S. aureus is the most clinically significant species of staphylococci. It is responsible for a number of infections
both relatively mild and life-threatening. S. aureus can be recovered from almost any clinical specimen and is an important cause of nosocomial infections.
Increasing drug resistance is an important concern with this common isolate.
The pathogenicity associated with S. aureus can be attributed to a number of virulence factors, such as enterotoxins, cytolytic toxins, and cellular components such as protein A. Five cytolytic toxins and two exfoliative toxins have been identified.
Staphylococcal enterotoxins are heat-stable exotoxins that cause diarrhea and vomiting in humans. Eight serologically distinct enterotoxins (A-E and G-I) have been identified. These toxins are produced by 30 ‘X,
to 50% of S. aureus isolates. Because the enterotoxins are stable at 100° C for 30 minutes, reheating contaminated food will not prevent disease. Enterotoxins Band C and sometimes G and I are associated with TSS.
Enterotoxin B has been linked to staphylococcal pseudomembranous enterocolitis, although the mechanism is not understood. These toxins, along with toxic shock syndrome toxin-l (fSST-l), are superantigens having the ability to activate a strong overreactive immune response.
Toxic Shock Syndrome Toxin
TSST-l causes nearly all cases of menstruatingassociated TSS. Previously referred to as enterotoxin F, this chromosomal-mediated toxin is also associated with approximately 50’X, of the nonmenstruatingassociated TSS cases. TSST-l is a superantigen stimulating T cell proliferation and the subsequent production of a large concentration of cytokines that are responsible for the symptoms. At a low concentration, TSST-l causes leakage by endothelial cells, and at higher concentration it is cytotoxic to these cells. TSST-l is absorbed through vaginal mucosa permitting the systemic effects seen in TSS.
Produced by phage group II, exfoliative toxin is also known as epidermolytic toxin. It causes the epidermal layer of the skin to slough off and is known to cause staphylococcal SSS, sometimes referred to as Ritter’s disease. This toxin has also been implicated in bullous impetigo.
S. Qureus produces other extracellular proteins that affect red blood cells (RBCs) and leukocytes. These hemolysins and leukocidins are cytolytic toxins with properties different from those of previously described toxins. S. Qureusproduces four hemolysins: alpha, beta, gamma, and delta. a-Hemolysin, in addition to lysing erythrocytes, can damage platelets, macrophages and cause severe tissue damage. f}-Hemolysin (sphingomyelinase C) acts on sphingomyelin in the plasma membrane of erythrocytes and is also called the “hotcold” lysin. The “hot-cold” feature associated with this toxin is seen as an enhanced hemolytic activity on incubation at 37° Cand subsequent exposure to cold (4° C). This hemolysin is exhibited in the CAMP (Christie, Atkins, and Munch-Petersen) test performed in the laboratory to identify group B streptococci. &-Hemolysin, although found in a higher percentage of S. Qureusstains and some coagulase negative staphylococci, is considered less toxic to cell structure than either a- or B- hemolysins. y-Hemolysin is often only found associated with Panton-Valentine leukocidin (PVL).
Staphylococcal leukocidin, PVL, is an exotoxin that is lethal to polymorphonuclear leukocytes. It has been implicated as contributing to the invasiveness of the organism by suppressing phagocytosis and has been associated with severe cutaneous infections and necrotizing pneumonia. Although produced by relatively few strains of S. Qureus, it has been associated with some cases of community-acquired staphylococcal infections.
Several enzymes are produced by staphylococci. Examples are coagulase, protease, hyaluronidase, and lipase.
Staphylocoagulase is produced mainly by S. Qureus. Although the exact role of coagulase in pathogenicity remains uncertain, it is considered a virulence marker.
Many strains of S. Qureus produce hyaluronidase. This enzyme hydrolyzes hyaluronic acid present in the intracellular ground substance that makes up connective tissues, permitting the easy spread of bacteria during infection. Lipases are produced by both coagulasepositive and coagulase-negative staphylococci. Lipases act on lipids present on the surface of the skin, particularly fats and oil secreted by the sebaceous glands.
