12. Tables

12. Tables

There are 4 sets of tables containing information essential for the performance of the CDS Test. The tables are: Calibrations, Surrogate Discs, Reference Strains and the ß-lactam Testing Guide.  These tables are intended for both Clinical and Veterinary laboratories. Antibiotics used in veterinary medicine only are flagged.

12.1. Calibrations

Calibration tables 1a, 1b, 1c & 1d collectively summarise much of the basic information used in the CDS Test. They list all the organisms and antimicrobials tested, the media used, conditions of incubation, disc potencies, cut off sizes for unusual annular radii and the MICs for susceptible strains (breakpoints). The tables are updated regularly and operators should ensure that they are using the latest versions of these tables. It is most important to pay particular attention to the footnotes included with each table as these highlight exceptions, restrictions and some specific directions. For organisms not included in the Table of Calibrations, extrapolate the testing from that established for similar organisms.

Table 12.1a Gram Positives

Corynebacterium species

(Blood Sensitest, CO2, 35‑37°C)b

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillinc 5 ≤ 2
Benzylpenicillin 0.5 u ≤ 0.125
Chloramphenicol 30 ≤ 8
Ciprofloxacin 2.5 ≤ 1
Clindamycinn 2 ≤ 0.5
Erythromycin 5 ≤ 0.5
Fusidic acid 2.5 ≤ 0.5
Marbofloxacink 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Quinupristin/dalfopristin 15 ≤ 2
Rifampicin 1 ≤ 0.5
Teicoplanin 15 2 mm ≤ 8
Tetracycline 10 ≤ 4
Tigecycline 15 ≤ 1
Vancomycin 5 2 mm ≤ 4

Enterococci

(Blood Sensitest, CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillind 5 4 mmd ≤ 4
Chloramphenicol 30 4 mm ≤ 8
Doxycyclinee 30 4 mm ≤ 16
Fosfomycine  200  ≤ 64
Gentamicin 200 ≤ 256
Linezolid 10 ≤ 4
Marbofloxacink 5 ≤ 2
Nitrofurantoine 200 4 mm ≤ 64
Quinupristin/dalfopristinf
15 ≤ 2
Streptomycin 300 4 mm ≤ 512
Teicoplanin 15 2 mm ≤ 8
Tetracycline 10 ≤ 4
Tigecycline 15 ≤ 1
Vancomycing 5   ≤ 4

Listeria species

(Blood Sensitest, CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillin 5 ≤ 1
Cotrimoxazole 25 ≤ 0.5/9.5
Gentamicin 10 ≤ 1

Staphylococci

(Sensitest, Air 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillinh 5 ≤ 0.5
Benzylpenicillini 0.5 u ≤ 0.06
Cefoxitin j 10 ≤ 4
Ceftarolineo 5 ≤ 1
Cephalexinh 100 ≤ 16
Chloramphenicol 30 ≤ 8
Ciprofloxacin 2.5 ≤ 1
Clindamycinn 2 ≤ 0.5
Cotrimoxazole 25 ≤ 1/19
Erythromycin 5 ≤ 0.5
Fusidic acid 2.5 ≤ 0.5
Gentamicin 10 ≤ 1
Kanamycin 50 ≤ 8
Linezolid 10 ≤ 4
Marbofloxacink 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Mupirocin 5 ≤ 2
Neomycink 30 4 mm ≤ 8
Nitrofurantoine 200 ≤ 32
Novobiocink 5 ≤ 1
Oxacillinl 1 ≤ 0.25
Quinupristin/dalfopristin 15 ≤ 2
Rifampicin 1 ≤ 0.5
Teicoplanin 15 2 mm ≤ 8
Tetracycline 10 ≤ 4
Tigecycline 15 ≤ 1
Trimethoprim 5 ≤ 4
Vancomycin 5 2 mm ≤ 4

Streptococci & Erysipelothrix species

(Blood Sensitest, CO2, 35‑37°C)m

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillinm 5 4 mm ≤ 2
Benzylpenicillin 0.5 u ≤ 0.125
Cefotaxime 0.5 4 mm ≤ 0.5
Cefotaximem 5 ≤ 2
Ceftriaxone 0.5 4 mm ≤ 0.5
Ceftriaxonem 5 ≤ 2
Chloramphenicol 30 ≤ 8
Clindamycinn 2 ≤ 0.5
Cotrimoxazole 25 ≤ 0.5/9.5
Erythromycin 5 ≤ 0.5
Marbofloxacink 5 ≤ 2
Moxifloxacin 2.5 4 mm ≤ 1
Nitrofurantoine 200 4 mm ≤ 32
Quinupristin/Dalfopristin 15 ≤ 2
Rifampicin 1 ≤ 0.5
Teicoplanin 15 2 mm ≤ 8
Tetracycline 10 ≤ 4
Tigecycline 15 ≤ 1
Vancomycin 5 2 mm ≤ 4

