Transcript Slide 1
Dr f.Rashedmarandi Glossary I (Part 1). β -lactams: Class and Subclass Designation and Generic Name a b c d e Agents Included; Generic Names Antimicrobial Subclass Antimicrobial Class penicillin amoxicillin ampicillin azlocillin mezlocillin piperacillin carbenicillin ticarcillin cloxacillin dicloxacillin methicillin nafcillin oxacillin mecillinam amoxicillin-clavulanic acid ampicillin-sulbactam piperacillin-tazobactam ticarcillin-clavulanic acid cefazolin cephalothin cephapirin cephradine cefamandole cefonicid cefuroxime (sodium) cefoperazone cefotaxime ceftazidime ceftizoxime ceftriaxone cefepime cefmetazole cefotetan cefoxitin moxalactam cefaclor cefadroxil cefdinir cefditoren cefetamet cefixime cefpodoxime cefprozil ceftibuten cefuroxime (axetil) cephalexin cephradine loracarbef aztreonam doripenem ertapenem imipenem meropenem faropenem penicillina aminopenicillina penicillins ureidopencillina carboxypenicillina penicillinase-stable penicillinsb amidinopenicillin β -lactam/β -lactamase inhibitor combinations cephalosporin Ic,e cephems (parenteral) cephalosporin IIc,e cephalosporin IIIc,e cephalosporin IVc,e cephamycind oxacephem cephalosporine cephems (oral) carbacephem carbapenem monobactams penems penem Penicillinase-labile; hydrolyzed by staphylococcal penicillinase. Not hydrolyzed by staphylococcal penicillinase. Cephalosporin I, II, III, and IV are sometimes referred to as 1st-, 2nd-, 3rd-, and 4th-generation cephalosporins, respectively. Cephalosporin III and IV are also referred to as “extended-spectrum cephalosporins.” This does not imply activity against ESBL-producing gramnegative bacteria. Although often referred to as a 2nd-generation cephalosporin, cephamycins are not included with the other cephalosporins with regard to reporting of ESBL- producing strains. For all confirmed ESBL-producing strains, the test interpretation should be reported as resistant for this antimicrobial class or subclass. Penicillins These penicillins are penicillinase-labile;hydrolyzed by staphylococcal penicillinase penicillin penicillina amoxicillin aminopenicillina azlocillin ureidopencillina mezlolin piperacillin carbenicillin ticarcillin carboxypenicillina Penicillins Penicillinase-stable penicillin Penicillinase-stable penicillin Cloxacillin Dicloxacillin Meticillin Nafcillin oxacillin ß- lactam /ß- lactamase inhibitor combinations ß- lactam /ß- lactamase inhibitor combinations amoxicillin-clavulanic acid ampicillin-sulbactam piperacillintazobactam ticarcillin-clavulanic acid Cephems cefazolin cephalothin cephapirin cephradine cefamandole cefonicid cefuroxime (sodium) cefoperazone cefotaxime ceftazidime ceftizoxime ceftriaxone cefepime cefmetazole cefotetan cefoxitin moxalactam cefaclor cefadroxil cefdinir cefditoren cefetamet cefixime cefpodoxime cefprozil ceftibuten cefuroxime (axetil) cephalexin cephradine cephalosporin Ic,e cephems (parenteral) cephalosporin IIc,e cephalosporin IIIc,e cephalosporin IVc,e cephamycind oxacephem cephalosporine cephems (oral) III and IV are also referred to as “extended-spectrum cephalosporins.” This does not imply activity against ESBL-producing gram-negative bacteria. Selecting Antimicrobial Agents for Testing and Reporting ,criteria • Considerations in the assignment of agents to specific test/report groups include clinical efficacy, prevalence of resistance, minimizing emergence of resistance, cost, FDA clinical indications for usage, in addition to the specific issues described. Reporting Results 1.Susceptible (S) The “susceptible” category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the recommended dosage is used for the site of infection. 2.Intermediate (I) The “intermediate” category includes isolates with antimicrobial agent MICs that approach usually attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates. The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated (e.g., quinolones and β-lactams in urine) or when a higher than normal dosage of a drug can be used (e.g., βlactams). This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major discrepancies in interpretations, especially for drugs with narrow pharmacotoxicity margins. 