Protease, lipase, and hyaluronidase are capable of destroying tissue and may facilitate the spread of infection to adjoining tissues.
Protein A is one of several cellular components that have been identified in the cell wall of S. aureus. Probably the most significant role of protein A in infections caused by S. aureus is its ability to bind the Fe portion of immunoglobulin G(IgG). Binding IgGin this manner can block phagocytosis.
The primary reservoir for staphylococci is the nares, with colonization also occurring in the axillae, vagina, pharynx, and other skin surfaces. Nasal carriage in patients admitted to the hospital is common. Because close contact among patients and hospital personnel is not unusual, transfer of organisms often takes place.
Consequently, increased colonization in patients and hospital workers frequently occurs. Hospital outbreaks may develop in nurseries, burn units, and among patients who have undergone surgery or other invasive procedures. Transmission of S. aureus may occur by direct contact with unwashed contaminated hands and by inanimate objects (fomites). Both hospital and community-acquired infections caused by drugresistant S. aureus have increased in the past 20 years.
Infections Caused by Staphylococcus au reus
As with most infections, the development of staphylococcal infection is determined by the virulence of the strain, size of the inoculum, and status of the host’s immune system. Infections are initiated when a breach of the skin or mucosal barrier allows staphylococci access to adjoining tissues or the bloodstream. Any event that compromises the host’s ability to resist infection encourages colonization and infection. Individuals with normal defense mechanisms are able to combat the infection more easily than those with impaired immune systems. Once the organism has crossed the initial barriers, it activates the host’s acute inflammatory response, which leads to the proliferation and activation of polymorphonuclear cells.
However, the organisms are able to resist the action of inflammatory cells by the production of toxins and enzymes, thereby establishing a focal lesion.
Skin and Wound Infections
Infections caused by S. aureus are suppurative. Typically the abscess is filled with pus and surrounded by necrotic tissues and damaged leukocytes. Some of the common skin infections caused by S. aureus are folliculitis, furuncIes, carbuncles, and bullous impetigo. These opportunistic infections usually occur as a result of previous skin injuries such as cuts, burns, and surgical wounds. Folliculitis is a relatively mild inflammation of a hair follicle or oil gland; the infected area is raised and red. Furuncles (boils), which can be an extension of folliculitis, are large, raised, superficial abscesses. Carbuncles occur when larger, more invasive lesions develop from multiple furuncles, which may progress into deeper tissues. Unlike furuncles, patients with carbuncles often present with fever and chills, indicating systemic spread of the bacteria.
Bullous impetigo caused by S. aureus is different from streptococcal non bullous impetigo in that staphylococcal pustules are larger and surrounded by a small zone of erythema. Bullous impetigo is a highly contagious infection that is easily spread by direct contact,
fomites, or autoinoculation.
Staphylococcal infections can also be secondary to skin diseases of different etiologies. Dry, irritated skin combined with poor personal hygiene encourages the development of infection. Some of these infections are manifested because of increased colonization of the organisms in blocked hair follicles, sebaceous glands, and sweat glands. Immunocompromised individualsparticularly those who are receiving chemotherapy, are debilitated by chronic illnesses, or have invasive devices-are predisposed to developing staphylococcal infections.
Scalded Skin Syndrome
Scalded skin syndrome (SSS), or Ritter’s disease, is an extensive exfoliative dermatitis that occurs primarily in newborns and previously healthy young children.
This syndrome is caused by staphylococcal exfoliative or epidermolytic toxin produced by S. aureus phage group II, which is probably present at a lesion distant from the site of exfoliation. The disease has also been recognized in adults. Cases of SSS in adults occur most commonly in patients with chronic renal failure and in those with compromised immune systems. Although the mortality rate is low (0% to 7%) in cases seen among children, the rate in adults is as high as 50%.
The severity of the disease varies from being a localized skin lesion in the form of bullous impetigo to a more extensive generalized condition. Bullous impetigo manifests as a localized lesion that contains purulent material. This lesion may progress to the generalized form, which is characterized by cutaneous erythema followed by profuse peeling of the epidermal layer of the skin. The typical pattern in which the erythema occurs is origination from the face, neck, axillae, and groin and then extension to the trunk and extremities. The duration of the disease is brief, about 2 to 4 days. The incidence of spontaneous recovery among children is high.
