Gordon L. Archer

Staphylococcus aureus: A Well-Armed Pathogen

Staphylococcus aureus is a virulent pathogen that is currently the most common cause of infections in hospitalized patients. S. aureus infection can involve any organ system. The success of S. aureus as a pathogen and its ability to cause such a wide range of infections are the result of its extensive virulence factors. The increase in the resistance of this virulent pathogen to antibacterial agents, coupled with its increasing prevalence as a nosocomial pathogen, is of major concern. The core resistance phenotype that seems to be most associated with the persistence of S. aureus in the hospital is methicillin resistance. Methicillin resistance in nosocomial S. aureus isolates has been increasing dramatically in United States hospitals and is also associated with resistance to other useful antistaphylococcal compounds. Possible ways to decrease the incidence of nosocomial S. aureus infections include instituting more effective infection control, decreasing nasal colonization, developing vaccines, and developing new or improved antimicrobials.

Staphylococcus epidermidis Causing Prosthetic Valve Endocarditis: Microbiologic and Clinical Observations as Guides to Therapy

Seventy-five episodes of prosthetic valve endocarditis from Staphylococcus epidermidis were studied retrospectively. Methicillin-resistant isolates caused 53 (87%) of 61 infections occurring within 1 year of surgery but only two of the nine after 1 year (p less than 0.001). Resistance to methicillin was heterogeneic and extended to the cephalosporins. Of 55 isolates, 43 (78%) were susceptible to gentamicin and all to vancomycin and rifampin. In 55 patients, prosthetic valve endocarditis was complicated by tissue invasion or valve dysfunction. Among these 55 patients, 30 of the 32 who were cured needed surgery. Prosthetic valve endocarditis from methicillin-resistant S. epidermidis was cured in 21 of 26 patients treated with vancomycin and 10 of 20 treated with beta-lactam antibiotic therapy (p = 0.055). Cure rates of patients treated with vancomycin but not beta-lactam antibiotics were increased by the addition of rifampin or gentamicin to therapy. Prosthetic valve endocarditis from methicillin-resistant S. epidermidis should be treated with vancomycin plus rifampin, or an aminoglycoside. Surgical intervention is important in treating complications of prosthetic valve endocarditis.

Related Clones Containing SCCmec Type IV Predominate among Clinically Significant Staphylococcus epidermidis Isolates

ABSTRACT SCCmec is a mobile genetic element that carries the gene (mecA) mediating methicillin resistance in staphylococci. For Staphylococcus aureus, four SCCmec types have been described, one (type IV) of which has been associated with newly identified community-acquired methicillin-resistant S. aureus. However, the distribution of SCCmec types among S. epidermidis is not known. SCCmec typing of a collection of 44 methicillin-resistant Staphylococcus epidermidis (MRSE) isolates recovered between 1973 and 1983 from the blood of patients with prosthetic valve endocarditis (PVE) was performed by PCR amplification of key genetic elements (mecA, mecI, IS1272, and ccrAB). Of the 44 isolates, 1 (2%) harbored SCCmec type I, 15 (34%) harbored type II, 12 (28%) harbored type III, and 16 (36%) harbored type IV. The complete nucleotide sequence of SCCmec type IV was determined for 16 isolates and found to be identical in size (24 kb) and 98% homologous to DNA sequences published for S. aureus. Type IV SCCmec was also common (5 of 10 isolates) among a geographically dispersed collection of 10 recent (1998 to 2001) S. epidermidis bloodstream isolates. Multilocus sequence typing (MLST) (using the same seven genes presently employed for S. aureus MLST) of these MRSE isolates and of 10 additional recent geographically dispersed methicillin-susceptible isolates demonstrated that all 16 PVE isolates and 2 of 5 recent isolates harboring type IV SCCmec were in three related clonal groups. All three MSSE PVE isolates recovered from patients between 1976 and 1979 were in the same clonal groups as type IV SCCmec MRSE isolates. These data support the hypothesis of intra- and interspecies transfer of type IV SCCmec and suggest that there are clonal associations in S. epidermidis that correlate with SCCmec type.