3.Resistant (R) The “resistant” category implies that isolates are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules, and/or that demonstrate zone diameters that fall in the range where specific microbial resistance mechanisms (e.g., beta- lactamases) are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies. Antibacterial Effect & Mechanism of action Antibiotic class Bacteria Inhibition of cell wall synthesis β lactams penicillins β lactamsβ lactamase inhibitors cephems(Cephalosporins, cephamycin,oxacephem, carbacephem) Mechanism non β Lactamase producing G+ Some fastidious aerobic Gsome anaerobic bacteria β lactamase producing bacteria Aerobic & anaerobic g+ ,g- Penems )Carbapenem,penem) Broad spectrum activity against many aerobic g+,g- Antibacterial Effect & Mechanism of action Monobactams)Aztreonam) Only against aerobic gGlicopeptide(vancomycin, teicoplanin) Primarily aerobic g+ Aminoglicosides Primarily g-rods,synergistic Inhibit protein synthesis at combination with cell wall ribosomal level active drugs(pen,amp,van) against resistant g+ bacteria Macrolides fastidious g- .for G+ only erythrmycin need to be tested. Inhibition of cell wall synthesis Inhibit protein synthesis at ribosomal level Tetracyclines G + , G- Inhibit protein synthesis at ribosomal level Quinolones(quinolones,flu oroquinolones) G + , G- Inhibiting DNA gyrase or topoisomerase Antibacterial Effect & Mechanism of action Antibiotic class Bacteria Sulfonamide /trimetoprim Folate pathway inhibitors G+,G- Lipopeptide G-(polymyxin B,colistin) G+ Daptomycin Mechanism Inhibition of folate pathway Cell membrane Single – Drug class chloramphenicol Inhibit protein synthesis clindamycin Inhibit protein synthesis Linezolid Inhibit protein synthesis streptogramins Inhibit protein synthesis telitromycin Inhibit protein synthesis tigecycline Inhibit protein synthesis rifampin RNA synthesis inhibitors nitrofurantoin Inhibit protein synthesis at ribosomal level “Warning”: The following antimicrobial agents should not be routinely reported for bacteria isolated from CSF .These antimicrobial agents are not the drugs of choice and may not be effective for treating CSF infections caused by these organisms agents administered by oral route only 1st- and 2nd-generation cephalosporins (except cefuroxime sodium) Clindamycin - macrolides - tetracyclines - fluoroquinolones Notice! “Warning”: The following antimicrobial agent/organism combinations may appear active in vitro, but are not effective clinically and should not be reported as susceptible. Antimicrobial Agents That Must Not Be Reported as Susceptible Organism penicillins, cephalosporins, and aztreonam ESBL-producing K. pneumoniae, K. oxytoca, E. coli, and P. 1st- and 2nd-generation cephalosporins, cephamycins, and aminoglycosides penicillins, β-lactam/β-lactamase inhibitor combinations, cephems, and carbapenems aminoglycosides (except high concentrations), cephalosporins, clindamycin, and trimethoprimsulfamethoxazole Salmonella spp., Shigella spp. mirabilis oxacillin-resistant Staphylococcus spp. Enterococcus spp. Suggested panel for routine testing & reporting cefazolin Enterobacteriaceae Enterobacteriaceae from Urine cefepime ampicillin gentamyci n Amoxicillinclavulanic acid Ampicillinsulbactam cefuroxime Cefotaxim Ceftizoxime ceftriaxone Ciprofloxacin levofloxacin Imipenem meropenem piperacilin Amikacin tetracycline SXT Chlor amphenicol Ciprpfloxacin ofloxacin Nitrofurantoin Salmonella and Shigella When fecal isolates of Salmonella and Shigella spp. are tested, only ampicillin, a quinolone, and trimethoprim-sulfamethoxazole should be reported routinely. In addition, chloramphenicol and a third- generation cephalosporin should be tested and reported for extraintestinal isolates of Salmonella spp. Pseudomonas aeroginosa Suggested panel for routine testing & reporting Urine Imipenem meropenem Ofloxacin gentamycin piperacilin Amikacin cefepime ceftazidi me Ciprofloxacin levofloxacin Suggested panel for routine testing & reporting Staphylococcus spp Urine Oxacilln (cefoxitin disk) penicillin clindamycin Chlor amphenicol Ciprofloxacin levofloxacin Gentamycin Ciproflox acin Nitrofurant oin SXT linezolid SXT tetracycline vancom ycin staphylococcus Penicillin-susceptible staphylococci are also susceptible to other penicillins, cephems, and carbapenems approved for use by the FDA for staphylococcal infections. Penicillin-resistant, oxacillin- susceptible strains are resistant to penicillinase-labile penicillins, but susceptible to other penicillinase-stable penicillins, β-lactam/β-lactamase inhibitor combinations, relevant cephems, and carbapenems. Oxacillin-resistant staphylococci are