Molecular epidemiology of trimethoprim resistance among coagulase-negative staphylococci

Plasmid-Mediated Antimicrobial Resistance in Staphylococci and Other Firmicutes

In staphylococci and other Firmicutes, resistance to numerous classes of antimicrobial agents, which are commonly used in human and veterinary medicine, is mediated by genes that are associated with mobile genetic elements. The gene products of some of these antimicrobial resistance genes confer resistance to only specific members of a certain class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents. The resistance mechanisms specified by the resistance genes fall into any of three major categories: active efflux, enzymatic inactivation, and modification/replacement/protection of the target sites of the antimicrobial agents. Among the mobile genetic elements that carry such resistance genes, plasmids play an important role as carriers of primarily plasmid-borne resistance genes, but also as vectors for nonconjugative and conjugative transposons that harbor resistance genes. Plasmids can be exchanged by horizontal gene transfer between members of the same species but also between bacteria belonging to different species and genera. Plasmids are highly flexible elements, and various mechanisms exist by which plasmids can recombine, form cointegrates, or become integrated in part or in toto into the chromosomal DNA or into other plasmids. As such, plasmids play a key role in the dissemination of antimicrobial resistance genes within the gene pool to which staphylococci and other Firmicutes have access. This chapter is intended to provide an overview of the current knowledge of plasmid-mediated antimicrobial resistance in staphylococci and other Firmicutes.

σBand SarA independently regulate polysaccharide intercellular adhesin production inStaphylococcus epidermidis

The production of polysaccharide intercellular adhesin (PIA) is an essential process in foreign body infections mediated by Staphylococcus epidermidis. Transcriptional regulation of the icaADBC operon, the genes responsible for production of enzymes that synthesize PIA, is multi-factorial and involves at least SarA and σB. Transcriptional and promoter fusion studies revealed that the decreased transcription of the icaADBC operon observed in a S. epidermidis 1457 sigB mutant is not mediated through a direct interaction of σB–RNA polymerase at the icaADBC promoter region but instead through the upregulation of IcaR, a known repressor of icaADBC transcription. Transcriptional analysis of a 1457 sigB–icaR double mutant confirmed that the decreased icaADBC transcript in 1457 sigB is IcaR dependent. Furthermore, primer extension studies suggest that the icaR promoter appears to be σAdependent, suggesting that σBindirectly controls icaR transcription through an unknown pathway. In addition, it was confirmed that the loss of SarA results in the loss of icaADBC transcription and PIA production in S. epidermidis. It was further demonstrated, through the over-production of SarA in 1457 sigB, that the loss of sarP1 promoter activity in 1457 sigB has little or no effect on the loss of PIA production in this mutant. Finally, it was demonstrated that PIA production could be restored in both 1457 sigB and 1457 sarA by complementing these mutants with a full-length icaADBC operon controlled by a cadmium-inducible noncognate promoter. It is concluded that σBand SarA operate independently of each other to regulate PIA production and biofilm development in S. epidermidis.Key words: Staphylococcus epidermidis, biofilm, σB, SarA, icaADBC.

Evolution of the antimicrobial resistance of Staphylococcus spp. in Spain: five nationwide prevalence studies, 1986 to 2002

Data regarding the evolution of Staphylococcus resistance in a whole country have a definite influence on the design of empirical treatment regimens. Nevertheless, incidence studies over long periods of time are expensive and very difficult to carry out. In order to ascertain the present situation of the antimicrobial resistance of Staphylococcus in Spain and the change of this resistance over time, we performed five point prevalence studies (1986 to 2002) in a large group of Spanish hospitals (from 68 institutions in 1986 to 143 in 2002) collecting all Staphylococcus strains isolated on a single selected day. All microorganisms were identified in the five studies at the same laboratory, and antimicrobial susceptibility testing was performed against 17 antimicrobial agents by the agar dilution method and a microdilution method. During this period, there was an overall increase in resistance to most antimicrobials among Staphylococcus aureus/coagulase-negative staphylococci, mainly to oxacillin (1.5%/32.5% in 1986 versus 31.2%/61.3% in 2002) (P < 0.001), erythromycin (7%/41.1% in 1986 versus 31.7%/63% in 2002) (P < 0.001), gentamicin (5.2%/25.4% in 1986 versus 16.9%/27.8% in 2002) (P < 0.001; P = 0.5), and ciprofloxacin (0.6%/1.1% in 1986 versus 33.9%/44.9% in 2002) (P < 0.001). All of the isolates were uniformly susceptible to glycopeptides, linezolid, and quinupristin/dalfopristin. Resistance of S. aureus to trimethoprim/sulfamethoxazole was very low (from 0.5% to 2.1%) (P = 0.152). Periodic performance of prevalence studies is a useful, inexpensive, and easy tool to know the nationwide situation of a microorganism and its resistance to antimicrobials; it also helps us assess the emergence and spread of antimicrobial resistance.

