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Table 10. MIC and zone diameter breakpoints for staphylococci Comments 1-3 relate to urinary tract infections (UTI) only. 1 These recommendations are for organisms associated with uncomplicated urinary tract infections only. For complicated infections and infections caused by Staphylococcus aureus and Staphylococcus epidermidis, which are associated with more serious infections, systemic recommendations should be used. 2 If an organism is isolated from multiple sites, for example from blood and urine, interpretation of susceptibility should be made with regard to the systemic site (e.g., if the blood isolate is resistant and the urine isolate susceptible, both should be reported resistant irrespective of the results obtained using interpretative criteria for urine isolates). 3 Direct susceptibility tests on urine samples may be interpreted only if the inoculum gives semi-confluent growth. Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters Amikacin for Staphylococcus aureus Amikacin for coagulase-negative staphylococci Gentamicin Tobramycin for Staphylococcus aureus Tobramycin for coagulase- negative staphylococci Neomycin For topical use only. The zone diameter breakpoint distinguishes the "wild type" susceptible population from isolates with reduced susceptibility. Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci β-Lactams
Most staphylococci are penicillinase-producers. The benzylpenicillin will mostly, but not unequivocally, separate β-lactamase producers. Isolates positive for β-lactamase are resistant to benzylpenicillin, phenoxymethylpenicillin, amino-,carboxy-and ureidopenicillins. Isolates negative for β-lactamase and susceptible to cefoxitin (cefoxitin is used to screen for "methicillin resistance") can be reported susceptible to these drugs. Isolates positive for β-lactamase and susceptible to cefoxitin are susceptible to penicillin- β-lactamase inhibitor combinations and penicillinase-resistant penicillins (oxacillin, cloxacillin, dicloxacillin and flucloxacin). Isolates resistant to cefoxitin are methicillin resistant and resistant to β-lactam agents, including β-lactamase inhibitor combinations, except for cephalosporins with approved anti-MRSA activity and clinical breakpoints. MIC breakpoint (mg/L) Interpretation of zone diameters Ampicillin UTI1-3 Staphylococci exhibiting resistance to oxacillin/cefoxitin Staphylococcus saprophyticus should be regarded as resistant to other penicillins, Cefoxitin Staphylococcus aureus cephalosporins, carbapenems and combinations of β- lactam and β-lactamase inhibitors.
Cefoxitin S. saprophyticus
- - - 10 19 - 20
For coagulase negative staphylococci with cefoxitin (Screen)
zone diameters of 22-26 mm, PCR for mecA is required Cefoxitin coagulase-negative to determine susceptibility for treatment of deep seated infection with any β-lactam. For oxacillin tests on Mueller–Hinton or Columbia agars Some hyper-producers of β-lactamase give zones within the range of 7-14 mm and if possible, should be
checked by a PCR method for mecA or a latex
agglutination test for PBP2a. Increase in oxacillin zone
size in the presence of clavulanic acid is not a reliable
test for hyper-producers of β-lactamase as zones of
inhibition with some MRSA also increase in the
presence of clavulanic acid. Rarely, hyper-producers of
β-lactamase give no zone in this test and would
therefore not be distinguished from MRSA.
For S. saprophyticus there is very little data for
resistant mecA strains.

With penicillin check for a heaped zone edge which
indicates β-lactamase mediated resistance.
Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters Quinolones
MIC breakpoints relate to high-dose therapy (750 mg BD). Ciprofloxacin UTI1-3 Staphylococcus saprophyticus Moxifloxacin 1 Disc diffusion for staphylococci does not give reliable Staphylococcus aureus results. An MIC method should be used to determine susceptibility, positive results requiring confirmation. Coagulase negative Population analysis is the most reliable method for confirming resistance and for distinguishing susceptible, hetero-GISA and GISA isolates. If, on clinical grounds, resistance to vancomycin is Staphylococcus aureus suspected, it is recommended that the organism be Vancomycin
sent to a specialist laboratory, such as Southmead Coagulase negative
Hospital in Bristol1 or the Antibiotic Research Laboratory in Cardiff2. Macrolides, lincosamides and streptogramins
The zone diameter breakpoint relates to an MIC of 1 mg/l as no data for the intermediate category are currently available. Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters Macrolides, lincosamides and streptogramins cont.
Erythromycin can be used to determine the susceptibility to azithromycin, clarithromycin and
roxithromycin.
Organisms that appear resistant to erythromycin, but
susceptible to clindamycin should be checked for the
presence of inducible resistance (see
http://www.bsac.org.uk/Resources/BSAC/Testing_for_d
issociated_resistance_in_staphylococc12.pdf. Inducible
clindamycin resistance can be detected only in the
presence of a macrolide antibiotic. If positive, report
as resistant to clindamycin or report as susceptible
with a warning that clinical failure during treatment
with clindamycin may occur by selection of
constitutively resistant mutants and the use of
clindamycin best avoided in severe infection.
The presence of blood has a marked effect on the activity of Quinupristin-dalfopristin. On the rare occasions when blood needs to be added to enhance the growth of staphylococci, susceptible ≥15 mm, resistant ≤14 mm. Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters The zone diameter breakpoint relates to an MIC of 1 mg/l as no data for the intermediate category are currently available. The zone diameter breakpoint relates to an MIC of 0.5 mg/l as no data for the intermediate category are currently available. The zone diameter breakpoint relates to an MIC of 1 mg/l as no data for the intermediate category are currently available. Staphylococci susceptible to tetracycline are also susceptible to doxycycline and minocycline. Some staphylococci resistant to tetracycline may be susceptible to minocycline and doxycycline. Strains with MIC values above the susceptible breakpoint are very rare or not yet reported. The identification and antimicrobial susceptibility tests on any such isolate must be repeated and if the result is confirmed the isolate must be sent to a reference laboratory. Until there is further evidence regarding clinical response for confirmed isolates with MIC above the current resistant breakpoint they should be reported as resistant. Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters Miscellaneous antibiotics
Strains with MIC values above the susceptible breakpoint are very rare or not yet reported. The identification and antimicrobial susceptibility tests on any such isolate must be repeated and if the result is confirmed the isolate sent to a reference laboratory. Until there is evidence regarding the clinical response for confirmed isolates with MIC above the current resistant breakpoint they should be reported resistant. Susceptibility testing by disc diffusion is not reliable. Susceptibility should be determined using a broth dilution method with Mueller Hinton broth or by an MIC method on Mueller Hinton agar. For advice on testing susceptibility to co-trimoxazole see Appendix 1. The MIC breakpoint is based on the trimethoprim concentration in a 1:19 combination with sulfamethoxazole. Breakpoints are epidemiological "cut-offs" based on distributions for the "wild type" population. However, there is no clear evidence correlating these breakpoints with clinical efficacy. Trimethoprim UTI1-3 Staphylococcus saprophyticus Fosfomycin Disc content indicates 200 µg fosfomycin/50 µg glucose-6-phosphate Version 11.1 May 2012 Table 10. MIC and zone diameter breakpoints for staphylococci MIC breakpoint (mg/L) Interpretation of zone diameters Miscellaneous antibiotics cont.
Nitrofurantoin UTI1-3 Staphylococcus saprophyticus Rifampicin 0.5 1 = Department of Microbiology, Lime Walk Building, Southmead Hospital Westbury–on-Trym, Bristol, BS10 5NB. 2 = Public Health Wales, Microbiology Cardiff, University Hospital of Wales, Heath Park, Cardiff, CF14 4XW. Version 11.1 May 2012

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