The toxin is metabolized and excreted by the kidneys. Investigators believe this may be the reason why the incidence of SSS is higher among children younger than 5 years old and among adults with chronic renal failure and impaired immune systems.
Toxic Epidermal Necrolysis
Toxic epidermal necrolysis (fEN) is a clinical manifestation with multiple causes; it is most commonly drug induced, but some cases may have been linked to infections and vaccines. The cause is unknown, but symptoms appear to be due to a hypersensitivity reaction. Although it has a very similar initial presentation to that of SSS, treatments differ. Whereas TEN can be resolved by the administration of steroids early in the initial stages of presentation, steroids aggravate SSS.The mortality rate associated with TEN is high, and administration of suspected offending drugs should be stopped as soon as possible.
Toxic Shock Syndrome
Toxic shock syndrome is a rare but potentially fatal, multisystem disease characterized by high fever, hypotension, and shock. It was first described by Todd in 1978 and was associated with highly absorbent tampon use, although some cases appeared in men, children, and non menstruating women. The two categories of TSS are menstruating-associated and nonmenstruating- associated. Although non menstruating TSS has been associated with nearly any staphylococcal infection, many cases have been seen with postsurgical infections and secondary to influenza virus infections.
During 1979 and 1980, 91% of TSS cases were menstruating associated; during 1987 to 1996, this percentage declined to 59%. The number of TSS cases has decreased from 482 cases being reported to the Centers for Disease Control and Prevention (CDC) in 1984 to only 133 confirmed cases reported in 2003, although underreporting may be occurring because of misdiagnosis or failure to meet the CDC case criteria. Staphylococcal TSS generally results from a localized infection by S. aureus; only the toxin TSST-l is systemic. The initial clinical presentation of TSS consists of high fever, rash, and signs of dehydration, particularly if the patient has had watery diarrhea and vomiting for several days. In extreme cases, patients may be severely hypotensive and in shock. The rash is found predominantly on the trunk but can spread over the entire body.
Laboratory findings include an elevated leukocyte count, with the differential blood count showing an increase in band forms and metamyelocytes. The number of platelets is decreased, and although there is no evidence of bleeding, disseminated intravascular coagulation is likely to occur. The effects of dehydration on the kidneys are manifested by elevations in creatinine S. aureus does not need to be isolated to confirm the diagnosis of TSS. Supportive therapy to replace vascular volume loss is given, along with appropriate antimicrobial therapy. Most patients with TSS recover, although 2%to 5%of the cases may be fatal. Preventive measures, such as the use of minimum absorbency tampons and warning-label requirements by the Food and Drug Administration (FDA) for tampon products have greatly decreased the risk of TSS.
S. aureus enterotoxins, most commonly A and D, have been identified and associated with gastrointestinal disturbances. The source of contamination is usually an infected food handler. Staphylococcal food poisoning is a type of intoxication resulting from ingestion of a preformed toxin. Disease occurs when food becomes contaminated with enterotoxin-producing strains of S. aureus by improper handling and is then improperly stored, which allows growth of the bacteria and resulting toxin production. An individual then ingests the food contaminated with enterotoxin. Foods that are often incriminated in staphylococcal food poisoning include salads, especially those containing mayonnaise and eggs, meat or meat products, poultry, egg products, bakery products with cream fillings, sandwich fillings, and dairy products. Foods that are kept at room temperature are especially susceptible to higher levels of toxin production when contaminated with toxin-producing staphylococci and are more commonly associated with food poisoning. The enterotoxins do not cause any detectable odor or change in the appearance or taste of the food. Symptoms appear rapidly (approximately 2 to 8 hours after ingestion of the food) and resolve within 24 to 48 hours. Although no fever is associated with this condition, nausea, vomiting, abdominal pain, and severe cramping are common.