 

a    The standard 6mm cut‑off applies where no exception has been specified.

b    Slow growers are incubated for 48h.

c    Corynebacterium species: benzylpenicillin 0.5 u/ampicillin 5 μg: If the inhibitory zone is < 6 mm with a benzylpenicillin 0.5 u disc and ≥ 4 mm with an ampicillin 5 μg disc, report the susceptibility as follows: “There is decreased susceptibility to penicillin, ampicillin and amoxycillin with the MIC between 0.25 mg/L and 2.0 mg/L”.

d    Perform a nitrocefin based test to detect β–lactamase activity if the zone of inhibition has a sharp edge and an annular radius > 4 mm. β–Lactamase–positive isolates are reported as resistant.

e    For testing urine isolates only.

f    Enterococcus faecalis is intrinsically resistant to Pristinamycin and Quinupristin/Dalfopristin.

g    A zone of inhibition with a hazy edge and a light growth towards the disc indicates a low level resistance to vancomycin (VanB type), irrespective of the size of the inhibitory zone. The light growth will be more evident if the plate is reincubated for 48 h.

h    For testing Staphylococcus saprophyticus only.

i     NOT for testing Staphylococcus saprophyticus.

j     For testing Staphylococcus aureus only.

k    Antibiotic calibrated for veterinary medicine.

l     For testing coagulase‑negative staphylococci (except Staphylococcus saprophyticus).

m   NOT for testing Streptococcus pneumoniae from CSF.  If Streptococcus pneumoniae or any other Streptococcus species from a site other than CSF is resistant to benzylpenicillin 0.5 u, cefotaxime 0.5 µg or ceftriaxone 0.5 µg, test ampicillin 5 µg, cefotaxime 5 µg and ceftriaxone 5 µg.

n     Isolates with inducible clindamycin resistance (iMLSB phenotype) will have a large zone to clindamycin, but should be reported as resistant. Inducible clindamycin resistance can be detected as described in section 4.8.

o   For testing MRSA only.

Table 12.1b Gram Negative

Acinetobacter species, Enterobacteriaceae & Vibrionaceae

(Sensitest, air, 35‑37°C)b

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Amikacin 30 4 mm ≤ 16c
Ampicillinc 25 ≤ 8
Apramicind 15 4 mm ≤ 8
Augmentine 60 ≤ 8/4
Azithromycin* 15 4 mm ≤ 16
Aztreonam 30 ≤ 8
Cefazolin 30 ≤ 4
Cefepime 10 ≤ 2
Cefotaxime 5 ≤ 1
Cefotetan 30 ≤ 8
Cefoxitin 30 ≤ 8
Cefpirome 10 ≤ 2
Cefpodoxime 10 ≤ 2
Ceftazidime 10 ≤ 4
Ceftriaxone 5 ≤ 1
Cefuroxime 30 ≤ 8
Cephalexinc,g 100 ≤ 1 6
Chloramphenicol 30 ≤ 8
Ciprofloxacin 2.5 ≤ 1
Cotrimoxazole  25  ≤ 1/19
Doripenem 10   ≤ 4
Enoxacin 10 ≤ 4
Ertapenemf 10 ≤ 4
Fosfomycing 200 ≤ 32
Gentamicin 10 4 mm ≤ 2
Imipenem 10 ≤ 4
Kanamycin 50 ≤ 8
Marbofloxacind 5 ≤ 2
Meropenem 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Nalidixic acidg 30 ≤ 4
Neomycind 30 4 mm ≤ 8
Nitrofurantoing 200 ≤ 32
Norfloxacing 10 ≤ 4
Polymyxin B 300 u 4 mm ≤ 1
Spectinomycind 25 ≤ 32
Streptomycind 25 ≤ 16
Tazocine 55 ≤ 16/2
Tetracycline 10 4 mm ≤ 4.0
Tigecycline 15 ≤ 1
Timentine 85 ≤ 16/2
Tobramycin 10 4 mm ≤ 2
Trimethoprim 5 ≤ 4

 

Branhamella catarrhalis (Moraxella catarrhalis)

(Blood Sensitest, 5% CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Benzylpenicillin 0.5 u ≤ 0.25
Cefaclor 30 ≤ 4
Cefpodoxime 10 ≤ 2
Cefuroxime 30 ≤ 4
Chloramphenicol 10 ≤ 4
Ciprofloxacin 2.5 ≤ 1
Cotrimoxazole 25 ≤ 1/19
Erythromycin 5 ≤ 0.5
Marbofloxacind 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Tetracycline 10 ≤ 4

 

Campylobacter species

(Blood Sensitest, microaerophilic, 42°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ciprofloxacin 2.5 ≤ 1
Erythromycin 5 4 mm ≤ 0.5
Gentamicin 10 ≤ 1
Tetracycline 10 ≤ 4

 