Genetic and phenotypic analysis of biofilm phenotypic variation in multiple Staphylococcus epidermidis isolates

Production of biofilm in Staphylococcus epidermidis is mediated through enzymes produced by the four-gene operon ica and is subject to phenotypic variation. The purpose of these experiments was to investigate the regulation of ica and icaR transcription in phenotypic variants produced by multiple unrelated isolates of S. epidermidis. Ten isolates were chosen for the study, four of which contained IS256. IS256 mediates a reversible inactivation of ica in approximately 30 % of phenotypic variants. All ten strains produced at least two types of phenotypic variant (intermediate and smooth) in which biofilm formation was significantly impaired. Reversion studies indicated that all phenotypic variants were stable after overnight growth, but began to revert to other phenotypic forms after 5 days of incubation at 37 degrees C. ica transcriptional analysis was performed on phenotypic variants from three IS256-negative isolates; 1457, SE5 and 14765. This analysis demonstrated that ica transcription was significantly reduced in the majority of phenotypic variants, although two variants from SE5 and 1457 produced wild-type quantities of ica transcript. Analysis of seven additional phenotypic variants from SE5 revealed that ica expression was only reduced in three. Expression of icaR transcript was unaffected in all smooth phenotypic variants. Mutations within ica were identified in two SE5 variants with wild-type levels of ica transcription. It is concluded that mutation and transcriptional regulation of ica are the primary mechanisms that govern phenotypic variation of biofilm formation within IS256-negative S. epidermidis.

Antimicrobial susceptibility of coagulase-negative staphylococci isolated from bovine mastitis in Argentina

A total of 123 isolates of coagulase-negative staphylococci isolated from bovine clinical and subclinical mastitis in Argentina from March 1998 to March 2000 was investigated for in vitro susceptibility to several antimicrobial agents. Minimum inhibitory concentrations that inhibit 90% of the isolates tested (reported in micrograms per milliliter) were: 4.40, 0.38, 4.00, 0.75, 0.75, 3.60, and 2.00 for penicillin, oxacillin, cephalothin, gentamicin, erythromycin, clindamycin, and ampicillin-sulbactam, respectively. Resistance was detected in 34 (27.6%), 4 (3.2%), 7 (5.7%), and 6 (4.8%) isolates for penicillin, oxacillin, erythromycin, and pirlimycin, respectively. No resistance was detected for gentamicin, cephalothin, or ampicillin-sulbactam. Results indicated that coagulase-negative staphylococci isolates in Argentina exhibited the highest degree of resistance to penicillin of all antimicrobial agents tested.

Methicillin resistant Staphylococcus aureus (MRSA) in the intensive care unit

Methicillin resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen that causes severe morbidity and mortality worldwide. MRSA strains are endemic in many American and European hospitals and account for 29%-35% of all clinical isolates. Recent studies have documented the increased costs associated with MRSA infection, as well as the importance of colonisation pressure. Surveillance strategies have been proposed especially in high risk areas such as the intensive care unit. Pneumonia and bacteraemia account for the majority of MRSA serious clinical infections, but intra-abdominal infections, osteomyelitis, toxic shock syndrome, food poisoning, and deep tissue infections are also important clinical diseases. The traditional antibiotic therapy for MRSA is a glycopeptide, vancomycin. New antibiotics have been recently released that add to the armamentarium for therapy against MRSA and include linezolid, and quinupristin/dalfopristin, but cost, side effects, and resistance may limit their long term usefulness.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Characterization of a chromosomal gene encoding type B beta-lactamase in phage group II isolates of Staphylococcus aureus