resistant to all currently available β-lactam antibiotics. Thus, susceptibility or resistance to a wide array of β-lactam antibiotics may be deduced from testing only penicillin and oxacillin. Routine testing of other penicillins, β-lactamase inhibitor combinations, cephems, and carbapenems is not advised. cefoxitin disk test The cefoxitin disk test is the preferred method for testing S. aureus, S. lugdunensis, and coagulase-negative staphylococci for resistance to the penicillinase-stable penicillins. Cefoxitin is used as a surrogate for detecting oxacillin resistance; report oxacillin as susceptible or resistant based on the cefoxitin result. Suggested panel for routine testing & reporting Penicillin Ampicillin linezolid ciproflox acin Nitrofu rantoin Enterococcus Urine vancomycin Gentamycin chloramphenicol strepto mycin . Warning: For Enterococcus spp., cephalosporins, aminoglycosides (except for high-level resistance screening), clindamycin, and trimethoprim-sulfamethoxazole may appear active in vitro, but are not effective clinically, and isolates should not be reported as susceptible. Enterococcus ß lactamase - enterococcus :susceptible to penicillin,ampicillin,amoxicillin,ampicillin sulbactam,amoxicillin-clavulanic acid,piperacillin,piperacillin tazobactam Prediction with: penicillin disk test ß lactamase + enterococcus : Resistant to penicillin,ampicillin,amoxicillin,ampicillin sulbactam,amoxicillinclavulanic acid,piperacillin,piperacillin tazobactam a β-lactamase test is the only reliable test for detecting β-lactamase-producing Enterococcus spp. Enterococcus For blood and cerebrospinal fluid isolates(Enterococci), a β-lactamase test is also recommended. Notice: combined therapy with high- dose penicillin or high-dose ampicillin or vancomycin or teicoplanin plus gentamicin or streptomycin for bactericidal action is usually indicated for serious enterococcal infections, such as endocarditis. Alternative drugs for VRE Because of limited alternatives, chloramphenicol, erythromycin, tetracycline (or doxycycline or minocycline), and rifampin may be tested for vancomycinresistant enterococci (VRE), And consultation with an infectious disease practitioner is recommended. Acinetobacter spp Suggested panel for routine testing & reporting Urine ceftazidime Imipenem meropenem Amikacin cefepime ciprofloxacin SXT gentamycin Piperacilin ticarcilin Cefotaxim ceftriaxon Burkholderia cepacia Suggested panel for routine testing & reporting ceftazidime meropenem SXT Stenotrophomonas maltophilia Suggested panel for routine testing & reporting SXT levofloxacin Minocyclin Suggested panel for routine testing & reporting S.pneumonia Penicillin (oxacilin) linezolid vancomycin clindamycin ofloxacin tetracycline Erythromycin chloramphenicol SXT Pneumococcus from CSF =MIC penicillin and cefotaxime or ceftriaxone or meropenem Vancomycin (MIC or Disk) Pneumococcus from other sites: Oxacillin Disk Screening Test zone ≤ 19 mm =» MIC of penicillin,cefotaxime,ceftriaxone . Susceptibility testing of penicillins and other β-lactams approved by FDA for treatment of Streptococcus pyogenes or Streptococcus agalactiae is not necessary for clinical purposes and need not be done routinely, since as with vancomycin, resistant strains have not been recognized. Interpretive criteria are provided for pharmaceutical development, epidemiology, or monitoring for emerging resistance. Any strain found to be nonsusceptible should be referred to a reference laboratory for confirmation. Streptococcus spp.other than S.pneumonia Suggested panel for routine testing & reporting Penicillin or ampicillin Linezolid vancomycin clindamycin ofloxacin Cefepime or cefotaxime Or ceftriaxone Erythromycin chloramphenicol Group B streptococci Recommendations for intrapartum prophylaxis for Group B streptococci are penicillin or ampicillin. While cefazolin is recommended for penicillin-allergic women at low risk for anaphylaxis, those at high risk for anaphylaxis may receive clindamycin or erythromycin. Group B streptococci are susceptible to ampicillin, penicillin, and cefazolin, but may be resistant to clindamycin and/or erythromycin. When a Group B streptococcus is isolated from a pregnant woman with severe penicillin allergy (high risk for anaphylaxis), clindamycin and erythromycin should be tested and reported. ESBLs Strains of Klebsiella spp. E. coli, and P. mirabilis that produce ESBLs may be clinically