Origin and evolution of DNA associated with resistance to methicillin in staphylococci

The gene mediating resistance to methicillin in staphylococci (mecA) and its flanking sequences (mec DNA) make up a chromosomal DNA locus that is unique to methicillin-resistant bacteria; no equivalent locus exists in methicillin-susceptible cells. The origin of mec DNA is not known, but evidence supports horizontal transfer of mec DNA between different staphylococcal species and of the mecA gene between different Gram-positive genera.

Improved Multilocus Sequence Typing Scheme for Staphylococcus epidermidis

ABSTRACT We evaluated three multilocus sequence typing (MLST) schemes for Staphylococcus epidermidis and selected the seven most discriminatory loci for the formation of a new, more powerful MLST scheme. This improved scheme gave 31 sequence types (STs) and 5 clonal complexes (CCs), whereas the other schemes delineate 16 to 24 STs and 1 to 3 CCs.

Antimicrobial Susceptibility and Selection of Resistance Among Staphylococcus epidermidis Isolates Recovered from Patients with Infections of Indwelling Foreign Devices

Twenty-seven isolates of Staphylococcus epidermidis from patients with prosthetic valve endocarditis or infected cerebrospinal fluid shunts were examined for susceptibility to antimicrobial agents. Subpopulations resistant to 20 and 100 μg of methicillin per ml were present in 63% of the isolates (methicillin-resistant isolates). Subpopulations resistant to 20 μg of nafcillin and cephalothin per ml were found in every methicillin-resistant isolate but with frequencies (10−5.0 ± 0.5 and 10−6.4 ± 0.9, respectively) which were not always detectable by susceptibility testing. Resistance to ≥1.6 μg of penicillin per ml was found in 80% of isolates. Cephalothin, cefazolin, and cefamandole were more active than cefoxitin or cephradine, and gentamicin was more active than tobramycin or amikacin; rifampin was the single most active agent against all isolates. There was no difference in susceptibility between prosthetic valve endocarditis and cerebrospinal fluid shunt infection isolates. Among methicillin-resistant isolates, the phenotypic expression of resistance to methicillin or nafcillin but not to cephalothin could be enhanced by 48 h of incubation with each drug. Isolates containing no methicillin-resistant subpopulations were killed by incubation with methicillin, nafcillin, or cephalothin. High-level resistance to rifampin emerged in both methicillin-resistant and methicillin-sensitive isolates after 8 to 24 h of incubation with this drug. The presence or absence of antibiotic-resistant subpopulations among S. epidermidis isolates and their selection during treatment should be considered when therapy is devised.

Potential Associations between Hematogenous Complications and Bacterial Genotype in Staphylococcus aureus Infection

BACKGROUND The impact of bacterial clonality on infections caused by Staphylococcus aureus is unclear. METHODS Three hundred seventy-nine S. aureus isolates (125 methicillin-resistant S. aureus [MRSA] and 254 methicillin-susceptible S. aureus [MSSA]) were genotyped by spa typing and multilocus sequence typing. For MRSA isolates, the staphylococcal chromosomal cassette mec (SCCmec) element was also typed. Three clinical categories were identified: nasal carriage only (n=118), uncomplicated infection (n=104), and bacteremia with hematogenous complications (n=157). RESULTS By use of eBURST, 18 clonal complexes (CCs) were found in 371 isolates. Eight CCs accounted for 89% of isolates and occurred in all clinical categories. CC5 (P=.0025) and CC30 (P=.0308) exhibited a significant trend toward more frequent hematogenous complications. Isolates within spa types 2 and 16 showed the same significant trend and grouped within CC5 and CC30, respectively. SCCmec II isolates also showed the same significant trend compared with SCCmec IV; 96% were CC5 or CC30. CONCLUSIONS Although most S. aureus genotypes exhibited the capacity to cause invasive disease, strains within CC5 and CC30 exhibited a significant trend toward increasing levels of hematogenous complications. Isolates within these CCs were also implicated by use of spa and SCCmec typing. The genetic determinants underlying these findings remain to be demonstrated.