In contrast to most Staphylococcus aureus isolates in which the gene for staphylococcal beta-lactamase (blaZ) is plasmid borne, isolates typeable by group II bacteriophages frequently carry blaZ on the chromosome. Furthermore, the chromosomal gene encodes the type B variant of staphylococcal beta-lactamase for which the nucleotide and deduced amino acid sequences have not yet been reported. To better understand beta-lactamase production among phage group II staphylococci and the nature of the type B beta-lactamase, we determined the type and amount of enzyme produced by 24 phage group II isolates. Of these isolates, 1 did not produce beta-lactamase, 8 produced the type B enzyme, and 15 produced the type C enzyme. In all eight type B beta-lactamase-producing isolates, blaZ was located on the chromosome. This was in contrast to the type C beta-lactamase-producing isolates, in which blaZ was located on a 21-kb plasmid. The nucleotide sequence corresponding to the leader peptide and the N-terminal 85% of the mature exoenzyme form of type B S. aureus was determined. The deduced amino acid sequence revealed 3 residues in the leader peptide and 12 residues in the exoenzyme portion of the beta-lactamase that differ from the prototypic type A beta-lactamase sequence. These include the serine-to-asparagine change at residue 216 found in the kinetically similar type C enzyme, a threonine-to-lysine change at residue 128 close to the SDN loop (residues 130 to 132), and several substitutions not found in any of the other staphylococcal beta-lactamases. In summary, modern isolates of S. aureus typeable by group II phages produce type B or type C staphylococcal beta-lactamase. The type B gene resides on the chromosome and has a sequence that, when compared to the sequences of the other staphylococcal beta-lactamases, corresponds well with its kinetic properties.

Trimethoprim resistance and susceptibility genes in Staphylococcus epidermidis

Genes encoding trimethoprim (TMP)-resistant and -susceptible dihydrofolate reductases (DHFR) in Staphylococcus epidermidis isolated in Saitama Prefecture were compared with the TMP-resistant DHFR gene of S. aureus, dfrA. The nucleotide sequences of TMPr and TMPs genes in five S. epidermidis isolates tested could be divided into three types: type 1, identical with the TMPr gene dfrA that had been found in S. aureus; type 3, identical with the TMPs gene dfrC in S. epidermidis; and type 2, having only two nucleotide substitutions to dfrC with no amino acid change. TMPr isolates carried either one of the type 2 or type 3 sequences in addition to the type 1 sequence. A Southern hybridization analysis revealed that, in TMPr S. epidermidis, the type 1 sequence was located on a 5.5 kb EcoRI-EcoRV restriction fragment together with the sequence for the gentamicin (GM)-resistant gene, while the type 2 or type 3 sequence was located on the 1.0 kb EcoRI-EcoRV fragment. No plasmid-carrying dfrA-homologous sequence was detected in the S. epidermidis isolates we tested. These results suggest that the TMPr and GMr genes are closely linked and located on the chromosome in S. epidermidis isolated in Japan.

Determination of the chromosomal relationship between mecA and gyrA in methicillin-resistant coagulase-negative staphylococci

