Methicillin-resistant S. aureus and S. epidermidis strains: modern hospital pathogens

Staphylococci

Staphylococci are among the leading causes of nosocomial infections. Increasing insusceptibility to beta-lactams and the glycopeptides complicates treatment of these infections. This review examines the current status and future perspectives for the therapy of infections caused by Staphylococcus aureus and coagulase-negative staphylococci.

Photodynamic effects of novel XF porphyrin derivatives on prokaryotic and eukaryotic cells

The worldwide rise in the rates of antibiotic resistance of bacteria underlines the need for alternative antibacterial agents. A promising approach to the killing of gram-positive antibiotic-resistant bacteria of the skin uses light in combination with a photosensitizer to induce a phototoxic reaction. Different concentrations (0 to 100 microM) of porphyrin-based photosensitizers (CTP1, XF70, and XF73) and different incubation times (5 min, 1 h, and 4 h) were used to determine phototoxicity against two methicillin-resistant Staphylococcus aureus strains, one methicillin-sensitive S. aureus strain, one methicillin-resistant Staphylococcus epidermidis strain, one Escherichia coli strain, and human keratinocytes and fibroblasts. Incubation with 0.005 microM XF70 or XF73, followed by illumination, yielded a 3-log10 (> or = 99.9%) decrease in the viable cell numbers of all staphylococcal strains, indicating that the XF drugs have high degrees of potency against gram-positive bacteria and also that the activities of these novel drugs are independent of the antibiotic resistance pattern of the staphylococci examined. CTP1 was less potent against the staphylococci under the same conditions. At 0.005 microM, XF70 and XF73 demonstrated no toxicity toward fibroblasts or keratinocytes. No inactivation of E. coli was detected at this concentration. XF73 was confirmed to act via a reactive oxygen species from the results of studies with sodium azide (a quencher of singlet oxygen), which reduced the killing of both eukaryotic and prokaryotic cells. When a quencher of superoxide anion and the hydroxyl radical was used, cell killing was not inhibited. These results demonstrate that the porphyrin-based photosensitizers had concentration-dependent differences in their efficacies of killing of methicillin-resistant staphylococcal strains via reactive oxygen species without harming eukaryotic cells at the same concentrations.

Effect of hospitalization and antimicrobial therapy on antimicrobial resistance of colonizing Staphylococcus epidermidis

Endogenous infections with multi-resistant S. epidermidis are among the leading causes of nosocomial infections. The effect of hospitalization and antimicrobial therapy on antimicrobial resistance of colonizing staphylococci was determined from swabs of the nose, hand, axilla and groin from 157 patients on one day. Hospitalization for >72 hours, compared with <72 hours, was associated with a higher percentage of isolates resistant to oxacillin (56% versus 19%), gentamicin (40% versus 15%), trimethoprim (36% versus 17%), clindamycin (56% versus 17%), and fusidic acid (20% versus 4%; p < 0.01 for all), but not to rifampicin (6% versus 1%) or fosfomycin (43% versus 34%, p > 0.05 for both). Concurrent antimicrobial therapy resulted in increased resistance to oxacillin (61% versus 28%), gentamicin (43% versus 20%), and clindamycin (60% versus 26%; p < 0.01 for all), but not to trimethoprim (39% versus 23%), fusidic acid (19% versus 9%), rifampicin (6% versus 3%), or fosfomycin (46% versus 38%, p > 0.05 for all). The increase in resistant isolates was not independent, since hospitalization and antimicrobial therapy were correlated (p < 0.001). After adjustment for potential risk factors such as diabetes mellitus, central venous catheters, and hemodialysis, the odds ratio for oxacillin resistance was 2.8-3.6. None of the risk factors showed statistically significant results, except for the presence of neoplastic disease, which had a significant interaction (P=0.035). The within-subgroup odds ratios for patients with and without neoplasm were 4.2 (95% CI, 2.3-5.7) and 2.1 (95% CI, 0.78-3.12), respectively. These results show that hospitalization for more than three days, with or without antimicrobial therapy, and the presence of neoplastic disease are associated with increased antimicrobial resistance in colonizing S. epidermidis.

