlinked resistance mechanism. No evidence was obtained by hybridization experiments that clinical isolates or spontaneous mutants expressing high-level Gmr carried more than one copy of the Gmr determinant, thus eliminating the possibility that a gene dosage effect was responsible for high-level resistance. Hybridization experiments with transductants and wild strains suggested that the Gmr determinant was located at homologous sites in wild strains from different groups, although restriction site differences were observed in flanking sequences. Electron microscope analysis of a cloned Gmr determinant and genetic evidence suggested that a Dublin clinical isolate harbored a transposon very similar to Tn4001.

Tn4201, a beta-lactamase transposon in Staphylococcus aureus

We report the preliminary characterization of a 6.7-kilobase transposon, Tn4201, encoding resistance to penicillin associated with the conjugative plasmid pCRG1600 in Staphylococcus aureus. Tn4201 is capable of inverting in orientation (frequently observed in derivatives of pCRG1600) and is expressed equally well in either orientation. The element is also capable of movement to alternative plasmid and chromosomal sites. Tn4201 exhibits rec-independent transposition in an apparent site-specific manner and may play an important role in the resistance of staphylococci to beta-lactam compounds.

Characterization of clinically significant isolates of Staphylococcus epidermidis from patients with endocarditis

Biotyping, slime production, bacteriophage typing, serotyping, antibiograms, and plasmid profiles were used to characterize 19 Staphylococcus epidermidis strains isolated from 12 patients with prosthetic valve endocarditis and from 7 patients with native valve endocarditis. With the API Staph battery, 12 different biocodes with, at the most, three differences were obtained. Slime production was found for 10 strains (53%). Agglutinogens investigated by agglutination with two specific sera were found for 12 strains (63.1%). Three strains were phage typable (15.2%). Against a panel of nine antimicrobial agents, 15 different profiles were found. Multiply antibiotic-resistant strains were isolated from patients with prosthetic valve endocarditis when disease onset occurred less than 18 months after heart surgery and from patients with native valve endocarditis who received antibiotics immediately prior to their illness. All of the strains were available for plasmid analysis, and all the DNA profiles were distinct. On gels run in Tris-borate buffer, 73.7% of the strains had large plasmids of more than 30 megadaltons. A small plasmid of 2.8 megadaltons was found in multiply resistant strains and in strains resistant only to tetracyclines. None of the isolates appeared to be the same strain, and the bacteriological differences between the strains were confirmed mainly by the antibiotic susceptibility profile and the plasmid pattern analysis. These bacteriological results were in agreement with the clinical data.

Multiply- and methicillin-resistant Staphylococcus aureus strains isolated in the German Democratic Republic in 1985 and 1986

Multiply- and methicillin-resistant Staphylococcus aureus (MRSA) strains have been isolated from five small outbreaks of nosocomial infection in five different hospitals. The MRSA were typed by phage patterns, biochemical traits, resistance phenotypes and plasmid patterns. Three different groups of strains can be distinguished. The MRSA from three outbreaks in one country share identical characters. Phage typing by the use of the International Basic Set for Phage Typing staphylococci as well as experimental phages does not completely discriminate between the strains. Attribution of several resistance determinants to plasmids in two of the described strain groups proved valuable for strain differentiation. These multiply-resistant strains are sensitive to vancomycin and to rifampicin.

Emergence of resistance in gram-negative bacteria during therapy with expanded-spectrum cephalosporins

To assess the clinical importance of emergence of beta-lactam resistance caused by stable derepression of chromosomal beta-lactamases, sequential cultures from patients treated with expanded-spectrum cephalosporins were monitored for the persistence of bacteria possessing these enzymes. Antibiotic susceptibilities and beta-lactamase production before and after cefoxitin induction were determined in sequential isolates of individual bacterial strains. Of 49 strains isolated from 44 patients, 25 strains (51%) were eradicated by cephalosporin therapy, 17 strains (35%) persisted with unchanged susceptibility in sequential cultures, and 7 strains (14%) from 7 patients developed multiple beta-lactam resistance during cephalosporin therapy. In 6 of the 7 strains, resistance was associated with stable derepression of beta-lactamases. In the patient group whose strains developed resistance, subsequent use of non-beta-lactam antibiotics was more frequent and mortality was higher.

Staphylococcus aureus strains of the 94/96 complex isolated in the German Democratic Republic: incidence and discrimination of strain clones

The incidence of S. aureus with a phage pattern of 94/96 rose from 9% in 1979 to 18% in 1985. The frequency of occurrence not only increased among isolates from inpatients and outpatients but also among those from healthy carriers. All of the 504 investigated strains of different origin exhibited a uniform pattern of biochemical characteristics. In each of 40 investigated strains, a plasmid with a molecular mass of 16 Md was found. Elimination experiments indicated that these plasmids determined resistance to penicillin and/or cadmium. Resistance to chloramphenicol was found to be determined by plasmids of 2.0 Md, resistance to oxytetracycline by plasmids of 2.7 Md. Clones could be discriminated by means of 7 experimental phages. The application of these phages for typing strains from infections in hospitals is demonstrated.

Transfer of the conjugal tetracycline resistance transposon Tn916 from Streptococcus faecalis to Staphylococcus aureus and identification of some insertion sites in the staphylococcal chromosome

The Streptococcus faecalis pheromone-dependent conjugative plasmid pAD1::Tn916 and the membrane filter-dependent conjugative plasmid pPD5::Tn916 were used to introduce Tn916 into Staphylococcus aureus by intergeneric protoplast fusions and intergeneric membrane-filter matings. In recombinants obtained by protoplast fusion where no plasmid DNA could be detected, tetracycline resistance resulted from transposition of Tn916 from pAD1 to the S. aureus chromosome. Transformation analyses showed that S. aureus Tn916 chromosomal insertions occurred near pig, ilv, uraA, tyrB, fus, ala, and the trp operon. DNA hybridization analyses of EcoRI- and HindIII-digested chromosomal DNAs confirmed the diversity of chromosomal sites involved and demonstrated that the inserts were Tn916 insertions rather than integrations of all or part of pAD1::Tn916. Both pAD1::Tn916 and pPD5::Tn916 were transferred to S. aureus by membrane-filter matings. These plasmids remained intact and expressed tetracycline resistance in S. aureus. S. aureus strains carrying pAD1::Tn916, but not a chromosomal insert of Tn916, and any one of several conjugal gentamicin-resistance plasmids lost their ability to serve as conjugal donors of the gentamicin-resistance plasmids.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Conjugative transfer of staphylococcal antibiotic resistance markers in the absence of detectable plasmid DNA

Eleven Staphylococcus aureus clinical isolates were tested for transfer of resistance markers by transduction and filter mating. The resistance markers of six of the strains could be transferred only by transduction; however, the five remaining strains transferred their resistance both by transduction and filter mating. The resistance markers that were cotransferred in filter matings (transfer of resistance to penicillin and streptogramin A was accompanied, in each case, by the transfer of one or more markers, i.e., resistance to aminoglycosides, cadmium, or tetracycline, depending on the donor) were not cotransduced. The filter mating transfers were recA independent and were observed with both Staphylococcus aureus and Staphylococcus epidermidis recipients. Experiments to elucidate the mechanism of transfer by filter mating suggested that conjugation requiring cell-to-cell contact may have been involved. These transfers occurred in the absence of detectable plasmid DNA.