Diarrhea and headaches may also occur. Death from staphylococcal food poisoning is rare, although such cases have occurred among elderly patients, infants, and severely debilitated persons.
Staphylococcal pneumonia has been known to occur secondary to influenza A virus infection. Although rare, staphylococcal pneumonia has a high mortality rate.
The pneumonia, which develops as either a contiguous, lower respiratory tract infection or a complication of bacteremia, is characterized by multiple abscesses and focal lesions in the pulmonary parenchyma. Infants and immunocompromised patients, such as elderly patients and those receiving chemotherapy or immunosuppressants, are most affected.
Staphylococcal bacteremia leading to secondary pneumonia and endocarditis has been observed among intravenous (IV) drug users. The organisms gain and blood urea nitrogen. Cultures of focal lesions may
yield S. aureus, but blood cultures are usually negative. entrance to the bloodstream via contaminated needles or from a focal lesion present on the skin or in the respiratory or genitourinary tract. Staphylococcal osteomyelitis occurs as a manifestation secondary to bacteremia. The infection develops when the organism is present in a wound or other focus of infection and gains entrance into the blood. Bacteria may lodge in the diaphysis of the long bones and establish an infection. Symptoms include fever, chills, swelling, and pain around the affected area.
Septic arthritis is frequently caused by S. aureus in children, especially with trauma to the extremities, and can occur in patients with a history of rheumatoid arthritis or IVdrug abuse. The organisms mayor may not be recovered from aspirated joint fluid.
The role of S. epidermidis as an etiologic agent of disease has become increasingly evident. Infections caused by S. epidermidis are predominantly hospital acquired. Some of the predisposing factors are instrumentation procedures such as catheterization, medical implantation, and immunosuppressive therapy. S. epidermidis is probably the most common cause of hospital-acquired urinary tract infections CUTis). Prosthetic valve endocarditis is most commonly caused by S. epidermidis, although other coagulase-negative staphylococci such as S. lugdunensis have also been recovered in these cases. S. epidermidis infections have been associated with intravascular catheters, cerebrospinal fluid shunts, and other prosthetic devices. Septicemia has been reported in immunocompromised patients.
Infections associated with the use of implants, such as indwelling catheters and prosthetic devices, are often caused by isolates shown to produce a biofilm. Biofilm production is a key component in bacterial pathogenesis and is a complex interaction between host, indwelling device, and bacteria (see Chapter 31). One of the bacterial factors involved in adherence of S. epidermidis may be poly-gamma-DL-glutamic acid (PGA), which provides a protective advantage against host defenses.
Staphylococcus saprophyticus has been associated with UTls in young, sexually active females. This species adheres more effectively to the epithelial cells lining the urogenital tract than other coagulase-negative staphylococci. It is rarely found on other mucous membranes or skin surfaces. When present in urine cultures, S. saprophytic us may be found in low numbers (d 0,000 colony forming units/mL) and still be considered significant.
Other Coagulase-Negative Staphylococci
Other species of CoNS are found as normal flora in humans and animals. Although they are not commonly seen as pathogens, their role in some infections is well established. Therefore they cannot be automatically discarded as contaminants in all cases.
S. haemolyticus is another commonly isolated CoNS. It has been reported in wounds, bacteremia, endocarditis, and UTls. Of notable interest is the existence of vancomycin resistance in some S. haemolyticus isolates. Other species not commonly seen but that have established themselves as opportunistic pathogens are S. lugdunensis, S. warneri, S. capitis, S. simulans, and S. schleiferi. A wide range of infections have been associated with these organisms, for example, endocarditis, septicemia, and wound infections.
Specimen Collection and Handling
Proper specimen collection, transport, and processing are essential elements in the correct diagnosis and interpretation of any bacterial culture result. Clinical materials collected from infected sites should be transported to the laboratory without delay to prevent drying, maintain the proper environment, and minimize the growth of contaminating organisms.
Although the recovery of staphylococci requires no special procedures, specimens should be taken from the site of infection after appropriate cleansing of the surrounding area to avoid contamination by the skin flora.