Epidemiology of methicillin-resistant staphylococci in Europe

Methicillin-resistant staphylococci are mostly resistant not only to all beta-lactams but also to a wide range of other antibiotics, and have emerged as major nosocomial pathogens during the past two decades. Considerable variations in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) exist between institutions and between geographic areas. In Europe, in general, a north-south gradient is observed, MRSA strains being rare in Scandinavian hospitals (<2%) and far more prevalent in Mediterranean hospitals (>40%). Whether low or high, the rates of MRSA prevalence in European countries have remained approximately the same during the last decade. Recent findings suggest that MRSA might also be emerging as a community-acquired pathogen. The first stage in the emergence of MRSA is its acquisition by methicillin-susceptible S. aureus, and the integration into its chromosome, of the mecA gene, which, together with the other mec genes, is carried on a mobile genetic element, the staphylococcal chromosomal cassette mec (SCCmec). The origin of SCCmec elements as well as the mechanisms of their acquisition remain unknown. Molecular epidemiology studies using different techniques clearly indicate that the massive geographic spread of MRSA results from the dissemination of relatively few highly epidemic clones. Five major lineages (the so-called Iberian, Brazilian, Hungarian, New York/Japan and pediatric pandemic MRSA clones) have been defined. In Europe, the Iberian clone has been reported in several countries; the Brazilian, pediatric and Hungarian clones have also been detected, but less frequently. A unique Italian clone is predominant in Italy. As with S. aureus, coagulase-negative staphylococci (CNS) represent a serious concern in hospital-acquired infections. Despite marked geographic variations, in some areas of Europe high proportions (60-70%) of CNS are methicillin resistant. The formation of biofilm is a key virulence factor of S. epidermidis, the prominent CNS pathogen, which is the most common cause of bacteremia in device-related infections. Another emerging nosocomial pathogen, S. hemolyticus, is characterized by a tendency to develop multiple antibiotic resistances, with a unique predisposition to glycopeptide resistance.

Evaluation of Staf-Sistem 18-R for identification of staphylococcal clinical isolates to the species level

The accuracy and efficiency of Staf-Sistem 18-R (Liofilchem s.r.l., Roseto degli Abruzzi, Teramo, Italy) were compared with those of conventional biochemical methods to identify 523 strains belonging to 16 different human Staphylococcus species. Overall, 491 strains (93.9%) were correctly identified (percentage of identification, > or = 90.0), with 28 (5.4%) requiring supplementary tests for complete identification. For 14 isolates (2.8%), the strains did not correspond to any key in the codebook and could not be identified by the manufacturer's computer service. Only 18 isolates (3.4%) were misidentified. The system is simple to use, is easy to handle, gives highly reproducible results, and is inexpensive. With the inclusion of more discriminating tests and adjustment in supplementary code numbers for some species, such as Staphylococcus lugdunensis and Staphylococcus schleiferi, Staf-Sistem 18-R is a suitable alternative for identification of human coagulase-positive and coagulase-negative Staphylococcus species in microbiological laboratories.

Evaluation of two commercial systems for identification of coagulase-negative staphylococci to species level

Bloodstream (224) and urine (nine) isolates of coagulase-negative staphylococci (CNS) were evaluated using MicroScan Pos ID and Rapid Pos ID panels. A modification of the conventional method of Kloos and Schleifer served as the reference method. The isolates were selected to include a broad range of CNS species, including 44 S. epidermidis, 50 S. hominis, 39 S. warneri, 33 S. capitis, 21 S. haemolyticus, 12 S. simulans, 11 S. saprophyticus, six S. cohnii, five S. lugdunensis, three S. xylosus, four S. auricularis, two S. schleiferi, two S. intermedius, and one S. sciuri. The Pos ID panel had an overall rate of agreement (correct plus probably correct) with the reference method of 79%, including 95% for S. epidermidis, 95% for S. haemolyticus, 64% for S. hominis, 67% for S. simulans, 79% for S. warneri, and 100% for S. saprophyticus. The Rapid Pos ID panel had an overall rate of agreement (correct plus probably correct) of 76%, including 91% for S. epidermidis, 90% for S. haemolyticus, 64% for S. hominis, 58% for S. simulans, 77% for S. warneri, and 100% for S. saprophyticus. Both systems are acceptable for the identification of the clinically significant species S. haemolyticus, S. saprophyticus, and S. epidermidis, but are less reliable for the infrequently isolated species of CNS.

Randomized double-blinded trial of rifampin with either novobiocin or trimethoprim-sulfamethoxazole against methicillin-resistant Staphylococcus aureus colonization: prevention of antimicrobial resistance and effect of host factors on outcome

