Patients with SSIs are five times more likely to be readmitted to hospital, 60% more likely to be admitted to the intensive care unit, twice as likely to die, be hospitalised for up to seven days more, and cost three times more to treat as non- infected patients.
In the USA, the Centers for Disease Prevention and Control (CDC) estimates that more than 27 million surgical procedures are performed annually, with approximately 290,000 SSIs and 8000 patient deaths being associated with SSI infections.
Across Europe, over 29 million surgical procedures are performed each year, with around 2.6% of patients developing an SSI during their recovery in hospital. However, an estimated 40–60% of SSI cases are thought to be preventable. Recently, in the USA, the Centers for Medicare and Medicaid Services created a list of hospital-acquired conditions that are non-reimbursable because they were deemed preventable, including a range of SSIs. Swift identification of SSI is therefore desirable to enable healthcare professionals to initiate appropriate patient care quickly and decisively, enabling better patient outcomes and reducing costs for the healthcare provider.
Resistant microorganisms
Most microorganisms have the potential to cause SSI, but resistant organisms pose a particular threat. Methicillin-resistant Staphylococcus aureus (MRSA) is a primary cause; however, there is increasing concern worldwide about vancomycin-resistant enterococci (VRE) and extended-spectrum beta-lactamase-producing (ESBL) organisms. The causative organism has to be identified as quickly as possible, along with an accurate assessment of its resistance status, to ensure an effective treatment programme can be implemented.
Resistance screening
The Brilliance range of chromogenic agars is now being adopted widely as the media of choice by laboratories around the world. Unlike some other rapid methods for the identification of resistant organisms that require expensive or specialised equipment, chromogenic culture media can be adopted easily by clinical laboratories of any size, without capital investment. Brilliance media can be supplied in prepoured agar plates that are ready to inoculate and give easy-to-read results within 24 hours or less. Speed, convenience and ease of use make chromogenic media of enormous value as routine screening tools for significant resistant microorganisms. Rapid and reliable results allow infection control procedures to be initiated in a timely manner and ensure that patients receive the most appropriate treatment at the earliest opportunity.
Methicillin-resistant Staphylococcus aureus
Oxoid Brilliance MRSA agar is a chromogenic medium that detects the phosphatase activity of MRSA. When cleaved, the chromogen produces distinctive denim blue colonies that are easily visible (Fig 1).
Antibacterial agents in the medium inhibit the growth of competing organisms, including methicillin-sensitive S. aureus (MSSA), while additional compounds suppress the expression of phosphatase activity in other staphylococci. These properties result in a highly selective medium that demonstrates excellent sensitivity and specificity and allows results to be obtained in as little as 18 hours, which is up to six hours earlier than alternative chromogenic media.
Oxoid Brilliance MRSA agar demonstrates high positive and negative predictive values (98.1% and 99.2%, respectively), rivalling those claimed by many molecular techniques (eg polymerase chain reaction [PCR] methods) and yet achieving them at a fraction of the cost. Good positive and negative predictive values are essential in hospitals where patient isolation facilities are in short supply. In such circumstances, it may be necessary to cohort MRSA-positive patients.
False-positive results could lead to MRSA-negative patients being put at increased risk of infection by prolonged stays in MRSA-cohorted wards, as well as the unnecessary and improper use of `last resort’ antibiotics. False-negative results could prevent an MRSA-positive patient from being isolated and receiving appropriate treatment, putting other patients at risk of infection.
In a comparative trial, it was concluded that the excellent selectivity of Oxoid Brilliance MRSA agar meant that fewer confirmatory tests were required, making it a reliable and economical option for clinical laboratories.
Rapid detection of ESBL-production
Oxoid Brilliance ESBL Agar is a chromogenic medium for the rapid (within 24 hours), presumptive identification of ESBL-producing microorganisms directly from clinical samples. Not only does this medium detect ESBL production, but it is also able to distinguish ESBL-producing Escherichia coli (eg CTX-M) and ESBL production in the KESC (Klebsiella, Enterobacter, Serratia and Citrobacter) group of bacteria.
This differentiation is achieved using two chromogenic compounds, one which targets the galactosidase activity of Escherichia coli and KESC bacteria, and another that detects the glucuronidase activity of E. coli. This results in green KESC colonies and blue E. coli colonies, which are easily distinguished on Brilliance ESBL agar plates (Fig 2).
Occasionally, E. coli will be galactosidase-negative, but these organisms produce pink colonies. Proteus, Morganella and Providencia do not utilise either chromogen, but produce tan-coloured colonies with a brown halo due to the deamination of tryptophan. The semi-opaque background of the medium contrasts with the brightly coloured colonies and allows clear and easy identification of target organisms.