mecA, the gene that mediates methicillin resistance, and its accompanying mec locus DNA, insert near the gyrA gene in Staphylococcus aureus. To investigate whether there is a similar relationship between mecA and gyrA in coagulase-negative staphylococci (CNS), mecA- and gyrA-specific DNA fragments were used to probe methicillin-resistant isolates of Staphylococcus epidermidis (MRSE) (n = 11) and Staphylococcus haemolyticus (MRSH) (n = 11). The gyrA probe hybridized to the same SmaI DNA fragment as the mecA probe in all strains tested. However, since the size of the SmaI fragments containing mecA and gyrA varied from 73 to 600 kb, the distance between the two genes was determined more precisely. Cloned mecA or gyrA fragments plus vector sequences each containing a SmaI site were introduced into the chromosome of three isolates each of MRSE and methicillin-resistant S. aureus (MRSA), and the sizes of the generated SmaI fragments were determined by pulsed-field gel electrophoresis. The distance between gyrA and mecA was found to be between 38 and 42 kb in both MRSE and MRSA, and the two genes were in the same relative orientation in all strains. Restriction fragment length polymorphism (RFLP) patterns around the gyrA gene in CNS were identical, but species specific, for all 10 MRSE and 10 MRSH isolates examined. In contrast, 8 of 11 methicillin-susceptible S. epidermidis isolates and 7 of 7 methicillin-susceptible S. haemolyticus isolates had different gyrA RFLP patterns. These data show that mecA is site and orientation specific, relative to gyrA, in both MRSE and MRSA. In addition, the local environment around gyrA in methicillin-resistant CNS, in contrast to methicillin-susceptible isolates, is similar, suggesting clonality or the requirement for specific DNA sequences with which the mec complex must interact for chromosomal integration to occur.

Antimicrobial resistance in staphylococci. Epidemiology, molecular mechanisms, and clinical relevance

Staphylococcal infections continue to pose important clinical problems in children and adults. Antibiotic resistance among the staphylococci has rendered therapy of these infections a therapeutic challenge. Despite early, uniform susceptibility to penicillin, staphylococci acquired a gene elaborating beta-lactamase that rendered penicillin inactive and that is borne by nearly all clinical isolates. "Penicillinase-resistant beta-lactams," such as methicillin, were introduced in the early 1960s, but resistance to them has become an increasing concern. The mechanism of the so-called "methicillin resistance" is complex. Moreover, once confined to the ecology of hospitals and other institutions, a recent increase in community-acquired methicillin-resistant S. aureus infections has been observed. Glycopeptides, until now the only uniformly reliable therapeutic modality, have been increasingly used for therapy of staphylococcal infections. The recent recognition of clinical isolates with reduced susceptibility to glycopeptides is of concern.

A single amino acid substitution in Staphylococcus aureus dihydrofolate reductase determines trimethoprim resistance

A single amino acid substitution, Phe98 to Tyr98, in dihydrofolate reductase (DHFR) is the molecular origin of trimethoprim (TMP) resistance in Staphylococcus aureus. This active site amino acid substitution was found in all S. aureus TMP-resistant clinical isolates tested. In order to explore the structural role of Tyr98 in TMP-resistance the ternary complexes of the chromosomal S. aureus DHFR (SaDHFR) with methotrexate (MTX) and TMP in the presence of nicotinamide adenine dinucleotide phosphate (NADPH) as well as that of mutant Phe98Tyr DHFR SaDHFR(F98Y) ternary folate-NADPH complex have been determined by X-ray crystallography. Critical evidence concerning the resistance mechanism has also been provided by NMR spectral analyses of 15N-labelled TMP in the ternary complexes of both wild-type and mutant enzyme. These studies show that the mutation results in loss of a hydrogen bond between the 4-amino group of TMP and the carbonyl oxygen of Leu5. This mechanism of resistance is predominant in both transferable plasmid-encoded and non-transferable chromosomally encoded resistance. Knowledge of the resistance mechanism at a molecular level could help in the design of antibacterials active against multi-resistant Staphylococcus aureus (MRSA), one of todays most serious problems in clinical infectology.

Molecular analysis of a mobilizable theta-mode trimethoprim resistance plasmid from coagulase-negative staphylococci

The Staphylococcus epidermidis plasmid pSK639 is the prototype of a newly described family of small plasmids identified in clinical staphylococcal isolates. pSK639 is 8 kb in length and possesses a composite structure consisting of an IS257-flanked segment mediating trimethoprim resistance (Tpr), and regions responsible for replication and mobilization of the plasmid. Comparative sequence analysis suggests that a pSK639-like plasmid may represent a progenitor of previously identified staphylococcal Tpr determinants related to the transposon-like structure, Tn4003. In contrast to the small staphylococcal plasmids characterized to date that all utilize a rolling circle mode of replication, the replication region of pSK639 was found to contain features typical of an iteron-controlled theta-mode replicon. pSK639 is the first small plasmid of this type to be identified in the staphylococci.