resistant to therapy with penicillins, cephalosporins, or aztreonam, despite apparent in vitro susceptibility to some of these agents. • Some of these strains will show zones of inhibition below the normal susceptible population, but above the standard breakpoints for certain extendedspectrum cephalosporins or aztreonam; such strains may be screened for potential ESBL production by using the screening breakpoints .Other strains may test intermediate or resistant by standard breakpoints to one or more of these agents. • In all strains with ESBLs, the zone diameters for one or more of the extended-spectrum cephalosporins should increase in the presence of clavulanic acid. Method Initial Screen Test Phenotypic Confirmatory Test Medium Mueller-Hinton Agar Mueller-Hinton Agar Antimicrobial Disk Concentration For K. pneumoniae, K. oxytoca, and E. coli: Cefpodoxime 10 µµg or ceftazidime 30 µµg or aztreonam 30 µµg or cefotaxime 30 µµg or ceftriaxone 30 µµg For P. mirabilisa: Cefpodoxime 10µ µg or ceftazidime 30 µµg or cefotaxime 30µ µg (The use of more than one antimicrobial agent for screening improves the sensitivity of detection.) Ceftazidime 30 µµg Ceftazidime-clavulanic acidb 30/10µ µg and Cefotaxime 30µ µg Cefotaxime-clavulanic acidb 30/10 µµg (Confirmatory testing requires use of both cefotaxime and ceftazidime, alone and in combination with clavulanic acid.) Results of initial screen test For K. pneumoniae, K. oxytoca, and E. Result of phenotypic confirmatory Test A ≥ 5-mm increase in a zone diameter for either antimicrobial agent tested in coli: Cefpodoxime zone ≤ 17 mm combination with clavulanic acid versus its Ceftazidime zone ≤ 22 mm Aztreonam zone ≤ 27 mm zone when tested alone = ESBL (e.g., ceftazidime Cefotaxime zone ≤ 27 mm Ceftriaxone zone ≤ 25 mm zone = 16; ceftazidimeFor P. mirabilisa: Cefpodoxime zone ≤ 22 mm Ceftazidime clavulanic acid zone = 21). zone ≤ 22 mm Cefotaxime zone ≤ 27 mm Zones above may indicate ESBL production Screening of Proteus mirabilis for ESBL • Screening of Proteus mirabilis for ESBL production is recommended only when it is deemed clinically relevant (e.g., a bacteremic isolate). • For all confirmed ESBL-producing strains, the test interpretation should be reported as resistant for all penicillins, cephalosporins, and aztreonam. staphylococcus Penicillin-susceptible staphylococci are also susceptible to other penicillins, cephems, and carbapenems approved for use by the FDA for staphylococcal infections. (slide4) MRSA • Penicillin-resistant, oxacillin- susceptible strains are resistant to penicillinase-labile penicillins, but susceptible to other penicillinase-stable penicillins, β-lactam/βlactamase inhibitor combinations, relevant cephems, and carbapenems. slides 5,6 MRSA • Oxacillin-resistant staphylococci are resistant to all currently available β-lactam antibiotics. Thus, susceptibility or resistance to a wide array of β-lactam antibiotics may be deduced from testing only penicillin and oxacillin. Routine testing of other penicillins, β-lactamase inhibitor combinations, cephems, and carbapenems is not advised. • Historically, resistance to the penicillinasestable penicillins has been referred to as “methicillin resistance,” thus the acronyms MRSA (for “methicillin-resistant S. aureus”) or MRS (for “methicillin-resistant staphylococci”) are still commonly used, even though methicillin is no longer the agent of choice for testing or treatment. Of the penicillinase-stable penicillins, oxacillin is preferred for in vitro testing. Oxacillin susceptibility test results can be applied to the other penicillinase-stable penicillins, i.e., cloxacillin,dicloxacillin,flucloxacillin,methicillin, and nafcillin. Most resistance to oxacillin in staphylococci is mediated by the mecA gene, which directs the production of a supplemental penicillin-binding protein, PBP 2a, and is expressed either homogeneously or heterogeneously. Homogeneous resistance is easily detected with standard testing methods, whereas heterogeneous expression may be more difficult to detect with some methods because only a fraction of the population (e.g., 1 in 100 000 cells) expresses the resistance phenotype Detection of oxacillin resistance • Detection of oxacillin resistance: Tests for mecA or for the protein expressed by mecA, the penicillin-binding protein 2a (PBP 2a, also called PBP2') are the most accurate methods for prediction of resistance to