Haemophilus species

(HTM agarh, 5% CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillin 5 ≤ 1
Augmentin 15 ≤ 2/1
Cefaclor 30 ≤ 4
Cefotaxime 0.5 ≤ 0.25
Cefotaxime 5 ≤ 1
Cefpodoxime 10 ≤ 2
Ceftriaxone 0.5 ≤ 0.25
Ceftriaxone 5 ≤ 1
Cefuroxime 30 ≤ 4
Chloramphenicol 10 ≤ 2
Ciprofloxacin 2.5 ≤ 1
Cotrimoxazole 25 ≤ 1/19
Marbofloxacind 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Tetracycline 10 ≤ 4

 

Helicobacter pylori

(Chocolate Columbia Blood Agar, microaerophilic, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Amoxycillin 2 ≤ 1
Ciprofloxacin 2.5 ≤ 1
Erythromycini 5 ≤ 0.5
Metronidazole 5 ≤ 4
Rifampicinj 5 ≤ 2
Tetracycline 10 ≤ 4

 

Neisseria meningitidis

(Blood Sensitest, 5% CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Benzylpenicillin 0.5 u 4 mm ≤ 0.25
Cefotaxime 0.5 ≤ 0.25
Ceftriaxone 0.5 ≤ 0.25
Chloramphenicol 10 ≤ 2
Ciprofloxacin 2.5 ≤ 1
Rifampicin 1 ≤ 0.5

 

Pasteurella species

(Blood Sensitest, 5% CO2, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Ampicillinm 5 ≤ 2
Benzylpenicillin 0.5 u 4 mm ≤ 0.25
Ciprofloxacin 2.5 ≤ 1
Erythromycind 5 4 mm ≤ 2
Marbofloxacind 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Tetracycline 10 ≤ 4

 

Pseudomonas species, Burkholderia species & Chryseobacterium species

(Sensitest, air, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Amikacin 30 4 mm ≤ 16
Augmentinn 60 ≤ 8/4
Aztreonam 30 ≤ 8
Cefepime 10 ≤ 2
Cefpirome 10 ≤ 2
Ceftazidime 10 ≤ 4
Ciprofloxacin 2.5 ≤ 1
Doripenem 10 ≤ 4
Ertapenem 10 ≤ 4
Fosfomycing 200 ≤ 32
Gentamicin 10 4 mm ≤ 4
Imipenem 10 ≤ 4
Marbofloxacind 5 ≤ 2
Meropenem 5 ≤ 2
Moxifloxacin 2.5 ≤ 1
Norfloxacing 10 ≤ 4
Polymyxin B 300 u 4 mm ≤ 1
Tazocin@ 55 ≤ 16/2
Ticarcillind 75 ≤ 32
Timentin& 85 ≤ 16/2
Tobramycin 10 4 mm ≤ 4
Trimethoprim 5 ≤ 4

 

Stenotrophomonas maltophiliao

(Sensitest, air, 35‑37°C)

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Cotrimoxazoleo 25 ≤ 2/38

 

*    For the testing of Salmonella typhi and other Salmonella isolated from blood culture.

a   The standard 6mm cut‑off applies where no exception has been specified.

b    Yersinia enterocolitica  is incubated in air at 30°C.

c    Test ampicillin 25 and cephalexin 100 in parallel. Strains resistant to cephalexin but appear susceptible to ampicillin (presence of chromosomal inducible AmpC β-lactamase) are reported as resistant to ampicillin/amoxicillin.

d    Antibiotic calibrated for veterinary medicine.

e    If an ESBL is present, report Augmentin, Timentin and Tazocin for isolates from urine only.

f     Acinetobacter species are considered resistant to ertapenem.

g    For testing urinary isolates only.

h    Haemophilus Test Medium Base containing 15 mg/L of freshly prepared haematin and NAD.

i     Erythromycin 5 μg is the surrogate disc for reporting the susceptibility to clarithromycin. The MIC of clarithromycin for susceptible strains is ≤ 0.5 mg/L.

j     Rifampicin 5 μg is the surrogate disc for reporting the susceptibility to rifabutin.

m   Pasteurella multocida is tested against ampicillin 5 μg and NOT benzylpenicillin 0.5 units.

n   Burkholderia pseudomallei is usually susceptible to Augmentin and can be tested against this antibiotic.

o    See Section 5.11 for notes on testing antibiotic susceptibilities of Sulphonamide resistant S. maltophilia

@    Piperacillin in the presence of 2mg/L tazobactam.

&    Ticarcillin in the presence of 2mg/L clavulanic acid

Table 12.1c Anaerobic organisms 

Anaerobic organismsb

(Supplemented Brucella Medium Base, anaerobic, 35‑37°C)c

Antibiotic Disc potency (µg) Exception to standard interpretationa MIC
Augmentin 3 4 mm ≤ 2/1
Benzylpenicillin 0.5 u ≤ 0.25
Cefoxitin 10 ≤ 4
Clindamycin 2 4 mm ≤ 0.5
Meropenem 5 ≤ 2
Metronidazole 5 4 mm ≤ 2
Moxifloxacin 2.5 4 mm ≤ 2
Tazocin 55 ≤ 8/2

a  The standard 6mm cut‑off applies where no exception has been specified.

b Slow growers should be incubated for 48 hrs.             

c  Brucella Medium Base containing 5% defibrinated horse blood, haemin 5 mg/L and vitamin K 1 mg/L.