Structure-function relationships among wild-type variants of Staphylococcus aureus beta-lactamase: importance of amino acids 128 and 216

beta-Lactamases inactivate penicillin and cephalosporin antibiotics by hydrolysis of the beta-lactam ring and are an important mechanism of resistance for many bacterial pathogens. Four wild-type variants of Staphylococcus aureus beta-lactamase, designated A, B, C, and D, have been identified. Although distinguishable kinetically, they differ in primary structure by only a few amino acids. Using the reported sequences of the A, C, and D enzymes along with crystallographic data about the structure of the type A enzyme to identify amino acid differences located close to the active site, we hypothesized that these differences might explain the kinetic heterogeneity of the wild-type beta-lactamases. To test this hypothesis, genes encoding the type A, C, and D beta-lactamases were modified by site-directed mutagenesis, yielding mutant enzymes with single amino acid substitutions. The substitution of asparagine for serine at residue 216 of type A beta-lactamase resulted in a kinetic profile indistinguishable from that of type C beta-lactamase, whereas the substitution of serine for asparagine at the same site in the type C enzyme produced a kinetic type A mutant. Similar bidirectional substitutions identified the threonine-to-alanine difference at residue 128 as being responsible for the kinetic differences between the type A and D enzymes. Neither residue 216 nor 128 has previously been shown to be kinetically important among serine-active-site beta-lactamases.

Cloning and characterization of a novel, plasmid-encoded trimethoprim-resistant dihydrofolate reductase from Staphylococcus haemolyticus MUR313

In recent years resistance to the antibacterial agent trimethoprim (Tmp) has become more widespread, and several trimethoprim-resistant (Tmpr) dihydrofolate reductases (DHFRs) have been described from gram-negative bacteria. In staphylococci, only one Tmpr DHFR has been described, the type S1 DHFR, which is encoded by the dfrA gene found on transposon Tn4003. In order to investigate the coincidence of high-level Tmp resistance and the presence of dfrA, we analyzed the DNAs from various Tmpr staphylococci for the presence of dfrA sequences by PCR with primers specific for the thyE-dfrA genes from Tn4003. We found that 30 or 33 isolates highly resistant to Tmp (MICs, > or = 512 micrograms/ml) contained dfrA sequences, whereas among the Tmpr (MICs, < or = 256 micrograms/ml) and Tmps isolates only the Staphylococcus epidermidis isolates (both Tmpr and Tmps) seemed to contain the dfrA gene. Furthermore, we have cloned and characterized a novel, plasmid-encoded Tmpr DHFR from Staphylococcus haemolyticus MUR313. The dfrD gene of plasmid pABU17 is preceded by two putative Shine-Dalgarno sequences potentially allowing for the start of translation at two triplets separated by nine nucleotides. The predicted protein of 166 amino acids, designated S2DHFR, encoded by the longer open reading frame was overproduced in Escherichia coli, purified, and characterized. The molecular size of the recombinant S2DHFR was determined by ion spray mass spectrometry to be 19,821.2 +/- 2 Da, which is in agreement with the theoretical value of 19,822 Da. In addition, the recombinant S2DHFR was shown to exhibit DHFR activity and to be highly resistant to Tmp.

Antimicrobial resistance in staphylococci

Staphylococci have developed a variety of strategies for dealing with the presence of antibiotics encountered in clinical environments. Resistance to beta-lactams and other antimicrobial agents has been accomplished by a diverse array of molecular mechanisms. Options available to treat infections caused by staphylococci resistant to methicillin are limited, and the next generation of antibiotics to be introduced, should glycopeptide resistance become an important clinical problem, is not yet on the horizon.

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.