oxacillin and can be used to confirm disk test results for isolates of staphylococci from serious infections. Isolates of staphylococci that carry the mecA gene, or that produce PBP 2a (the mecA gene product), should be reported as oxacillin resistant. Isolates that do not carry mecA or do not produce PBP 2a should be reported as oxacillin susceptible. Because of the rare occurrence of resistance mechanisms other than mecA, if MIC tests are performed in addition to disk diffusion, isolates for which oxacillin MICs are ≥4 µg/mL and are mecA negative or PBP 2a negative should be reported as oxacillin resistant. • Report isolates of staphylococci that carry mecA, or that produce PBP 2a, the mecA gen product, as oxacillin resistant. • Routine testing of urine isolates of S. saprophyticus is not advised, because infections respond to concentrations achieved in urine of antimicrobial agents commonly used to treat acute, uncomplicated urinary tract infections (e.g., nitrofurantoin, trimethoprim ± sulfamethoxazole, or a fluoroquinolone). • • WARNING: For oxacillin-resistant S. aureus and coagulase-negative staphylococci (MRS), other βlactam agents, i.e., penicillins, β-lactam/β-lactamase inhibitor combinations, cephems, and carbapenems, may appear active in vitro, but are not effective clinically. Results for these drugs should be reported as resistant or should not be reported. This is because most cases of documented MRS infections have responded poorly to β-lactam therapy, or because convincing clinical data have yet to be presented that document clinical efficacy for those agents. • For oxacillin-susceptible Staphylococcus aureus and coagulase-negative staphylococci, results for parenteral and oral cephems, βlactam/β-lactamase inhibitor combinations, and carbapenems, if tested, should be reported according to the results generated using routine interpretive criteria Penicillin-resistant,oxacillin-susceptible strains of Staphylococcus aureus produce β-lactamase, and the testing of the 10-unit penicillin disk instead of the ampicillin disk is preferred. Penicillin should be used to test the susceptibility of all βlactamase-labile penicillins, such as ampicillin, amoxicillin, azlocillin, carbenicillin, mezlocillin, piperacillin, and ticarcillin. Likewise, a positive β-lactamase test predicts resistance to these agents. For oxacillin-resistant staphylococci, report as resistant or do not report. Vancomycin resistant Staphylococcus Resistant strains are reliably detected by the microdilution reference method, the disk diffusion method and the vancomycin agar screen test when the tests are incubated for a full 24 hours at 35 ± 2 ºC. All staphylococcal isolates for which vancomycin zone diameters are 14 mm or less should be tested by a reference MIC method. The disk diffusion procedure will not differentiate strains with reduced susceptibility to vancomycin (MICs 4-8 µg/mL) from susceptible strains (MIC range 0.5 to 2 µg/mL) even when incubated for 24 hours. Additionally, vancomycin- resistant S. aureus (VRSA) strains (MICs ≥16 µg/mL) may produce only subtle growth around a vancomycin disk. A BHI vancomycin agar screen plate containing 6 µg/mL of vancomycin, such as that used for detection of vancomycin-resistant enterococci, may be inoculated to enhance the sensitivity of detecting vancomycin intermediate and vancomycin-resistant strains of S. aureus. Send any staphylococci determined to have an elevated MIC to vancomycin (≥4 µg/mL) to a reference laboratory. Staphylococcus spp. may develop resistance during prolonged therapy with quinolones. Therefore, isolates that are initially susceptible may become resistant within three to four days after initiation of therapy. Testing of repeat isolates may be warranted. Cefoxitin Disk Diffusion Testa The results of disk diffusion tests using a 30-μg cefoxitin disk and alternate breakpoints can be used to predict mecAmediated oxacillin resistance in staphylococci. The cefoxitin disk test is equivalent to oxacillin MIC tests in sensitivity and specificity for S. aureus. For coagulasenegative staphylococci, the cefoxitin disk test has equivalent sensitivity to oxacillin MIC tests but greater specificity, i.e., the cefoxitin disk test more accurately identifies oxacillinsusceptible strains than the oxacillin MIC test. Cefoxitin Disk Diffusion Testa for Prediction of mecA-Mediated Resistance in Staphylococci Antimicrobial