 

12.2. Surrogate Discs

Surrogate disc testing tables 2a and 2b list those antimicrobials where the susceptibility can be inferred from the results obtained with a closely related agent, the “surrogate disc”. The table is arranged according to bacterial species and the relationship between the antimicrobials is valid only for the species indicated. This table is also updated regularly as data are accumulated which either invalidate the relationship or enable us to add agents to the list. Laboratories may find it useful to include a comment on the susceptibility report that the result reported with a particular antimicrobial indicates the susceptibility to another.

Table 12.2a Gram Positive

Corynebacterium species

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Ampicillin/Penicillin V Benzylpenicillin 0.5 u
Azithromycin/Clarithromycin/Roxithromycin Erythromycin 5
Carbapenems Benzylpenicillin 0.5 u
Cefovecina/Ceftiofur/other Cephalosporinsb Benzylpenicillin 0.5 u
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Lincomycin Clindamycin 2
Norfloxacinc Ciprofloxacin 2.5
Pristinamycin Quinupristin / Dalfopristin 15
Tetracyclines Tetracycline 10
Tylosina Erythromycin. 5.

Enterococci

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Benzylpenicillin Ampicillin 5
Enrofloxacina/Orbifloxacina/Pradofloxacina Moxifloxacin 2.5
Pristinamycinf Quinupristin / Dalfopristin 15

Listeria species

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Benzylpenicillin Ampicillin 5

Staphylococci (except S. saprophyticus from urine)

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Ampicillin/Penicillin V Benzylpenicillin 0.5 u
Augmentin Oxacillind/Cefoxitine
1 / 10
Azithromycin/Clarithromycin/Roxithromycin Erythromycin 5
Carbapenems Oxacillind/Cefoxitine 1 / 10
Cefovecina/Ceftiofura/other Cephalosporinsb Oxacillind/Cefoxitine 1 / 10
Cloxacillin/Dicloxacillin/Flucloxacillin Oxacillind/Cefoxitine 1 / 10
Enrofloxacina/Orbifloxacina/Pradofloxacina Moxifloxacin 2.5
Framycetina Neomycin 30
Lincomycin Clindamycin 2
Norfloxacinc/Ofloxacina Ciprofloxacin 2.5
Pristinamycin Quinupristin/Dalfopristin 15
Tetracyclines Tetracycline 10
Tylosina Erythromycin. 5.

Staphylococcus saprophyticus from urine

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Benzylpenicillin/Penicillin V Ampicillin 5
Augmentin Cephalexin 100
Cefovecina/Ceftiofura/other Cephalosporinsb Cephalexin 100
Cloxacillin/Dicloxacillin/Flucloxacillin Cephalexin 100
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Norfloxacinc/Ofloxacina Ciprofloxacin 2.5
Tetracyclines Tetracycline 10

Streptococcig

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Ampicillin/Augmentin/Penicillin V Benzylpenicillin 0.5 u
Amoxycillin/Augmentin/Benzylpenicillin Ampicillinh 5
Augmentinj Benzylpenicillin 0.5 u
Augmentinj Ampicillin 5
Azithromycin/Clarithromycin/Roxithromycin Erythromycin 5
Carbapenems Benzylpenicillin 0.5 u
Cefovecina/Ceftiofura Benzylpenicillin 0.5 u
Cefovecina/Ceftiofura Cefotaximei 5
Cephalosporins (except Ceftiofur) Cefotaxime/Ceftriaxonek 0.5/5
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Flucloxacillin Benzylpenicillin 0.5 u
Lincomycin Clindamycin 2
Pristinamycin Quinupristin/Dalfopristin 15
Tetracyclines Tetracycline 10
Tylosina Erythromycin. 5.

a    Antibiotic used in veterinary medicine only.  