Microscopic examination of stained smears prepared directly from clinical samples (Figure 14-2) provides information that is helpful in the early diagnosis and treatment of the infection and should always be performed on appropriate specimens. Numerous grampositive cocci, along with polymorphonuclear cells in purulent exudates, joint fluids, aspirated secretions, and other body fluids, are easily seen when these sites are infected with staphylococci. A culture should be done regardless of the results of the microscopic examination, because the genus or species cannot be appropriately identified by microscopic morphology alone (Figure 14-3). An aspirate is the best sample, whereas a single swab will be less satisfactory for both culture and smear results. Clinicians should consider sending two swabs if both a Gram stain and culture are requested.
Isolation and Identification
Staphylococci grow easily on routine laboratory culture media, particularly sheep blood agar (SBA). A selective medium such as mannitol salt agar, Columbia colistin-nalidixic acid agar (CNA), or
phenyl ethyl alcohol (PEA) agar can be used for heavily contaminated specimens.
Staphylococci produce round, smooth, white, creamy colonies on SBA after 18 to 24 hours of incubation at 35° to 37° C. S. aureus may produce hemolytic zones around the colonies (Figure 14-4)and may exhibit pigment production (yellow) with extended incubation.
S. epidermidis colonies are usually small- to mediumsized,
nonhemolytic, white colonies (Figure 14-5). S. saprophyticus forms slightly larger colonies, with about 50% of the strains producing a yellow pigment.
Identification of staphylococci on the basis of colony morphology alone is not recommended.
Staphylococci have been traditionally differentiated from micrococci on the basis of oxidation-fermentation (O/F) reactions produced in OIF glucose medium.
Staphylococci ferment glucose, whereas micrococci fail to produce acid under anaerobic conditions. However, the OIF tests do not sufficiently discern certain weak acid producers, such as Micrococcuskristinae, and those staphylococci that fail to grow or produce acid anaerobically: S. saprophyticus, S. auricularis, S. hominis, S. xylosus, and S. cohnii. Tests to differentiate micrococci from staphylococci are shown in Table 14-1. A modified
oxidase test such as the Microdase ~isk (Remel, Lenexa, Kan) can be used to differentiate staphylococci from micrococci rapidly. Most staphylococci will be negative, whereas micrococci will be positive. Table 14-3 outlines other key characteristics for differentiating staphylococci from other gram-positive cocci. Many commercial systems have incorporated these traditional biochemical tests. In addition, molecular testing, plasmid typing, and fatty acid analysis have been used for species and strain identification.
S. aureus is often identified by the coagulase tests.
Clumping factor, formerly referred to as cell-bound coagulase, causes agglutination in human, rabbit, or pig plasma and is considered a major marker for S. aureus. Clumping factor on the surface of the bacterial cells directly converts fibrinogen to fibrin, which precipitates onto the cell surface, causing agglutination. Clumping factor is easily detected by the slide method and is used to screen catalase-positive colonies that morphologically resemble S. aureus. A heavy suspension of the suspected organism is prepared on a glass
slide in water or saline and is mixed with a drop of plasma. If agglutination occurs, the isolate can be identified as S. aureus. Because about 5% of S. aureus organisms do not produce clumping factor, any negative slide coagulase test result must be confirmed with the tube method, which detects staphylocoagulase, or free coagulase. Staphylocoagulase is an extracellular molecule that causes a clot to form when bacterial cells are incubated with plasma (Figure 14-6). Staphylocoagulase The clot formed in the tube may have a tendency to undergo autolysis, giving the appearance of a negative result. The clinical laboratory scientist should look for clot formation after 4 hours of incubation at 37° C. If no clot appears the tube should be left at room temperature to incubate overnight and checked the following day. Table 14-4 lists the coagulase-positive staphylococci and identifies their clinical source and significance. The clinical laboratory scientist must be aware that staphylococci other than S. aureus produce bound or free coagulase (Table 14-5).