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen in hospitals. Current antimicrobial regimens for eradicating colonizing strains are not well defined and are often complicated by the emergence of resistance. The combination of novobiocin plus rifampin in vitro and in vivo was found to prevent the emergence of resistant populations of initially susceptible strains of MRSA, particularly resistance to rifampin. We therefore studied, in a randomized, double-blind, multicenter comparative trial, the combination of novobiocin plus rifampin versus trimethoprim-sulfamethoxazole (T/S) plus rifampin in order to determine the efficacy of each regimen in eradicating MRSA colonization and to further characterize the host factors involved in the response to this antimicrobial therapy. Among the 126 individuals enrolled in the study, 94 (80 patients; 14 hospital personnel) were evaluable. Among the 94 evaluable subjects, no significant demographic or medical differences existed between the two treatment groups. Successful clearance of the colonizing MRSA strains was achieved in 30 of 45 (67%) subjects receiving novobiocin plus rifampin, whereas successful clearance was achieved in 26 of 49 (53%) subjects treated with T/S plus rifampin (P = 0.18). The emergence of resistance to rifampin developed more frequently in 14% (7 of 49) of subjects treated with T/S plus rifampin than in 2% (1 of 45) of subjects treated with novobiocin plus rifampin (P = 0.04). Restriction endonuclease studies of large plasmid DNA demonstrated that the same strain was present at pretherapy and posttherapy in most refractory cases (24 of 29 [83%] subjects). Among the 56 successfully treated subjects, clearance of MRSA was age dependent: 29 of 36 (80%) subjects in the 18- to 49-year-old age group, 19 of 35 (54%) subjects in the 50- to 69-year-old age group, and 8 of 23 (35%) in the 70- to 94-year-old age group (P < 0.01). Clearance was also site dependent; culture-positive samples from wounds were related to a successful outcome in only 22 (48%) of 46 subjects, whereas culture-positive samples from sites other than wounds (e.g., nares, rectum, and sputum) were associated with a success rate of 34 of 48 (71%) subjects (P = 0.02). Foreign bodies in wounds did not prevent the eradication of MRSA by either regimen. T/S plus rifampin was less effective in clearing both pressure and other wounds, whereas novobiocin plus rifampin was equally effective in clearing both pressure and other wounds. There were no significant differences in toxicity between the two regimens. Thus, the combination of novobiocin plus rifampin, in comparison with T/S plus rifampin, was more effective in preventing the emergence of resistance to rifampin and demonstrated a trend toward greater activity in clearing the MRSA carrier state. The response to either combination depended on host factors, particularly age and the site of MRSA colonization.

Evaluation of MicroScan Rapid Pos Combo panels for identification of staphylococci

MicroScan Rapid Pos Combo panels (Baxter Diagnostics, Inc., MicroScan, West Sacramento, Calif.) contain substrates conjugated with fluorophores and substrates with a fluorescent pH indicator. AutoSCAn W/A, an automated panel processor equipped with a fluorometer, reads the panels after 2 h of incubation and can identify staphylococci to the species level. We tested 239 strains belonging to 17 species of staphylococci. All the strains were identified by conventional methods (W.E. Kloos and K.H. Schleifer, J. Clin. Microbiol. 1:82-88, 1975) and by the MicroScan Rapid ID system. The system correctly identified 219 (91.6%) strains; nine (3.8%) identification results were probably correct, and six (2.5%) results were incorrect. The system designated five (2.1%) strains as rare biotypes. The automated MicroScan Rapid ID system is useful and reliable in identifying most human isolates of staphylococci encountered in the clinical laboratory.

The impact of converting to a biphasic blood-culture system on the overall cost and the incidence of pseudobacteremia

After converting from a conventional broth (CB) system to a biphasic (BP) agar-slide blood-culture system (Septi-Chek), our laboratory noted an increase in positive blood cultures in general, and in coagulase-negative staphylococci (CNS) in particular. To investigate these findings, we compared all blood cultures collected over a 21-month period using CB and then BP systems, totaling 28,199 blood cultures. The frequency of positive blood cultures increased from 9.2% to 12.7% (p less than 0.0001), whereas CNS isolation increased from 2.6% to 5.2% (p less than 0.0001). There was no significant change in the incidence of true primary or secondary bacteremia due to CNS (p = 0.9). The isolation of other pathogens, including Staphylococcus aureus, Candida albicans, Bacteroides species, and Gram-negative bacilli increased from 6.5% to 7.1% (p less than 0.05). We estimated the cost of processing 28,000 blood cultures by both CB and BP systems, using positivity rates of 9.2% and 12.7%, respectively, and standards provided by the College of American Pathologists (CAP, 1991) for workload hours of technologist time. We calculated a higher overall cost for the BP system. However, the use of this system eliminated the use of needles and syringes for subculture of bottles showing no growth, thus decreasing the risk of technologist exposure to body fluids. Despite the increased cost and more frequent occurrence of pseudobacteremia, the enhanced sensitivity and increased safety of the BP system justified its use in the prompt identification of patients with true bacteremia.

Morbidity and mortality of an endemic pathogen: methicillin-resistant Staphylococcus aureus

Over an 8-year period, two epidemics of methicillin-resistant Staphylococcus aureus (MRSA) occurred in a burn unit. Sources of sepsis were the burn wound and lung. Fourteen percent of the patients colonized with MRSA became bacteremic. The mean postburn day of bacteremia was 19 and the mortality rate was 5 percent. MRSA was introduced to the burn unit when a patient was transferred from another unit, on readmission of a previously infected patient, or heavy burn census when MRSA was epidemic in the hospital. Although the morbidity rate associated with MRSA infections was high, the mortality rate was low. Gram-negative sepsis has continued to be more lethal.

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.