Ceftazidime is inactive against Gram‑positive organisms.

c  Reporting of norfloxacin is for urine isolates only.

d   Oxacillin is used for testing coagulase‑negative staphylococci (exceptStaphylococcus saprophyticus) only.

e    Cefoxitin is used for testing Staphylococcus aureus only.

f     Enterococcus faecalis is intrinsically resistant to Pristinamycin and Quinupristin/Dalfopristin.

g    For streptococci groups A, B, C, G and Streptococcus milleri group, the susceptibility to benzylpenicillin, ampicillin, amoxycillin and cephalosporin antibiotics (except ceftazidime) is extrapolated from the testing of benzylpenicillin 0.5 u.

h    NOT for testing Streptococcus pneumoniae from CSF. Test if isolate is resistant to benzylpenicillin 0.5 u, cefotaxime 0.5 μg or ceftriaxone 0.5 μg and report as having a decreased susceptibility.

i     If the organism is resistant to benzylpenicillin 0.5 u and susceptible to cefotaxime 5 ug, report as having a decreased susceptibility to Cefovecina/Ceftiofur a.

j     For the reporting of all Streptococcus species including S. pneumoniae from sites other than CSF. If the organism is resistant to benzylpenicillin 0.5 u and susceptible to Ampicillin 5 µg, report as having a decreased susceptibility

k    If the organism is resistant to cefotaxime/ceftriaxone 0.5 and susceptible to cefotaxime/ceftriaxone  5 µg, report as having a decreased susceptibility.

Table 12.2b Gram Negatives

Acinetobacter , Enterobacteriaceae & Vibrionaceae species

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin Ampicillin 25
Cefovecina/Ceftiofura Cefotaxime 5
Cephalothinb Ampicillin 25
Ceftriaxone Cefotaxime 5
Cefotaxime Ceftriaxone 5
Colistin Polymyxin 300 u
Enrofloxacina/Orbifloxacina/Pradofloxacina Moxifloxacin 2.5
Framycetina Neomycin 30
Ofloxacina Ciprofloxacin 2.5
Piperacillin Ampicillin 25
Tetracyclines Tetracycline 10
Ticarcillina Ampicillin 25

Branhamella catarrhalis (Moraxella catarrhalis)

Antibiotic reported Surrogate disc used Disc potency (µg)
Azithromycin/Clarithromycin/Roxithromycin Erythromycin 5
Amoxycillin/Ampicillin/Penicillin V Benzylpenicillin 0.5 u
Augmentin Cefuroxime/Cefaclor 30
Cefovecina/Ceftiofura Cefuroxime/Cefaclor 30
Cephalosporins Cefuroxime/Cefaclor 30
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Ofloxacina Ciprofloxacin 2.5
Tetracyclines Tetracycline 10

Campylobacter species

Antibiotic reported Surrogate disc used Disc potency (µg)
Azithromycin/Clarithromycina/Roxithromycina Erythromycin 5
Enrofloxacina/Ofloxacina/Orbifloxacina Ciprofloxacin 2.5
Tetracyclines Tetracycline 10

Haemophilus influenzae/Haemophilus species

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin Ampicillin 5
Cefepime Cefotaxime/Ceftriaxone 0.5
Cefotaxime Ceftriaxone 0.5
Cefpirome Cefotaxime/Ceftriaxone 0.5
Ceftazidime Cefotaxime/Ceftriaxone 0.5
Cefovecina/Ceftiofura Cefuroxime/Cefaclor 30
Ceftriaxone Cefotaxime 0.5
Cephalexin Cefuroxime/Cefaclor 30
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Ofloxacina Ciprofloxacin 2.5
Tetracyclines Tetracycline 10

Helicobacter pylori

Antibiotic reported Surrogate disc used Disc potency (µg)
Clarithromycin Erythromycin 5
Rifabutin Rifampicin 5

Neisseria meningitidis

Antibiotic reported Surrogate disc used Disc potency (µg)
Ampicillin/Amoxycillin Benzylpenicillin 0.5 u
Cefotaxime Ceftriaxone 0.5
Cefovecina/Ceftiofura Benzylpenicillin 0.5u
Ceftriaxone Cefotaxime 0.5
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5

Pasteurella species

Antibiotic reported Surrogate disc used Disc potency (µg)
Amoxycillin/Benzylpenicillin Ampicillin 5
Ampicillinc/amoxycillin Benzylpenicillin
0.5 u
Augmentin Ampicillin  5
Cephalexin Ampicillin  5
Cefovecina/Ceftiofura Ampicillin 5
Enrofloxacina/Orbifloxacina/Pradofloxacina Moxifloxacin 2.5
Norfloxacin/Ofloxacina Ciprofloxacin 2.5
Tetracyclines Tetracycline 10

Pseudomonas species,  Burkholderia species & Chryseobacterium species

Antibiotic reported Surrogate disc used Disc potency (µg)
Colistin Polymyxin 300 u
Enrofloxacina/Orbifloxacina Moxifloxacin 2.5
Norfloxacin/Ofloxacina/Pradofloxacina Ciprofloxacin 2.5

a    Antibiotic used in veterinary medicine only.

b   Not for Acinetobacter species.

c    Pasteurella multocida is tested against ampicillin 5 µg and NOT benzylpenicillin 0.5 u.

12.3. Reference Strains

Quality assurance tables 3a, 3b, 3c & 3d list the 13 reference strains, the media, incubation conditions and the expected range of zone sizes observed with each disc of a stated potency. The footnotes explain how the acceptable ranges of zone sizes were derived and recommend the indications for and the frequency of testing the reference strains.

