Identification and molecular characterization of mannitol salt positive, coagulase-negative staphylococci from nasal samples of medical personnel and students

The identification of mannitol salt positive, coagulase-negative staphylococci (CNS) is often disregarded when Staphylococcus aureus is screened in clinical samples using mannitol salt agar. However, the emergence of CNS as important human pathogens has indicated that reliable methods for the identification of clinically significant CNS are of great importance in understanding the epidemiology of infections caused by them. The identification and molecular characterization of mannitol salt positive CNS from nasal samples of medical personnel and students is reported here. A total of 84 mannitol salt positive staphylococcal isolates were obtained from 240 nasal samples, of which 15 were CNS. The API STAPH system classified the CNS isolates into six species, and one-third of the isolates were identified with confidence levels of <80 %. 16S-23S rRNA intergenic spacer length polymorphism analysis (ITS-PCR) identified only two species (Staphylococcus haemolyticus and Staphylococcus saprophyticus). This identification was confirmed by antibiotyping, species-specific PCR and PFGE. The results from this study indicate that ITS-PCR is a potentially useful and reliable tool, enabling hospital laboratories to obtain rapid, full and accurate identification of CNS at the species level.

Comparison of genotypic and phenotypic methods for species-level identification of clinical isolates of coagulase-negative staphylococci

To compare commonly used phenotypic methods with genotypic identification methods 47 clinical isolates of coagulase-negative staphylococci (CONS), 10 CONS ATCC strains, and a Staphylococcus aureus clinical isolate were identified using the API Staph ID test, BD Phoenix Automated Microbiology System, and 16S rRNA gene and tuf gene sequencing. When necessary part of the sodA gene was sequenced for definitive identification. The results show that tuf gene sequencing is the best method for identification of CONS, but the API Staph ID test is a reasonably reliable phenotypic alternative. The performance of the BD Phoenix Automated Microbiology System for identification of CONS is poor. The present study also showed that although genotypic methods are clearly superior to phenotypic identifications, a drawback of sequence-based genotypic methods may be a lack of quality of deposited sequences in data banks. In particular, 16S rRNA gene sequencing suffers from the lack of high quality among sequences deposited in GenBank. Furthermore, genotypic identification based on 16S rRNA sequences has limited discriminating power for closely related Staphylococcus species. We propose partial sequencing of the tuf gene as a reliable and reproducible method for identification of CONS species.

Isolation and molecular characterization of multiresistant Staphylococcus sciuri and Staphylococcus haemolyticus associated with skin and soft-tissue infections

The isolation, molecular identification and genotyping of multiresistant Staphylococcus sciuri and Staphylococcus haemolyticus from skin and soft-tissue infections are reported. Accurate and full identification of three coagulase-negative staphylococcal isolates was achieved using PCR, while the API STAPH method failed to identify an isolate of S. haemolyticus fully. The PCR assay, which detects polymorphism in the 16S-23S rRNA spacer region, is shown to be potentially useful for rapid and accurate identification of coagulase-negative staphylococci. Identical PFGE type and antibiotic-resistance profiles of two methicillin-resistant S. haemolyticus isolates in this study suggest the existence of a multiresistant community clone.

rpoB gene sequence-based identification of Staphylococcus species

The complete sequence of rpoB, the gene encoding the beta subunit of RNA polymerase was determined for Staphylococcus saccharolyticus, Staphylococcus lugdunensis, S taphylococcus caprae, and Staphylococcus intermedius and partial sequences were obtained for an additional 27 Staphylococcus species. The complete rpoB sequences varied in length from 3,452 to 3,845 bp and had a 36.8 to 39.2% GC content. The partial sequences had 71.6 to 93.6% interspecies homology and exhibited a 0.08 to 0.8% intraspecific divergence. With a few exceptions, the phylogenetic relationships inferred from the partial rpoB sequences were in agreement with those previously derived from DNA-DNA hybridization studies and analyses of 16S ribosomal DNA gene sequences and partial HSP60 gene sequences. The staphylococcal rpoB sequence database we established enabled us to develop a molecular method for identifying Staphylococcus isolates by PCR followed by direct sequencing of the 751-bp amplicon. In blind tests, this method correctly identified 10 Staphylococcus isolates, and no positive results were obtained with 10 non-Staphylococcus gram-positive and gram-negative bacterial isolates. We propose partial sequencing of the rpoB gene as a new tool for the accurate identification of Staphylococcus isolates.