Cefpodoxime and additional antibacterial agents in the medium inhibit non-ESBL-producing Enterobacteriaceae and suppress the growth of other non-ESBL flora, including Stenotrophomonas maltophilia. This reduces the incidence of false-positive results, compared to traditional culture media, and minimises the need for confirmatory testing.
An evaluation of Brilliance ESBL agar was performed using a variety of isolates, including CTX-M, TEM, SHV and K1-hyperproducing strains, from clinical and other sources. Results indicated that K1-hyperproducing (non-ESBL) strains were inhibited while all representative ESBL strains grew.
Enhanced VRE detection
Brilliance VRE agar is a new chromogenic screening plate for the detection of VRE. This medium provides presumptive identification of VRE (vancomycin-resistant Enterococcus faecium and E. faecalis) from faecal samples, swabs, isolates or suspensions, within just 24 hours.
Differentiation between these two VRE species is achieved using two chromogenic compounds – one that targets phosphatase activity (present in both species) and another that targets alpha-galactosidase activity (only present in E. faecium). E. faecalis colonies appear light blue and are easily distinguished from the indigo/purple colonies of E. faecium (Fig. 3).
The growth of competing flora, including E. gallinarum and E. casseliflavus (both of which are intrinsically resistant to vancomycin), is suppressed by the inclusion of vancomycin and additional antibiotics in the medium. Brilliance VRE agar was evaluated in a clinical trial, using a panel of 120 clinical isolates. The medium demonstrated a sensitivity of 94.7% and 100% at 24 hours and 48 hours, respectively.
Minimum inhibitory concentration testing made easy
Oxoid M.I.C.Evaluator (M.I.C.E.) strips combine the simplicity and ease of use of the diffusion method with the accuracy of a minimum inhibitory concentration (MIC) test, to provide important information for the treatment of SSIs.
M.I.C.E. strips carry a stabilised antimicrobial compound, covering 15 doubling dilutions, on the polymer strip. The distinctive scale format provides an excellent contrast with the agar and the increased font size makes reading easier.
The strips are firm but flexible, making them easy to handle when applying them to pre-inoculated agar plates. Upon application, the antimicrobial agent is released from the M.I.C.E. strip, forming a defined concentration gradient in the surrounding agar.
After appropriate incubation, a lawn of growth develops with a clear zone around the M.I.C.E. strip where the concentration gradient in the medium is sufficient to inhibit growth. The MIC is easily read where the interface between zone and growth of the organism touches the strip.
Individually foil wrapped with desiccant to maintain integrity until use, M.I.C.E. strips minimise wastage and are available in stackable boxes of 10 and 50 strips. M.I.C.E. strips are available for a wide selection of antibiotics, including teicoplanin, in the concentration range 0.015–256 µg/mL. Specialist high- and low-level concentration strips are also available.
Confirmation and differentiation of MRSA and MRCoNS
The Oxoid PBP2’ latex agglutination test differentiates MRSA and methicillin-resistant coagulase-negative staphylococci (MRCoNS) from borderline MRSA and modified MRSA, identifying only true MRSA and MRCoNS, allowing prescription of vancomycin only when appropriate.
The test is easy to use and results (that require no expert interpretation) are available within 15 minutes, which is 24 hours earlier than by conventional methods. No further confirmation steps are required. The test is an accurate and cost-effective alternative to expensive and labour-intensive molecular methods (Fig 4). High sensitivity and specificity have been demonstrated in worldwide evaluations, and the test conforms to Clinical and Laboratory Standards Institute (CLSI) guidelines for the confirmation of oxacillin resistance.
RapID identification
Oxoid RapID test panels provide accurate and reliable identification of a broad range of SSI-associated microorganisms in just four hours (as little as two hours for certain organisms). Each panel incorporates a carefully selected series of biochemical tests directed towards a particular group of organisms (Fig 5).
Unlike some alternative methods, the RapID range does not depend on the growth of the organisms but detects specific constitutive enzymes. As a result, every panel can be incubated aerobically, without the need for an oil overlay, saving both time and materials. The special inoculation method is especially easy to use and allows each test well to be inoculated simultaneously, simplifying the procedure and allowing further time savings.
The RapID range is complemented by Windows-based Electronic RapID Compendium (ERIC) software. This powerful package ranks organism identification by probability. Greater than 95% probability is required for identification, ensuring excellent accuracy and reliability, and accessory tests and methods are offered to resolve any identification overlaps. Collectively, the extensive ERIC databases (one for each RapID panel) provide identifications for around 400 different microorganisms. Identification reports can be saved and printed, with reporting options that allow the user to track identification patterns and organism trends.
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