Table 12.3a Gram Positives

Enterococcus faecalis POW 1994

(Blood Sensitest, CO2 35°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Ampicillin 5            5.9 –  9.2
Chloramphenicol 30            6.3 –  8.7
Doxycycline  30            7.0 –11.0
Fosfomycin 200            7.1– 10.7
Gentamicin 200            6.6 –  9.9
Linezolid 10            6.6 –  9.0
Nitrofurantoin 200            6.1 –  8.7
Streptomycin 300            5.4 –  7.9
Teicoplanin 15            3.1 –  5.3
Tetracycline 10            7.0 –10.2
Tigecycline 15            6.6 –  9.5
Vancomycin 5            2.0 –  3.7

Staphylococcus aureus NCTC 6571

(Sensitest, air 35°C)

Antibiotic reported Disc potency (µg)  Annular radii (mm)
  Sensitest Blood Sensitest
Amoxycillinb 2 9.1 –11.9 9.0 – 11.6
Ampicillin 5 12.1–18.1 10.8 –14.5
Benzylpenicillin 0.5 u 8.7 –13.5 9.5 – 12.7
Cefoxitin 10 7.1 –10.1 7.9 – 9.7
Ceftaroline 5 10.5–14.1 10.2 –13.2
Cephalexin 100 10.7–15.5 11.9 – 14.7
Chloramphenicol 30 7.8 – 11.4 8.6 – 10.7
Ciprofloxacin 2.5 9.2 – 12.4 9.2 – 11.8
Clindamycin 2 8.5 – 12.9 8.1 – 11.2
Cotrimoxazole 25 10.1–13.3 10.9 –13.1
Erythromycin 5 8.0 –10.8 9.0 – 10.6
Fusidic acid 2.5 8.6 –12.6 6.3 – 8.0
Gentamicin 10 6.6 – 9.4 7.5 – 8.9
Linezolid 10 7.9 –13.1 8.5– 11.5
Marbofloxacinc 5 8.9– 11.0 8.9– 10.5
Moxifloxacin 2.5 10.9–14.5
10.8 –13.0
Mupirocin 5 7.4 – 12.2 8.1 – 11.6
Neomycinc 30 7.5 – 9.0 7.6 – 9.3
Nitrofurantoin 200 6.7 –10.3 6.3 –8.1
Novobiocinc 5 6.1 –12.5 6.4 –9.2
Oxacillin 1 7.4 –10.4 7.9 –10.6
Quinupristin/dalfopristin 15 9.2 – 12.4 7.0 –10.0
Rifampicin 1 9.3 – 12.5 9.4 –11.6
Rifampicin b
5 12.7–15.0 10.5 –14.2
Teicoplanin 15 3.4  –  6.1 4.1 –6.0
Tetracycline 10 11.3–14.4 10.1 –13.3
Tigecycline 15 10.3–13.9 9.2 –11.6
Trimethoprim 5 8.5 –11.3 9.2 –11.0
Vancomycin 5 2.8 –  4.9 3.0 –4.8

Streptococcus pneumoniae ARL 10582

(Blood Sensitest, 5% CO2, 35–37˚C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Ampicillin 5       10.8 –  15.2
Benzylpenicillin 0.5 u          8.3 – 14.8
Cefotaxime 0.5          9.3 –  14.8
Cefotaxime 5       10.9  –  15.3
Ceftriaxone 0.5          9.1 –  14.3
Ceftriaxone 5       11.5  –  15.2
Chloramphenicol 30          8.0 –  13.2
Clindamycin 2            7.1 –  9.9
Cotrimoxazole 25            7.0 –  9.2
Erythromycin 5           7.1 – 12.9
Moxifloxacin 2.5            5.6 –  8.6
Quinupristin/dalfopristin 15            6.4 –  9.2
Rifampicin 1           7.5 – 10.8
Teicoplanin 15            5.1 – 8.0
Tetracycline 10          9.5 –  11.5
Tigecycline 15          9.7 –  12.6
Vancomycin 5            5.1 –  8.6

Note: The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar and discs. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.

a    Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.

b   The 2 μg amoxycillin and 5 μg Rifampicin discs are used for the susceptibility testing of H. pylori only.

c   Antibiotic used in veterinary medicine only.