Transcription of the gene mediating methicillin resistance in Staphylococcus aureus (mecA) is corepressed but not coinduced by cognate mecA and beta-lactamase regulators.

Resistance to beta-lactam antibiotics in staphylococci is mediated by mecA and blaZ, genes encoding a penicillin-binding protein (PBP2a) with low beta-lactam affinity and beta-lactamase, respectively. The mec and bla regulators, mecR1-mecI and blaR1-blaI, respectively, encode inducer-repressors with sufficient amino acid homology to suggest that they could coregulate PBP2a production. In order to test this hypothesis, plasmids containing mec and bla regulatory sequences were introduced into Staphylococcus aureus containing a chromosomal mecA-lacZ transcriptional fusion. Corepression was confirmed by demonstrating a gene dosage-dependent reduction in beta-galactosidase activity by either MecI or BlaI and additive repression when both were present. Both MecI-MecI and BlaI-BlaI homodimer and MecI-BlaI heterodimer interactions were demonstrated in the yeast two-hybrid assay, and purified MecI and BlaI protected the same mec promoter-operator sequences. However, MecI was approximately threefold more effective at mecA-lacZ transcriptional repression than was BlaI. While MecI and BlaI displayed similar activity as repressors of mecA transcription, there was a marked difference between MecR1 and BlaR1 in the rate and specificity of induction. Induction through BlaR1 by a beta-lactam was 10-fold greater than through MecR1 at 60 min and was 81% of maximal by 2 h, while induction through MecR1 never exceeded 20% of maximal. Furthermore, complementation studies showed that MecI- or BlaI-mediated mecA transcriptional repression could be relieved by induction through homologous but not heterologous sensor-inducer proteins, demonstrating the repressor specificity of induction.

Native valve endocarditis due to coagulase-negative staphylococci: Clinical and microbiologic features

Twenty-one patients with native valve endocarditis caused by coagulase-negative staphylococci were studied; 14 had pre-existing valvular or congenital heart disease. Although commonly subacute in presentation, complications of endocarditis were frequent: arterial emboli in five patients, new electrocardiographic conduction system abnormalities in nine, congestive heart failure in eight, annular or myocardial abscesses in five, and disruption of valve leaflets in three. Cures were achieved in 10 of 12 patients treated medically and seven of nine treated surgically. In microbiologic studies of 16 coagulase-negative staphylococci from patients with endocarditis, only eight were identified as Staphylococcus epidermidis. All isolates were susceptible to vancomycin. Antibiotic resistance (methicillin, four isolates; gentamicin, two isolates; rifampin, one isolate) was usually associated with nosocomial acquisition of endocarditis. Rather than representing contamination, coagulase-negative staphylococci in blood cultures may indicate life-threatening endocarditis. However, with careful attention to the selection of antibiotics for therapy and to the occurrence of heart failure due to intracardiac complications, treatment of this form of endocarditis is frequently successful. Organisms must always be tested for cryptic resistance to beta-lactam antibiotics. Valve replacement may be required frequently.

Microarray Transcription Analysis of Clinical Staphylococcus aureus Isolates Resistant to Vancomycin

The transcriptomes of vancomycin intermediate-resistance Staphylococcus aureus (VISA) clinical isolates HIP5827 and Mu50 (MIC = 8 micro g/ml) were compared to those of highly vancomycin-resistant S. aureus (VRSA; MIC = 32 micro g/ml) passage derivatives by microarray. There were 35 genes with increased transcription and 16 genes with decreased transcription in common between the two VRSAs compared to those of their VISA parents. Of the 35 genes with increased transcription, 15 involved purine biosynthesis or transport, and the regulator (purR) of the major purine biosynthetic operon (purE-purD) was mutant. We hypothesize that increased energy (ATP) is required to generate the thicker cell walls that characterize resistant mutants.