Isolates that do not produce either clumping factor or staphylocoagulase are reported as coagulasenegative staphylococci or CoNS.Urine isolates that are coagulase negative are further tested to identify presumptively S. saprophyticus. Presumptive identification of S. saprophyticus is accomplished by testing for novobiocin susceptibility using a 5-!-lgnovobiocin disk (Figure 14-7).S. saprophyticus is resistant to novobiocin, but most other coagulase-negative staphylococci are susceptible. Figure 14-8 shows the schema for the identification of clinically significant staphylococci.
Although S. epidermidis and S. saprophytic us are the most clinically significant species of CoNS, other species are becoming more important clinically.
Table 14-5 outlines some key tests for identification of the clinically significant species of Staphylococcus, including coagulase-negative isolates.
Rapid Methods of Identification
Numerous rapid agglutination test kits are on the market for differentiating S. aureus from the CoNS. Among these are the BBLStaphyloslide (BO-BBL,Sparks, Mcl), Seradyn Color Slide (Remel), Staphaurex (Reme!), and the Bacti Staph (Remel) (Figure 14-9).These kits utilize reacts with a thermostable, thrombinlike molecule called coagulase-reacting factor (CRF) to form coagulase-CRF complex. The complex resembles thrombin and indirectly converts fibrinogen to fibrin. plasma-coated carrier particles, such as latex. The plasma detects both clumping factor (with fibrinogen) and protein A in the cell wall of S. aureus (with IgG).
These kits often have a higher specificity and sensitivity than the traditional slide test and are commonly used in clinical laboratories. Users should be aware that some strains of S. saprophytic us, S. sciuri, S. lugdunensis, and Micrococcus spp. may produce positive tests with this method, but they would be tube coagulase negative on further testing. Careful consideration of source, colony morphology, and susceptibility pattern can eliminate most errors.
Although numerous automated and rapid multitest systems for the identification of staphylococci are available, their accuracy varies. Most systems are able to identify S. aureus, most S. epidermidis, and S. saprophytic us strains accurately as well as some of the other staphylococcal species such as S. capitis, S. haemolyticus, S. simulans, and S. intermedius. However, the accuracy varies with less frequently recovered staphylococci.
Routine testing of staphylococcal isolates can be easily performed in the laboratory using standard guidelines issued by Clinical and Laboratory Standards Institute (CLSQ, formally known as National Committee for
Clinical Laboratory Standards (NCCLS). When using commercial tests, laboratories need to adhere to the manufacturers’ recommended procedures.
Testing of CoNS is dependent on source and determination if the isolate is a contaminant or a likely pathogen. Current CLSIguidelines do not require routine reporting of susceptibilities on S. saprophyticus.
Serious infections with S. aureus require susceptibility testing. Because of the production of j}-Iactamases (penicillinases), which break down the f3-lactamring of many penicillins, most S. aureus isolates are resistant to penicillin. Increasing resistance to alternative antimicrobial agents is a major concern.
Penicillin-resistant strains require treatment with penicillinase-resistant penicillins, such as nafcillin or oxacillin. Isolates that are resistant have been traditionally termed methicillin-resistant staphylococci, with S. aureus being called MRSAand S. epidermidis referred to as MRSE.The incidence of MRSAhas been on the rise for the past 20 years. MRSAin hospital-acquired (nosocomial) infections is costly and poses a serious threat to health institutions. Control of MRSArequires strict adherence to infection-control practices such as barrier protection, contact isolation, and handwashing compliance.
In the past MRSA was associated mainly with hospitalized or nursing home patients. Since the late 1990s, however, MRSA has been found associated with community-acquired infections and outbreaks. The use of vancomycin for MRSA remains the treatment of choice, but concerns with rising resistance to glycopeptides
call for the restrictive use of these drugs.
For laboratory purposes, oxacillin is generally used for detection of methicillin resistance. For S. lugdunensis and S. aureus, cefoxitin results are comparable to oxacillin results, and the results may be easier to read.