Possible role of insertion sequence IS257 in dissemination and expression of high- and low-level trimethoprim resistance in staphylococci

The transposon-like structure Tn4003 and related elements were found to encode high- and low-level trimethoprim resistance (Tpr) in Staphylococcus aureus and coagulase-negative staphylococci. By using transcriptional fusions in Escherichia coli, the variation in resistance levels was found to correlate with the transcriptional activity of the region presumed to carry the promoter for the operon containing the Tpr dihydrofolate reductase gene, dfrA, encoded by these elements. The reduced transcriptional activities exhibited by elements encoding low-level Tpr appear to be a consequence of deletions adjacent to the copy of IS257 which normally encodes the -35 sequences of these promoters. The data obtained not only support the involvement of IS257 in the transcription of the proposed thyE-dfrA-orf-140 operon of Tn4003 but may also implicate this insertion sequence in the mechanisms resulting in the variation in Tpr levels observed in staphylococci.

Hospital-acquired infections: diseases with increasingly limited therapies

About 5% of patients admitted to acute-care hospitals acquire nosocomial infections. A variety of factors contribute: increasing age of patients; availability, for treatment of formerly untreatable diseases, of extensive surgical and intensive medical therapies; and frequent use of antimicrobial drugs capable of selecting a resistant microbial flora. Nosocomial infections due to resistant organisms have been a problem ever since infections due to penicillinase-producing Staphylococcus aureus were noted within a few years of the introduction of penicillin. By the 1960s aerobic Gram-negative bacilli had assumed increasing importance as nosocomial pathogens, and many strains were resistant to available antimicrobials. During the 1980s the principal organisms causing nosocomial bloodstream infections were coagulase-negative staphylococci, aerobic Gram-negative bacilli, S. aureus, Candida spp., and Enterococcus spp. Coagulase-negative staphylococci and S. aureus are often methicillin-resistant, requiring parenteral use of vancomycin. Prevalence of vancomycin resistance among enterococcal isolates from patients in intensive care units has increased, likely due to increased use of this drug. Plasmid-mediated gentamicin resistance in up to 50% of enterococcal isolates, along with enhanced penicillin resistance in some strains, leaves few therapeutic options. The emergence of Enterobacteriaceae with chromosomal or plasmid-encoded extended spectrum beta-lactamases presents a world-wide problem of resistance to third generation cephalosporins. Control of these infections rests on (i) monitoring infections with such resistant organisms in an ongoing fashion, (ii) prompt institution of barrier precautions when infected or colonized patients are identified, and (iii) appropriate use of antimicrobials through implementation of antibiotic control programs.

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.

Phage typing of coagulase-negative staphylococci

Seventy-nine staphylococcal strains isolated from blood cultures (57 coagulase-negative staphylococci (CNS) and 22 S. aureus) and 308 CNS isolated from the skin of healthy donors were phage typed. S. epidermidis and S. capitis were readily typed with 91 strains out of 124 and 24 strains out of 43 strains being successful. Species such as S. haemolyticus, S. hominis and S. simulans could be moderately phage typed. Others gave only a few strains capable of being typed, such as S. saprophyticus and S. sciuri. Under our experimental conditions the S. warneri, S. xylosus and S. cohnii could not be typed with our set of phages.

Conjugative transfer genes in staphylococcal isolates from the United States

Staphylococcus aureus and coagulase-negative staphylococcal isolates from various geographic areas in the United States were examined by using a conjugative transfer gene DNA probe in dot-blot hybridization assays. Of 175 S. aureus isolates, 47 (27%) hybridized with the probe, while 24 of 208 (11.5%) coagulase-negative staphylococci hybridized. However, among methicillin-resistant S. aureus 52% (45 of 89) were probe positive while only 2% (2 of 86) of methicillin-susceptible S. aureus were probe positive. In contrast, 12.5% (22 of 176) of methicillin-resistant and 6% (2 of 32) of methicillin-susceptible coagulase-negative staphylococci contained transfer genes. All but one of the staphylococci containing transfer genes were resistant to gentamicin; 91.5% of S. aureus and 65% of coagulase-negative staphylococci containing transfer genes transferred gentamicin resistance to a S. aureus recipient. Of the 12 isolates that hybridized with the probe but did not transfer resistance, 10 (6 coagulase-negative staphylococci and 4 S. aureus) carried both gentamicin resistance and conjugative transfer genes on the same plasmid. Of these 10, 6 contained plasmid target fragments of sizes different from that of the probe, suggesting additions or deletions of DNA essential for transfer, while in 4 no such alterations could be detected. In two coagulase-negative staphylococci the entire transfer region was apparently integrated into the chromosome. Thus, staphylococci carrying conjugative transfer genes are widely disseminated in the United States and are usually found in multiresistant isolates on plasmids that also encode gentamicin resistance.