Molecular tracking of coagulase-negative staphylococcal isolates from catheter-related infections

Three molecular typing methods (pulsed-field electrophoresis, localization of the mecA gene, and probing the vicinity of mec) have been used for the characterization of 40 catheter-related isolates of coagulase-negative staphylococci (CNS) in 14 patients admitted to the same hospital. The 40 isolates yielded 14 different SmaI banding patterns and corresponding unique localizations of mecA, each associated with a unique ClaI mecA polymorph. In 6 of the 14 patients the contaminated skin at the catheter entry site was the source of 4 local infections and 2 cases of bacteremia. A contaminated hub was the origin of 2 local infections and 4 cases of bacteremia in 6 more patients. The remaining 2 patients had positive cultures from both skin and catheter hub. In each bacteremic patient, the CNS recovered from catheter-related sites (tip, skin, and/or hub) and the CNS recovered from blood were identical, but each of these matching isolates was unique to the particular patients, indicating a low rate of cross-infection from patient to patient. Although classical methods for typing CNS (e.g., biotype and antibiotype) are readily available for most hospital laboratories, they have limitations concerning reproducibility and discriminatory power. Molecular epidemiologic techniques can provide powerful support to traditional techniques in determining the etiologic role of CNS in the disease process.

Reproducibility of antibiogram of bovine mammary gland staphylococci under conditions of repeated subculturing

The reproducibility of antibiogram profiles of 10 staphylococcal isolates of bovine mammary gland origin was tested under conditions of repeated subculturing. Prototype (original or index) antibiogram profiles were determined by subculturing these isolates from stock cultures stored at -70 degrees C. The isolates were then subcultured four times on blood agar and antibiogram profiles determined at each subculture on Mueller-Hinton agar. The antibiogram profiles of each isolate at each subculture were compared with the prototype profiles of that isolate. At repeat antibiogram determinations, deviations of < or = 5 mm in the individual zones of inhibition of penicillin, ampicillin and streptomycin from the prototype antibiogram profiles, resulted in a shift of only three isolates from the resistant to intermediate and one isolate from intermediate to resistant status of antibiotic susceptibility classification. It is suggested that in the interpretation of antibiogram, susceptibility classification (resistant, intermediate, or susceptible) variations accruing from a few millimetres differences in the diameter of the zones of inhibition should probably be disregarded.

Comparison of an improved RAPD fingerprinting with different typing methods for discriminating clinical isolates of Staphylococcus spp

Different epidemiological markers were used to characterize 2 Staphylococcus epidermidis and 8 Staphylococcus aureus strains isolated from patients with severe infections. We compared random amplified polymorphic DNA (RAPD) fingerprints, biotypes, antibiotic assays, plasmid profiles and chromosomal DNA restriction endonuclease analysis (REA). Data analysis based on numerical taxonomy methods indicates that RAPD and REA give similar results allowing a good discrimination of the two species and of each isolate. The RAPD method is easier and faster than REA, but the reproducibility of RAPD fingerprints obtained in independent experiments can be problematic. We have found simple technical devices to improve the reproducibility of the RAPD procedure which is therefore a very useful tool in epidemiology for identification and characterization of Staphylococcus spp.

Randomly amplified polymorphic DNA analysis provides rapid differentiation of methicillin-resistant coagulase-negative staphylococcus bacteremia isolates in pediatric hospital

Coagulase-negative staphylococci (CoNS) are now recognized as the most common cause of nosocomial bacteremia in pediatric patients. Randomly amplified polymorphic DNA analysis was used to study the relationships among 12 isolates of CoNS obtained from eight patients with catheter-related bacteremia in two distinct wards of our hospital and 6 epidemiologically unrelated strains. With this method, we were able to discriminate isolates that otherwise were indistinguishable by conventional criteria such as biochemical typing and antibiotic susceptibility patterns. Our results indicate that there were episodes of cross-infections among four patients in one ward but independent infectious episodes among four patients in the other ward. Randomly amplified polymorphic DNA analysis is a rapid method which seems particularly well suited to the epidemiological study of CoNS isolates.

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.

Efficacy of microbial identification system for epidemiologic typing of coagulase-negative staphylococci

The lack of an adequate typing system hampers our understanding of the epidemiology of infections caused by coagulase-negative staphylococci (CoNS). CoNS have become recognized as important nosocomial pathogens and the principal cause of infections associated with invasive devices. Sensitive, specific, and convenient methods are needed to evaluate whether implementing infection control guidelines reduces the risk of nosocomial infections from CoNS and other pathogens. The Microbial Identification System (MIDI) (Microbial ID Inc., Newark, Del.), a semiautomated system for fatty acid methyl ester analysis, shows considerable promise for clinical and epidemiologic applications. Its predictive accuracy and reliability were tested by using epidemiologically related and replicated CoNS isolates as well as CoNS from epidemiologically unrelated clinical infections, which were obtained from five established hospital culture collections in diverse geographic locations. Two hundred isolates were fully characterized in 5 days by one person using MIDI, and the results were similar to those produced by more expensive and time-consuming conventional typing methods. MIDI appears to be a useful screening tool that could be used before more expensive and labor-intensive molecular methods. It offers important advantages to hospital epidemiologists and clinical microbiologists who must identify and type CoNS isolates.