 

Table 12.3b Gram Negatives

Bacteroides fragilis ATCC 25285

(Blood Sensitest, anaerobic, 35‑37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Metronidazole         5     7.1 –  13.5

Campylobacter jejuni NCTC 11168

(Blood Sensitest, microaerophilic, 42°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)
Ciprofloxacin 2.5            9.2 –16.9
Erythromycin 5           6.4 – 12.4
Gentamicin 10            7.0 –11.0
Tetracycline 10          10.8 – 16.8

Escherichia coli NCTC 10418d

(Sensitest, air, 35–37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Amikacin 30          6.7 –  10.3
Ampicillin 25          7.5 –  10.7
Apramicind 15            5.3 –  7.9
Azithromycin 15            5.4 –  7.0
Aztreonam 30       13.7 –  15.9
Cefazolin 30          6.7 –  12.7
Cefepime 10       11.9 –  15.3
Cefotaxime 5          9.7 –  13.7
Cefotetan 30       11.9 –  14.8
Cefoxitin 30          9.8 –  13.0
Cefpirome 10       11.9 –  14.6
Cefpodoxime 10       10.3 –  12.7
Ceftazidime 10          9.3 –  14.1
Ceftriaxone 5       10.5 –  14.3
Cefuroxime 30          8.3 –  11.1
Cephalexin 100          6.9 –  10.9
Chloramphenicol 30          8.7 –  11.9
Ciprofloxacin 2.5       12.4 –  15.8
Cotrimoxazole 25         10.2 – 13.0
Doripenem 10         11.4 – 14.7
Enoxacine 10         9.7 –  15.7
Ertapenem 10       12.1 –  16.1
Fosfomycin 200            5.7 –  9.7
Gentamicin 10            6.2 –  9.4
Imipenem 10       10.3 –  13.5
Kanamycin 50          6.2 –  11.8
Marbofloxacine 5       13.2 –  15.3
Meropenem 5       11.0 –  14.4
Moxifloxacin 2.5       10.0 –  13.4
Nalidixic acid 30          8.9 –  12.1
Neomycine 30            6.0 –  8.6
Nitrofurantoin 200            6.3 –  9.5
Norfloxacin 10       10.4 –  16.4
Polymyxin B 300 u             5.1–  7.5
Spectinomycind 25            5.0 –  7.8
Streptomycine 25            6.2 –  7.8
Tetracycline 10            4.5 –  8.6
Tigecycline 15          9.7 –  12.6
Tobramycin 10            6.4 –  8.4
Trimethoprim 5          8.8 –  13.6

Escherichia coli NCTC 11560

(Sensitest, air, 35‑37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Augmentin 60            6.4 –  9.6
Timentin 85            6.0 –  8.4
Tazocin 55            7.4 –  9.2

Haemophilus influenzae NCTC 4560

(HTMf agar, 5% CO2, 35‑37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Ampicillin 5          7.0 –  11.1
Cefaclor 30          8.1 –  12.1
Cefotaxime 0.5          9.2 –  12.8
Cefotaxime 5        12.6  – 16.1
Cefpodoxime 10       10.9 –  14.1
Ceftriaxone 0.5          9.1 –  12.9
Ceftriaxone 5       13.3 –  16.3
Cefuroxime 30          8.3 –  12.8
Chloramphenicol 10       11.1 –  14.3
Ciprofloxacin 2.5       11.1 –  15.9
Cotrimoxazole 25         9.0 –  12.5
Moxifloxacin 2.5      10.6 –  15.2
Tetracycline 10         8.2 –  13.0

Haemophilus influenzae NCTC11315

(HTMf agar, 5% CO2, 35‑37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Augmentin 15         7.7 –  10.1

Pseudomonas aeruginosa NCTC 10662d

(Sensitest, air, 35‑37°C)

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Amikacin 30         7.4 –  10.6
Aztreonam 30         8.3 –  13.1
Cefepime 10         8.1 –  11.3
Cefpirome 10          8.1 –  10.6
Ceftazidime 10          7.5 –  11.9
Ciprofloxacin 2.5          8.9 –  14.5
Doripenem 10         12.6 – 16.6
Ertapenemg 10                 –
Fosfomycin 200        8.0 –    10.8
Gentamicin 10         5.5 –    9.5
Imipenem 10          7.9 –  10.3
Meropenem 5          9.7 –  14.8
Moxifloxacing 2.5               –
Norfloxacin 10       10.1 –  12.6
Polymyxin B 300 u          5.2 –    7.2
Ticarcillin 75          7.3 –  12.1
Tobramycin 10          7.0 –  10.6

a    Reference strain testing must be performed: (i) In conjunction with the clinical isolate (for Neisseria gonorrhoeae), or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.

b  The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar and discs. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.

c   Bacteroides fragilis (ATCC 25285) may be used as the reference strain when testing H. pylori against metronidazole. When testing anaerobic organisms,Clostridium perfringens (POW 2006) should be used as the reference organism.

d  If antibiotic discs are tested with Escherichia coli NCTC 11560, there is no need to test these against Pseudomonas aeruginosa NCTC 10662 as well and vice versa.

e    Antibiotic used in veterinary medicine only

f     Haemophilus Test Medium Base containing 15 mg/L freshly prepared Haematin and NAD.

g    Ertapenem and Moxifloxacin should be tested against E. coli NCTC 11560

 

 