Lack of homology of enterococci which have high-level resistance to trimethoprim with the dfrA gene of Staphylococcus aureus

Multiresistant enterococci were tested for susceptibility to trimethoprim (TMP). Although most enterococci are inhibited by less than or equal to 1.0 microgram/ml, the MICs for 7 of 29 selected multiresistant isolates were greater than or equal to 8 micrograms/ml, including for two beta-lactamase positive (Bla+) strains, for which the MICs of TMP were greater than 1,000 micrograms/ml, and for another Bla+ strain, for which the MIC was 128 micrograms/ml. None of five isolates tested transferred TMP resistance and none of the resistant isolates hybridized to the dfrA gene of Staphylococcus aureus. Whether TMP resistance in enterococci is due to a mutation(s) or to acquisition of a new gene is not known. Acquisition of resistance to TMP is another example of the multiple antimicrobial resistance typically displayed by enterococci.

Detection of methicillin resistance in staphylococci by using a DNA probe

A DNA probe derived from the PBP 2a gene of the methicillin-resistant Staphylococcus aureus COL was compared with phenotypic microbiologic tests for its ability to identify methicillin-resistant and -susceptible staphylococci. Lysates were applied to nitrocellulose with a dot blot apparatus. Isolates tested were both S. aureus and coagulase-negative staphylococci that had been recovered from a variety of geographic and clinical sources. When compared with a spread plate phenotypic test, the DNA probe gave sensitivity, specificity, and predictive values for both positive and negative tests of 100% for 204 S. aureus isolates (103 positive, 101 negative) and 99, 95, 99, and 95%, respectively, for 249 coagulase-negative staphylococci (210 positive, 39 negative). The probe was more sensitive than broth microdilution and more specific than agar dilution in identifying methicillin-resistant and -susceptible coagulase-negative staphylococci; all tests were equally accurate in identifying the methicillin susceptibility of S. aureus. DNA probe analysis for determining the methicillin susceptibility of staphylococci was rapid, easily interpretable, and equally accurate with radioactive and nonradioactive probes, and it gave results equivalent to the most sensitive microbiologic test for all staphylococcus species studied.

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.

Antimicrobial resistance in nosocomial isolates of Staphylococcus haemolyticus

Staphylococcus haemolyticus is frequently cultured from hospitalized patients and is characterized by resistance to multiple antimicrobial agents. We found that S. haemolyticus represented 70 of 524 (13%) coagulase-negative staphylococcal isolates identified by the clinical microbiology laboratories of two hospitals over 2 months. S. haemolyticus isolates were recovered from wounds (44%), urine (26%), blood (10%), and other sources (20%). All S. haemolyticus isolates were tested for susceptibility to six antimicrobial agents; 77% were resistant to three or more agents, and 41% were resistant to five or six agents. In addition, among 47 multiply resistant isolates, high MICs (greater than or equal to 6.25 micrograms/ml) of vancomycin (62% of isolates) and teicoplanin (91% of isolates) were found. DNA probes which were derived from S. epidermidis or S. aureus and which contained sequences associated with resistance to antimicrobial agents were used to detect specific genes in the total cellular and plasmid DNAs of 10 resistant S. haemolyticus isolates. Resistance gene probes and the numbers of resistant isolates hybridizing were as follows: methicillin, 10 of 10; gentamicin, 9 of 10; erythromycin, 7 of 10; and trimethoprim, 0 of 10. Genes for resistance to methicillin were found only in chromosomal locations, genes for resistance to gentamicin were found in both chromosomal and plasmid locations, and genes for resistance to erythromycin were found in plasmid locations only. With the exception of trimethoprim resistance determinants, similar genes were found among concurrently isolated multiply resistant S. epidermidis isolates from our hospitals. S. haemolyticus is a potentially important nosocomial species which readily acquires antimicrobial resistance genes and which shares, to some extent, in a common gene pool with S. epidermidis.