Agent (Disk Content) Cefoxitin (30 µg) Organism Group Zone Diameter, Nearest Whole mm Comme nts ≤ 19 ≥ 20 (30) S. aureus for which cefoxitin disk diffusion zones are ≤ 19 mm should be reported as oxacillin resistant. Those for which cefoxitin zones are ≥ 20 mm should be reported as oxacillin susceptible. Coagulase-negative ≤ 24 staphylococci except S. lugdunensis ≥ 25 (31) Coagulasenegative staphylococci for which cefoxitin disk diffusion zones are ≤ 24 mm should be reported as oxacillin resistant. Those for which cefoxitin zones are ≥ 25 mm should be reported as oxacillinsusce. S. aureus and S. lugdunensis WARNING: For Enterococcus spp., cephalosporins, aminoglycosides (except for high-level resistance screening), clindamycin, and trimethoprim- sulfamethoxazole may appear active in vitro, but are not effective clinically, and isolates should not be reported as susceptible. Synergy between ampicillin, penicillin, or vancomycin and an aminoglycoside can be predicted for enterococci by using a high-level aminoglycoside (gentamicin and streptomycin) screening test. Other aminoglycosides need not be tested, because their activities against enterococci are not superior to gentamicin and streptomycin. high-level( AG) resistance screening(HLAR): Gentamycin 120µ :6=R 7-9=Inconclusive ≥10=S Because of limited alternatives, chloramphenicol, erythromycin, tetracycline (or doxycycline or minocycline), and rifampin may be used for vancomycin- resistant enterococci (VRE), and consultation with an infectious disease practitioner is recommended. non-β-lactamase-producing enterococci Penicillin susceptibility may be used to predict the susceptibility to ampicillin, amoxicillin, ampicillinsulbactam, amoxicillin-clavulanic acid,piperacillin,and piperacillin-tazobactam for non-β-lactamaseproducing enterococci. Ampicillin susceptibility can be used to predict imipenem susceptibility, provided the species is confirmed to be E.faecalis. Ampicillin is the class representative for ampicillin and amoxicillin.Ampicillin results may be used to predict susceptibility to amoxicillin-clavulanic acid, ampicillinsulbactam, piperacillin, and piperacillintazobactam among non-β-lactamaseproducing enterococci. The “susceptible” category for penicillin or ampicillin implies the need for high-dose therapy for serious enterococcal infections. Enterococcal endocarditis requires combined therapy with high-dose penicillin or high-dose ampicillin, or vancomycin or teicoplanin plus gentamicin or streptomycin for bactericidal action. Because ampicillin or penicillin resistance among enterococci due to β-lactamase production is not reliably detected using routine disk or dilution methods, a direct, nitrocefin-based β-lactamase test is recommended for blood and cerebrospinal fluid isolates. A positive β-lactamase test predicts resistance to penicillin, as well as amino-, carboxy-, and ureidopenicillins. When testing vancomycin against enterococci, plates should be held a full 24 hours and examined using transmitted light; the presence of a haze or any growth within the zone of inhibition indicates resistance. Organisms with intermediate zones should be tested by an MIC method . Oxacillin Screening Plates Oxacillin Screening Plates The oxacillin salt-agar screening-plate procedure can be used in addition to the dilution methods described above for the detection of MRSA. Perform the test by inoculating a S. aureus isolate onto Mueller-Hinton agar that has been supplemented with NaCl (4% w/v; 0.68 mol/L) and that contains 6 μg oxacillin/mL. Inoculate the agar from a direct colony suspension equivalent to a 0.5 McFarland standard using either a 1-μL loop or a swab. Using a 1-μL loop, spread the inoculum in an area 10 to 15 mm in diameter. Alternatively, using a swab, express excess fluid as for the disk diffusion test and then spot an area at least 10 to 15 mm in diameter or streak an entire quadrant. Incubate the plate at temperatures at 35 ± 2 °C (testing at temperatures above 35 °C may not detect MRS) for 24 hours and examine carefully with transmitted light for evidence of small colonies (>1 colony) or a light film of growth, indicating oxacillin resistance .Do not reuse plates after incubation. •Based on the cefoxitin result, report oxacillin as susceptible or resistant. ; • report oxacillin as susceptible or resistant based on the cefoxitin result. • ESBL Thank you 69 Health Reference Laboratory