For testing other coagulase-negative staphylococci, the CLSI MlOO-SI5 document recommends a cefoxitin disk be used as the preferred method of detecting methicillin
resistance. At a minimum, the laboratory should report susceptibilities for penicillin and oxacillin (or cefoxitin). The use of an oxacillin-salt agar plate, such as the Oxacillin Resistance Screening Agar (ORSA;Oxoid, Basingstoke, UK), can be used as a screening test for MRSA in clinical samples. A high salt concentration (5.5% NaCI) and polymixin B make the medium selective for staphylococci. Oxacillin resistance is indicated by the formation of blue colonies via acid formation from the fermentation of mannitol. The pH indicator is aniline blue.
Most oxacillin resistance is due to the gene mecA, which codes for a new penicillin-binding protein (PBP) called PBP2a, also designated PBP2′. Latex agglutination tests are available to detect these altered penicillinbinding
proteins, and they provide an alternative method for testing and confirmation of methicillin resistance.
The gold standard for MRSA detection is the detection of the mecA gene by using nucleic acid probes or polymerase chain reaction (PCR) amplification. A number of protocols have been published, and at least one
gene probe system, the EVIGENE MRSA Detection Kit (Statens Serum Institut, Copenhagen, Denmark) is commercially available. The EVIGENE system detects staphylococcal specific 16S rRNA, mecA, and nuc gene sequences. For those samples that may be mixed with both mecA-positive CoNS and S. aureus, a system, such as the IDl-MRSA Onfectio Diagnostic, Inc., Sainte-Foy, Quebec, Canada) could be performed on nasal swabs using real-time PCR with Smart Cycler technology (Cepheid, Sunnyvale, California).
Vancomycin is the drug of choice, and sometimes the only drug available for serious staphylococcal infections, and thus the development of vancomycin resistance has been a serious concern for the medical community. In 1996 the first vancomycin-intermediate S. aureus (VISA) strains were recovered in Japan. In 2002 isolates of true vancomycin-resistant S. aureus (VRSA) were reported in the United States, isolated from patients undergoing long-term vancomycin treatment. Automated antimicrobial susceptibility testing methods are not reliable in detecting these isolates. Screening using a vancomycin agar plate as described by the CLSI performance guidelines should enhance detection of VISAand VRSA.Detection of these isolates should be confirmed by a reference method or laboratory, and reporting should follow CDC guidelines.
Adherence to infection-control practices and CDC guidelines for vancomycin resistance may limit the emergence of this highly resistant organism.
Resistance in other categories of antimicrobials such as macrolides might not always be readily apparent by routine testing. Clindamycin, a macrolide, is frequently used in staphylococcal skin infections; additional testing using a modified disk diffusion test CDtest) might be useful when discrepant macrolide test results are obtained (e.g., erythromycin resistant, clindamycin susceptible).
Erythromycin and clindamycin susceptibility results are normally the same. However, staphylococcal resistance to clindamycin is occasionally inducible, meaning it is only detectable in vitro when the bacteria are also exposed to erythromycin. Inducible clindamycin resistance can be detected by disk diffusion by placing an erythromycin disk near a clindamycin disk. If an isolate possesses inducible clindamycin resistance, the bacteria will grow around the erythromycin disk and in the area of the agar where the two drugs overlap. However, a zone of inhibition will be observed on the side of the clindamycin disk farther away from the erythromycin disk. It is important for laboratories to keep up with the latest trends in antimicrobial resistance and to be aware of limitations that can occur with susceptibility testing.
The isolation of S. aureus from any source can usually be considered clinically significant. The recovery of coagulasenegative staphylococci from sterile sites and those sites associated with indwelling devices should be considered potential pathogens. Communication with the health care providerwillassist with determining the necessity for complete identification and susceptibility testing. The identification of S. saprophyticus from urinary tract infections should be made, especially if they are predominant, because even lower numbers can be significant.
The performance of antimicrobial susceptibility testing remains a crucial component of the laboratory, especially in the tracking of nosocomial infections caused by MRSA.
The addition of molecular methods may aid in the identification and control of these serious pathogens. Hospitalized patients with MRSA, VISA, or VRSA should be placed in isolation to minimize the spread of these drug-resistant strains. The detection of emerging resistance will continue to be a challenge as resistance mechanisms evolve. Laboratories must continue to stay aware of the latest testing strategies and techniques.
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