Table 12.3c Anaerobes

Clostridium perfringens POW 2006

(Supplemented Brucella Medium Base, anaerobic, 35‑37°C)c

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Benzylpenicillin 0.5 u            6.3 –  7.9
Cefoxitin 10            7.3 –  9.7
Clindamycin 2            6.1 –  8.1
Meropenem 5       10.1 –  12.9
Metronidazole 5            4.3 –  6.7
Moxifloxacin 2.5            4.7 –  6.3

Bacteroides fragilis ATCC 25285

(Supplemented Brucella medium Base, anaerobic, 35‑37°C)c

Antibiotic reported Disc potency (µg) Annular radii (mm)b
Augmentin 3            5.1 –  8.3
Tazocin 55          9.0 –  12.2
Timentin 85       11.6 –  16.0

a    Reference strain testing must be performed: (i) In conjunction with the clinical isolate, or at least once weekly; (ii) When a new batch of medium is used; (iii) When a new batch of discs is used.

b The acceptable range (95% confidence limits) is the mean ± 2 standard deviations. The mean was derived from > 120 measurements with different operators using different batches of both agar and discs. It is statistically acceptable to use one hundred measurements to represent the “normal distribution” and this gives a confidence limit of 95%, meaning an in‑built MU of 5% for the test.

c  Brucella Medium Base containing 5% defibrinated horse blood, haemin 5 mg/L and vitamin K 1 mg/L

 

12.4. β‑lactam Testing Guide

Table 4 is a guide through the maze of testing and reporting the susceptibility of Gram‑negative aerobic species to β‑lactam antibiotics. The production of one or more β‑lactamases is an important and common mechanism of resistance in these species but, for several reasons, resistance may be difficult or impossible to demonstrate by the usual methods of antibiotic susceptibility testing. The table sets out, in some detail, those species where resistance should be assumed on the basis of previous documentation of the presence of a stable mechanism of resistance and those where susceptibility can be reliably demonstrated by the disc test. This table also relies heavily on the footnotes to draw attention to exceptions and special circumstances.

Table 12.4    A guide to the testing and reporting of β‑lactam antibiotics for Gram-negative organisms.

  Antibioticb,c
Organism AMP AMC ATM CAZ CXM,CL,CPD
CRO,CTX,KZ
CPO
FEP
CTT FOX IPM,MEM
ETP, DOR
PRL TIM TZP
Enterobacteriaceae of the EEC groupd R R R R R T R R T Na R R
Serratia marcescens R R T T R T R R T Na R T
Aeromonas with
A1 cephalosporinase
and A2 carbapenemase
R R T* T* R T R R Re Na R R
Aeromonas sobria/veroni with
A2 carbapenemase only
R R T T T T T T R  Na T T
Morganella morganii
R T*  T* T* T* T* T* T* T* Na T* T
Proteus vulgaris/pennerif R U T T T T T T Na U U
C.amalonaticus R U R T T T T T Na U U
Enterobacteriaceae with ESBL R U R R R R T R T Na U U
Enterobacteriaceae with an MBL R R T R R R R R R Na R R
Enterobacteriaceae with an AMP Ci R R  T T R T T T T Na T T
Pseudomonas species
Burkholderia
speciesg /
Chryseobacterium  species
R Rg T T R T R R T T T T
P. aeruginosa with ESBL R R R R R R R R T R U U
P. aeruginosa with MBL R R T R R R R R T R R R
Stenotrophomonas maltophiliah R R Rh Rh R R R R R Rh Rh Rh

 

a N = Not calibrated for this species.
b

R = The isolate is resistant to the antibiotic because it possesses a mechanism of resistance that may not be shown by a disc test.

T = Can be tested and reported. T* = Can be tested and reported. See sects 5.2 on Aeromonas &  5.5.7 on Morganella.

U = Test isolates from uncomplicated UTI only.  Isolates from other sites are  considered  as RESISTANT.

c AMP ampicillin CL cephalexin DOR doripenem KZ cefazolin
  AMC Augmentin CPD cefpodoxime ETP ertapenem MEM meropenem
  ATM aztreonam CPO cefpirome FEP cefepime PRL piperacillin
  CAZ ceftazidime CRO ceftriaxone FOX cefoxitin TIM Timentin
CXM cefuroxime CTT cefotetan IPM imipenem TZP Tazocin
  CTX cefotaxime
d Enterobacter aerogenes, Enterobacter cloacae complex, Citrobacter freundii complex.
e A. caviae does not possess a carbapenemase (A2) and can be tested against imipenem, meropenem, ertapenem and doripenem.
f Isolates with high β‑lactamase activity may give no zone around CTX 5 μg but show a “key‑hole” effect that may be mistaken as an indication of the presence of an ESBL. However, unlike ESBL producers, they may be susceptible to ceftazidime.
g B. pseudomallei is usually susceptible to Augmentin and can be tested against this antibiotic.
h See Section 5.10 for notes on testing S. maltophilia against these and other antibiotics.
i This group varies in susceptibility to betalactams – see sections 5.5.3 & 5.5.7.