Low-level methicillin resistance in strains of Staphylococcus aureus

Two strains of Staphylococcus aureus expressing borderline or low-level methicillin resistance by one or more in vitro test methods were examined for resistance in vivo and for biochemical and genetic markers of methicillin resistance. In vivo, nafcillin was equally effective against experimental aortic valve endocarditis in rabbits, regardless of whether they were infected by a fully susceptible or a low-level-resistant strain. Resistance did not emerge during therapy. For the more resistant of the two low-level-resistant strains, methicillin was as effective as nafcillin. Nafcillin was ineffective against endocarditis caused by a truly methicillin-resistant strain, and resistance emerged on therapy. The low-level-resistant strains did not produce the low-affinity penicillin-binding protein 2a that is associated with methicillin resistance and did not contain DNA that hybridized with probes that recognized the methicillin resistance determinant. Low-level resistance in S. aureus is a phenomenon that is biochemically and genetically distinct from true methicillin resistance. These strains actually are susceptible to beta-lactam antibiotics. The clinical problem posed by these strains is not a therapeutic one but, instead, one of how to differentiate them from those that are truly methicillin resistant.

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.

Trimethoprim resistance transposon Tn4003 from Staphylococcus aureus encodes genes for a dihydrofolate reductase and thymidylate synthetase flanked by three copies of IS257

Trimethoprim resistance mediated by the Staphylococcus aureus multi-resistance plasmid pSK1 is encoded by a structure with characteristics of a composite transposon which we have designated Tn4003. Nucleotide sequence analysis of Tn4003 revealed it to be 4717 bp in length and to contain three copies of the insertion element IS257 (789-790 bp), the outside two of which are flanked by directly repeated 8-bp target sequences. IS257 has imperfect terminal inverted repeats of 27-28 bp and encodes for a putative transposase with two potential alpha-helix-turn-alpha-helix DNA recognition motifs. IS257 shares sequence similarities with members of the IS15 family of insertion sequences from Gram-negative bacteria and with ISS1 from Streptococcus lactis. The central region of the transposon contains the dfrA gene that specifies the S1 dihydrofolate reductase (DHFR) responsible for trimethoprim resistance. The S1 enzyme shows sequence homology with type I and V trimethoprim-resistant DHFRs from Gram-negative bacteria and with chromosomally encoded DHFRs from Gram-positive and Gram-negative bacteria. 5' to dfrA is a thymidylate synthetase gene, designated thyE.

N-terminal amino acid sequence of the chromosomal dihydrofolate reductase purified from trimethoprim-resistant Staphylococcus aureus

The existence of two distinct dihydrofolate reductases (DHFR) in highly trimethoprim-resistant clinical isolates has been unequivocally demonstrated. The enzymes have been characterized with regard to the affinity for substrates and sensitivity to inhibitors. The chromosomal, trimethoprim-sensitive DHFR was purified to homogeneity by a new simple two-step procedure. Its N-terminal amino acid sequence, determined up to the first 35 amino acids, showed 69% homology with the Escherichia coli DHFR.

Laboratory, clinical, and epidemiological aspects of coagulase-negative staphylococci

Coagulase-negative staphylococci, particularly Staphylococcus epidermidis, are increasingly important causes of nosocomial infection. Microbiologists and clinicians no longer can afford to disregard clinical isolates of coagulase-negative staphylococci as contaminants. Accurate species identification and antimicrobial susceptibility testing, in a clinically relevant time frame, are important aids in the diagnosis and management of serious coagulase-negative staphylococcal infections. Emphasis in the clinical laboratory should be placed on the routine identification of S. epidermidis and Staphylococcus saprophyticus, with identification of other species of coagulase-negative staphylococci as clinically indicated. The application of newer techniques, such as plasmid analysis and tests for slime production and adherence, contribute to our understanding of the epidemiology and pathogenesis of coagulase-negative staphylococci and may also be helpful in establishing the diagnosis of infection.